Note: Descriptions are shown in the official language in which they were submitted.
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Method and Device far Measuring Systolic and Diastolic Iiload Pressure and
lHeart Rate in
an Environment with Extreme Levels of Noise and Vibrations
Field of the Invention
[001] This invention relates generally to the field of blood pressure
monitoring methods and
devices and more particularly to auscultatory blood pressure monitoring
methods and devices
employing means for removing noise and vibration effects from audible blood
flow sounds.
Background of the Invention
[002] The blood pressure in the brachial artery is not constant, but varies
with time in relation
to the beating of the heart. Following a contraction of the heart to pump
blood through the
circulatory system, the blood pressure increases to a maximum level known as
the systolic blood
pressure. The minimum blood pressure between heartbeats is known as the
diastolic blood
pressure.
[003] The traditional technique for measuring the blood pressure of a patient
employs an
inflatable pressure cuff wrapped around an upper arm of a patient whose blood
pressure is to be
determined. As the pressure cuff is inflated, cuff pressure and pressure
applied to the arm of the
patient increases. If the pressure applied to the arm is increased beyond the
highest blood
pressure in the brachial artery located in the arm beneath the pressure cuff,
the artery will be
forced to close.
[004] As the pressure in the inflatable cuff is reduced from a high level
above the systolic
blood pressure, where the brachial artery is permanently closed, to a level
below the systolic
blood pressure level, the brachial artery beneath the cuff will begin to open
and close with each
heart beat as the blood pressure first exceeds the cuff pressure and then
falls below the cuff
pressure. As the blood pressure exceeds the cuff pressure, the artery will
open, and a low
frequency blood pressure sound, the so-called "Korotkoff sound" can be
detected. This sound is
detected using a stethoscope or microphone placed near the down-stream end of
the cuff on the
patient's arm. The highest cuff pressure at which the Korotkoff sounds are
detectable thus
corresponds to the systolic blood pressure of the patient.
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(005] As the cuff pressure is reduced further, the cuff pressure ~~ill be
brought below the
diastolic blood pressure. At this pressure level, the brachial artery beneath
the cuff remains open
throughout the heart beat cycle. Blood pressure sounds, caused by the opening
of the artery will,
therefore, not be produced. The lowest cuff pressure at which the blood
pressure sounds can be
detected thus corresponds to the diastolic blood pressure of the patient. The
determination of
blood pressure based on the detection of the onset and disappearance of blood
pressure sounds as
varying pressures are applied to an artery, is known as auscultatory blood
pressure
determination.
(006] In manual auscultatory blood pressure measltrement methods, a
stethoscope is used to
detect the onset and disappearance of the blood pressure sounds. Thus, the
blood pressure
measurement is highly dependent on the skill and hearing ability of the person
taking the
measurement. To overcome this dependence on hmnan skill and judgement, and to
automate the
process of determining a patient°s blood pressure, automatic blood
pressure monitoring systems
based on the auscultatory method of blood pressure determination have been
developed. These
automatic systems employ one or more microphones placed in or under an
inflatable cuff to
detect blood pressure sounds.
(007] However, it is almost impossible to detect the blood pressure sounds in
a noisy
environment such as a moving ambulance, helicopter, airplanes, or naval
vessels.
(008] Pneumatic systems measuring pressure variations caused by blood flowing
through the
artery instead of sound are not sensitive to noise, but extremely sensitive to
movement and
vibrations. Pressure variations caused by patient movement and any vibrations
present are
generally much larger than the pressure variations by the blood flow thus
rendering these
systems useless in the environments mentioned above.
(009] Some blood pressure monitoring systems employ twa microphones for
detecting blood
pressure sounds. Per example, two microphones may be placed under the
inflatable cuff
separated by a distance such that a low frequency blood pressure sound will
reach the first
microphone 180 degrees out of phase from the second microphone. Noise signals
v~=ill tend to
reach each microphone essentially simultaneously, and in phase. Therefore,
subtracting the two
microphone signals from each other will tend to enhance the useful data and
diminish unwanted
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noise. The two microphone signals can be added and subtracted from each other
to create signal
and noise detection thresholds. Microphone signals are considered to be valid
blood pressure
sound detections if they meet the detection thresholds. These blood pressure
monitoring methods
tend obtain useful data in moderately noisy environments. However, these
systems are less
effective when confronted with significant noise levels.
[0010] In US Patent No. 5,680,868 issued October 28, 1997 to Kahn et al. on a
method and
apparatus for monitoring the blood pressure of a patient by detecting low
frequency blood
pressure sounds in the presence of significant noise levels is disclosed. Kahn
discloses two
microphones placed over the brachial artery of a patient to detect the onset
and disappearance of
blood pressure sounds in the artery as the pressure on the artery is varied.
The microphones are
placed on the patient separated by a distance such that a true blood pressure
sound will
preferably be picked up at the second microphone approximately 180 degrees out
of phase with
respect to the blood pressure sound picked up by the first microphone. The
shift in phase
between the signals from the two microphones is used to indicate the detection
of a blood
pressure sound in the presence of~ significant noise levels. However, the
phase detection method
is still affected by vibrations detected out of phase at the two microphones.
This method is based
on the assumption that noise and vibrations are detected at both microphones
without a phase
shift whereas the blood pressure sound has a phase shift of approximately 180
degrees.
Vibrations due to body motion such as shivering or ambient vibrations imposed
on the body will
generally be detected out of phase at the two microphones making it difficult
to detect the
beginning and end of a blood pressure sound signal as the pressure cuff
deflates. Another
disadvantage of this method is that it is not possible to obtain directly from
the processed signals
a heart rate, which provides live saving information in emergency situations.
Summary of the Invention
(0011] It is an object of the invention to provide a method and a device for
measuring systolic
and diastolic blood pressure in environments comprising extreme levels of
noise anal vibration,
which overcomes the aforementioned problems.
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[0012] It is further an object of the invention to provide a method and a
device for measuring
systolic and diastolic blood pressure in environments comprising extreme
levels of noise and
vibration having enhanced signal detection and signal isolation.
[0013] In accordance with the present invention there is provided, a method
and device for
measuring systolic and diastolic blood pressure and heart rate in environments
with extreme
levels of noise and vibrations. Blood pressure signals corresponding to
Korotkoff sounds are
detected using an array of primary acoustic sensors, placed on a patient's
skin over the brachial
artery. The signals provided by the primary acoustic sensors are then
processed using a
combination of focused beamforming and plane wave beamforming. Use of an array
of acoustic
sensors in combination with beamforming substantially enhances signal
detection as well as
accurate isolation of the signal source whichis highly beneficial for blood
pressure measurements
performed under extreme levels of noise and vibrations.
[0014] In accordance with the present invention there is provided, a method
for measuring
systolic and diastolic blood pressure of a patient comprising the steps of:
sensing Korotkoff sounds using an array of primary acoustic sensors placed on
skin of a
patient's limb over an artery occluded by applying pressure thereupon, the
primary acoustic
sensors being placed in rows and columns forming a planar array, the planar
array being placed
on the patient's skin such that the rows are oriented approximately
perpendicular to the artery
and the columns are oriented approximately parallel to the artery, each
primary acoustic sensor
for producing a first acoustic signal in dependence upon the Koi-otkoff
sounds;
sensing noise and vibrations using a secondary acoustic sensor for producing a
secondary acoustic signal in dependence upon noise and vibrations;
sensing the pressure applied to the occluded artery using a pressure
transducer for sensing
pressure and for providing a pressure signal in dependence thereupon;
providing the. first acoustic signal of each primary acoustic sensor, the
secondary
acoustic signal and the pressure signal to a processor;
using the processor beamforming the first acoustic signals based on a focused
beamformer for beamforming approximately perpendicular to the artery and a
plane wave
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beamformer for beamforming approximately parallel to the artery in order to
produce an output
beam time series in dependence thereupon;
processing the first acoustic signals for removing interference due to noise
and
vibrations using the secondary acoustic signal in an adaptive interferer
canceller;
detecting the Korotkoff sounds based on the beam time series; and,
determining systolic and diastolic pressure by relating the detected Koratkoff
sounds to
the pressure signal.
[0015] In accordance with the present invention there is further provided, a
method for
measuring systolic and diastolic blood pressure of a patient comprising the
steps of:
sensing Korotkoff sounds using an array of primary acoustic sensors placed on
skin of a
patient's limb over an artery occluded by applying pressure thereupon, each
primary acoustic
sensor for producing a first acoustic signal in dependence upon the Korotkoff
sounds;
providing the first acoustic signal of each primary acoustic sensor to a
processor;
using the processor beamforming the first acoustic signal produced by each
primary
acoustic sensor in order to produce a beam time series in dependence
thereupon; and,
detecting the Korotkoff sounds based on the beam time series.
[0016] In accordance with the present invention there is further provided, a
device for
measuring systolic and diastolic blood pressure of a patient comprising:
an array of primary acoustic sensors for being placed on skin of a patient's
limb over an
artery occluded by applying pressure thereupon, each primary acoustic sensor
for producing a
first acoustic signal in dependence upon Korotkoff sounds;
a pressure transducer for sensing the pressure applied to the occluded artery
and for
providing a pressure signal in dependence thereupon; and,
a processor for receiving the first acoustic signals and the pressure signal,
for beamforming the
first acoustic signals in order to produce a beam time series in dependence
thereupon, for
detecting Korotkoff sounds based on the beam time series and for determining
systolic and
diastolic blood pressure using the detected Korotkoff sounds and the pressure
signal.
[0017] In accordance with the present invention there is further provided, a
device for
detecting Korotkoff sounds of a patient comprising:
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an array of primary acoustic sensors for being placed on skin of a patientis
limb over an
axtery, the primary acoustic sensors being placed in rows and columns forming
a planar array,
the planar array for being placed on the patient's skin such that the rows are
oriented
approximately perpendicular to the artery and the columns are oriented
approximately parallel to
the artery, each primary acoustic sensor for producing a first acoustic signal
in dependence upon
the Korotkoff sounds; and,
a processor for receiving the first acoustic signals, for beamforming the
first acoustic
signals based on a focused beamformer for beamforming approximately
perpendicular to the
artery and a plane wave beamformer for beamforming approximately parallel to
the artery in
order to produce a beam time series in dependence thereupon and for detecting
Korotlcoff sounds
based on the beam time series.
Brief Description of the Drawings
[0018] Exemplary embodiments of the invention will now be described in
conjunction with the
follov,.ring drawings, in which:
[0019] Fig. I a is a simplified diagram of a device for measuring systolic and
diastolic blood
pressure in environments comprising extreme levels of noise and vibration
according to the
invention;
[0020] Fig. 1 b illustrates schematically a processed Korotkoff sound signal
and its envelope
for determining systolic and diastolic blood pressure;
[0021] Fig. 2 is a simplified diagram of a signal processing structure
according to the
invention;
[0022] Fig. 3 illustrates results of the signal processing according the
invention of real signals
captured in a relatively noiseless environment;
[0023] Fig. 4 illustrates results of°the signal processing according
the invention of real signals
captured in presence of intense noise and vibrations;
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[0024] Fig. 5a is a simplified diagram of a pressure cuff illustrating the
positioning of acoustic
sensors of the device for measuring systolic and diastolic blood pressure in
enviromnents
comprising extreme levels of noise and vibration according to the invention;
[0025) Fig. 5b is a simplified diagram of a pressure cuff illustrating the
positioning of acoustic
sensors of the device for measuring systolic and diastolic blood pressure in
environments
comprising extreme levels of noise and vibration according to the invention;
[0026] Fig. 6a illustrates a processed Korotkoff sound signal for a patient
with arrhythmia;
[0027] Fig. 6b illustrates a pressure deflation curve corresponding to the
Korotkoff sound
signal illustrated in Fig. 6a;
[0028] Fig. 7 is a perspective view of an arrangement of primary acoustic
sensors according to
the invention;
[0029] Fig. 8 is a sectional view illustrating the arrangement of the primary
acoustic sensors
with respect to the brachial artery;
[0030] Fig. 9 is another sectional view illustrating the arrangement of the
primary acoustic
sensors with respect to the brachial artery;
[0031) Fig. 10 is a simplified diagram illustrating groupings of the primary
acoustic sensors for
the beamforming process according to the invention; and,
[0032] Figs. 11 a to 11 c are simplified flow diagrams illustrating various
embodiments of the
signal processing according to the invention.
Detailed Description
[0033] Fig. 1 a illustrates schematically a device 100 for measuring systolic
and diastolic blood
pressure - sphygmomanometer - in environments comprising high levels of noise
and vibration
according to the invention. The device 100 comprises a pressure cuff 1 to be
scrapped around a
limb of a patient whose blood pressure is to be determined. When wrapped
around the patient's
limb the pressure cuff 1 substantially forms a cylinder having an inside
surface 22 and an outside
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surface 24. Within the pressure eLaff 1 is mounted a primary acoustic sensor
3, a secondary
acoustic sensor 5 and a pressure transducer 7. The primary acoustic sensor 3
captures blood
pressure sounds and provides an electromagnetic blood pressure signal in
dependence thereupon.
The secondary acoustic sensor 5 captures ambient noise and vibrations and
provides an
electromagnetic signal in dependence thereupon. The pressure transducer 7
measures cuff
pressure, i.e. pressure exerted by the inflated pressure cuff 1 onto a
brachial artery of the patient.
The primary acoustic sensor 3 is located on the inside 22 of the pressure cuff
1 for positioning on
the skin of the patients limb over the brachial artery at the downstream end
of the pressure cuff 1
with respect to blood flow in the brachial artery. The secondary acoustic
transducer 5 is located
away from the brachial artery in order to capture noise and vibrations
superposed to the blood
pressure sound signal detected by the primary acoustic sensor ~. The pressure
cuff 1 is connected
via a communication link 8 to a housing 1 S comprising means for signal
conditioning 9 such as
filtering, an AID converter 11 and a processor 12.
[0034] In operation the pressure cuff 1 wrapped around an upper ann of the
patient is inflated
to a pressure beyond the highest blood pressure in the brachial artery forcing
the artery to close.
The pressure cuff 1 is inflated manually or by a motor driven pump.
[0035] As the pressure in the inflatable cuff is reduced to a level below the
systolic blood
pressure level, the brachial artery beneath the cuff will begin to open and
close with each heart
beat as the blood pressure first exceeds the cuff pressure and then falls
below the cuff pressure.
The arterial wall acts in a non-linear fashion with respect to the blood
pressure level. Thus, as the
blood pressure exceeds the cuff pressure, the artery will open, producing a
low frequency blood
pressure sound corresponding to the heart beat. This sound is tl2en detected
using the primary
acoustic sensor 3. Therefore, the pressure detected by the pressure transducer
7 at the time
instance when a first blood pressure sound is detected is the systolic blood
pressure. More
specifically, the systolic pressure is defined as the occurrence of the point
of greatest magnitude
of the positive derivative of the processed signals envelope - surrounding the
detected Korotkoff
sounds - as indicated by the dashed curve in Fig, 1b. This processing step is
synonymous with
the blood pressure when the first Korotkolf is heard, at the marked increase A
in the envelope
shown in Fig. 1 b.
8
,: " , . ; .. , , ~rr ~~ , , , . w .,. . ~,x . x... _ ,.... ... . . . .._. . .
...._ . . _ __ . .. . . ._....
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[0036] As the cuff pressure is reduced further, the cuff pressure will be
brought below the
diastolic blood pressure. At this pressure level, the brachial artery beneath
the cuff remains open
throughout the heart beat cycle. Blood pressure sounds, caused by the opening
of the artery will,
therefore, not be produced. The lowest cuff pressure at which the blood
pressure sounds are
detected thus corresponds to the diastolic blood pressure. This marked
decrease B is the point of
greatest negative slope of the envelope surrounding the Korotko ff sounds, as
indicated by the
dashed curve in Fig. 1 b.
[0037] During deflation of the pressure cuff 1 ambient noise and vibrations
are detected using
the secondary acoustic sensor 5. Sensor signals produced by the acoustic
sensors 3 and 5 and the
pressure transducer 7 are transmitted via the communication link 8 to the
processor 12 for
processing.
[0038) The signals are then processed according to the invention as shown in
Fig. 2. In a first
optional step the sensor signals are processed in signal conditioning means 9
such as a band pass
filter. Since the frequency range of the acoustic signal of interest is well
localized using a band
pass filter is a useful step fox removing excess noise outside this frequency
range of interest. The
filtered signals are then converted into corresponding digital signals using
an AID converter 11
for provision to the processor 12 such as a microprocessor. In the processor
12 the acoustic
sensor signals are then processed using an adaptive interferes canceller,
indicated in Fig. la by
dotted lines, in order to remove any interference n~j~t~ - detected by the
secondary acoustic
sensor 5 - from the noisy measured signal s~, j~t~ - detected by the primary
acoustic sensor 3. The
noisy measured signal s( jOt~ is provided to the adaptive interferes canceller
as input signal. The
signal 3z( jat) provided by the secondary acoustic sensor 5 is provided to an
adaptive filter of the
adaptive interferes canceller as an interference noise signal. The output of
the adaptive filter
u( jet) for the interferes input n(j~t) is given by equation (1):
L
u(jOt) _ ~w°' x nCCj+i- 2)Ot~' (i = I,2,...L), ~j = 1,2,...K~, (I)
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[0039] wherein L is the number of adaptive weights ~w,,w~ ,...w, ) at time jet
and K is the
maximum number of samples to be processed. The adaptive weights for the
adaptive weight
algorithm (1) are given by the adaptive weight update equations (2):
w(.i+1)~r = w~a' + a+ n x ~C~ +i 2~~t~ x ~t ~ ~i = 1 2 ...L) ~ = 1 2 ...K~ (2)
~ I ' O> ) > » > .o » >
[0040] wherein ~, is an adaptive step size parameter, a is a stability
parameter and ~n~ is the
Euclidean norm of the vector:
r~~Cj + 1- 2)~t~'n~C~ +2- 2)~t~,...n~Cj + 2)At)
[0041) The output of the adaptive interferes canceller is then given by
y~j~t) = s~jat) - u~jAt) . In order to calculate the adaptive weight for a
sample ~j + 1)~t to be
processed the output of the interferes canceller of the previous sample jet is
used as can be seen
in the adaptive weight update equation {2).
[0042] This algorithm is an ideal tool for removing any noise and vibration
effects in a
measured signal if an interferes is accurately measured. The noise measured by
the second
acoustic sensor 5 placed away from the brachial artery is treated as the
interferes ~~ jAt~ and an
adaptive weighted signal u~ jOt) is then subtracted from the measured acoustic
signal of~ the
blood pressure sound s~jAt) . Detailed information concerning the adaptive
interferes canceller
are disclosed by the inventor in "Implementation of Adaptive and Synthetic-
Aperture Processing
Schemes in Integrated Active-Passive Sonar Systems", published in Proceedings
of the IEEE,
86(2), 358-396, February 1998.
[0043] The adaptive interferes canceller has been found to be a powerful tool
for removing
interference noise from a "noisy" signal if the interference is accurately
measured. Furthermore,
the adaptive interferes canceller as applied in the device and method for
measuring blood
pressure according to the invention requires only a minimum amount of
computation in order to
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provide good results even for signals detected in environments with extreme
noise and vibration
levels.
[0044] Optionally, to further reduce noise effects the output signal y~jOty
may be band pass
filtered.
[0045] The output signal y~ j~t~ is then provided to a peak discriminator in
order to extract
valid peaks resulting fram heartbeats in the acoustic signal y~j~t) from any
background noise.
In a first step peaks greater than a noise floor level determined by the peak
discriminator are
isolated. The isolated peaks are then further examined in order to determine
if they satisfy
periodicity and constancy in repetition, that is beats are not missing, as is
expected from
heartbeats. Peaks not satisfying these constraints are discarded. The output
of the peak
discriminator is a series of constantly repeating periodic peaks. This process
also eliminates
random peaks due to strong transient noise effects. As is obvious to a person
of skill in the art,
there are numerous methods for detecting peaks. The method described above has
been found to
produce good results even in enviromnents with extreme high noise and
vibration levels while
the required computation is kept to a minimum.
[0046] From the results of the peak discriminator a pulse rate estimator
determines the
immediately available pulse rate of the patient.
[0047] The output of the peak discriminator is also provided to a blood
pressure estimator. The
systolic blood pressure is defined as the blood pressure when the first
heartbeat is detected as the
pressure cuff 1 is deflating. The diastolic blood pressure is defined as the
blood pressure when
the last heartbeat is detected. From the results of the peak discriminator the
time instances where
these two pulse peaks occur are determined and then used as a reference to the
signal acquired by
the pressure transducer 7. The signal acquired by the pressure transducer 7
provides a
measurement of the pressure in the deflating pressure cuff 1 as a function of
time. The
corresponding pressures at these tithe instances are the systolic blood
pressure and the diastolic
blood pressure, respectively.
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[0048 Alternatively, the measurements are taken while the pressure cuff 1 is
being inflated.
This method has the advantage that the pressure cuff 1 is inflated to a
pressure only slightly
above the systolic blood pressure whereas in the above method the pressure
cuff is inflated to a
pressure much higher than the actual systolic blood pressure in order to
ensure closure of the
brachial artery.
[0049) Furthermore, a plurality of acoustic sensors, or array of sensors, may
be used for each
of detecting blood pressure signals and interference noise in order to further
improve signal
quality.
[0050] Fig. 3 shows results of the device and method for measuring blood
pressure and heart
rate according to the invention in an almost noiseless environment. The top
curve indicates
pressure deflation of the pressure cuff 1 as function of time. The second
curve from top shows
the acoustic signal measured by the primary acoustic sensor 3. Periodic pulses
resulting from the
Korotkoff sound are clearly visible and the first and last pulse are well
defined. The next curve
shows the acoustic signal after it has been processed by the adaptive
interferer canceller. It is
evident that the noise level is lower in this signal and residual Korotkolf
sounds present in the
unprocessed signal have been removed. The bottom curve sho~,vs the peaks
discriminated by the
peak discriminator from the noise. Small areas at the beginning and the end
are discarded due to
their non-periodic nature and the fact that they are not constant over a
period that could be
deemed to be a series of heart beats. The remaining sequence is retained and
used to determine
blood pressure and heart rate. In this case the systolic blood pressure is 123
psi, the diastolic
blood pressure is 83 psi and the heart rate is 84 beats per minute.
[0051] Fig. 4 shows results for measurements taken aboard a helicopter - an
environment
comprising extreme noise and vibration levels. It is evident that the signal
detected by the
primary acoustic sensor is very noisy and the first and the last pulse cannot
directly be identified.
However, after processing the signal using the adaptive interferer canceller
and the first stage of
the peak discrimination the first and the last pulse are readily identified.
The systolic and
diastolic blood pressures are 132 psi and 108 psi, respectively, and the heart
rate is 92 beats per
minute. These measurements, as well as those taken under noiseless conditions
compare
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favorably with measurements taken by the traditional auscultatory method by a
medical
practitioner immediately before the experiments using the device according to
the invention.
[0052] The device and method for measuring the blood pressure and heart rate
according to the
invention is highly advantageous to the prior art by providing good results in
environments with
extreme levels of noise and vibration. In many emergency situations it is
essential for saving the
live of a victim to obtain accurate measurements of blood pressure and heart
rate in order to
provide first emergency treatment. Unfortunately, in many cases this has to be
done in a very
noisy environment such as an ambulance, a helicopter or a naval vessel. This
invention provides
the means to obtain accurate results under such conditions and allows
measurements of blood
pressure and heart rate even if the victim is under cardiogenic shock. The
signal processing
requires only a minimum of computation, therefore, the device for measuring
blood pressure and
heart rate may be battery operated and assembled in a small portable housing.
For example, such
a device allows measurement of the blood pressure while the victim is
transported on a stretcher
to an ambulance, thus saving valuable time.
[0053] In another embodiment the device according to the invention is used to
monitor the
heart rate during transportation. In this case the pressure cuff 1 is inflated
only slightly above the
diastolic pressure in order to be able to detect the Korotkoff sound bul not
to interrupt the blood
flow through the artery.
[0054] For the implementation of the adaptive interferer canceller it is
critical to strategically
place the two sensors, the primary acoustic sensor 3 far producing a first
acoustic signal in
dependence upon the blood pressure signals - Korotkoff sounds - and the
secondary acoustic
sensor 5 for producing a second acoustic signal in dependence upon noise and
vibrations. The
secondary acoustic sensor 5 is placed such that the phase characteristics and
properties of the
interference signal are substantially identical to the phase characteristics
and properties of the
interference signal sensed by the primary acoustic sensor 3. Fu~~thermore, if
there is a correlation
coupling term for the signal of interest between the two acoustic sensors the
adaptive
interference cancellation process will remove the signal of interest.
Therefore, the secondary
acoustic sensor 5 is placed at a location where it is unlikely that components
of the signal of
interest - Korotkoff sounds - are sensed by the secondary acoustic sensor 5.
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[005] The following description of the signal processing using adaptive
interference
cancellation disclosed by B. Widrow and S. Steams ''Adaptive Signal
Processing", Prentice Hall,
Englewood Cliffs, NJ 07632, 1985 illustrates the need for strategically
placing the secondary
acoustic sensor 5.
[0056] The first acoustic signal is defined by y~j~t~= s(jat~+n(j,~t~, where
s(jat) and
~~jOt~ are signal and noise components, respectively. In the adaptive
interference cancellation
process with performance feedback it is essential that the noise component is
sensed
simultaneously with the received noisy signal. Assume that the noise
measurements provided by
the secondary acoustic sensor 5 to the input of the adaptation process shown
in Fig. 1 a are
defined by the input vector ~ with terms ~s(~t), ~(2~t),..., s(Mnt)1'~ ,
wherein M is the
maximum number of samples to be processed. Furthermore, the output of a least
mean square
adaptation process is a linear combination of the input measurements s and the
weight
coefficients [w(~t~, w~2~t~,..., w(M~t~~~~ of the vector W that are derived by
descending toward
the minimum of the surface of the performance feedback. The output of the
adaptive interferer
canceller is then given by:
a = Y - F '~W (3)
[0057] where, Y is the input vector of the noisy measurements from the first
acoustic sensor,
y( jOt) for j =1,2,..., M . The adaptive interference cancellation process is
based on the
minimization of the square of equation (3), E",;" [(e)'',= E,il", L(S + N~z ~-
E,~1;~~ l(~' W )21. From this
results that when E~e~ is minimized then the signal power E~~f ~ is unaffected
and the term
EL(N - ~ '~W ~2 jis minimized. Therefore, the output of the adaptive
interference cancellation
process provides estimates of the signal vector, i.e. E,n;" ~e ~ = E~S ~ .
[0058] However, if the second sensor senses components of l:he noise s and the
signal S ,
then the adaptive interference cancellation process will cancel the signal of
interest and the
output of the adaptive interference cancellation process will be white noise
E",;" [e ~ = E[e ~ .
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[0059] Referring to Figs. 5a and Sb a detailed view of two embodiments of the
device for
measuring systolic and diastolic blood pressure according to the invention are
shown illustrating
the positioning of the acoustic sensors. During clinical trials it has been
found that some signal
cancellation occurred when the secondary acoustic sensor 5 was in contact with
the skin of a
patient's limb or housed on inside surface 22 of the pressure cuff 1. In order
to overcome the
signal cancellation the secondary sensor 5 is placed on outside surface 24 of
the pressure cuff 1
as shown in embodiments 200 and 300. Clinical trials have been performed to
determine the
positions for mounting the secondary acoustic sensor ~ on the pressure cuff 1
such that the
second acoustic signal satisfies the above conditions.
[0060] During the clinical trials a best position for mounting the secondary
acoustic sensor 5
on the pressure cuff 1 has been found to be the position shown in embodiments
200 and 300. As
illustrated in embodiment 200 the primary acoustic; sensor 3 is placed on the
skin of a patient's
limb over the brachial artery on the inside sL~rface 22 of the pressure cuff 1
at the downstream
end of the pressure cuff 1 with respect to the blood flow in the brachial
artery. Alternatively, an
array of primary acoustic sensors 3 is placed on the skin of a patient's limb
across the brachial
artery to ensure detection of the Korotkoff sounds. The secondary acoustic
sensor 5 is placed on
the outside surface 24 of the pressure cuff 1 at a location approximately
diametrically opposite
the location of the primary acoustic sensor 3 and at the upstream end of the
pressure cuff 1.
[0061 Furthermore, the Korotkoff sounds are modulated with the deflated
pressure of the
pressure cuff 1. If the secondary acoustic sensor 5 has acoustic contact with
the pressure cuff 1
the secondary acoustic sensor 5 mounted on the outside of the pressure cuff 1
senses the
Korotkoff sounds, resulting in a cancellation of the signal of interest.
Therefore, the secondary
acoustic sensor S is mounted to the pressure cuff 1 in an acoustically
isolating fashion. This is
accomplished, for example, by housing the acoustic sensor in a thin bell using
an elastomer
isolator. The acoustic sensors housed in the bell are then attached to
pressure cuff l, the primary
acoustic sensor 3 on the inside and the secondary acoustic sensor on the
outside. Optionally, the
acoustic sensors axe removable attached using, for example, hook and loop
fasteners for optimal
placement of the same by a health care practitioner. This allows the health
care practitioner to
ensure that the primary acoustic sensor 3 is placed directly over the brachial
artery while the
secondary acoustic sensor 5 is placed approximately diametrically opposite.
further optionally,
l~
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an array of, for example, three primary acoustic sensors 3 are housed in the
bell to ensure
detection of the Korotkoff sounds.
[0062] Figs. 6a and 6b illustrate a processed signal and a corresponding
pressure deflation
curve for a patient with arrhythmia. Arrhythmia is an alteration in rhythm of
the heart beat that
causes Korotkoff sound pulses to disappear within the pressure deflation
curve. During the peak
discrimination process according to the invention the heart rate estimate is
updated with each
pulse found during searching for the systolic and diastolic pressure.
Furthermore, the process
keeps track of the number of "missed" heartbeats and determines a new value
for the systolic and
diastolic pressures if it finds a new sequence of heartbeats after the gap, as
shown in Fig. 6a. It is
noted that the peak discrimination process according to the invention is
highly advantageous
since most commercially available systems fail to estimate systolic and
diastolic blood pressure
for patients with arrhythmia.
[0063] In one embodiment of the blood pressure measurement system according to
the
invention the systolic and diastolic blood pressures are determined based on
an electronic
equivalent of the corresponding medical definitions of systolic and diastolic
pressures, i.e.
systolic pressure occurring at the onset of a train of pulses perceptible to
the acoustic sensors and
diastolic pressure when these pulses disappeared completely. This results in a
bias between
measurements taken with the system according to the invention using the above
signal
processing and measurements taken using the conventional auscultatory method.
Therefore, in
another embodiment of the signal processing according to the invention the
above definitions of
the systolic and diastolic pressures are specified as the appearance and
disappearance of a train
of auditory pulses perceptible to an average human ear.
[0064] During the clinical trials systolic and diastolic pressure measured
using the blood
pressure measurement system according to the invention were repeatedly lower
than systolic and
diastolic pressure measured using the conventional auscultatory method.
Furthermore, the
difference was larger for the systolic pressure than the diastolic pressure.
The consistent
difference in blood pressure measurements resulted from the positioning of the
bell housing the
primary acoustic sensor 3 underneath the inflated pressure cuff exerting
additional pressure on
the artery. This effect is more apparent at higher pressures, affecting the
systolic pressure to a
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greater degree than the diastolic pressure. To overcome this difference in the
measurements
taken using the system according to the invention and the conventional
auscultatory method the
bell housing of the primary sensor 3 has a surface area for contacting a
patients skin that is large
enough to substantially reduce the additional pressure exerted on the artery.
Alternatively, the
bell housing of the primary sensor 3 is attached to a non inflatable extension
32 of the pressure
cuff 1 as illustrated in embodiment 300.
[0065] In the following a modified sensor configuration replacing the primary
acoustic sensor
3 with an array of primary acoustic sensors and signal processing methods
based on the same
according to the invention will be disclosed. Use of an array of primary
acoustic sensors as ~~ill
be disclosed in the following allo~rs employment of beamforming processing
steps in
combination with adaptive interference cancellation on all primacy acoustic
sensor signals in
order to further enhance the detection of the Korotkoff sounds which is highly
beneficial
especially in noisy environments.
[0066] Referring to Fig. 7, a perspective view of an embodiment 400 according
to the
invention schematically illustrates positioning of a plurality of primary
acoustic sensors 405 in
sensor strips 403 placed, for example, on the skin 402 of a patient's limb
over the brachial artery
401. As shown in Fig. 7 the primary acoustic sensors 405 are, for example,
arranged in a
plurality of sensor strips 403 oriented perpendicular to the brachial artery
401 and the sensor
strips 403 are placed successively along the same thus forming a planar array.
The arrangement
of the primary acoustic sensors 405 in a planar array allows use of planar
array beamforming
processes such as the planar array adaptive beam forming process disclosed by
the first inventor
in US Patent No. 6,482,160 issued November 19, 2002.
[0067] In operation, the source of the signal of interest - the Korotkoff
sound signal - is
traveling along the occluded brachial artery 40I, hence the signal of interest
is traveling along a
line as depicted in Fig. 7. Further, the Korotkoff sound signal is emitted
from the occluded
brachial artery 401 and travels through human tissue such as muscle, fat and
skin to the primary
acoustic sensors 405 where it is sensed as signal of interest. Based on the
progression of the
signal source and the signal transmission to the primary acoustic sensors 405
the beam forming
process according to the invention as described below is applied. The signals
of interest sensed
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by the primary acoustic sensors 405 in each sensor strip 403 are bearnformed
using a linear
focused beamformer based on the geometry illustrated in Fig. 8. In this
beamforming process
beam steering angles O", of each of the primary acoustic sensors 405 in a
sensor strip 403 are
varied to find a maximum response which occurs when the steering is directed
from the primary
acoustic sensor to the brachial artery 401 with an appropriate focal distance
F", as shown in
Fig. 8. Generally, the range of the beam steering angles Om and the focal
distances F", is small
for this application of beamforming. Further, the objective of this
beamforming process is
optimal signal detection and not optimal signal localization. Therefore, based
on the size of the
planar array, i.e. the number of primary acoustic sensors 405, the beamforming
process is very
fast allowing real time signal processing using portable signal processing
equipment. Based on
the geometry illustrated in Fig. 8 the beamformer response for the primary
acoustic sensors 405
within a sensor strip 403 is given by:
N
_ ~ e-.IZnlz"
8~~~ Onv ~ Fns ~ Xn ~~)
n=1
T 1
Fn' + Cn - ~ 2 l ~d~, + 2F", ~n - ~ 2 ~ ~d", cos O,» - Fz,
zn = ~, -~ ~ ~4)
[0068] where F", is the focal distance, O", is the steering angle, dn, is the
spacing of the
primary acoustic sensors 405 within a sensor strip 403 as illustrated in Fig.
8, and C is speed of
sound within human tissue. X" ~ f ) is the frequency domain representation of
a signal xn ~t~
received from primary acoustic sensor n , and B~ f , Om , F", ) is the
frequency beamformer
response, where N is the total number of primary acoustic sensors 405 in one
sensor strip 403.
The inverse Fourier transform of B~, f , O ", , Fn, ) yields the beam time
series b~t, O ", , Fn, ) .
Preferably, the primary acoustic sensor spacing d~, is dimensioned to be less
than'/2 the
wavelength of the signal of interest. The array gain of the ~'V pri.mary
acoustic sensors in a
sensor strip is given bylOlog(N). The illustration in Fig. 8 indicates a total
number of !V = 3
primary acoustic sensors within a sensor strip in a preferred embodiment of
the invention.
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[0069] Optimal values of Fn, and O", yield a maximum response. The beam time
series
obtained from the primary acoustic sensors 405 of each sensor strip 403 is
then treated as a
directional acoustic sensor signal. The directional acoustic sensor signals
are then beamformed
based on a geometry illustrated in Fig. 9. Based on the assumption that in
general the brachial
artery 401 is not parallel to the skin 402 an angular beam steering, indicated
by O,. , is applied to
provide maximum signal response. The final beam response obtained from
beamforming the
directional acoustic sensor signals of the sensor strips 403 is given by:
a / \
~~.f~~r~ ~Xr~~yG-..,~Z~~~~.
r=1
~y. _ ~2.1~d,, cos0,.
z,. _
[0070] where O,. is the beam steering angle, dr is the acoustic sensor spacing
between
acoustic sensors in two successive sensor strips 403 as illustrated in Fig. 9,
and C is speed of
sound within human tissue. X r (, f ~ is the frequency domain reps esentation
of the directional
acoustic sensor signal xr ~t~ of a sensor strip Y , which is equivalent to
b~t, O ", , F", ) . Bt f ~, O,. ~ is
the beamformer response in frequency domain. The inverse Fourier transform of
B~, f , O r ~ yields
the beam time series b(t, O r ~ as the final output of the beamforming process
outlined above
where the primary acoustic sensor signals are first beamformed within the
sensor strips and these
beam time series are then beamformed across the sensor strips in a second
stage. Preferably, the
acoustic sensor spacing dr between primary acoustic sensors in two successive
sensor strips is
dimensioned to be less than t/z the wavelength of the signal of interest. The
array gain derived
from the beamforming across the R sensor strips is given by 10 log~.R~ . The
illustration in Fig. 9
indicates a total number of R = 3 sensor strips 403 in a preferred embodiment
of the invention.
Optionally, use of more sensor strips 403 provides higher signal to noise
ratios due to higher
array gains, and hence better signal detection, however, at the cost of
increased hardware and
software complexity. Together, the two stages of beamforming provide a total
array gain
of101og~NR~.
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[0071] Optionally, the conventional beamformers described by equations (4) and
(5) above are
replaced with adaptive beamfonrners as disclosed, for example, by the first
inventor in US Patent
No. 6,482,160 issued November 19, 2002, providing enhanced array gain, and
improved signal
detection. Implementation of the adaptive beamformers has shown provision of
near
instantaneous convergence and provision of coherent beam time series which is
highly beneficial
for the present application of measuring blood pressure in noisy environments.
[0072] As described above the beamforming process is divided into two stages,
a first stage of
beamforming the primary acoustic sensor signals within each sensor strip 403,
as shown in
Fig. 10, using a focused beamformer and a second stage of beamforming across
the sensor strips
403, indicated by ovals, using a planar wave beamformer.
[0073] Alternatively, these stages are interchanged, i.e. the primary acoustic
sensor signals of
acoustic sensors 405 across the sensor strips 403, indicated by the ovals in
Fig. 10, are
beamformed in a first stage using a plane wave beam former and the results are
then beamformed
in a second stage within each sensor strip 403 using a focused beamformer.
[0074] Figs. 11 a to 11 c illustrate various embodiments for the
implementation of the
beamforming process according to the present invention into the signal
processing using adaptive
interference cancellation for blood pressure measurements as disclosed in US
Patent Application
No. 09!718,515, filed November 24, 2000. In a first embodiment, sho~m in Fig.
11 a the adaptive
interference cancellation is applied on each of the microphone signals
followed by the two stages
of beamforming. Alternatively, the adaptive interference cancellation is
applied on the beam time
series output of the first stage of the beamforming process, as shown in Fig.
1 1b. Further
alternatively, and preferably, the adaptive interference cancellation is
performed on the final
beam time series, as shown in Fig. l lc. Performing the adaptive interference
cancellation
processing on the final beam time series is the most efficient implementation
with respect to
processing complexity and does not sacrifice accuracy since the beamforming
process is a linear
operation.
[0075] Using multiple receiving acoustic sensor arranged in a planar array in
combination with
a planar beamforming process for signal processing is highly beneficial for
blood pressure
measurements in noisy environments. The inherent array gain of multiple
receiving acoustic
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sensors and the ability of the multiple acoustic sensors to act as directional
acoustic sensors
substantially enhance signal detection as well as aGCUrate isolation of the
signal source.
[0076] Of course, numerous other embodiments may be envisaged without
departing from the
spirit and scope of the invention.
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