Note: Descriptions are shown in the official language in which they were submitted.
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EFFICIENTLY ENCODING AND COMPRESSING ECG DATA OPTIMIZED FOR USE
IN AN AMBULATORY ECG MONITOR
TECHNICAL FIELD
This application relates in general to electrocardiographic monitoring and, in
particular,
to a method for efficiently encoding and compressing ECG data optimized for
use in an
ambulatory electrocardiography monitor.
BACKGROUND ART
The first electrocardiogram (ECG) was invented by a Dutch physiologist, Willem
Einthoven, in 1903, who used a string galvanometer to measure the electrical
activity of the
heart. Generations of physicians around the world have since used ECGs, in
various folliis, to
diagnose heart problems and other potential medical concerns. Although the
basic principles
.. underlying Dr. Einthoven's original work, including his naming of various
waveform deflections
(Einthoven's triangle), are still applicable today, ECG machines have evolved
from his original
three-lead ECG, to ECGs with unipolar leads connected to a central reference
terminal starting in
1934, to augmented unipolar leads beginning in 1942, and finally to the 12-
lead ECG
standardized by the American Heart Association in 1954 and still in use today.
Further advances
in portability and computerized interpretation have been made, yet the
electronic design of the
ECG recording apparatuses has remained fundamentally the same for much of the
past 40 years.
Essentially, an ECG measures the electrical signals emitted by the heart as
generated by
the propagation of the action potentials that trigger depolarization of heart
fibers.
Physiologically, transmembrane ionic currents are generated within the heart
during cardiac
activation and recovery sequences. Cardiac depolarization originates high in
the right atrium in
the sinoatrial (SA) node before spreading leftward towards the left atrium and
inferiorly towards
the atrioventricular (AV) node. After a delay occasioned by the AV node, the
depolarization
impulse transits the Bundle of His and moves into the right and left bundle
branches and Purkinje
fibers to activate the right and left ventricles.
During each cardiac cycle, the ionic currents create an electrical field in
and around the
heart that can be detected by ECG electrodes placed on the skin. Cardiac
electrical activity is
then visually represented in an ECG trace by PQRSTU-waveforms. The P-wave
represents atrial
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electrical activity, and the QRSTU components represent ventricular electrical
activity.
Specifically, a P-wave represents atrial depolarization, which causes atrial
contraction.
P-wave analysis based on ECG monitoring is critical to accurate cardiac rhythm
diagnosis and focuses on localizing the sites of origin and pathways of
arrhythmic conditions. P-
wave analysis is also used in the diagnosis of other medical disorders,
including imbalance of
blood chemistry. Cardiac arrhythmias are defined by the morphology of P-waves
and their
relationship to QRS intervals. For instance, atrial fibrillation (AF), an
abnormally rapid heart
rhythm, can be confirmed by an absence of P-waves and an irregular ventricular
rate. Similarly,
sinoatrial block is characterized by a delay in the onset of P-waves, while
junctional rhythm, an
abnormal heart rhythm resulting from impulses coming from a locus of tissue in
the area of the
AV node, usually presents without P-waves or with inverted P-waves. Also, the
amplitudes of P-
waves are valuable for diagnosis. The presence of broad, notched P-waves can
indicate left atrial
enlargement. Conversely, the presence of tall, peaked P-waves can indicate
right atrial
enlargement. Finally, P-waves with increased amplitude can indicate
hypokalemia, caused by
low blood potassium, whereas P-waves with decreased amplitude can indicate
hyperkalemia,
caused by elevated blood potassium.
Cardiac rhythm disorders may present with lightheadedness, fainting, chest
pain,
hypoxia, syncope, palpitations, and congestive heart failure (CHF), yet rhythm
disorders are
often sporadic in occurrence and may not show up in-clinic during a
conventional 12-second
ECG. Continuous ECG monitoring with P-wave-centric action potential
acquisition over an
extended period is more apt to capture sporadic cardiac events. However,
recording sufficient
ECG and related physiological data over an extended period remains a
significant challenge,
despite an over 40-year history of ambulatory ECG monitoring efforts combined
with no
appreciable improvement in P-wave acquisition techniques since Dr. Einthoven's
original
pioneering work over a 110 years ago.
Electrocardiographic monitoring over an extended period provides a physician
with the
kinds of data essential to identifying the underlying cause of sporadic
cardiac conditions,
especially rhythm disorders, and other physiological events of potential
concern. A 30-day
observation period is considered the "gold standard" of monitoring, yet a 14-
day observation
period is currently pitched as being achievable by conventional ECG monitoring
approaches.
Realizing a 30-day observation period has proven unworkable with existing ECG
monitoring
systems, which are arduous to employ; cumbersome, uncomfortable and not user-
friendly to the
patient; and costly to manufacture and deploy. Still, if a patient's ECG could
be recorded in an
ambulatory setting over a prolonged time periods, particularly for more than
14 days, thereby
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allowing the patient to engage in activities of daily living, the chances of
acquiring meaningful
medical information and capturing an abnormal event while the patient is
engaged in normal
activities are greatly improved.
The location of the atria and their low amplitude, low frequency content
electrical signals
make P-waves difficult to sense, particularly through ambulatory ECG
monitoring. The atria are
located posteriorly within the chest, and their physical distance from the
skin surface adversely
affects current strength and signal fidelity. Cardiac electrical potentials
measured dermally have
an amplitude of only one-percent of the amplitude of transmembrane electrical
potentials. The
distance between the heart and ECG electrodes reduces the magnitude of
electrical potentials in
proportion to the square of change in distance, which compounds the problem of
sensing low
amplitude P-waves. Moreover, the tissues and structures that lie between the
activation regions
within the heart and the body's surface alter the cardiac electrical field due
to changes in the
electrical resistivity of adjacent tissues. Thus, surface electrical
potentials, when even capable of
being accurately detected, are smoothed over in aspect and bear only a general
spatial
relationship to actual underlying cardiac events, thereby complicating
diagnosis. Conventional
12-lead ECGs attempt to compensate for weak P-wave signals by monitoring the
heart from
multiple perspectives and angles, while conventional ambulatory ECGs primarily
focus on
monitoring higher amplitude ventricular activity that can be readily sensed.
Both approaches are
unsatisfactory with respect to the P-wave and the accurate, medically
actionable diagnosis of the
myriad cardiac rhythm disorders that exist.
Additionally, maintaining continual contact between ECG electrodes and the
skin after a
day or two of ambulatory ECG monitoring has been a problem. Time, dirt,
moisture, and other
environmental contaminants, as well as perspiration, skin oil, and dead skin
cells from the
patient's body, can get between an ECG electrode's non-conductive adhesive and
the skin's
surface. These factors adversely affect electrode adhesion and the quality of
cardiac signal
recordings. Furthermore, the physical movements of the patient and their
clothing impart
various compressional, tensile, bending, and torsional forces on the contact
point of an ECG
electrode, especially over long recording times, and an inflexibly fastened
ECG electrode will be
prone to becoming dislodged. Moreover, dislodgment may occur unbeknownst to
the patient,
making the ECG recordings worthless. Further, some patients may have skin that
is susceptible
to itching or irritation, and the wearing of ECG electrodes can aggravate such
skin conditions.
Thus, a patient may want or need to periodically remove or replace ECG
electrodes during a
long-term ECG monitoring period, whether to replace a dislodged electrode,
reestablish better
adhesion, alleviate itching or irritation, allow for cleansing of the skin,
allow for showering and
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exercise, or for other purpose. Such replacement or slight alteration in
electrode location
actually facilitates the goal of recording the ECG signal for long periods of
time.
Conventionally, multi-week or multi-month monitoring can be performed by
implantable
ECG monitors, such as the Reveal LINQ insertable cardiac monitor, manufactured
by Medtronic,
Inc., Minneapolis, MN. This monitor can detect and record paroxysmal or
asymptomatic
arrhythmias for up to three years. However, like all forms of implantable
medical device (IMD),
use of this monitor requires invasive surgical implantation, which
significantly increases costs;
requires ongoing follow up by a physician throughout the period of
implantation; requires
specialized equipment to retrieve monitoring data; and carries complications
attendant to all
surgery, including risks of infection, injury or death.
Holter monitors are widely used for extended ECG monitoring. Typically, they
are often
used for only 24-48 hours. A typical Holter monitor is a wearable and portable
version of an
ECG that include cables for each electrode placed on the skin and a separate
battery-powered
ECG recorder. The leads are placed in the anterior thoracic region in a manner
similar to what is
done with an in-clinic standard ECG machine using electrode locations that are
not specifically
intended for optimal P-wave capture. The duration of monitoring depends on the
sensing and
storage capabilities of the monitor. A "looping" Holter (or event) monitor can
operate for a
longer period of time by overwriting older ECG tracings, thence "recycling"
storage in favor of
extended operation, yet at the risk of losing event data. Although capable of
extended ECG
monitoring, Holter monitors are cumbersome, expensive and typically only
available by medical
prescription, which limits their usability. Further, the skill required to
properly place the
electrodes on the patient's chest precludes a patient from replacing or
removing the sensing leads
and usually involves moving the patient from the physician office to a
specialized center within
the hospital or clinic.
U.S. Patent No. 8,460,189, to Libbus et al. ("Libbus") discloses an adherent
wearable
cardiac monitor that includes at least two measurement electrodes and an
accelerometer. The
device includes a reusable electronics module and a disposable adherent patch
that includes the
electrodes. ECG monitoring can be conducted using multiple disposable patches
adhered to
different locations on the patient's body. The device includes a processor
configured to control
collection and transmission of data from ECG circuitry, including generating
and processing of
ECG signals and data acquired from two or more electrodes. The ECG circuitry
can be coupled
to the electrodes in many ways to define an ECG vector, and the orientation of
the ECG vector
can be determined in response to the polarity of the measurement electrodes
and orientation of
the electrode measurement axis. The accelerometer can be used to determine the
orientation of
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the measurement electrodes in each of the locations. The ECG signals measured
at different
locations can be rotated based on the accelerometer data to modify amplitude
and direction of the
ECG features to approximate a standard ECG vector. The signals recorded at
different locations
can be combined by summing a scaled version of each signal. Libbus further
discloses that inner
ECG electrodes may be positioned near outer electrodes to increase the voltage
of measured
ECG signals. However. Libbus treats ECG signal acquisition as the measurement
of a simple
aggregate directional data signal without differentiating between the distinct
kinds of cardiac
electrical activities presented with an ECG waveform, particularly atrial (P-
wave) activity.
The ZIOTM XT Patch and ZIOTM Event Card devices, manufactured by iRhythm
Tech.,
Inc., San Francisco, CA, are wearable monitoring devices that are typically
worn on the upper
left pectoral region to respectively provide continuous and looping ECG
recording. The location
is used to simulate surgically implanted monitors, but without specifically
enhancing P-wave
capture. Both of these devices are prescription-only and for single patient
use. The ZIOTM XT
Patch device is limited to a 14-day period, while the electrodes only of the
ZIOTM Event Card
device can be worn for up to 30 days. The ZIOTM XT Patch device combines both
electronic
recordation components and physical electrodes into a unitary assembly that
adheres to the
patient's skin. The ZIOTM XT Patch device uses adhesive sufficiently strong to
support the
weight of both the monitor and the electrodes over an extended period and to
resist disadherence
from the patient's body, albeit at the cost of disallowing removal or
relocation during the
monitoring period. The Z101"1 Event Card device is a form of downsized Holter
monitor with a
recorder component that must be removed temporarily during baths or other
activities that could
damage the non-waterproof electronics. Both devices represent compromises
between length of
wear and quality of ECG monitoring, especially with respect to ease of long
term use, female-
friendly fit, and quality of cardiac electrical potential signals, especially
atrial (P-wave) signals.
ECG signals contain a large amount of information that requires large storage
space,
large transmission bandwidth, and long transmission time. Long-term ECG
monitoring further
increases the amount of information to be stored and processed. Data
compression is useful in
ECG applications, especially long-term monitoring. Data compression can reduce
the
requirement for data storage space, reduce power consumption, and extends
monitoring time.
ECG compression can be evaluated based on compression ratio, signal error
loss, and time of
= execution. A good ECG data compression preferably should preserve the
useful diagnostic
information while compressing a signal to a smaller acceptable size.
Currently, many Holter
monitors use some types of compression algorithm; however, compression ratios
are not
satisfactory.
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Therefore, a need remains for a low cost extended wear continuously recording
ECG
monitor that is low power and storage space and data transmission efficient,
therefore
contributing to long-teim use
DISCLOSURE OF THE INVENTION
Physiological monitoring can be provided through a lightweight wearable
monitor that
includes two components, a flexible extended wear electrode patch and a
reusable monitor
recorder that removably snaps into a receptacle on the electrode patch. The
wearable monitor
sits centrally (in the midline) on the patient's chest along the sternum
oriented top-to-bottom
The ECG electrodes on the electrode patch are tailored to be positioned
axially along the midline
of the sternum for capturing action potential propagation in an orientation
that corresponds to the
aVF lead used in a conventional 12-lead ECG that is used to sense positive or
upright P-waves.
The placement of the wearable monitor in a location at the sternal midline (or
immediately to
either side of the sternum), with its unique narrow "hourglass"-like shape,
significantly improves
the ability of the wearable monitor to cutaneously sense cardiac electrical
potential signals,
particularly the P-wave (or atrial activity) and, to a lesser extent, the QRS
interval signals
indicating ventricular activity in the ECG waveforms
Moreover, the electrocardiography monitor offers superior patient comfort,
convenience
and user-friendliness. The electrode patch is specifically designed for ease
of use by a patient (or
caregiver); assistance by professional medical personnel is not required. The
patient is free to
replace the electrode patch at any time and need not wait for a doctor's
appointment to have a
new electrode patch placed. Patients can easily be taught to find the familiar
physical landmarks
on the body necessary for proper placement of the electrode patch. Empowering
patients with
the knowledge to place the electrode patch in the right place ensures that the
ECG electrodes will
be correctly positioned on the skin, no matter the number of times that the
electrode patch is
replaced. In addition, the monitor recorder operates automatically and the
patient only need snap
the monitor recorder into place on the electrode patch to initiate ECG
monitoring. Thus, the
synergistic combination of the electrode patch and monitor recorder makes the
use of the
electrocardiography monitor a reliable and virtually foolproof way to monitor
a patient's ECG
and physiology for an extended, or even open-ended, period of time.
Furthermore, the ECG data collected during the long-term monitoring are
compressed
through a two-step compression algorithm executed by the electrocardiography
monitor. The
algorithm generates a compression ratio significantly higher than other Holter-
type monitors
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Requirement for storage space and power cell consumption are reduced,
contributing to the long-
term availability of the monitor and efficient transmission of recorded data
post-processing.
One embodiment provides a computer-implemented method for encoding and
compressing electrocardiography values. A series of electrocardiography values
is obtained. A
plurality of bins are defined, each bin comprising a lower threshold ECG
value, an upper
threshold ECG value, and a code for the bin. A serial accumulator is set to a
pre-determined
value, such as a center value of an ECG recorder. For each of the
electrocardiography values
remaining in the series of the electrocardiography values, a recursive process
is performed that
includes the following processes: selecting the electrocardiography value next
remaining in the
series of the electrocardiography values; taking a difference of the selected
electrocardiography
value and the serial accumulator; identifying the bin in the plurality of the
bins corresponding to
the difference; representing the selected electrocardiography value by the
code for the identified
bin; and adjusting the serial accumulator by a value derived from the
identified bin. The
resulting string of represented codes are written into a sequence in a non-
volatile memory.
Another embodiment provides a computer-implemented method for encoding and
compressing electrocardiography values. A series of electrocardiography values
is obtained. A
plurality of bins are defined, each bin comprising a lower threshold ECG
value, an upper
threshold ECG value, and a code for the bin. A serial accumulator is set to a
pre-determined
value, such as a center value of an ECG recorder. For each of the
electrocardiography values
remaining in the series of the electrocardiography values, a recursive process
is performed that
includes the following processes: selecting the electrocardiography value next
remaining in the
series of electrocardiography values; taking a difference of the selected
electrocardiography
value and the serial accumulator; identifying the bin in the plurality of the
bins corresponding to
the difference; representing the selected electrocardiography value by the
code for the identified
bin; and adjusting the serial accumulator by a value derived from the
identified bin. The
recursive process generates a sequence of represented codes. The sequence of
represented codes
is further encoded in the form of a single number between 0 and 1, through the
following steps:
setting a range for an initial code from the sequence of the represented
codes, processing each of
the codes remaining in the sequence of the represented codes, by a recursive
process of:
obtaining an estimation of probabilities of next codes; dividing the range
into sub-ranges, each
sub-range representing a fraction of the range proportional to the
probabilities of the next codes;
obtaining a next code; selecting the sub-range corresponding to the next code;
representing the
next code by the selected sub-range; substituting the selected sub-range in
place of the range and
continuing the steps of the process using the selected sub-range in place of
the range. During the
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process, strings of codes represented by the selected sub-ranges are encoded
into part of the
single number between 0 and 1 and can be periodically or continually stored
into the non-volatile
memory, can be stored on-demand or as-needed, or can be queued up and stored
en masse upon
completion of the process.
When only the first compression step is performed, unnecessary data in ECG
data are
filtered out, and the number of codes needed to encode the ECG data are
reduced. When the
second compression step is combined with the first step, further memory
consumption is
avoided.
The foregoing aspects enhance ECG monitoring performance and quality by
facilitating
long-term ECG recording, which is critical to accurate arrhythmia and cardiac
rhythm disorder
diagnoses.
The monitoring patch is especially suited to the female anatomy, although also
easily
used over the male sternum. The narrow longitudinal midsection can fit nicely
within the inter-
mammary cleft of the breasts without inducing discomfort, whereas conventional
patch
electrodes are wide and, if adhered between the breasts, would cause chafing,
irritation,
discomfort, and annoyance, leading to low patient compliance.
In addition, the foregoing aspects enhance comfort in women (and certain men),
but not
irritation of the breasts, by placing the monitoring patch in the best
location possible for
optimizing the recording of cardiac signals from the atrium, particularly P-
waves, which is
another feature critical to proper arrhythmia and cardiac rhythm disorder
diagnoses
Still other embodiments will become readily apparent to those skilled in the
art from the
following detailed description, wherein are described embodiments by way of
illustrating the
best mode contemplated. As will be realized, other and different embodiments
are possible and
the embodiments' several details are capable of modifications in various
obvious respects, all
without departing from their spirit and the scope. Accordingly, the drawings
and detailed
description are to be regarded as illustrative in nature and not as
restrictive.
DESCRIPTION OF THE DRAWINGS
FIGURES 1 and 2 are diagrams showing, by way of examples, an extended wear
electrocardiography monitor, including an extended wear electrode patch, in
accordance with
one embodiment, respectively fitted to the sternal region of a female patient
and a male patient.
FIGURE 3 is a front anatomical view showing, by way of illustration, the
locations of the
heart and lungs within the rib cage of an adult human
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FIGURE 4 is a perspective view showing an extended wear electrode patch in
accordance
with one embodiment with a monitor recorder inserted.
FIGURE 5 is a perspective view showing the monitor recorder of FIGURE 4.
FIGURE 6 is a perspective view showing the extended wear electrode patch of
FIGURE
4 without a monitor recorder inserted.
FIGURE 7 is a bottom plan view of the monitor recorder of FIGURE 4.
FIGURE 8 is a top view showing the flexible circuit of the extended wear
electrode patch
of FIGURE 4.
FIGURE 9 is a functional block diagram showing the component architecture of
the
circuitry of the monitor recorder of FIGURE 4.
FIGURE 10 is a functional block diagram showing the circuitry of the extended
wear
electrode patch of FIGURE 4.
FIGURE 11 is a schematic diagram showing the ECG front end circuit of the
circuitry of
the monitor recorder of FIGURE 9.
FIGURE 12 is a flow diagram showing a monitor recorder-implemented method for
monitoring ECG data for use in the monitor recorder of FIGURE 4.
FIGURE 13 is a graph showing, by way of example, a typical ECG waveform
FIGURE 14 is a functional block diagram showing the signal processing
functionality of
the microcontroller.
FIGURE 15 is a functional block diagram showing the operations performed by
the
download station
FIGURES 16A-C are functional block diagrams respectively showing practical
uses of
the extended wear electrocardiography monitors of FIGURES 1 and 2.
FIGURE 17 is a perspective view of an extended wear electrode patch with a
flexile wire
electrode assembly in accordance with a still further embodiment.
FIGURE 18 is perspective view of the flexile wire electrode assembly from
FIGURE 17,
with a layer of insulating material shielding a bare distal wire around the
midsection of the
flexible backing.
FIGURE 19 is a bottom view of the flexile wire electrode assembly as shown in
FIGURE
17.
FIGURE 20 is a bottom view of a flexile wire electrode assembly in accordance
with a
still yet further embodiment.
FIGURE 21 is a perspective view showing the longitudinal midsection of the
flexible
backing of the electrode assembly from FIGURE 17
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FIGURE 22 is a flow diagram showing a monitor recorder-implemented method for
ECG
signal processing and ECG data compressing for use in the monitor recorders of
FIGURE 4.
FIGURE 23 is a flow diagram showing a monitor recorder-implemented method for
encoding ECG values.
FIGURE 24 is an example of a panel of codes or encodings with each code
covering a
range defined by a lower threshold ECG value and an upper threshold ECG value.
FIGURE 25 is an illustrating the encoding and compression scheme in accordance
with
method and parameters as described with reference to in FIGURES 23 and 24.
FIGURE 26 is a flow diagram showing a monitor recorder-implemented method for
further compressing the encodings.
BEST MODE FOR CARRYING OUT THE INVENTION
ECG and physiological monitoring can be provided through a wearable ambulatory
monitor that includes two components, a flexible extended wear electrode patch
and a removable
reusable (or single use) monitor recorder. Both the electrode patch and the
monitor recorder are
optimized to capture electrical signals from the propagation of low amplitude,
relatively low
frequency content cardiac action potentials, particularly the P-waves
generated during atrial
activation. FIGURES 1 and 2 are diagrams showing, by way of examples, an
extended wear
electrocardiography monitor 12, including a monitor recorder 14, in accordance
with one
embodiment, respectively fitted to the sternal region of a female patient 10
and a male patient 11.
The wearable monitor 12 sits centrally, positioned axially along the sternal
midline 16, on the
patient's chest along the sternum 13 and oriented top-to-bottom with the
monitor recorder 14
preferably situated towards the patient's head. In a further embodiment, the
orientation of the
wearable monitor 12 can be corrected post-monitoring, as further described
infra, for instance, if
the wearable monitor 12 is inadvertently fitted upside down.
The electrode patch 15 is shaped to fit comfortably and conformal to the
contours of the
patient's chest approximately centered on the sternal midline 16 (or
immediately to either side of
the sternum 13). The distal end of the electrode patch 15, under which a lower
or inferior pole
(ECG electrode) is adhered, extends towards the Xiphoid process and lower
sternum and,
depending upon the patient's build, may straddle the region over the Xiphoid
process and lower
sternum. The proximal end of the electrode patch 15, located under the monitor
recorder 14,
under which an upper or superior pole (ECG electrode) is adhered, is below the
manubrium and,
depending upon patient's build, may straddle the region over the manub ri um .
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During ECG monitoring, the amplitude and strength of action potentials sensed
on the
body's surface are affected to varying degrees by cardiac, cellular,
extracellular, vector of current
flow, and physical factors, like obesity, dermatitis, large breasts, and high
impedance skin, as can
occur in dark-skinned individuals. Sensing along the sternal midline 16 (or
immediately to either
side of the sternum 13) significantly improves the ability of the wearable
monitor 12 to
cutaneously sense cardiac electric signals, particularly the P-wave (or atrial
activity) and, to a
lesser extent, the QRS interval signals in the ECG waveforms that indicate
ventricular activity by
countering some of the effects of these factors.
The ability to sense low amplitude, low frequency content body surface
potentials is
directly related to the location of ECG electrodes on the skin's surface and
the ability of the
sensing circuitry to capture these electrical signals. FIGURE 3 is a front
anatomical view
showing, by way of illustration, the locations of the heart 4 and lungs 5
within the rib cage of an
adult human. Depending upon their placement locations on the chest, ECG
electrodes may be
separated from activation regions within the heart 4 by differing combinations
of internal tissues
and body structures, including heart muscle, intracardiac blood, the
pericardium, intrathoracic
blood and fluids, the lungs 5, skeletal muscle, bone structure, subcutaneous
fat, and the skin, plus
any contaminants present between the skin's surface and electrode signal
pickups. The degree of
amplitude degradation of cardiac transmembrane potentials increases with the
number of tissue
boundaries between the heart 4 and the skin's surface that are encountered.
The cardiac
electrical field is degraded each time the transmembrane potentials encounter
a physical
boundary separating adjoining tissues due to differences in the respective
tissues' electrical
resistances. In addition, other non-spatial factors, such as pericardial
effusion, emphysema or
fluid accumulation in the lungs, as further explained infra, can further
degrade body surface
potentials.
Internal tissues and body structures can adversely affect the current strength
and signal
fidelity of all body surface potentials, yet low amplitude cardiac action
potentials, particularly
the P-wave with a normative amplitude of less than 0.25 microvolts (mV) and a
normative
duration of less than 120 milliseconds (ms), are most apt to be negatively
impacted. The atria 6
are generally located posteriorly within the thoracic cavity (with the
exception of the anterior right
atrium and right atrial appendage), and, physically, the left atrium
constitutes the portion of the heart
4 furthest away from the surface of the skin on the chest. Conversely, the
ventricles 7, which
generate larger amplitude signals, generally are located anteriorly with the
anterior right ventricle and
most of the left ventricle situated relatively close to the skin surface on
the chest, which contributes
to the relatively stronger amplitudes of ventricular waveforms. Thus, the
quality of P-waves (and
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other already-low amplitude action potential signals) is more susceptible to
weakening from
intervening tissues and structures than the waveforms associated with
ventricular activation.
The importance of the positioning of ECG electrodes along the sternal midline
15 has
largely been overlooked by conventional approaches to ECG monitoring, in part
due to the
inability of their sensing circuitry to reliably detect low amplitude, low
frequency content
electrical signals, particularly in P-waves. In turn, that inability to keenly
sense P-waves has
motivated ECG electrode placement in other non-sternal midline thoracic
locations, where the
QRSTU components that represent ventricular electrical activity are more
readily detectable by
their sensing circuitry than P-waves. In addition, ECG electrode placement
along the sternal
midline 15 presents major patient wearability challenges, such as fitting a
monitoring ensemble
within the narrow confines of the inter-mammary cleft between the breasts,
that to large extent
drive physical packaging concerns, which can be incompatible with ECG monitors
intended for
placement, say, in the upper pectoral region or other non-sternal midline
thoracic locations. In
contrast, the wearable monitor 12 uses an electrode patch 15 that is
specifically intended for
extended wear placement in a location at the sternal midline 16 (or
immediately to either side of
the sternum 13). When combined with a monitor recorder 14 that uses sensing
circuitry
optimized to preserve the characteristics of low amplitude cardiac action
potentials, especially
those signals from the atria, as further described infra with reference to
FIGURE 11, the
electrode patch 15 helps to significantly improve atrial activation (P-wave)
sensing through
placement in a body location that robustly minimizes the effects of tissue and
body structure.
Referring back to FIGURES 1 and 2, the placement of the wearable monitor 12 in
the
region of the sternal midline 13 puts the ECG electrodes of the electrode
patch 15 in locations
better adapted to sensing and recording low amplitude cardiac action
potentials during atrial
propagation (P-wave signals) than placement in other locations, such as the
upper left pectoral
region, as commonly seen in most conventional ambulatory ECG monitors. The
sternum 13
overlies the right atrium of the heart 4. As a result, action potential
signals have to travel through
fewer layers of tissue and structure to reach the ECG electrodes of the
electrode patch 15 on the
body's surface along the sternal midline 13 when compared to other monitoring
locations, a
distinction that is of critical importance when capturing low frequency
content electrical signals,
such as P-waves.
Moreover, cardiac action potential propagation travels simultaneously along a
north-to-
south and right-to-left vector, beginning high in the right atrium and
ultimately ending in the
posterior and lateral region of the left ventricle. Cardiac depolarization
originates high in the
right atrium in the SA node before concurrently spreading leftward towards the
left atrium and
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inferiorly towards the AV node. The ECG electrodes of the electrode patch 15
are placed with
the upper or superior pole (ECG electrode) along the sternal midline 13 in the
region of the
manubrium and the lower or inferior pole (ECG electrode) along the sternal
midline 13 in the
region of the Xiphoid process 9 and lower sternum. The ECG electrodes are
placed primarily in
a north-to-south orientation along the sternum 13 that corresponds to the
north-to-south
waveform vector exhibited during atrial activation. This orientation
corresponds to the aVF lead
used in a conventional 12-lead ECG that is used to sense positive or upright P-
waves.
Furthermore, the thoracic region underlying the sternum 13 along the midline
16 between
the manubrium 8 and Xiphoid process 9 is relatively free of lung tissue,
musculature, and other
internal body structures that could occlude the electrical signal path between
the heart 4,
particularly the atria, and ECG electrodes placed on the surface of the skin.
Fewer obstructions
means that cardiac electrical potentials encounter fewer boundaries between
different tissues. As
a result, when compared to other thoracic ECG sensing locations, the cardiac
electrical field is
less altered when sensed dermally along the sternal midline 13. As well, the
proximity of the
sternal midline 16 to the ventricles 7 facilitates sensing of right
ventricular activity and provides
superior recordation of the QRS interval, again, in part due to the relatively
clear electrical path
between the heart 4 and the skin surface.
Finally, non-spatial factors can affect transmembrane action potential shape
and
conductivity. For instance, myocardial ischemia, an acute cardiac condition,
can cause a
transient increase in blood perfusion in the lungs 5. The perfused blood can
significantly
increase electrical resistance across the lungs 5 and therefore degrade
transmission of the cardiac
electrical field to the skin's surface. However, the placement of the wearable
monitor 12 along
the sternal midline 16 in the inter-mammary cleft between the breasts is
relatively resilient to the
adverse effects to cardiac action potential degradation caused by ischemic
conditions as the body
surface potentials from a location relatively clear of underlying lung tissue
and fat help
compensate for the loss of signal amplitude and content. The monitor recorder
14 is thus able to
record the P-wave morphology that may be compromised by myocardial ischemia
and therefore
make diagnosis of the specific arrhythmias that can be associated with
myocardial ischemia more
difficult.
During use, the electrode patch 15 is first adhered to the skin along the
sternal midline 16
(or immediately to either side of the sternum 13). A monitor recorder 14 is
then snapped into
place on the electrode patch 15 using an electro mechanical docking interface
to initiate ECG
monitoring. FIGURE 4 is a perspective view showing an extended wear electrode
patch 15 in
accordance with one embodiment with a monitor recorder 14 inserted. The body
of the electrode
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patch 15 is preferably constructed using a flexible backing 20 formed as an
elongated strip 21 of
wrap knit or similar stretchable material about 145mm long and 32mm at the
widest point with a
narrow longitudinal mid-section 23 evenly tapering inward from both sides. A
pair of cut-outs
22 between the distal and proximal ends of the electrode patch 15 create a
narrow longitudinal
midsection 23 or "isthmus" and defines an elongated "hourglass"-like shape,
when viewed from
above, such as described in commonly-assigned U.S. Design Patent application,
entitled
"Extended Wear Electrode Patch," Serial No. 29/472,045, filed November 7,
2013,
The upper part of the "hourglass" is sized to
allow an electrically non-conductive receptacle 25, sits on top of the outward-
facing surface of
the electrode patch 15, to be affixed to the electrode patch 15 with an ECG
electrode placed
underneath on the patient-facing underside, or contact, surface of the
electrode patch 15; the
upper part of the "hourglass" has a longer and wider profile (but still
rounded and tapered to fit
comfortably between the breasts) than the lower part of the "hourglass," which
is sized primarily
to allow just the placement of an ECG electrode of appropriate shape and
surface area to record
the P-wave and the QRS signals sufficiently given the inter-electrode spacing.
The electrode patch 15 incorporates features that significantly improve
wearability,
performance, and patient comfort throughout an extended monitoring period. The
entire
electrode patch 15 is lightweight in construction, which allows the patch to
be resilient to
disadhesing or falling off and, critically, to avoid creating distracting
discomfort to the patient,
even when the patient is asleep. In contrast, the weight of a heavy ECG
monitor impedes patient
mobility and will cause the monitor to constantly tug downwards and press on
the patient's body
that can generate skin inflammation with frequent adjustments by the patient
needed to maintain
comfort.
During everyday wear, the electrode patch 15 is subjected to pushing, pulling,
and
torsional movements, including compressional and torsional forces when the
patient bends
forward, or tensile and torsional forces when the patient leans backwards. To
counter these
stress forces, the electrode patch 15 incorporates crimp and strain reliefs,
such as described in
commonly-assigned U.S. Patent application, entitled "Extended Wear
Electrocardiography
Patch," Serial No. 14/080,717, filed November 14, 2013.
In addition, the cut-outs 22 and longitudinal midsection 23 help
minimize interference with and discomfort to breast tissue, particularly in
women (and
gynecomastic men). The cut-outs 22 and longitudinal midsection 23 further
allow better
conformity of the electrode patch 15 to sternal bowing and to the narrow
isthmus of flat skin that
can occur along the bottom of the inter-mammary cleft between the breasts,
especially in buxom
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women. The cut-outs 22 and narrow and flexible longitudinal midsection 23 help
the electrode
patch 15 fit nicely between a pair of female breasts in the inter-mammary
cleft. In one
embodiment, the cut-outs 22 can be graduated to form the longitudinal
midsection 23 as a narrow
in-between stem or isthmus portion about 7mm wide. In a still further
embodiment, tabs 24 can
respectively extend an additional 8mm to 12mm beyond the distal and proximal
ends of the
flexible backing 20 to facilitate with adhering the electrode patch 15 to or
removing the electrode
patch 15 from the sternum 13. These tabs preferably lack adhesive on the
underside, or contact,
surface of the electrode patch 15. Still other shapes, cut-outs and
conformities to the electrode
patch 15 are possible.
The monitor recorder 14 removably and reusably snaps into an electrically non-
conductive receptacle 25 during use. The monitor recorder 14 contains
electronic circuitry for
recording and storing the patient's electrocardiography as sensed via a pair
of ECG electrodes
provided on the electrode patch 15, as further described infra beginning with
reference to
FIGURE 9. The non-conductive receptacle 25 is provided on the top surface of
the flexible
backing 20 with a retention catch 26 and tension clip 27 molded into the non-
conductive
receptacle 25 to conformably receive and securely hold the monitor recorder 14
in place.
The monitor recorder 14 includes a sealed housing that snaps into place in the
non-
conductive receptacle 25. FIGURE 5 is a perspective view showing the monitor
recorder 14 of
FIGURE 4. The sealed housing 50 of the monitor recorder 14 intentionally has a
rounded
isosceles trapezoidal-like shape 52, when viewed from above, such as described
in commonly-
assigned U.S. Design Patent application, entitled "Electrocardiography
Monitor," Serial No.
29/472,046, filed November 7, 2013.
The edges 51 along the top and bottom surfaces are rounded for patient
comfort. The
sealed housing 50 is approximately 47 mm long, 23 mm wide at the widest point,
and 7 mm
high, excluding a patient-operable tactile-feedback button 55. The sealed
housing 50 can be
molded out of polycarbonate, ABS, or an alloy of those two materials. The
button 55 is
waterproof and the button's top outer surface is molded silicon rubber or
similar soft pliable
material. A retention detent 53 and tension detent 54 are molded along the
edges of the top
surface of the housing 50 to respectively engage the retention catch 26 and
the tension clip 27
molded into non-conductive receptacle 25. Other shapes, features, and
conformities of the sealed
housing 50 are possible.
The electrode patch 15 is intended to be disposable, while the monitor
recorder 14 is
designed for reuse and can be transferred to successive electrode patches 15
to ensure continuity
of monitoring, if so desired. The monitor recorder 14 can be used only once,
but single use
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effectively wastes the synergistic benefits provided by the combination of the
disposable
electrode patch and reusable monitor recorder, as further explained infra with
reference to
FIGURES 16A-C. The placement of the wearable monitor 12 in a location at the
sternal midline
16 (or immediately to either side of the sternum 13) benefits long-term
extended wear by
removing the requirement that ECG electrodes be continually placed in the same
spots on the
skin throughout the monitoring period. Instead, the patient is free to place
an electrode patch 15
anywhere within the general region of the sternum 13.
As a result, at any point during ECG monitoring, the patient's skin is able to
recover from
the wearing of an electrode patch 15, which increases patient comfort and
satisfaction, while the
monitor recorder 14 ensures ECG monitoring continuity with minimal effort. A
monitor
recorder 14 is merely unsnapped from a worn out electrode patch 15, the worn
out electrode
patch 15 is removed from the skin, a new electrode patch 15 is adhered to the
skin, possibly in a
new spot immediately adjacent to the earlier location, and the same monitor
recorder 14 is
snapped into the new electrode patch 15 to reinitiate and continue the ECG
monitoring.
During use, the electrode patch 15 is first adhered to the skin in the sternal
region.
FIGURE 6 is a perspective view showing the extended wear electrode patch 15 of
FIGURE 4
without a monitor recorder 14 inserted. A flexible circuit 32 is adhered to
each end of the
flexible backing 20. A distal circuit trace 33 from the distal end 30 of the
flexible backing 20
and a proximal circuit trace (not shown) from the proximal end 31 of the
flexible backing 20
electrically couple ECG electrodes (not shown) with a pair of electrical pads
34. In a further
embodiment, the distal and proximal circuit traces are replaced with
interlaced or sewn-in
flexible wires, as further described infra beginning with reference to FIGURE
17. The electrical
pads 34 are provided within a moisture-resistant seal 35 formed on the bottom
surface of the
non-conductive receptacle 25. When the monitor recorder 14 is securely
received into the non-
conductive receptacle 25, that is, snapped into place, the electrical pads 34
interface to electrical
contacts (not shown) protruding from the bottom surface of the monitor
recorder 14. The
moisture-resistant seal 35 enables the monitor recorder 14 to be worn at all
times, even during
showering or other activities that could expose the monitor recorder 14 to
moisture or adverse
conditions.
In addition, a battery compartment 36 is formed on the bottom surface of the
non-
conductive receptacle 25. A pair of battery leads (not shown) from the battery
compartment 36
to another pair of the electrical pads 34 electrically interface the battery
to the monitor recorder
14. The battery contained within the battery compartment 35 is a direct
current (DC) power cell
and can be replaceable, rechargeable or disposable.
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The monitor recorder 14 draws power externally from the battery provided in
the non-
conductive receptacle 25, thereby uniquely obviating the need for the monitor
recorder 14 to
carry a dedicated power source. FIGURE 7 is a bottom plan view of the monitor
recorder 14 of
FIGURE 4. A cavity 58 is formed on the bottom surface of the sealed housing 50
to
accommodate the upward projection of the battery compartment 36 from the
bottom surface of
the non-conductive receptacle 25, when the monitor recorder 14 is secured in
place on the non-
conductive receptacle 25. A set of electrical contacts 56 protrude from the
bottom surface of the
sealed housing 50 and are arranged in alignment with the electrical pads 34
provided on the
bottom surface of the non-conductive receptacle 25 to establish electrical
connections between
the electrode patch 15 and the monitor recorder 14. In addition, a seal
coupling 57
circumferentially surrounds the set of electrical contacts 56 and securely
mates with the
moisture-resistant seal 35 formed on the bottom surface of the non-conductive
receptacle 25.
The battery contained within the battery compartment 36 can be replaceable,
rechargeable or
disposable. In a further embodiment, the ECG sensing circuitry of the monitor
recorder 14 can
be supplemented with additional sensors, including an Sp02 sensor, a blood
pressure sensor, a
temperature sensor, respiratory rate sensor, a glucose sensor, an air flow
sensor, and a volumetric
pressure sensor, which can be incorporated directly into the monitor recorder
14 or onto the non-
conductive receptacle 25.
The placement of the flexible backing 20 on the sternal midline 16 (or
immediately to
either side of the sternum 13) also helps to minimize the side-to-side
movement of the wearable
monitor 12 in the left- and right-handed directions during wear. However, the
wearable monitor
12 is still susceptible to pushing, pulling, and torqueing movements,
including compressional
and torsional forces when the patient bends forward, and tensile and torsional
forces when the
patient leans backwards or twists. To counter the dislodgment of the flexible
backing 20 due to
compressional and torsional forces, a layer of non-irritating adhesive, such
as hydrocolloid, is
provided at least partially on the underside, or contact, surface of the
flexible backing 20, but
only on the distal end 30 and the proximal end 31. As a result, the underside,
or contact surface
of the longitudinal midsection 23 does not have an adhesive layer and remains
free to move
relative to the skin. Thus, the longitudinal midsection 23 forms a crimp
relief that respectively
facilitates compression and twisting of the flexible backing 20 in response to
compressional and
torsional forces. Other forms of flexible backing crimp reliefs are possible.
Unlike the flexible backing 20, the flexible circuit 32 is only able to bend
and cannot
stretch in a planar direction. The flexible circuit 32 can be provided either
above or below the
flexible backing 20. FIGURE 8 is a top view showing the flexible circuit 32 of
the extended
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wear electrode patch 15 of FIGURE 4 when mounted above the flexible backing
20. A distal
ECG electrode 38 and proximal ECG electrode 39 are respectively coupled to the
distal and
proximal ends of the flexible circuit 32 to serve as electrode signal pickups.
The flexible circuit
32 preferably does not extend to the outside edges of the flexible backing 20,
thereby avoiding
gouging or discomforting the patient's skin during extended wear, such as when
sleeping on the
side. During wear, the ECG electrodes 38, 39 must remain in continual contact
with the skin. A
strain relief 40 is defined in the flexible circuit 32 at a location that is
partially underneath the
battery compartment 36 when the flexible circuit 32 is affixed to the flexible
backing 20. The
strain relief 40 is laterally extendable to counter dislodgment of the ECG
electrodes 38, 39 due to
bending, tensile and torsional forces. A pair of strain relief cutouts 41
partially extend
transversely from each opposite side of the flexible circuit 32 and continue
longitudinally
towards each other to define in 'S'-shaped pattern, when viewed from above.
The strain relief
respectively facilitates longitudinal extension and twisting of the flexible
circuit 32 in response
to tensile and torsional forces. Other forms of circuit board strain relief
are possible.
ECG monitoring and other functions performed by the monitor recorder 14 are
provided
through a micro controlled architecture. FIGURE 9 is a functional block
diagram showing the
component architecture of the circuitry 60 of the monitor recorder 14 of
FIGURE 4. The
circuitry 60 is externally powered through a battery provided in the non-
conductive receptacle 25
(shown in FIGURE 6). Both power and raw ECG signals, which originate in the
pair of ECG
electrodes 38, 39 (shown in FIGURE 8) on the distal and proximal ends of the
electrode patch
15, are received through an external connector 65 that mates with a
corresponding physical
connector on the electrode patch 15. The external connector 65 includes the
set of electrical
contacts 56 that protrude from the bottom surface of the sealed housing 50 and
which physically
and electrically interface with the set of pads 34 provided on the bottom
surface of the non-
conductive receptacle 25. The external connector includes electrical contacts
56 for data
download, microcontroller communications, power, analog inputs, and a
peripheral expansion
port. The arrangement of the pins on the electrical connector 65 of the
monitor recorder 14 and
the device into which the monitor recorder 14 is attached, whether an
electrode patch 15 or
download station (not shown), follow the same electrical pin assignment
convention to facilitate
interoperability. The external connector 65 also serves as a physical
interface to a download
station that permits the retrieval of stored ECG monitoring data,
communication with the monitor
recorder 14, and performance of other functions. The download station is
further described infra
with reference to FIGURE 15.
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Operation of the circuitry 60 of the monitor recorder 14 is managed by a
microcontroller
61, such as the EFM32 Tiny Gecko 32-bit microcontroller, manufactured by
Silicon Laboratories
Inc., Austin, TX. The microcontroller 61 has flexible energy management modes
and includes a
direct memory access controller and built-in analog-to-digital and digital-to-
analog converters
(ADC and DAC, respectively). The microcontroller 61 also includes a program
memory unit
containing internal flash memory that is readable and writeable. The internal
flash memory can
also be programmed externally. The microcontroller 61 operates under modular
micro program
control as specified in firmware stored in the internal flash memory. The
functionality and
firmware modules relating to signal processing by the microcontroller 61 are
further described
infra with reference to FIGURE 14. The microcontroller 61 draws power
externally from the
battery provided on the electrode patch 15 via a pair of the electrical
contacts 56. The
microcontroller 61 connects to the ECG front end circuit 63 that measures raw
cutaneous
electrical signals using a driven reference that eliminates common mode noise,
as further
described infra with reference to FIGURE 11.
The circuitry 60 of the monitor recorder 14 also includes a flash memory 62,
which the
microcontroller 61 uses for storing ECG monitoring data and other physiology
and information.
The flash memory 62 also draws power externally from the battery provided on
the electrode
patch 15 via a pair of the electrical contacts 56. Data is stored in a serial
flash memory circuit,
which supports read, erase and program operations over a communications bus.
The flash
memory 62 enables the microcontroller 61 to store digitized ECG data The
communications bus
further enables the flash memory 62 to be directly accessed externally over
the external
connector 65 when the monitor recorder 14 is interfaced to a download station.
The microcontroller 61 includes functionality that enables the acquisition of
samples of
analog ECG signals, which are converted into a digital representation, as
further described infra
with reference to FIGURE 14. In one mode, the microcontroller 61 will acquire,
sample,
digitize, signal process, and store digitized ECG data into available storage
locations in the flash
memory 62 until all memory storage locations are filled, after which the
digitized ECG data
needs to be downloaded or erased to restore memory capacity. Data download or
erasure can
also occur before all storage locations are filled, which would free up memory
space sooner,
albeit at the cost of possibly interrupting monitoring while downloading or
erasure is performed.
In another mode, the microcontroller 61 can include a loop recorder feature
that will overwrite
the oldest stored data once all storage locations are filled, albeit at the
cost of potentially losing
the stored data that was overwritten, if not previously downloaded. Still
other modes of data
storage and capacity recovery are possible.
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The circuitry 60 of the monitor recorder 14 further includes an actigraphy
sensor 64
implemented as a 3-axis accelerometer. The accelerometer may be configured to
generate
interrupt signals to the microcontroller 61 by independent initial wake up and
free fall events, as
well as by device position. In addition, the actigraphy provided by the
accelerometer can be used
during post-monitoring analysis to correct the orientation of the monitor
recorder 14 if, for
instance, the monitor recorder 14 has been inadvertently installed upside
down, that is, with the
monitor recorder 14 oriented on the electrode patch 15 towards the patient's
feet, as well as for
other event occurrence analyses.
The microcontroller 61 includes an expansion port that also utilizes the
communications
.. bus. External devices, separately drawing power externally from the battery
provided on the
electrode patch 15 or other source, can interface to the microcontroller 61
over the expansion
port in half duplex mode. For instance, an external physiology sensor can be
provided as part of
the circuitry 60 of the monitor recorder 14, or can be provided on the
electrode patch 15 with
communication with the microcontroller 61 provided over one of the electrical
contacts 56. The
physiology sensor can include an Sp02 sensor, blood pressure sensor,
temperature sensor,
respiratory rate sensor, glucose sensor, airflow sensor, volumetric pressure
sensing, or other
types of sensor or telemetric input sources. In a further embodiment, a
wireless interface for
interfacing with other wearable (or implantable) physiology monitors, as well
as data offload and
programming, can be provided as part of the circuitry 60 of the monitor
recorder 14, or can be
provided on the electrode patch 15 with communication with the microcontroller
61 provided
over one of the electrical contacts 56.
Finally, the circuitry 60 of the monitor recorder 14 includes patient-
interfaceable
components, including a tactile feedback button 66, which a patient can press
to mark events or
to perfoun other functions, and a buzzer 67, such as a speaker, magnetic
resonator or
piezoelectric buzzer. The buzzer 67 can be used by the microcontroller 61 to
output feedback to
a patient such as to confirm power up and initiation of ECG monitoring. Still
other components
as part of the circuitry 60 of the monitor recorder 14 are possible.
While the monitor recorder 14 operates under micro control, most of the
electrical
components of the electrode patch 15 operate passively. FIGURE 10 is a
functional block
diagram showing the circuitry 70 of the extended wear electrode patch 15 of
FIGURE 4. The
circuitry 70 of the electrode patch 15 is electrically coupled with the
circuitry 60 of the monitor
recorder 14 through an external connector 74. The external connector 74 is
terminated through
the set of pads 34 provided on the bottom of the non-conductive receptacle 25,
which electrically
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mate to corresponding electrical contacts 56 protruding from the bottom
surface of the sealed
housing 50 to electrically interface the monitor recorder 14 to the electrode
patch 15.
The circuitry 70 of the electrode patch 15 perfoiins three primary functions.
First, a
battery 71 is provided in a battery compartment formed on the bottom surface
of the non-
conductive receptacle 25. The battery 71 is electrically interfaced to the
circuitry 60 of the
monitor recorder 14 as a source of external power. The unique provisioning of
the battery 71 on
the electrode patch 15 provides several advantages. First, the locating of the
battery 71
physically on the electrode patch 15 lowers the center of gravity of the
overall wearable monitor
12 and thereby helps to minimize shear forces and the effects of movements of
the patient and
clothing. Moreover, the housing 50 of the monitor recorder 14 is sealed
against moisture and
providing power externally avoids having to either periodically open the
housing 50 for the
battery replacement, which also creates the potential for moisture intrusion
and human error, or
to recharge the battery, which can potentially take the monitor recorder 14
off line for hours at a
time. In addition, the electrode patch 15 is intended to be disposable, while
the monitor recorder
14 is a reusable component. Each time that the electrode patch 15 is replaced,
a fresh battery is
provided for the use of the monitor recorder 14, which enhances ECG monitoring
performance
quality and duration of use. Also, the architecture of the monitor recorder 14
is open, in that
other physiology sensors or components can be added by virtue of the expansion
port of the
microcontroller 61. Requiring those additional sensors or components to draw
power from a
source external to the monitor recorder 14 keeps power considerations
independent of the
monitor recorder 14. This approach also enables a battery of higher capacity
to be introduced
when needed to support the additional sensors or components without effecting
the monitor
recorders circuitry 60
Second, the pair of ECG electrodes 38, 39 respectively provided on the distal
and
proximal ends of the flexible circuit 32 are electrically coupled to the set
of pads 34 provided on
the bottom of the non-conductive receptacle 25 by way of their respective
circuit traces 33, 37.
The signal ECG electrode 39 includes a protection circuit 72, which is an
inline resistor that
protects the patient from excessive leakage current should the front end
circuit fail.
Last, in a further embodiment, the circuitry 70 of the electrode patch 15
includes a
cryptographic circuit 73 to authenticate an electrode patch 15 for use with a
monitor recorder 14.
The cryptographic circuit 73 includes a device capable of secure
authentication and validation.
The cryptographic device 73 ensures that only genuine, non-expired, safe, and
authenticated
electrode patches 15 are permitted to provide monitoring data to a monitor
recorder 14 and for a
specific patient.
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The ECG front end circuit 63 measures raw cutaneous electrical signals using a
driven
reference that effectively reduces common mode noise, power supply noise and
system noise,
which is critical to preserving the characteristics of low amplitude cardiac
action potentials,
especially those signals from the atria. FIGURE 11 is a schematic diagram 80
showing the ECG
front end circuit 63 of the circuitry 60 of the monitor recorder 14 of FIGURE
9. The ECG front
end circuit 63 senses body surface potentials through a signal lead ("Si") and
reference lead
("REF") that are respectively connected to the ECG electrodes of the electrode
patch 15. Power
is provided to the ECG front end circuit 63 through a pair of DC power leads
("VCC" and
"GND"). An analog ECG signal ("ECG") representative of the electrical activity
of the patient's
heart over time is output, which the micro controller 11 converts to digital
representation and
filters, as further described infra.
The ECG front end circuit 63 is organized into five stages, a passive input
filter stage 81,
a unity gain voltage follower stage 82, a passive high pass filtering stage
83, a voltage
amplification and active filtering stage 84, and an anti-aliasing passive
filter stage 85, plus a
reference generator. Each of these stages and the reference generator will now
be described.
The passive input filter stage 81 includes the parasitic impedance of the ECG
electrodes
38, 39 (shown in FIGURE 8), the protection resistor that is included as part
of the protection
circuit 72 of the ECG electrode 39 (shown in FIGURE 10), an AC coupling
capacitor 87, a
termination resistor 88, and filter capacitor 89. This stage passively shifts
the frequency
response poles downward there is a high electrode impedance from the patient
on the signal lead
Si and reference lead REF, which reduces high frequency noise.
The unity gain voltage follower stage 82 provides a unity voltage gain that
allows current
amplification by an Operational Amplifier ("Op Amp") 90. In this stage, the
voltage stays the
same as the input, but more current is available to feed additional stages.
This configuration
allows a very high input impedance, so as not to disrupt the body surface
potentials or the
filtering effect of the previous stage.
The passive high pass filtering stage 83 is a high pass filter that removes
baseline wander
and any offset generated from the previous stage. Adding an AC coupling
capacitor 91 after the
Op Amp 90 allows the use of lower cost components, while increasing signal
fidelity.
The voltage amplification and active filtering stage 84 amplifies the voltage
of the input
signal through Op Amp 91, while applying a low pass filter. The DC bias of the
input signal is
automatically centered in the highest performance input region of the Op Amp
91 because of the
AC coupling capacitor 91.
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The anti-aliasing passive filter stage 85 provides an anti-aliasing low pass
filter. When
the microcontroller 61 acquires a sample of the analog input signal, a
disruption in the signal
occurs as a sample and hold capacitor that is internal to the microcontroller
61 is charged to
supply signal for acquisition.
The reference generator in subcircuit 86 drives a driven reference containing
power
supply noise and system noise to the reference lead REF. A coupling capacitor
87 is included on
the signal lead Si and a pair of resistors 93a, 93b inject system noise into
the reference lead REF.
The reference generator is connected directly to the patient, thereby avoiding
the thermal noise
of the protection resistor that is included as part of the protection circuit
72.
In contrast, conventional ECG lead configurations try to balance signal and
reference
lead connections. The conventional approach suffers from the introduction of
differential
thermal noise, lower input common mode rejection, increased power supply
noise, increased
system noise, and differential voltages between the patient reference and the
reference used on
the device that can obscure, at times, extremely, low amplitude body surface
potentials.
Here, the parasitic impedance of the ECG electrodes 38, 39, the protection
resistor that is
included as part of the protection circuit 72 and the coupling capacitor 87
allow the reference
lead REF to be connected directly to the skin's surface without any further
components. As a
result, the differential thermal noise problem caused by pairing protection
resistors to signal and
reference leads, as used in conventional approaches, is avoided.
The monitor recorder 14 continuously monitors the patient's heart rate and
physiology.
FIGURE 12 is a flow diagram showing a monitor recorder-implemented method 100
for
monitoring ECG data for use in the monitor recorder 14 of FIGURE 4. Initially,
upon being
connected to the set of pads 34 provided with the non-conductive receptacle 25
when the monitor
recorder 14 is snapped into place, the microcontroller 61 executes a power up
sequence (step
101). During the power up sequence, the voltage of the battery 71 is checked,
the state of the
flash memory 62 is confirmed, both in terms of operability check and available
capacity, and
microcontroller operation is diagnostically confirmed. In a further
embodiment, an
authentication procedure between the microcontroller 61 and the electrode
patch 15 are also
performed.
Following satisfactory completion of the power up sequence, an iterative
processing loop
(steps 102-110) is continually executed by the microcontroller 61. During each
iteration (step
102) of the processing loop, the ECG frontend 63 (shown in FIGURE 9)
continually senses the
cutaneous ECG electrical signals (step 103) via the ECG electrodes 38, 29 and
is optimized to
maintain the integrity of the P-wave. A sample of the ECG signal is read (step
104) by the
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microcontroller 61 by sampling the analog ECG signal that is output by the ECG
front end
circuit 63. FIGURE 13 is a graph showing, by way of example, a typical ECG
waveform 120.
The x-axis represents time in approximate units of tenths of a second. The y-
axis represents
cutaneous electrical signal strength in approximate units of millivolts. The P-
wave 121 has a
smooth, normally upward, that is, positive, waveform that indicates atrial
depolarization. The
QRS complex often begins with the downward deflection of a Q-wave 122,
followed by a larger
upward deflection of an R-wave 123, and terminated with a downward waveform of
the S-wave
124, collectively representative of ventricular depolarization. The T-wave 125
is normally a
modest upward waveform, representative of ventricular depolarization, while
the U-wave 126,
often not directly observable, indicates the recovery period of the Purkinje
conduction fibers.
Sampling of the R-to-R interval enables heart rate information derivation. For
instance,
the R-to-R interval represents the ventricular rate and rhythm, while the P-to-
P interval
represents the atrial rate and rhythm. Importantly, the PR interval is
indicative of atrioventricular
(AV) conduction time and abnormalities in the PR interval can reveal
underlying heart disorders,
thus representing another reason why the P-wave quality achievable by the
ambulatory
electrocardiography monitoring patch optimized for capturing low amplitude
cardiac action
potential propagation described herein is medically unique and important. The
long-term
observation of these ECG indicia, as provided through extended wear of the
wearable monitor
12, provides valuable insights to the patient's cardiac function symptoms, and
overall well-being
Referring back to FIGURE 12, each sampled ECG signal, in quantized and
digitized
form, is processed by signal processing modules as specified in firmware (step
105), as described
infra, and temporarily staged in a buffer (step 106), pending compression
preparatory to storage
in the flash memory 62 (step 107). Following compression, the compressed ECG
digitized
sample is again buffered (step 108), then written to the flash memory 62 (step
109) using the
communications bus. Processing continues (step 110), so long as the monitoring
recorder 14
remains connected to the electrode patch 15 (and storage space remains
available in the flash
memory 62), after which the processing loop is exited (step 110) and execution
terminates. Still
other operations and steps are possible.
The microcontroller 61 operates under modular micro program control as
specified in
firmware, and the program control includes processing of the analog ECG signal
output by the
ECG front end circuit 63. FIGURE 14 is a functional block diagram showing the
signal
processing functionality 130 of the microcontroller 61. The microcontroller 61
operates under
modular micro program control as specified in firmware 132. The firmware
modules 132
include high and low pass filtering 133, and compression 134. Other modules
are possible. The
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microcontroller 61 has a built-in ADC, although ADC functionality could also
be provided in the
firmware 132.
The ECG front end circuit 63 first outputs an analog ECG signal, which the ADC
131
acquires, samples and converts into an uncompressed digital representation.
The microcontroller
61 includes one or more firmware modules 133 that perform filtering. In one
embodiment, three
low pass filters and two high pass filters are used. Following filtering, the
digital representation
of the cardiac activation wave front amplitudes are compressed by a
compression module 134
before being written out to storage 135, as further described infra beginning
with reference to
FIGURE 22.
The download station executes a communications or offload program ("Offload")
or
similar program that interacts with the monitor recorder 14 via the external
connector 65 to
retrieve the stored ECG monitoring data. FIGURE 15 is a functional block
diagram showing the
operations 140 performed by the download station. The download station could
be a server,
personal computer, tablet or handheld computer, smart mobile device, or
purpose-built
programmer designed specific to the task of interfacing with a monitor
recorder 14. Still other
forms of download station are possible, including download stations connected
through wireless
interfacing using, for instance, a smart phone connected to the monitor
recorder 14 through
Bluetooth or Wi-Fi.
The download station is responsible for offloading stored ECG monitoring data
from a
monitor recorder 14 and includes an electro mechanical docking interface by
which the monitor
recorder 14 is connected at the external connector 65. The download station
operates under
programmable control as specified in software 141. The stored ECG monitoring
data retrieved
from storage 142 on a monitor recorder 14 is first decompressed by a
decompression module
143, which converts the stored ECG monitoring data back into an uncompressed
digital
representation more suited to signal processing than a compressed signal. The
retrieved ECG
monitoring data may be stored into local storage for archival purposes, either
in original
compressed form, or as uncompressed.
The download station can include an array of filtering modules. For instance,
a set of
phase distortion filtering tools 144 may be provided, where corresponding
software filters can be
provided for each filter implemented in the firmware executed by the
microcontroller 61. The
digital signals are run through the software filters in a reverse direction to
remove phase
distortion. For instance, a 45 Hertz high pass filter in firmware may have a
matching reverse 45
Hertz high pass filter in software. Most of the phase distortion is corrected,
that is, canceled to
eliminate noise at the set frequency, but data at other frequencies in the
waveform remain
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unaltered. As well, bidirectional impulse infinite response (IIR) high pass
filters and reverse
direction (symmetric) IIR low pass filters can be provided. Data is run
through these filters first
in a forward direction, then in a reverse direction, which generates a square
of the response and
cancels out any phase distortion. This type of signal processing is
particularly helpful with
improving the display of the ST-segment by removing low frequency noise.
An automatic gain control (AGC) module 145 can also be provided to adjust the
digital
signals to a usable level based on peak or average signal level or other
metric. AGC is
particularly critical to single-lead ECG monitors, where physical factors,
such as the tilt of the
heart, can affect the electrical field generated. On three-lead Holter
monitors, the leads are
oriented in vertical, horizontal and diagonal directions. As a result, the
horizontal and diagonal
leads may be higher amplitude and ECG interpretation will be based on one or
both of the higher
amplitude leads. In contrast, the electrocardiography monitor 12 has only a
single lead that is
oriented in the vertical direction, so variations in amplitude will be wider
than available with
multi-lead monitors, which have alternate leads to fall back upon.
In addition, AGC may be necessary to maintain compatibility with existing ECG
interpretation software, which is typically calibrated for multi-lead ECG
monitors for viewing
signals over a narrow range of amplitudes. Through the AGC module 145, the
gain of signals
recorded by the monitor recorder 14 of the electrocardiography monitor 12 can
be attenuated up
(or down) to work with FDA-approved commercially available ECG interpretation.
AGC can be implemented in a fixed fashion that is uniformly applied to all
signals in an
ECG recording, adjusted as appropriate on a recording-by-recording basis.
Typically, a fixed
AGC value is calculated based on how an ECG recording is received to preserve
the amplitude
relationship between the signals. Alternatively, AGC can be varied dynamically
throughout an
ECG recording, where signals in different segments of an ECG recording are
amplified up (or
down) by differing amounts of gain.
Typically, the monitor recorder 14 will record a high resolution, low
frequency signal for
the P-wave segment. However, for some patients, the result may still be a
visually small signal.
Although high resolution is present, the unaided eye will normally be unable
to discern the P-
wave segment. Therefore, gaining the signal is critical to visually depicting
P-wave detail. This
technique works most efficaciously with a raw signal with low noise and high
resolution, as
generated by the monitor recorder 14. Automatic gain control applied to a high
noise signal will
only exacerbate noise content and be self-defeating.
Finally, the download station can include filtering modules specifically
intended to
enhance P-wave content. For instance, a P-wave base boost filter 146, which is
a form of pre-
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emphasis filter, can be applied to the signal to restore missing frequency
content or to correct
phase distortion. Still other filters and types of signal processing are
possible.
Conventional ECG monitors, like Holter monitors, invariably require
specialized training
on proper placement of leads and on the operation of recording apparatuses,
plus support
equipment purpose-built to retrieve, convert, and store ECG monitoring data.
In contrast, the
electrocardiography monitor 12 simplifies monitoring from end to end, starting
with placement,
then with use, and finally with data retrieval. FIGURES 16A-C are functional
block diagrams
respectively showing practical uses 150, 160, 170 of the extended wear
electrocardiography
monitors 12 of FIGURES 1 and 2. The combination of a flexible extended wear
electrode patch
and a removable reusable (or single use) monitor recorder empowers physicians
and patients
alike with the ability to readily perform long-teiin ambulatory monitoring of
the ECG and
physiology.
Especially when compared to existing Holter-type monitors and monitoring
patches
placed in the upper pectoral region, the electrocardiography monitor 12 offers
superior patient
comfort, convenience and user-friendliness. To start, the electrode patch 15
is specifically
designed for ease of use by a patient (or caregiver); assistance by
professional medical personnel
is not required. Moreover, the patient is free to replace the electrode patch
15 at any time and
need not wait for a doctor's appointment to have a new electrode patch 15
placed. In addition,
the monitor recorder 14 operates automatically and the patient only need snap
the monitor
recorder 14 into place on the electrode patch 15 to initiate ECG monitoring.
Thus, the
synergistic combination of the electrode patch 15 and monitor recorder 14
makes the use of the
electrocardiography monitor 12 a reliable and virtually foolproof way to
monitor a patient's ECG
and physiology for an extended, or even open-ended, period of time.
In simplest form, extended wear monitoring can be performed by using the same
monitor
recorder 14 inserted into a succession of fresh new electrode patches 15. As
needed, the
electrode patch 15 can be replaced by the patient (or caregiver) with a fresh
new electrode patch
15 throughout the overall monitoring period. Referring first to FIGURE 16A, at
the outset of
monitoring, a patient adheres a new electrode patch 15 in a location at the
sternal midline 16 (or
immediately to either side of the sternum 13) oriented top-to-bottom (step
151). The placement
of the wearable monitor in a location at the sternal midline (or immediately
to either side of the
sternum), with its unique narrow "hourglass"-like shape, significantly
improves the ability of the
wearable monitor to cutaneously sense cardiac electrical potential signals,
particularly the P-
wave (or atrial activity) and, to a lesser extent, the QRS interval signals
indicating ventricular
activity in the ECG waveforms.
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Placement involves simply adhering the electrode patch 15 on the skin along
the sternal
midline 16 (or immediately to either side of the sternum 13). Patients can
easily be taught to find
the physical landmarks on the body necessary for proper placement of the
electrode patch 15.
The physical landmarks are locations on the surface of the body that are
already familiar to
patients, including the inter-mammary cleft between the breasts above the
manubrium
(particularly easily locatable by women and gynecomastic men), the sternal
notch immediately
above the manubrium, and the Xiphoid process located at the bottom of the
sternum.
Empowering patients with the knowledge to place the electrode patch 15 in the
right place
ensures that the ECG electrodes will be correctly positioned on the skin, no
matter the number of
times that the electrode patch 15 is replaced.
A monitor recorder 14 is snapped into the non-conductive receptacle 25 on the
outward-
facing surface of the electrode patch 15 (step 152). The monitor recorder 14
draws power
externally from a battery provided in the non-conductive receptacle 25. In
addition, the battery
is replaced each time that a fresh new electrode patch 15 is placed on the
skin, which ensures that
the monitor recorder 14 is always operating with a fresh power supply and
minimizing the
chances of a loss of monitoring continuity due to a depleted battery source.
By default, the monitor recorder 14 automatically initiates monitoring upon
sensing body
surface potentials through the pair of ECG electrodes (step 153). In a further
embodiment, the
monitor recorder 14 can be configured for manual operation, such as by using
the tactile
feedback button 66 on the outside of the sealed housing 50, or other user-
operable control. In an
even further embodiment, the monitor recorder 14 can be configured for
remotely-controlled
operation by equipping the monitor recorder 14 with a wireless transceiver,
such as described in
commonly-assigned U.S. Patent application, entitled "Remote Interfacing of an
Extended Wear
Electrocardiography and Physiological Sensor Monitor," Serial No. 14/082,071,
filed November
15, 2013. The wireless transceiver
allows wearable or mobile communications devices to wirelessly interface with
the monitor
recorder 14.
A key feature of the extended wear electrocardiography monitor 12 is the
ability to
monitor ECG and physiological data for an extended period of time, which can
be well in excess
.. of the 14 days currently pitched as being achievable by conventional ECG
monitoring
approaches. In a further embodiment, ECG monitoring can even be performed over
an open-
ended time period, as further explained infra. The monitor recorder 14 is
reusable and, if so
desired, can be transferred to successive electrode patches 15 to ensure
continuity of monitoring.
At any point during ECG monitoring, a patient (or caregiver) can remove the
monitor recorder
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14 (step 154) and replace the electrode patch 15 currently being worn with a
fresh new electrode
patch 15 (step 151). The electrode patch 15 may need to be replaced for any
number of reasons.
For instance, the electrode patch 15 may be starting to come off after a
period of wear or the
patient may have skin that is susceptible to itching or irritation. The
wearing of ECG electrodes
can aggravate such skin conditions. Thus, a patient may want or need to
periodically remove or
replace ECG electrodes during a long-term ECG monitoring period, whether to
replace a
dislodged electrode, reestablish better adhesion, alleviate itching or
irritation, allow for cleansing
of the skin, allow for showering and exercise, or for other purpose.
Following replacement, the monitor recorder 14 is again snapped into the
electrode patch
15 (step 152) and monitoring resumes (step 153). The ability to transfer the
same monitor
recorder 14 to successive electrode patches 15 during a period of extended
wear monitoring is
advantageous not to just diagnose cardiac rhythm disorders and other
physiological events of
potential concern, but to do extremely long term monitoring, such as following
up on cardiac
surgery, ablation procedures, or medical device implantation. In these cases,
several weeks of
monitoring or more may be needed. In addition, some IMDs, such as pacemakers
or implantable
cardioverter defibrillators, incorporate a loop recorder that will capture
cardiac events over a
fixed time window. If the telemetry recorded by the IMD is not downloaded in
time, cardiac
events that occurred at a time preceding the fixed time window will be
overwritten by the IMD
and therefore lost. The monitor recorder 14 provides continuity of monitoring
that acts to
prevent loss of cardiac event data. In a further embodiment, the firmware
executed by the
microcontroller 61 of the monitor recorder 14 can be optimized for minimal
power consumption
and additional flash memory for storing monitoring data can be added to
achieve a multi-week
monitor recorder 14 that can be snapped into a fresh new electrode patch 15
every seven days, or
other interval, for weeks or even months on end.
Upon the conclusion of monitoring, the monitor recorder 14 is removed (step
154) and
recorded ECG and physiological telemetry are downloaded (step 155). For
instance, a download
station can be physically interfaced to the external connector 65 of the
monitor recorder 14 to
initiate and conduct downloading, as described supra with reference to FIGURE
15.
In a further embodiment, the monitoring period can be of indeterminate
duration.
Referring next to FIGURE 16B, a similar series of operations are followed with
respect to
replacement of electrode patches 15, reinsertion of the same monitor recorder
14, and eventual
download of ECG and physiological telemetry (steps 161-165), as described
supra with
reference to FIGURE 16A. However, the flash memory 62 (shown in FIGURE 9) in
the
circuitry 60 of the monitor recorder 14 has a finite capacity. Following
successful downloading
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of stored data, the flash memory 62 can be cleared to restore storage capacity
and monitoring can
resume once more, either by first adhering a new electrode patch 15 (step 161)
or by snapping
the monitor recorder 14 into an already-adhered electrode patch 15 (step 162).
The foregoing
expanded series of operations, to include reuse of the same monitor recorder
14 following data
download, allows monitoring to continue indefinitely and without the kinds of
interruptions that
often affect conventional approaches, including the retrieval of monitoring
data only by first
making an appointment with a medical professional.
In a still further embodiment, when the monitor recorder 14 is equipped with a
wireless
transceiver, the use of a download station can be skipped. Referring last to
FIGURE 16C, a
similar series of operations are followed with respect to replacement of
electrode patches 15 and
reinsertion of the same monitor recorder 14 (steps 171-174), as described
supra with reference to
FIGURE 16A. However, recorded ECG and physiological telemetry are downloaded
wirelessly
(step 175), such as described in commonly-assigned U.S. Patent application,
Serial No.
14/082,071, cited supra. The recorded ECG and physiological telemetry can even
be
.. downloaded wirelessly directly from a monitor recorder 14 during monitoring
while still snapped
into the non-conductive receptacle 25 on the electrode patch 15. The wireless
interfacing
enables monitoring to continue for an open-ended period of time, as the
downloading of the
recorded ECG and physiological telemetry will continually free up onboard
storage space.
Further, wireless interfacing simplifies patient use, as the patient (or
caregiver) only need worry
.. about placing (and replacing) electrode patches 15 and inserting the
monitor recorder 14 Still
other forms of practical use of the extended wear electrocardiography monitors
12 are possible.
The circuit trace and ECG electrodes components of the electrode patch 15 can
be
structurally simplified. In a still further embodiment, the flexible circuit
32 (shown in FIGURE
5) and distal ECG electrode 38 and proximal ECG electrode 39 (shown in FIGURE
6) are
.. replaced with a pair of interlaced flexile wires. The interlacing of
flexile wires through the
flexible backing 20 reduces both manufacturing costs and environmental impact,
as further
described infra. The flexible circuit and ECG electrodes are replaced with a
pair of flexile wires
that serve as both electrode circuit traces and electrode signal pickups.
FIGURE 17 is a
perspective view 180 of an extended wear electrode patch 15 with a flexile
wire electrode
assembly in accordance with a still further embodiment. The flexible backing
20 maintains the
unique narrow "hourglass"-like shape that aids long term extended wear,
particularly in women,
as described supra with reference to FIGURE 4. For clarity, the non-conductive
receptacle 25 is
omitted to show the exposed battery printed circuit board 182 that is adhered
underneath the non-
conductive receptacle 25 to the proximal end 31 of the flexible backing 20.
Instead of
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employing flexible circuits, a pair of flexile wires are separately interlaced
or sewn into the
flexible backing 20 to serve as circuit connections for an anode electrode
lead and for a cathode
electrode lead.
To folin a distal electrode assembly, a distal wire 181 is interlaced into the
distal end 30
of the flexible backing 20, continues along an axial path through the narrow
longitudinal
midsection of the elongated strip, and electrically connects to the battery
printed circuit board
182 on the proximal end 31 of the flexible backing 20. The distal wire 181 is
connected to the
battery printed circuit board 182 by stripping the distal wire 181 of
insulation, if applicable, and
interlacing or sewing the uninsulated end of the distal wire 181 directly into
an exposed circuit
trace 183. The distal wire-to-battery printed circuit board connection can be
made, for instance,
by back stitching the distal wire 181 back and forth across the edge of the
battery printed circuit
board 182. Similarly, to form a proximal electrode assembly, a proximal wire
(not shown) is
interlaced into the proximal end 31 of the flexible backing 20. The proximal
wire is connected to
the battery printed circuit board 182 by stripping the proximal wire of
insulation, if applicable,
and interlacing or sewing the uninsulated end of the proximal wire directly
into an exposed
circuit trace 184. The resulting flexile wire connections both establish
electrical connections and
help to affix the battery printed circuit board 182 to the flexible backing
20.
The battery printed circuit board 182 is provided with a battery compartment
36 A set of
electrical pads 34 are formed on the battery printed circuit board 182. The
electrical pads 34
electrically interface the battery printed circuit board 182 with a monitor
recorder 14 when fitted
into the non-conductive receptacle 25. The battery compartment 36 contains a
spring 185 and a
clasp 186, or similar assembly, to hold a battery (not shown) in place and
electrically interfaces
the battery to the electrical pads 34 through a pair battery leads 187 for
powering the
electrocardiography monitor 14. Other types of battery compartment are
possible. The battery
contained within the battery compartment 36 can be replaceable, rechargeable,
or disposable.
In a yet further embodiment, the circuit board and non-conductive receptacle
25 are
replaced by a combined housing that includes a battery compartment and a
plurality of electrical
pads. The housing can be affixed to the proximal end of the elongated strip
through the
interlacing or sewing of the flexile wires or other wires or threads.
The core of the flexile wires may be made from a solid, stranded, or braided
conductive
metal or metal compounds. In general, a solid wire will be less flexible than
a stranded wire with
the same total cross-sectional area, but will provide more mechanical rigidity
than the stranded
wire. The conductive core may be copper, aluminum, silver, or other material.
The pair of the
flexile wires may be provided as insulated wire. In one embodiment, the
flexile wires are made
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from a magnet wire from Belden Cable, catalogue number 8051, with a solid core
of AWG 22
with bare copper as conductor material and insulated by polyurethane or nylon.
Still other types
of flexile wires are possible. In a further embodiment, conductive ink or
graphene can be used to
print electrical connections, either in combination with or in place of the
flexile wires.
In a still further embodiment, the flexile wires are uninsulated. FIGURE 18 is
perspective view of the flexile wire electrode assembly from FIGURE 17, with a
layer of
insulating material 189 shielding a bare uninsulated distal wire 181 around
the midsection on the
contact side of the flexible backing. On the contact side of the proximal and
distal ends of the
flexible backing, only the portions of the flexile wires serving as electrode
signal pickups are
electrically exposed and the rest of the flexile wire on the contact side
outside of the proximal
and distal ends are shielded from electrical contact. The bare uninsulated
distal wire 181 may be
insulated using a layer of plastic, rubber-like polymers, or varnish, or by an
additional layer of
gauze or adhesive (or non-adhesive) gel. The bare uninsulated wire 181 on the
non-contact side
of the flexible backing may be insulated or can simply be left uninsulated.
Both end portions of the pair of flexile wires are typically placed
uninsulated on the
contact surface of the flexible backing 20 to form a pair of electrode signal
pickups. FIGURE 19
is a bottom view 190 of the flexile wire electrode assembly as shown in FIGURE
17. When
adhered to the skin during use, the uninsulated end portions of the distal
wire 181 and the
proximal wire 191 enable the monitor recorder 14 to measure dermal electrical
potential
differentials. At the proximal and distal ends of the flexible backing 20, the
uninsulated end
portions of the flexile wires may be configured into an appropriate pattern to
provide an
electrode signal pickup, which would typically be a spiral shape formed by
guiding the flexile
wire along an inwardly spiraling pattern. The surface area of the electrode
pickups can also be
variable, such as by selectively removing some or all of the insulation on the
contact surface.
For example, an electrode signal pickup arranged by sewing insulated flexile
wire in a spiral
pattern could have a crescent-shaped cutout of uninsulated flexile wire facing
towards the signal
source.
In a still yet further embodiment, the flexile wires are left freely riding on
the contact
surfaces on the distal and proximal ends of the flexible backing, rather than
being interlaced into
the ends of the flexible backing 20. FIGURE 20 is a bottom view 200 of a
flexile wire electrode
assembly in accordance with a still yet further embodiment. The distal wire
181 is interlaced
onto the midsection and extends an exposed end portion 192 onto the distal end
30. The
proximal wire 191 extends an exposed end portion 193 onto the proximal end 31.
The exposed
end portions 192 and 193, not shielded with insulation, are further embedded
within an
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electrically conductive adhesive 201. The adhesive 201 makes contact to skin
during use and
conducts skin electrical potentials to the monitor recorder 14 (not shown) via
the flexile wires.
The adhesive 201 can be formed from electrically conductive, non-irritating
adhesive, such as
hydrocolloid.
The distal wire 181 is interlaced or sewn through the longitudinal midsection
of the
flexible backing 20 and takes the place of the flexible circuit 32. FIGURE 21
is a perspective
view showing the longitudinal midsection of the flexible backing of the
electrode assembly from
FIGURE 17. Various stitching patterns may be adopted to provide a proper
combination of
rigidity and flexibility. In simplest form, the distal wire 181 can be
manually threaded through a
plurality of holes provided at regularly-spaced intervals along an axial path
defined between the
battery printed circuit board 182 (not shown) and the distal end 30 of the
flexible backing 20.
The distal wire 181 can be threaded through the plurality of holes by
stitching the flexile wire as
a single "thread." Other types of stitching patterns or stitching of multiple
"threads" could also
be used, as well as using a sewing machine or similar device to machine-stitch
the distal wire
181 into place, as further described infra. Further, the path of the distal
wire 181 need not be
limited to a straight line from the distal to the proximal end of the flexible
backing 20.
An effective ECG compression solution can reduce battery power consumption,
ameliorate storage restriction, and extend monitoring time. The effectiveness
of an ECG
compression technique is evaluated mainly through compression ratio, degree of
error loss, and
execution time. The compression ratio is the ratio between the bit rate of the
original signal and
the bit rate of the compressed one. The error loss is the error and loss in
the reconstructed data
compared to non-compressed data. The execution time is the computer processing
time required
for compression and decompression. A lossless compressions may provide exact
reconstruction
of ECG data, but usually cannot provide a significant compression ratio, thus
may not be a good
choice when high compression ratio is required. In addition, analysis of ECG
data does not
require exact reconstruction; only certain feature of the ECG signal are
actually important.
Therefore, lossy compression, or techniques that introduce some error in the
reconstructed data,
is useful because lossy compression may achieve high compression ratios.
The ECG signal captured by the monitor recorder 14 is compressed by the
compression
module 134 as part a firmware 132 located on microcontroller 61 prior to being
outputted for
storage, as shown in FIGURE 14. FIGURE 22 is a flow diagram showing a monitor
recorder-
implemented method for ECG signal processing and ECG data compressing for use
in the
monitor recorders of FIGURE 4. A series of ECG signals are sensed through the
front end
circuit 63, which converts analog ECG signals into an uncompressed digital
representation. The
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compressing module 134 first read the digital presentation of the ECG signals
or ECG values
(step 201). The compressing module 134 subsequently encodes the ECG value
(step 202). This
encoding step achieves one level of compression and in one embodiment is a
lossy compression,
as further discussed infra in FIGURE 23. The compressing module 134 also
perfoim a second
level of compression by further encoding and compressing the sequence of codes
resulting from
the encoding process of step 202 (step 203). The compressed data is stored
into a non-volatile
memory, such as the flash memory 62.
Monitoring ECG (step 201) is described in FIGURE 12. Encoding ECG values (step
202) is performed by translating each sample data into one of codes, or
encodings, further
described with reference to in FIGURE 24. By encoding ECG data in the form of
a series of
codes, a level of compression is achieved. FIGURE 23 is a flow diagram showing
a monitor
recorder-implemented method for encoding ECG values. FIGURE 24 is an example
of a panel
of codes or encodings with each code covering a range defined by a lower
threshold ECG value
and an upper threshold ECG value, to be referenced to during the encoding
process described in
FIGURE 23. In one embodiment, a series of ECG values are obtained, which
constitute a
datastream (step 211). The series of ECG value can be one of raw
electrocardiography value,
processed electrocardiography value, filtered electrocardiography value,
averaged
electrocardiography value, or sampled electrocardiography value. The
compression module 134
defines a plurality of bins, each bin comprising a lower threshold ECG value,
an upper threshold
ECG value, and an encoding or code (step 212). One example of such a panel of
the bins is
shown in the Table in Figure 24, with the first column denoting the lower
threshold ECG value,
the second column denoting the upper threshold ECG value, and the third column
denoting the
code of a bin. An ECG data value is assigned to a corresponding bin, based
upon the difference
between the data value and a serial accumulator. The first serial accumulator
is set to a pre-
determined value such as a center value of an ECG recorder (step 213), each
succeeding serial
accumulator is a function of a previous serial accumulator and the actual ECG
reading and will
be described infra. For the series of the ECG values, the following encoding
steps are performed
by the compression module (steps 214 to 220). These steps includes: selecting
the ECG value
next remaining in the series to be processed (step 215); taking a difference
between the selected
ECG value and the serial accumulator (step 216); identifying the bin in the
plurality of the bins
corresponding to the difference (step 217), which will be further described
infra; representing the
selected ECG value by the encoding for the identified bin (218); and adjusting
the serial
accumulator by a value derived from the identified bin (step 219). Through
this process, each
ECG value is represented, or encoded, by one of the bins. As a result, one
level of data
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compression is achieved since the limited number of bins requires less storage
space compared to
the actual ECG data values.
Several ways of executing the step 217, i.e., identifying the bin in the
plurality of the bins
corresponding to the difference between the selected ECG value and the serial
accumulator, or
assigning a difference between the selected ECG value and the serial
accumulator to a proper
bin. In one embodiment, a difference is assigned to a bin when the difference
lies between the
lower threshold ECG value and the upper threshold ECG value of the bin. There
are two options
to assign a bin when a difference between the selected ECG value and the
serial accumulator
falls onto the lower threshold ECG value or the upper threshold ECG value of a
bin. In one
option, a bin is identified when the difference is equal to or larger than the
lower threshold ECG
value and smaller than the upper threshold ECG value of the identified bin. In
the other option, a
bin is identified when the difference is larger than the lower threshold ECG
value and equal to or
smaller than the upper threshold ECG value of the identified bin.
During the step 219, the value derived from the identified bin can be the
lower threshold
ECG value, the higher threshold ECG value, or a number derived from the lower
threshold ECG
value, upper threshold ECG value, or both. The derivation can be an addition
or subtraction of
the lower or upper threshold ECG value by a constant number or an offset. The
derivation can
also be an adaptive process wherein the offset may be adjusted to input ECG
data, and varies
from one bin to another bin.
Converting ECG values into a limited numbers of codes facilitate a further
compression
step which will be described infra. Some data error loss is introduced by the
encoding process;
however, proper bin setup minimizes the ECG data error loss and preserves
useful data essential
for accurate diagnosis, including P-wave signal. The number of codes and the
lower and upper
threshold ECG value of the codes are determined to achieve both efficient
encoding and
sufficient data reconstruction, especially for P-wave signals. The number of
codes and the lower
and upper threshold ECG value of the codes are flexible and can be adjusted to
adapt to ECG
data input and storage space. In one embodiment, the number of the bins are
chosen from 23 to
210. A higher number of bins usually results in less ECG data error loss but
more storage space
and battery power use.
The proper demarcation of upper and lower thresholds also reduces error loss
and
contributes to accurate re-construction of ECG value and graph shape. The
number of bins and
the thresholds for these bins are carefully selected to keep essential
information of the ECG
signals and filter away non-essential information, with a special emphasis to
accurately
representing the P-wave. Normally, each successive bin continues forward from
a previous bin
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so as to cover a contiguous range of electrocardiography values. In one
embodiment, the size of
the bins, i.e., the interval between the higher threshold ECG value and the
lower threshold ECG
value, are not equal thought the contiguous range; instead, areas of high
frequency calls for a
smaller size of bins. The size of the bins is partly deteunined by the
frequency of the ECG
.. values falling into the bin.
In one embodiment, 24= 16 bins are used, as described with reference to in
FIGURE 24
where the lower threshold ECG value and upper threshold ECG value for each bin
are also
provided. This setup provides minimum error loss and a significant compression
ratio, among
other considerations. The first, second, and third columns represent the lower
threshold ECG
value, the upper threshold ECG value, and the coding of the bins. The bin that
an ECG data will
fall into depends on the difference between the raw ECG data value and
corresponding serial
accumulator compared to the range that the bin covers. If an ECG raw data
falls into a particular
bin, the raw ECG data can be represented by the code of the bin. In this
example, the codes are
encoded with a four-bit storage space, with one bit to encode sign and three
bits to encode
magnitude. Similar, up to 32 codes can be encoded with a five-bit storage
space, with one bit to
encode sign and 4 bits to encode magnitude.
The minimum (Min) and maximum (Max) values in FIGURE 24 defines an inclusive
range of ECG values for each ECG code. An input ECG value that fall within the
range defined
by Min and Max value will be encoded by the code in the third column in FIGURE
24. The Min
and Max ranges can be the same for all of the bins or can be tailored to
specific ranges of ECG
values, to emphasize higher or lower density. For example, the Min and Max
value 5,001-
50,000 correspond to code +7 is low density and reflects the expectation that
few actual ECG
values exceeding 5001 [tV will occur. The density of the Min and Max value can
be adjusted to
enhance ECG signal detection such as P-wave signal. as a further example, the
Min and Max
ECG value ranges can be evenly defined throughout, or be doubled each of the
successive bin.
In one embodiment, the number of bins is selected to be a power of two,
although a
power of two is not strictly required, particularly when a second stage
compression as further
described below with reference to FIGURE 26.
FIGURE 25 is an example illustrating the encoding and compression scheme in
accordance with method and parameters as described with reference to in
FIGURES 23 and 24.
The first three ECG values of an ECG datastream, 12000, 11904, and 12537, are
shown in
column Ito show a recursive process. Remaining values are omitted since they
are processed
through the same recursive process. The initial ECG value, 12000, is
equivalent to the center
value of the ECG recorder. The initial serial accumulator is assigned to the
center value of the
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ECG recorder, 12000. The difference between the initial ECG value to the
initial serial
accumulator is 0, which falls within the lower and upper threshold of bin 0.
Thus the initial ECG
value is encoded with the code 0. 12000 is transferred to next row as the
serial accumulator for
next ECG value. The next ECG value is 11904. The difference between the next
ECG value and
the serial accumulator for the second value is 11904 - 12000 = -96. The
difference of -96 falls
into the bin with the code of -3, where the lower threshold of the bin is -41
and the upper
threshold of the bin is -150. Thus, the second ECG value is encoded with the
code of -3, which
is the bin identification. For the purpose of decoding the second value, an
encoder first refers to
the assigned bin, which is bin -3; the encoder then reads the lower threshold
ECG value of the
assigned bin -3, which is -41; and the encoder finally add the lower threshold
ECG value of the
assigned bin to the decoded value of the first ECG value, which is 12000, to
arrive at a decoded
value of 11959. The decoded value 11959 in turn serves as the serial
accumulator for the next
ECG value, in this case the next ECG value is the third one of 12537. The
difference between
the third value and its corresponding serial accumulator is 12537 - 11959=578.
This difference,
578, falls into the bin with a code of +5, which has a lower threshold ECG
value of 301 and
upper threshold ECG value of 1500. Thus the third ECG value is encoded with
the code of +5.
The third ECG value is decoded by adding the lower threshed ECG value of the
assigned bin +5,
which is 301, to the decoded value of second ECG value, which is 11959, to
arrive at the
decoded value of 12260. The decoded value of 12260 in turn will serve as the
serial accumulator
for the next ECG value The encoding process continue until the last reading is
taken. The
encoder keeps track of the accumulated encoded value as the encoding process
progresses along.
The encoding process described above is also a lossy compression process that
encodes
raw ECG signals with a finite number of codes. This process captures essential
information
while achieving significant data compression. In one embodiment, an other
compressing step is
performed. The other compression step may be performed independently. The
other
compression step may also be performed on top of the encoding process
described above to
achieve a higher level compression than one step alone. The second compression
step can be a
lossless compression performed on the codes from the first step. In one
embodiment, the
compression ratio of the second compression is in the range of 1.4 to 1.6,
increasing the data
storage capacity of a non-volatile memory by more than 41-66%. In another
embodiment, the
compression ratio of the second compression is in excess of 1.6, increasing
the data storage
capacity of a non-volatile memory by more than 66%. Thus, the combination of
the lossy
compression and the lossless compression serves to achieve both high fidelity
of the ECG signal
preservation and high compression ratio, which translate into increased data
storage capacity and
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reduced power consumption for the ambulatory electrocardiography monitor,
resulting in
extended wear time of the monitor.
In one embodiment, the second compression is effected by encoding a sequence
of codes
obtained from the first compression into a single number between 0 and 1, with
frequently used
.. codes using fewer bits and not-so-frequently occurring codes using more
bits, resulting in
reduced storage space use in total. FIGURE 26 is a flow diagram showing a
monitor recorder-
implemented method 230 for further compressing the codes. A sequence of the
codes
corresponding to the series of the ECG values is provided to the compressing
module 134. The
compressing module 134 set a range of 0 to 1 to an initial sequence of the
codes (step 231). The
compressing module 134 further performs recursive steps of assigning each
successive codes
into a sub-range within a previous range according to the probabilities of the
codes appearing
after a code (steps 232-239). In order to do so, the compressing module 134
obtains an
estimation of probabilities of next codes, given a current code (step 233).
Several variations of
calculating and adjusting the probabilities of the next codes will be
described infra. The
compressing module 134 divides the range of the current code into sub-ranges,
each sub-range
representing a fraction of the range proportional to the probabilities of the
next codes (step 234).
These sub-ranges are contiguous and sequential. The compressing module 134
reads the next
code (step 235) and selects the sub-range corresponding to the read next code
(step 236). The
read next code is represented, or encoded, by the corresponding sub-range
(step 237). The sub-
range corresponding to the read next code is assigned to be the range for the
code next to the
read next code (step 238), and the range is further divided into sub-ranges
with each sub-range
representing a fraction of the range proportional to the probabilities of
codes next to the read next
code (step 239). In this way, each code in the sequence of the codes is
represented by, or
encoded through, its location within a sub-range through a recursive process.
During the
recursive process, strings of codes represented by the selected sub-ranges are
encoded into part
of the single number between 0 and 1 and can be periodically or continually
stored into the non-
volatile memory, can be stored on-demand or as-needed, or can be queued up and
stored en
masse upon completion of the process. One example of the non-volatile memory
is the flash
memory 62.
The compressing module 134 uses a statistical model to predict what the next
code is,
given a current encoding (Step 233). In one embodiment, a total of 16 codes or
bin numbers are
used, thus the statistical model uses 16 tables, one for each current code.
Within each table,
numeric possibilities for 16 possible next codes given the particular current
code are generated.
In one embodiment, the probabilities of the next codes can be calculated from
sample ECG
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values. In another embodiment, the probabilities of the next codes can be
modified by ECG data
including recorded ECG data and data presently recorded. In still another
embodiment, the
probabilities of next codes can be adaptive, i.e, adjusted or varied along the
recursive
compression steps. Finally, in yet another embodiment, the compressing module
134 may use a
statistical model to arrive at the estimation of probabilities of next codes,
given two or more
consecutive preceding codes. When two consecutive preceding codes are used, 16
x 16 =256
different pairs of consecutive codes are possible. The compressing module 134
generates 256
tables, each tables containing numeric possibilities for 16 possible next
codes given a particular
pair of previous codes. When three consecutive preceding codes are used, 16 x
16 x 16 = 4096
different trios of consecutive codes are possible. The compressing module 134
generates 4096
tables, each tables containing numeric possibilities for 16 possible next
codes given a particular
trio of previous codes. Using two or more consecutive preceding codes further
enhances
compression ratio compared to using one preceding code, but also demands more
processing
power from the microcontroller.
While the invention has been particularly shown and described as referenced to
the
embodiments thereof, those skilled in the art will understand that the
foregoing and other
changes in form and detail may be made therein without departing from the
spirit and scope.
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