Note: Descriptions are shown in the official language in which they were submitted.
I~ Rec'd P~TIPTO O 3 APR 199~
r~s ~ 9~
ENHANCED ARTERIAL OXYGEN SATURATION DETERMINATION
AND ARTERIAL BLOOD PRESSURE MONITORING
BACKGROUND
lo The Field of the Invention
The present invention is related to noninvasive systems
and methods which are used to monitor the physiological
condition of a patient'~ circulatory system. More
particularly, the present invention is related to an
enhanced noninvasive system and method for monitoriny a
patient's arterial oxygen saturation, and which also
provides continuous measurement o lood pressure.
:~ ,
2. The Backqround Art
The proper utilization of many lifesaving medical
techniques and treatments depends upon the attending
physician obtaining accurate and continually updated
information reyarding various bodily functions of the
patient. Perhaps the most critical information to be
obtained by a physician, and that which will often tell the
physician a great deal concerning what course of treatment
should be immediately instituted, are heart rate, blood
pressure, and arterial oxygen saturation.
In settings such as operating rooms and in intensive
care units, monitoring and recording these indicators of
bodily functions is particularly important. For example,
when an anesthetized patient undergoes surgery, it is
generally the anesthesiologist's role to monitor the
general condition of the patient while the surgeon proceeds
with his tasks. If the anesthesiologist has knowledge of
the patlent's arterial oxygen saturation, heart rate, and
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16 Res1d PCTIPTO O 3 APR l't~
blood pressure, the general condition of the patient's
circulatory system can be assessed~
Arterial oxygen saturation (abbreviated herein as S,02)
is expressed as a percentage of the total hemoglobin in the
patient's blood which is bound to oxygen. The hemoglobin
whi~h is bound to oxygen is referred to as oxyhemoglobin.
In a healthy patient, the S~02 value is above 95% since
blood traveling through the arteries has just passed
through the lungs and has been oxygenated. As blood
courses through the capillaries, oxygen is off-loaded into
the tissues and carbon dioxide is on-loaded into the
hemoglobin. Thus, the oxygen saturation levels in the
capillaries (abbreviated herein as Sc02) is lower than in
the arteries. Furthermore, the blood oxygen saturation
levels in the veins i5 even lower, being about 75% in
healthy patients.
Importantly, if the patient's arterial oxygen saturation
level is too high or too low, the physician may take action
such as reducing or increasing t:he amount of oxygen being
administered to the patient. Proper management of S~02 is
particularly important in neonates where S12 must be
maintained high enough to support cell metabolism but low
enough to avoid damaging oxygen-sen~itive cel}s in the eye
and causing impairment or complete loss o~ vision.
Blood pressure monitoring includes at least three values
which are o~ interest to a physician. First, the systolic
pressure is the high pressur2 generated in the arteries
during contraction (or systole) o~ the left ventricle o~
the heart. Second, the diastolic pressure is the pressure
maintained in the arteries during relaxation (or diastole)
of the left ventricle. Due to the elastic natuxe of the
walls of the arteries, the diastolic pressure is above zero
but less than the systolic pressure.
A third value o~ interest to a physician is the mean
arterial pressure. The mean arterial pressure may be
simply described as the arithmetic average o~ all the blood
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16 Rec'd P~/PT0 0 3 APR l992
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pressure values between, and including, the systolic and
diastolic pre~sures. In addition to the just mentioned
three discrete blood pressure values, a physician is also
interested in obtaining the blood pressure waveform. As is
well known, patients having identical systolic and
diastolic values may have very different mean arterial
pressures and their blood pressure waveforms may be
dramatically different. Having the blood pressure wave~orm
at hand allows the physician to ~ore accurately assess the
patient's condition.
Blood pressure is generally measured quantitatively in
millimeters of mercury (mmHg) referenced against
atmospheric pressure ~about 760 mmHg). Thus, in a normal
person the blood pressure may be 120 mmHg above atmospheric
pressure during systole and 70 mmHg above atmospheric
pressure during diastole. Such values are commonly recorded
as "120 over 70" (120/70l.
Continuous monitoring of arterial oxygen saturation
levels (S,02) and arterial blood pressures each present
unique problems.
One method of determining S,O? is to withdraw blood from
an artery and analyz~ the same l:o determine the amount of
oxyhemoglobin present. Whils in vitro analysi provides
the most accuratQ blood gas determinations, the
disadvantages o~ drawing a blood sample each time an S,02
determi~ation i~ desired by the physician is readily
apparent. Significantly, even in the operating room in
vitro S~O~ deter~inations may take up to several minutes.
Since nerve cells deprived of su~icient oxygen begin to
die in a matter of minute~, the time taken to obtain the
results of an in vitro S,02 analysis may seriously
compromise patient safety.
Particularly in the case of a patient undergoing routine
surgery, the difficulties of withdrawing blood samples
throughout the surgical procedure for S,2 determinations is
generally too great to be adopted as a general practice.
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Still, monitoring of S,02 during all surgeries where general
anesthesia is used and in intensive care units is expected
to have a significant positive ef~ect on the well-being of
patients. Thus, past efforts have been directed to
providing noninvasive systems and m~thod~ for determining
arterial S,0~.
The term "oximetry" has been adopted in the art to refer
to noninvasive apparatus and methods for determininy blood
oxygen saturation levels~ Pre~lously available oximetry
systems make use of the fact that the absorption
characteristics of different blood components, namely, HbO2
and Hb and also referred to as the coefficient o~
extinction, differ depending upon which wavelength of light
(e.q., infrared or visihle portion~ of the spectrum) is
being used.
Thus, previously available noninvasive oximetric systems
impinge at least both visible and infrared light upon a
body part, such as a finger, and then estimate the SO2 level
Z using the relative proportions of visible and infrared
i 20 light which was transmitted or reflected. Undesirablv,
such systems inherently include some inaccuracy, which
increases to a substantial error for low (50-70%) S2
levels, due to, among other things, the inclusio~ of
capillary blood as well as arterial blood in the reading. ;;
In an effort to improve the accuracy of the S02 values
obtained using only two wavelengths of light, rather than
the buIky and expensive ear oximeter previously available,
, whic~ impinged light of eight different wavelengths on the
body part, other apparatus have utilized the pulsatile
component of the transmitted or reflected light beam to
distinguish variations in the detected intensity of the
light beam which are due to chan~es in blood components
from other causes. Generally referred to as pulse
oximetry, using the pulsatile signal modulating the light
beams for S,02 estimate provides a siynificant improvement
in accuracy over nonpul~e oximetry system~ yet still does
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not distinguish between arterial blood oxygen saturation
and capillary blood oxygen saturation.
The previously available systems and methods of
monitoring blood pressure also all have a variety of
disadvantages. The most commonly performed method, the
auscultatory sphygmomanometer method (utilizing a pressure
cu~f, mercury manometer, and a stethoscope), often provides
reasonable estimates of systolic and diastolic blood
pressure. But the method doe~ not provide any information
concerning the mean blood pressure or the pre~sure
waveform. Moreover, a trained professional must take one
Gr more minutes to carry out the method and even then may
be unsuccessful.
Arterial catheterization provides very accurate blood
pressure measurements and waveforms in critical care
situations. The extreme invasiveness and the risks of
catheterization, including infection, thrombus formation,
hemorrhage, and cerebral mbolization precludes the method
from being routinely used on patients.
In an attempt to provide noninvasive blood pressure
monitoring devices, several methods have been suggested in
the past. Devices incorporatinq a constantly inflated
finger cuff which tracks the pressure changes within the
finger disadvantageously may cause pain to the patient,
interference with the pressure measurement, and/or tissue
damageO
In an effoxt to avoid the disadvantages of using a
constantly inflated pr~s~ure cuXf, various devices
utilizing photoplysmography have been introduced. While
such davices utilize a light beam directed at the finger,
or other body part, to sense changes in blood vessel volume
in order to determine changes in pressure and thus avoid
- the use of a constantly inflated pressure cuff, such
devices still suffer from inaccurate readings, particularly
when determining the diastolic pressure, and such devices
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16 Rec'd P~T/PTO O ~ ~PR 199
~T~lS 9 ~
still cannot provide an accurate representation of the
arterial pressure waveform.
In view of the disadvantages and drawbacks of the
previously available apparatus and methods, it would be an
advance in the art to provide a system and method for
noninvasively measuring arterial blood oxygen saturation
levels while minimizing the effect of capillary oxygen
saturation on the measurement. It would be another advance
to provide a system for measuring both arterial oxygen
saturation levels and blood pressure using no more hardware
than necessary to measure oxygen saturation. It would also
be an advance in the art to provide a system and method ~or
noninvasively measuring blood oxygen~saturation levels and
blood pressure which minimizes contact with, ~nd the
pressure applied to, the body of the patient. It would be
a further advance in the art to provide a system for
noninvasive blood oximetry or blood pressure monitoring
~hich may be applied to any one of several parts of the
patient's body.
It would also be an advance in the art to provide both
a method and system for blood oximetry and blood pressure
monitoring which may be implemented using little
specialized hardwarsO It would bQ yet another advance in
the art to provide a noninvasive blood pressure monitoring
system and method which can provide systolic, diastolic,
and mean arterial pressure measurements as well a an
accurate representation of the pressure waveform. Still
another advanc~ in the art would be to provide a
noninva~ive system and method for measuring arterial blood
oxygen saturation levels which enhances the arterial
contribution and reduces the influence of the capillary
contribution to the oxygen saturation measurement.
OBJECTS AND BRIEF SUMMARY OF THE INVENTION
In view o~ the prior stats of the art, it i5 a primary
object of the present invention to provide a noninvasive
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system and method to determine arterial blood oxygen
saturation levels while minimizing the interference of the
capillary blood oxygen saturation levels with the
determination of arterial blood oxygen saturation levels.
Another object of the present invention is to implement
a noninvasive system and method for carrying out arterial
blood oxim~try which i5 more accurate than previously
available apparatus and methods and which i5 also capabie
of being used on more than one body part of the patient.
lo It is another object of the present invention to provide
a system and method which allows both blood pressure
monitoring and blood oximetry to be concurrently ~arried
out by the same apparatus. Still another object of the
present invention is to provide a system and method for
noninvasive blood oximetry which can be operated in both a
transmi~sion and reflection mode and can be backed on any
one of a plurality of body parts.
It is a still urther object of the present invention to
provide a noninvasive blood oximetry and blood pressure
monitoring system and method which does not require that
pressure be applied to the patient's body during the
monitoring interval and that occlusive pressure is applied
`~ for only brie~ durations during calibration intervals.
Yet another object of the present invention is to
provide a noninvasive system and method for both blood
oximetryiand accurately determining a patient's systolic,
diastolic, and mean arterial blood pressure and displaying
the patient's blood pressure waveform.
Additional objects and advantages will be apparent from
the description which follows, or may bP learned by the
~, practice of the invention.
Consistent with the foregoing objects, the present
invention provides a noninvasive system and method ~or
enhanced monitoring of arterial oxygen saturation (S,02)
which may be used alone or in co~bination with a method for
continuously and noninvasively monitoring blood pressure.
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When used, the monitoring of blood pressure provides
determinations of systolic pressure, diastolic pressure,
mean arterial pressure, and perhaps most siynificantly,
producing an accurate arterial pressure waveform. ~ost
advantageously, the present invention allows the same
hardware to be used for both monitoring of arterial oxygen
saturation and monitoring of arterial blood pressure.
The apparatus of the presently preferred embodiment of
the present invention includes-a light means comprising two
or more light emitting devices which are positioned to
direct at least two light beams into a body part of tha ~ i
patient. The two light beams are comprised of two
different wavelengths, preferably a~re~erence light beam,
which is absorbed substantially equally by both
oxyhemoglobin and reduced hemoglobin, preferably having a
wavelength in the infrared portion of the spectrum and a
measurement light beam, which is absorbed unequally by
oxyhemog~obin and reduced hemoglobin, preferably having a
wavelength in the visible red portion of the spectrum.
Other portions of the spectrum m21y al80 be used within the
scope of the claimed invention.
Also provided is a detection mean~, transducer means, or
a photodeteGtor which detects th~ amount of the light beams
which are absorbed by the blood. The detection means and
equivalent devices may be positioned to detect either the
light transmitted through, or reflected by, the body part.
Importantly, the visible red light beam which will be
transmitted or re~lected will vary according to the ratio
o~ oxyhemoglobin (HbO2), to reduced hemoglobin (Hb) in the
blood. Oxyhemoglobin i5 the component of blood responsible
for carrying almost all of the oxygen to the body tissues.
In contrast, the intensity of the detected infrared light
beam will not vary significantly with the ratio of HbO2 to
Hb. This is due to the fact that the amount of infrared
light absorbed by the body part is affected relatiYely
little by the changing proportions of HbO2 and Hb.
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In accordance with the present invention, an enhancement
means is provided to increas2 the arterial contribution of
the pulsatile component of the light beams which are
detected by the phototransducer means. The enhancement
means comprises a pressure means for imposing an increased
pressure on the body part.
With each heartbeat the volumP of the arteries varies
slightly which modulates the intensity of the detected
light beams. The pulsatile co~ponent may also be referred
to a~ the "AC component" of the light beam "signal." The
pulsatile c~mponent is impressed upon a relatively steady
light beam "signal'~ referred to as the "DC" "signal." The
importanc~ of the pulsatile component is known to those
skilled in the art and will be further explained later in
lS this disclosure.
The enhancement means operates by applying an increased
enhancement pressure onto the body part into which the
light beam~ are directed. By applying an enhancement
pressure to the body part, the enhancement pressure being
approximately equal to the mean arterial pressure of the
major artery or arteries locat:ed in the body part, the
arterial pulsatile component of the light beam detected by
i the phototransducer means will be maximized due to
unloading of the translu~inal pressure which results in
maximizing arterial compliance. Generally,~the increase in
the pulsatile component will be about an order of magnitude
greater than the pulsatile component of the detected light
beams without application of the enhancement pressure.
Importantly, application of the enhancement pressure
decreases the relative contribution of the capillary blood
oxygen saturation (Sc02) to the intensity of the detected
light beams. Thus, the increased enhancement pressure both
increases the modulation of the light beam due to the
increase in amplitude of the arterial pulses and by
reducing the amount o~ capillary blood in the body part.
'
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The imposition of the enhancement pressure on the body
part may be considered a "physiological calibration."
Having carried out such a "physiological calibration" by
enhancing the contribution of the pulsatile arterial oxygen
saturation level to the light detected by the
. .
phototransducer means, a processor means, for example a
microprocessor or other computing device, may derive a
calibration factor representing the contribution of the
capillary oxygen saturation to t~e total light detected by
the phototransducer means.
The processor means, or microprocessor, controls the
operation of the system to carry out the method of the
present invention to completion and thus continually
update~ and displays the arterial oxygen saturation level
of the patiant on a display means such as a video monitor.
The enhancement pressure may be imposed by a device such as
an inflatable pressure cuf~, accompanied by a controllable
pressure pump, adapted for placement on a finger, forehead,
or some other body part.
The enhancement pressure is only applied during a first
interval of the calibration period. During a second
interval of the calibration period, the enhancement
pressure i~ released and a calibration factor is obtained
which reflects the ratio of S,02 to Sc02 After the
calibration period is completed, the monitoring period is
begun and the calibration information is used to determine
the proportion of the pulsatile signal detected ~y the
phototransducer means which is caused by the arterial
oxygen saturation level rather th~n the capillary oxygen
saturation level.
The present invention also includeR utilizing the above
described hardware for continual blood pressure monitoring
and wa~eform display. The pressure monitoring function is
carried out by determining the mean arterial pressure and
the systolic blood pressure using the oscillometric method.
In the oscillometric method the mean arterial pressure is
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det~rmined by adjusting the inflation of a pressure cuff
placed around a body part until the pulsatile signal is
maximized. once the amplitude o~ the pulsatile signal is
maximized, the pressure within the cuff is approximately
equal to the mean arterial pressure.
The oscillometric method determines the systolic
pressure by increasing the pressure applied to a body part
to above the systolic pressure, i.e., completely occluding
the artery so that no pulsatile signal is present, and then
gradually reducing the pressure within the cuff until a
pulsatile signal appears, providing a data point which can
be used to calculate the patient' s systolic pressure using
a procedure described herein.
Advantageously/ the present invention also provides for
calculation of a complete pressure wave~orm and diastolic
pressure. With the mean arterial pressure and the systolic
pressure being known, the present invention allows the
change in volume of the artery, which is proportional to
the pressure within the artery, to be detected by the
phototransducer means as a modulation of the intensity of
the measurement (red or infrared) light beam directed into
the body part.
The pressure-volume relationship of an artery is not
linear or the same ~rom patient t:o patient or from hour to
hour. The pressure-volume relationship of the patient's
artery may be described and predicted using a model known
as the "Hardy model compliance curve." The information
needed to datermine the pressure-volume relationship,
including the systolic pressure and the mean arterial
pressure, are obtained using the oscillometric method
during the calibration period when the pressure cuff is
inflated in the below-described manner.
During the monitoring period, the pressure within the
cuf~ is released and the volume change in the artery is
detected by the phototransducer means. The present
in~ention then uses a recursive procedure wherein an
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3~r~ PCT/PTO ~ 3 APR ~992
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estimated diastolic pressure and the Hardy model compliance
curve is used to derive a calculated mean arterial
pressure. If the difference between the calculated mean
arterial pressure and the measured mean arterial pressure
is within a predetermined standard, then the estimated
diastolic pressure is displayed on the display means as the
patient's diastolic pressure. If the calculated mean
arterial pressure and the measured mean arterial pressure
do not agree within predetermined limits, a new estimated
lo diastolic pressure is chosen and'the calculations repeated
until the estimated diastolic pressure produces a
calculated mean arterial pressure substantially the same as
the measured mean arterial pressure.
As the diastolic blood pressure is being calculated/
three parameters required to determine the pressure-volume
relationship in the artery using the Hardy model are being
calculated. The three parameters include:
k = compliance index for the arterial blood vessels
of the patient;
V~ = maximum volume of the arterial blood ves~els in
the patient's body part; and
VO = volum~ of the arteri.al blood vessels in the
patient's body part al: zero pressure
Importantly, using the described method, the value of
any point on a blood pressure waveform between the systolic
and diastolic pre sures may ba calculated. Thus, a
continuous and complete blood pressure waveform may be
generated using the method. The ability to calculate a
complete and accurate representation o~ the patient's
arterial blood pressure waveform is a great advance over
previously available systems using photoplethysmography.
' 35 - Further information concerning the pressure monitoring
function of the present invention will be provided later in
this di~closure as well as being provided in United States
Patent Application Serial No. 07/068,107 (now U.S. Patent
No. 4,846,189) entitled "Noncontactiv~ Arterial Blood
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16 Re~'d PCT/PTO 0 3 APR 199~ 99~
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Pressure Monitor and Measuring Method" filed on June 29,
1987, which is incorporated herein by referenceO
As will be more fully appreciated during a dPscription
of the remainder of this disclosure, the blood oximetry
functions of the present invention may be carried out alone
or a system can be designed to carry out the oximetry
function as well as the blood pressure monitoring function
without requiring any hardware in addition to that used to
carry out the oximetry functiQn of the present invention.
BRIEF DESCRIPTION QF THE DRAWINGS
Figure 1 is a perspective view o~ the presently
preferred embodiment of the present invention which is
configurad to pro~ide both blood pressure monitoring and
arterial oxygen saturation monitoring functions.
Figure 2 is a block diagram of the system of the
presently preferred embodiment of the present ~nvention.
Figure 2A is a cross sectional view o~ another preferred
embodiment of the pressure cuff represented in Figure 2.
Figures 3A and 3B are flow charts representing the steps
of one presently preferred method of the present invention
for determining arterial blood oxygen saturation levels.
Figure 4 is a waveform diagram showing the application
and release of pressure on the patient's body by the
pressure cuff of the descxibed embodiment and its effect on ~ ;
the detected light beams.
Figures 5A and 5B are flow charts representing the steps
of another presently preferred method of the present
invention for determining arterial blood oxygen saturation
levels. -
Reference will now be made to the drawings to describe
the presently preferred embodiment of the present
invention. While the embodiment described herein performs
both blood oxygen sa'curation and blood pressure monitoring
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components. It will be appreciated that components which
are equivalent to many of the functional blocks represented
in Figure 2 are contained within the structures illustrated
in Figure 1 and thus are not separately represented in
Figure 1.
Shown in Figure 1 is a patient's finger 36 and the
presently preferred embodiment of the present invention
being used to determine the patient' 5 Sl02 level at the
numerical display represente~ generally at 120 The
patient's blood pressure is also being monitored with the
systolic, mean, and diastolic blood pressure values being
provided at numerical displays represented generally at 20,
1~, and 16, respectively. The pa~ient's blood pressure
wavsform i~ also being shown on the visual display
indicated at 22.
The illustrated embodiment, as well as other embodiments
of the present invention, have application in many
circumstances. Such circumstanceR may include patients
undergoing anesthesia during surgery, critical and
intensive care units, exercise and sleep studies, as well
as other applications.
In Figure 1 the sensing elements of the embodiment,
including the pressure cuff 34 which surrounds the light
emitting diodes~ the photodetector, and the pressure
transducer, are located between the first and second
knuckle of the patient's index finger. While this position
is illustrated for purpos~s of describing the presently
pre~erred embodiment, other positions on the body may be
used in specific circumstances as will be discussed later.
Also, the specific arrangement of the sensinq elements in
relation to the body part will be described as appropriate
in the description of the preferred embodiment.
Figure 2 illustrates the major functional blocks of the
embodiment illustrated in Figure 1 and described herein.
It is to be understood that the hardware represented by the
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16 R~'d PCT/PTO O 3 APR 1992
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functional blocks illustrated in Figure 2 may be
implemented in many different ways.
In the presently preferred embodiment, the microcomputer
may be a general purpose microcomputer 40 such as an IBM
Personal Computer or an equivalent device. Alternativelv,
it may be desirablP to utilize a more power~ul
microcomputer or to devise a microprocessor-based apparatus
specifically designed to carry out the data processing
functions incidental to this-invention. When choosing a
microcomputer, if both the blood oximetry and the blood
prPssure monitoring (including waveform display) are to be
carried out and displayed in real time, the microcomputer
40 or other processor means must carry out a large number
of computation~ very quickly.
Importantly, the hardware which embodies the processor
means of the present invention must function to perform the
operations essential to the invention and any device
capable of performing the necessary operations should be
considered an equivalent of the processor means. As will
be appreciated, advances in the art of modern ele~tronic
devices may allow the processor means to carry out
internally many of the ~unctiona carried out by hardware
illustrated in Figure 2 as being independent of the
processor means. The practical considerations of cost and
performance o~ the system will generally determine the
delegation o~ functions between the processor means and the
remaining dedicated hardware.
As can be seen in Figure 2, in the presently preferred
embodiment microcomputer 40 is interconnected with the
remaining apparatus hardware by way of I/0 ports 44 and a
plurality of analog to digital converters 460 Also, a
visual display 42 is connected to the microcomputer 40.
Visual display 42 perform~ the function of a display
mean~. As intended herein, the display means may be any
device which enables the operating personnel to observe the
values and waveforma calculated by the microcomputer.
8lJ8SF~TUTE St~E~T
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Thus, the display means may be a device such as a cathode
ray tube, an LCD display, a chart recorder, or any other
device performing a similar function.
The method o~ the present invention is carried out under
the control of a program resident in the microcomputer.
Those skilled in the art, using the information given
herein, will readily be able to assemble the necessary
hardware, e~ither by purchasing it off-the-shelf or by
fabricating it and properly pro~ram the microcomputer in
lU either a low level or a high level programming language.
While it is desirable to utilize clock rates that are as
high as possible and as many bits as possible in the A/D
converters 46, the application o~ the embodiment and
economic con~ideration will allow one skilled in the art
to choose appropriate hardware ~or interfacing the
microcomputer with the remainder of the embodiment. Also,
it should be understood that for reasons of simplifying the
diagrams, power supply connections, as well as other
necessary structures, are not explicitly shown in the
figures, but are provided in actuality using conventional
technigues and apparatus.
As represented in Figure 2, an LED current driver 48 is
provided. The LED current driver 48 controls the amount of
current directed to the infrared LED and the red LED.
Since LEDs ar~ current controlled devices, the amount of
current passed through the devices determines, within
device limits, the intensity of the light bean emitted
thereby.
Schematically shown in Figure 2 is a side view of a
patient' finger 36 with the pressure cuff 34 shown in
cross section, also referred to as the enhancement means,
which surrounds the finger. Disposed on the interior of
the pressure cuff are the infrared LED 56, the red LED 54,
and a photodiode 64.
Both the infrared LED 56 and the red LED 54 may be
devices which are commonly available in the semiconductor
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industry. They provide high power outputs and relatively
stable operation at a reasonable cost per device. The red
LED 54 preferably emits a light beam having a wavelength of
660 nanometers ~also preferably in the range from about 600
to about 725 nanometers) and the infrared LED 56 preferably
emits a light beam having a wavelength of ~30 nanometers
(also preferably in the range from about 875 to about 1,000
nanometers).
Light emitting devices other than those mentioned above
could be used and are intended to be within the scope of
the inventive concepts claimed herein. The light emitting
devices may be placed outside of the pressure cuff 34 with
a fiber optic pathway provided to the interior of the
pressure cuff. Furthermore, othPr wavelengths o~ light may
be used as suitable devices for generating such wavelengths
become available.
As used herein, the phrase light means is intended to
include the above-mentioned LEDs as well as any devices
which perform functions e~uivalent to those performed by
the LEDS. As will be appreciated by considering the
foregoing discussion, any source or sources of light
capable o~ emittlng light having two differing and
appropriate wavelengths may function as the light means.
Thus, for example, unitary light emitting devices capable
25 of emitting two or more wavelength~ of light, or devices
emitting wavelengths of light other than those specified
above, are within the intended scope of the phrase
structure defined by light means.
The photodiode 64 disposed withln the pressure cuff 34
is preferably one having a spectral response which is
substantially equal at the wavelengths emitted by the
infrared LED 56 and the red LED 54 and which, like the
LEDS, is sapable of stable operation over a long period of
time. It may be desirable to include a temperature sensing
device (not shown) adjacent the LEDS and the photodiode to
provide the microcomputer 40 data on the temperature
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dependent variations in the operations of LEDs 54 and 56
and the photodiode 64. It is preferable that the LEDS and
the photodiode be readily replaceable so that any drift
which occurs in the operating parameters o~ the devices
(possibly due to the effects of aging) may be remedied by
replacing old components with new ones.
The function~ carried out by photodiode 64 may be best
labeled by the phrases detection means, light detection
means, and transducer means. Import~ntly, any device which
performs the function of detecting the amount of light
transmitted through, or reflected fro~, a body part and
creating an electrical signal of some kind which contains
inormation on the intensity of the light striking the
device may function a~ the detection m2ans, light detection
mean~, or transducer means~
As will be appreciated by those skilled in the art,
phototransducers such as phototransistors and many other
devices now available, or available in the future, have
application within the scope of the present invention.
Methods for determining arterial blood oxygen levels using
either light beams passed through, or reflected from, a
body part will be described later in this disclosure.
It is presently pre~erred that the LEDs 54 and 56 be
positioned about the fing~r so that the light ~eams pass
through the digital arteries on each side o~ the phalanx
bone. Thus, the arterial blood's contribution to the
modulation of the light beam~ is maximized rather than the
light beams being absorbed by tissue and bone~ Also,
rather than having a single LED located on each side of the
phalanx bone, a pair of LEDS, each pair including a red LED
and an infrared L~D, ~ay be positioned immediately adjacent
each other. Each pair of LEDs is positioned on the
interior of the pressure cuf~ so tha~ the respective light
beams pass through one o~ the arterie~ located on each side
of the phalanx bone o~ the finger. This provides that both
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an infrared and a red light beam will be equally modulated
by the same artery.
Also represented in Figure 2 is a pressure transducer
58. The pressure transducer 58 is used when determining the
patient's blood pressure but is not necessary to the blood
oximetry function o~ the present invention. Pressur~
transducer 58 acts as a pressure detection means or a
pressure transducer means and functions to generate an
electrical signal which is proportional to the pressure
being imposed upon the body part by the pressure cuff.
Thus, any device perfor~ing the same, or an equivalent
~unction, should be considered a pressure detection means
or pressure transducer means.
Alternatlvely, rather than locating the sensing elements
15 on the patient's finger, the sensing elements may be
located on body parts such as on a toe, ear, the web of the
hand, or over the temporal artery on the patient' 5
forehead. of course, each of these locations will require
a different arrangement for the pressure cuff or other
structure for imposing the enhancement pressure.
In particular, locating the sensing structures over the
temporal artery on the forehead requires that the LEDs and
photodiode be positioned so that the photodiode senses the
light beam~ which are reflected from, rather than
transmitted through, the body part. Furthermore, a
strUCtUrQ other than a pressure cuff must b~ used to apply
pres~ure to the temporal artery and to hold the pressure
imposing device in place. Still, the temporal artery may
be the most preferred location for the sensing structures
in many cases due to the fact that perfusion at the
temporal artery is affected less by vascular disease and
drugs than the arteries found in the extremities. Thus,
use of the temporal artery may provide more accurate S,o2
determinations than a location on a patient's extremities,
in some cases.
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As shown in Figure 2, an LED multiplexer 52, driven by
a clock 50, alternately connects the current driver 48 to
either the infrared LED 56 or the red LED 54. The
operation of the clock 50 and the LED multiplexer 52
ensures that only one of either the red LED 54 or the
infrared LED 56 will operate at one time. The output of
clock 50 is also input to channel multiplexer 74 to provide
synchronized operation.
The pressure cuff 34 should be opaque so that the
photodiode 64 is shielded from any stray ambient light.
The pressure cuf~ 34 i5 inflated and deflated by a pump 68
which operates under the control of the pump driver 20
which is in turn controlled by the microcomputPr 40.
As suggested earlier, if the embodiment is to be used
only for determinations of S~O2, the pump 68 need only be
capable of inflating the pressure cuff 34 to a pressure
equal to the mean arterial pressure. I~ the embodiment is
to be used to also determine blood pressure, the pump 68
should be capable of inflating the pressure cuff 34 to a
pressure well above the patient'æ systolic pressure so that
the arteries may be completely occluded and the systolic
pressure determined as explained earlier.
The pressure cuf~ 34, pump S8, and pump driver 70
comprise the enhancement mean3 or pressure means of the
present invention. As will be appreciated from the
previou~ discus~ion concer~ing the application of mean
arterial pressure on an artery and its effect on the
arterial pulsatile signal, any structure which functions to
partially or fully occlude a patient's artery should be
considered the equivalent of the enhancement means or
pressure means. The body part which is used as a sensing
location will often dictate the best devices and structures
used as the enhancement or pressure means.
As illustrated in Figure 2, a preamplifier 66 receives
the output of the photodiode 64. The preamplifier 66
boosts the photodiode output to a level usable by the
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automatic gain control (AGC) 72. The automatic gain
control 72 ~unctions to limit the dynamic range o~ the
voltaqe signal output from the preamplifier 66 to that
which is appropriate ~or the circuits which ~ollow.
The gain-controlled output from the AGC 72 is applied to
a channel multiplexer 74 which is also driven by the clock
50. Thus, when the LED multiplexer 52 causes the red LED
54 to operate, the output of the AGC 72 is directed to
Channel 1 (red) as reprasented at 76 in Figure 2.
Conversely, when the LED multiplexer 52 causes the infrared
LED 56 to operate, the output of the AGC 72 is directed to
Channel 2 (infrared) as represented at 78 in Figure 2.
Each channel 76 and 78 include a low pass ~ilter 80 and
82 to reduce high frequency (e.q., > 40 Hz) noise. The
signal output from each of the low pass filters 80 and 82
is applied to pulsatile signal amplifiers 84 and 86,
respectively, which include high-pass ~ilters to prevent
passage of direct current and very low frequencies (e.a.,
> 1 Hz). Thus, the pulsatile signal amplifiers 84 and 86
can be thought of as AC amplifiers. The output of the
pulsatile signal amplifiers provide ~ signal and, ~VR
signal to the microprocessor by way o~ the A/D converters
46. The ~ and ~VR signals re~lect only the AC, i.e.,
pulsatile, component o~ the light beams passed through the
patient's body part.
The total signal amplifiers 88 and 90, one providsd for
each channel, are not frequency limited and thus pass to
thair output~ an amplified waveform containing both the DC
and AC components o~ the V~ and VR signals which wPre outpu~
from the low pass filtPrs 80 and 82, respectively.
With the hardware assembled as illustrated in Figure 2,
data concerning all of the variables which must be
considered to determine both the patient's S,02 level and
blood pressure i5 presented to the microcomputer for
processlng according to the method of the present
invention. In summary, the microcomputer 40 controls the
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intensity of the LEDs 54 and 56, the inflation of the
pressure cuff 34, and the gain of the output from the
photodiode 64. The microcomputer receives as input data,
the QV~ and ~VR signals (pulsatile compon nt of the signals)
and the V~ and VR signals (the total signals including both
the AC and DC components).
The presently preferred method of the present invention
is carried out by the system illustrated in Figure 2 and
comprises those steps illustrated in the ~low chart of
Figure 3. In order to explain one method of tAe preferred
embodiment~ Figures 3 and 3B will be used with reference to
the wavefo~m diagrams of Figure 4 as well as the block
diagram of Figura 2.
The flow chart of Figures 3 and 3B represents just one
of the many embodiments which may be used to carry ou~ the
method defined in the claims. Particularly, with the
widespread availability of powerful microprocessors, the
present invention requires little specialized hardware and
the data acquisition and manipulations steps described
herein may be varied and yet st:ill be within the scope of
the invention as defined in the claims. In order to
- clarify the following description, the blood oximetry
function of the present invention will first be ex~lained
and then the combination o~ the blood oximetry function and
th~ blood pressure monitoring function will be explained.
It should bo not d that the flow chart of Figure 3 is
divided into three principal periods: the initialization
period, the calibration period, and the monitoring period.
Furthermore, the calibration period is divided into an
enhancement pressure-on interval when the enhancement
pxessure is applied to the patient's body part and an
enhancement pressure-off interval when the enhancement
pressure is not applied.
Briefly, the 5tep8 carried out during the initialization
period include thos2 pertaining to determining certain s~t
up parameters, and implementing any software routines which
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must be running while data is being acquired. The steps
carried out during the calibration period include imposing
an increased enhancement pressure on the body part,
acquiring data, determining the S1O2 with the enhancement
pressure on, and then with the enhancement pressure off,
continuing to acquire data which can be used to determine
a "physiological calibration factor" which is used during
the monitoring period. During the monitoring period no
pressure is applied to the body part and further data is
obtained to determine the patient's SO2 level. The data
previously acquired and the resulting calculated values are
used according to the method described herein to determine
the S1O2 level during the monitoring period.
As shown in the flow chart of Figures 3 and 3B, the
method of the present invention begins during the
initialization period with the initialization of the
hardware and software of the system as represented at step
100. Those skilled in the application of microprocessors
to medical monitoring situations will understand the
various software routines which should be run after power
is applied, but before data is acquired. For example, as
represented at step 102, it is very desirable to implement
a conventional noise discrimination routine.
In the present case, such a noise discrimination routine
may be one known to those skilled in the art which includes
an algorithm to distinguish information associated with
each pulse and heart beat from noise, which in the present
system, may be due to ambient light temporarily striking
the photodiode or artifacts in the signals caused by notion
of the patient. During such a noise discrimination routine,
the patient's heart rate will be determined and may be
displayed for the information of the attending medical
professional.
As mentioned earlier, the calibration period includes an
"enhancement pressure-on interval" and an "enhanced
pressure-off interval" which is followed by a monitoring
~ ~c~'d Pl~T/pTo O ~ APR 1992
-25-
period. The length o~ each of these periods (T~, T~, and
TMON~ respectively) ~re determined at step 104 according to
the eriteria discussed below. While not represented in the
flow chart of Figure 3A, in some embodiments it may be
S desirable to include a software routine which will vary T~,
T~, and TMON according to the physiological condition of the
patient.
It is known that application of pressure on a body part
which causes even partial occlUsion of blood vessels and
lo capillaries to some extent has an effect on perfusion in
the body part. Significantly, if pressure is applied to a
body paxt long enough, the actual blood pr2ssure found in
the blood vessels will begin to change due to changes in
the blood vessels involved. Further~ore, determinations of
S~0~ become more difficult and less reliable the longer the
pressure is applied. Moreover, fro~ the view point of the
unanesthetized patient, application of pressure on a body
part wil~ result in pain.
Thus, it is important that the time that the enhancement
pressure is imposed be limited to avoid pain in the
unanesthetized patient and in all patients to avoid
altering the patient ' s blood pressure and S,02. In general
cases, T~ will be less than or equal to about 002 to about
O.5 of th~ sum o~ T~ a~d T~ON resulting in a pressure
imposed du~y cycle of less than about 20% to a~out 50%.
With the above considerations in mind, i~ is necessary
to deter~ine how long the calibration period (T~ ~ T~j
should be in relation to the length o~ the monitoring
period which will also determine how often the steps of the
calibration period are carried out. Importantly, the
calibration period must be long enou~h to allow accurate
data to be collected. Additionally, since physiological
parametsrs change o~er time, and may change rapidly due to
stress, iniury to the patient, drug~, or other treatment
3S administered to the patient, the steps of the calibration
period must be carried out regularly.
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For example, if a patient's condition is rapidly
changing and the patient i3 unconscious, it may be
desirable to carry out the steps of the calibration period
f or as long as the steps o~ the monitoring period are
carried out in order to obtain the most accurate and
constantly updated information to the attending physician.
Moreover, in many patients suffering from vascular disease,
poor perfusion may cause reliable S,02 determinations to be
available only when the enhan~ement pressure is imposed
upon the body part.
Once the initialization period steps have been
completed, the enhancement pressure is applied to the body
part a-q represented at step 106. As explained earlier, the
enhancement pressure may be applied to one of several body
parts containing a significant artery. As explained
earlier, the imposition of the enhancement pressure
accomplishes two primary results: Increasing the amplitude
of the AC (or pulsatile) component of the arterial pulse
component of the transmitted (or reflected in the case o~
the method reprasented in Figures 5A and 5B) light beams;
and Decrea~ing the absorption of the light bea~s by blood
in the capillaries increasing the amplitude of the AC (or
pulsatile) component o~ the arterial pulse o~ the artery.
~oth of these results allows more accurate noninvasive S~02
determinations than previously possibla. Such accurate S.02
determinations are even possible under conditions o~
relatively low per~usion. As will now be recognized, the
enhaneement pressure is so named because the contribution
of the arterial blood to the SO2 determination is enhanced.
The result of increasing the amplitude of the pulse of
the artery is brought about by the well known effect that
the amplitude o~ the blood pressure pulse3 is maximized as
the pressure imposed upon the artery equals the mean
arterial pressure. The increase in artery pulses, i.e.,
the pulsatile signal detected by the system, allows more
accurate SAO2 determinations even under conditions o~ low
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perfusion. Because the difference between S~O2 and ScO2 may
vary dramatically from patient to patient and from hour to
hour, the "physiological calibration" carried out by the
present invention is essential to improving the accuracy of
S~02 determinations.
In practice, it is not necessary for the blood oximetry
system to hold the enhancement pressure at exactly the mean
arterial pressure for the entire enhancement pressure-on
interval. As shown in Figure 4 at waveform A, when the
enhancement pressure is increased to, for example, loO mmHg
(assuming th~ mean arterial pressure is 100 mmHg) the
pulsatile signals ~Va and ~V~ (waveforms B and D,
respectively) increase by about a~ order of magnitude.
Thus, the enhancement pressure need only be about equal to
the mean arterial pressure to cause the desired increase in
the pulsatile signals (~VR and ~V~).
Rather than holding the enhancement pressure exactly on
the mean arterial pressure, it may be useful to 910wly ramp
the enhancement pressure (e,~., 5 mmHg/sec), particularly
when a ranping pressure must be imposed to accurately
determine the mean arterial pressure for use in blood
pressure.
As shown at step 108 in Figure 3A, after th~ enhancement
pressure ha~ been imposed, it is generally necessary to
wait at least two heart beats so that the physiological
parameters can stabilize after changing the pressure
imposed upon the body part. once the physiological
parameters have stabilized, it is necessary to determine
values for the following variables as shown at 110 in
30Figure 3: ^
~V _ the pulsatile signal output from the
~ photodiode when th~ red LED i~ operating
during the enhancement pressure-on interval
av~ = the pulsatile signal output from the
~ photodiode when the infraxed LED is
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operating during the enhancement prsssure-on
interval
VR = the average of the total signal output from
~P the photodiode when the red LED is operating
during the enhancement pressure-on interval
V~ = the average of the total signal output from
~ the photodiode when the infrared LED is
operating during the enhancement pressure-on
interval
The ~VR and ~V~ are input-t~ the microcomputer by way
of the appropriate channel ampliflers and analog to digital
converters. The VR and V~ value~ are calculated by the
microcomputer by the data received from the total signal
amplifiers ~8 and 90 and the analog to digital converters
46. Figure 4 provide representative waveforms suggesting
relative values of the listed variables.
~0 In practice, the waveform~ are not continuous but are
time division multiplexed with Channel 1 tthe red channel)
and Channel 2 ~the infrared channel) each having a voltage
from the photodiode gated to the channel amplifiers an
equal amount of time. HoweYer, the gating of the output of
the photodiode is not represented in waveforms B, C, D, and
- E in order to increase the clarity of the waveforms.
Mor over, the operation o~ the clock represented in Figure
2 desirably may be synchronized with the operation of the
analog-to-digital converters and also sho~ld be fast enough
that a very accurate representation of the waveforms may be
preserved.
Each o these waveforms i~ represented in Figure 4. As
shown at waveforms B and D during T~, the ~VR and V~
waveform~ include only the C or pulsatile component of the
photodiode ~ignal as processed by, and output from, the
pulsatile signal amplifiers of each channel. The VR and the
V~ represented by waveforms C and E, respectively, of
Figure 4, are an average, or more specifically a mean, of
the total signal output from the photodiode.
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It will be appreciated that in the described embodiment
the signal output from photodiode 64 will be expressed and
processed in terms of a voltage, hence the label 1'V.i-
In particular, the V~ and the V~ signals are not
directly measured but are determined mathematically by themicrocomputer hardware and software from the signal output
from the total signal amplifiers 88 and 90 of each channel
and digitized by the analog-to-digital converters 46. It
will be appreciated that much .of the signal processing
hardware may be eliminated by assigning more of the signal
processing to the microcomputer without departing from the
spirit and essential characteristics of the system and
method of tha present invention. N~rtheless, in order to
arrive at an appropriat~ balance between speed of
operation, flexibility, accuracy, and cost o~ the system,
the dedicated hardware, such as the amplifiers 84, 86, 88,
and 90, which is illustrated and described is preferably
included.in the system.
Next, as represented at step 112, the average (mean~ of
multiple determinations of ~V~ V~ , VR , and V~ are
each calculated and stored until the elapsed time of the
enhancement pressure on interval (t~) is equal to or
greater than the preset enhance~lent pressure interval T~.,
as represented at step 114. It will be realized that in
some circumstances it may be d~sirable to express T~, and
the other periods and intervals discussed herein, in terms
o~ the number of heartbeats which have occurred rathPr than
on a set period of time. Still ~urther, it may be useful
in some cases to include algorithms in the embodied method
of the present invention which may switch between using
heartbeat~ and set time periods for the intervals and which
may also vary the length, whether time or heartbeats, of
the intervals.
Each averag~ determined from the ~VR, ~V~, VR, and
V~ signals are individually stored in the microcomputer's
Er
memory.
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16 3~e~d P~T/PTO O 3 APR 1992
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Next, as shown at Step 116, a value for RLOG~ using
equation (1) is determined u~ing the qtored average values:
log (1 ~ ~VR /VR
RLOG~ =
log (1 ~ ~V~ /V~ )
Equation (1) is applied to a data obtained by
transmitting the light beams through a body part since the
transmission of light through whole blood only somewhat
follows the LambertBeers law~ Equation (1) requires that
the log of the pertinent value~ be calculated. This
equation i familiar to thosa skilled in the art and may be
easily carried out by the microcomputer.
However, since transmission of light through whole blood
results in values which deviate significantly from the
LambertBeers law once a value for RLO&~ i5 calculated and
stored, the S,02 corresponding to the RLOG~ value is found
by reference to a RLOG~ look-up table as indicated at step
118. The RLOG~ look up table i5 derived from empirical data
~athered during use of the system described herein. For
example, once a red LED, infrared LED, photodiode, and
other hardware items have been configured to provide the
system descri~ed herein, the values obtained for ~LOG~ may
be correlated with the S~02 value obtained using another S,02
determination method, for example,,an in vitro method.
Alternatively, tha subject's S,02 may be altered by altering
the compositio~ of the inspired ga~e and monitoring the
composition o~ the expired gases. Once the look up table
has been completed, it can ba used in the case of any
number of patients if the performance of the apparatus
hardware is maintained within appropriate parameters
considering the e~fects o~ age, temperature, and
variability of mass produced components.
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The S,02 which was determined from the RLOG~ look-up
table at step 118 is displayed as represented at step 120
in Figura 3 on the display means 42 represented in Figure
2. It should be appreciated that the 5~2 value displayed
at step 120 during the enhancement pressure on interval is
more ~ccurate and reliable than S~O2 values provided by
previously available pulse oximetry sys~ems due to the
enhancement of the arterial pulsatile signal output from
the photodiode and the-decrease of the capillary oxygen
10 saturation contribution to the same signal.
Nevertheless, the interval during which the enhancement
pressure is imposed must be limited due to several
considerations including avoiding pain for the patient and
affecting the physiology of the patient so that the
15 measurements obtained are altered in any significant
fashion. Thus, the enhancement pressure is released from
the body part for the remainder of the calibration period
and monitoring period as represented at step 122 as shown
in Figure 3B.
As shown in Figure 4, the enhancement pressure-off
interval of th~ calibration period begins when the
enhancement pressure is released and the pressure on ~he
body part returns to the ambient pressure. Again, as
represe~ted at step 124, it is necessary to wait at least
25 two heartbeats begore measuring any variables.
Continuing to refer to Figure 3B and similarly to the
step~ taken during the enhancement pressure-on interval,
the snhancement pressure-off interval includes steps to
determine four variables as shown at Step 126.
3 O ~VB = the pulsatile signal output from the
photodiode when the red LED is operating
during the enhancement pressure-off interval
. ~V~ - the pulsatile signal output from the
. 35 ~ photodiode when the infrared LED i5 operating
. during the enhancement pressure-o~f interval
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v~ = the average o~ the total signal output from
the photodiode when the red LED is operating
during thQ enhancement pressure-o~f interval
V~ = the average of the total ~ignal output from
the photodiode when the in~rared LED is
operating during the enhancement pressure-off
interval
Also, similarly to the step~ taken during the
enhancement pressure-on interval, ths average of multiple
determinations of the enhancement pressure-off interval
lS variables (step 12~) is calculated until the length of the
enhancement pres~ure-G~ interval (t~) is equal to or
greater than the time previously set for the enhancement
pressure off interval (T~) a represented at step 130 in
Figure 3B.
A value for RLOG~ is then obtained as represented at
step 132 in accordance wlth equation (2) shown below:
log (1 + ~V~ / R~)
RLO~ . (2)
~og (1 + ~V~ /V~
Then, having calculated and stored both RLOG~ and
RLO~, R may be calculated accordi~g to equation (3) below:
R = (RLOG~/RLOG~ ~3)
Where C i5 a calibration function given by equation (4)
below:
C = ~(5O2~ (SO2)~ F(SO2)~ (4)
SlJ~S~lTUTE SHEET
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- . .. . .. .. . . .. .
; : ,
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~6 ~c'd DCT/P~a 0 3 ~PR 199~
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where:
F(SO2)~ = the inverse o~ the look-up table fun~-tion
for funotio~al oxygen saturation without
the enhancement pressure imposed
F(SO~)~ = the invers~ of the look-up table function
for functional oxygen saturation with the
enhancement pres~ure imposed
~o
Thus, C in equation (4) I-epresents a calibration factor
which must be introduced to maintain accuracy of the system
because of the dif~erence~, which may be very small, : .
between the look-up tables for RLOG~ and RLOGMoN~ Having
calculated R in accordance with equ~tion (3), corrections
can be made to subsequent S~02 measurements to account ~or
ths effect of Sc02 and to reduce or eliminata the
contribution of Sco2 on the S~o2 determination leaving just
the 5-2 level to be displayed to the physician. Having
carried out these steps, the calibration period is
completed.
The first step in the monitoring period (tMoN) shown at
.136 in Figure 3B, require~ that the values for the
~ollowing variable~ be determinecl: .:
AVR = the pulsatile signal output from the :
MON photodiode when the xed LED is operating
during the ~onitoring period ~.
~V~ - the pulsatile signal output ~rom the
MO~ photodiode when the infrared LED is operating ~'
during the monitoriny period -
- = the average o~ the total signal output from ~:
MON the photodiode when the red LED is operating ~ -
during the monitoring period
~0
V~ = the average of the total signal output from
MON the photodiode when the infrared LE~ is
. operating during th~ monitoring period
- .
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16 ~ec'd PCT/PTO O 3 aPR 1992
,; ~e~/lJ5 ~0/ ~5 ~ ~
-34-
Next, at step 138, a running average of the ~our
variables is calculated. It may be desirable to allow the
physician using the system of the present invention to
determine how heavily past values ~or the four variables
will be weighted in subsequent calculations.
As will be appreciated, weighing previously obtained
determinations of the four variable~ will result in a
displayed S,02 value whi~h is more immune to motion
artifacts, noise, and spurious signals but which is less
responsive to rapid changes in S~2 levels. Alternatively,
if the previously obtained values for the four variables
are weighted little or not at all, then the system will be
very re~ponsive to rapid changes in~S~O2 levels but motion
; artifactsr noise, and supuriou~ cignals may cause the
display of an occasional inaccurate SAO2 value. When such
an inaccurate S,2 value is displayed, the physician will
need to judge whether the display is an accurate reflection
of the patient's condition or is caused by sources other
than thP patient's S,02 levels. ~-
Next as shown at step 140, values for ~VaR and ~Va~ are
calculated according to equations (5) and (6), provided
below:
~Va~ = av~(l aR) (5)
25
~Va~ = av~(l-aR) (6)
where a equals the capillary pulse volume fraction.
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1~ Rec'd PCT/PT0 0 3 APR 19~2
~ / 00 5 1 8
-35-
Next, at step 142, RLOGa i5 calculated according to
equation (7):
log (1 + ~VaR/VR)
RLOG. = _
s
log (1 ~ ~Va~/V~)
Having calculated RLOG" the S,02 level may be determined
by obtaining a value ~rom the RLOG, look-up table as
MON
representPd at step 144. The RLOG, look-up table is
derived empirically in a fashion similar to that described
earlier for the RLOG~ look-up table. Significantly, the
value obtained from the RLOG, look-up table represents the
S,02 value since the Sc02 contribution ha~ already been
"calibrated out" by the step~ used to arrive at RLOG,. ThP
value obtained from the RLOGl look-up table is displayed
as indicated at step 146. The steps of the monitoring
period are repeated until tMoN ~ ~MON as shown at step 148.
Alternative steps may be substituted to or added to the
method of the invention without departing from its intended
scope. For example, it is possible to arrive at a
calibration factor by comparing the F(SO2~ and F(SO22)~
values to determine what percentage of the SO2MON value
represents tha S,02 level. However~ the above described
step~ are presently preferred in order to obtain the most
accurate S~02 detsrminations when the photodetection means
is configured to operate in a transmission mode such as is
the case in the embodiment represented in Figure 2.
Significantly, the inventive conçepts taught herein may
also be carried out by configuring the light emi~ting means
and the photo detection means to operate in a reflective
mode. A structure adapted for operating in a reflective
mode i~ repres2nted in Figure 2A which is a cross sectional
view showing LED 54A and LED 56A positioned within a
pressure cu~ 34A adjacent the photodiode 64A. Positioning
the LEDs 35~ and 56A adjacent to the photodiod~ 64A, or in
another similar position, allows the photodiode 64A to
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1~ R~c'd ~CJ/PT~ O 3 APR 159~ :
,,"" ~; ~ ~ / 0 ~
-36-
receive that portion of the light beams re~lected from the
blood, tissue, and bone of the patient's finger 36Ao It
will be appreciated that it is necessary to operate the
embodiment in such a reflective mode to best utilize body
parts such as the patient's forehead as a sensing location.
When an apparatus which embodies the inventive concepts
taught herein is operated in a reflective mode, it is
necessary to alter the method set forth in the flow charts
o~ Figures 3A and 3B somewhat. ~hus, the flow chart shown
in Figure 5A and 5B provide the step~ carried out when
using the presently pre~erred structure represented in
Figure 2A.
The steps shown in the flow char~ o~ Figures 5A and 5B
closely parallel the steps previously described in
connection with Figures 3A and 3B except where departures
are necessary to allow operation in a reflective mode.
When the photodetector is positioned to receive light which
is reflected from the patient's body part, it is necessary
to calculate and store Y~ (rather than RLOG~ when operating
in the transmission mode~. A value for Y~ is derived ~rom
the stored average valves according to equation (8)
provided below..
~VR
eP
Y~ (81
VR
~P
Thos~ skilled in the art will appreciate that the
calculation of Y~, and the other c~iculations repxesented
in Figures 5A and SB, may be readily carried out by a
microcomputer as previously explained.
Once a value for Y~ is calculated and stored, the S,O2
corresponding to the calculated value of Y~ is found by
referenc~ to a Y~ lo~k-up ~able as indicated at step 218A.
The Y~ look-up table i derived ~rom empirical data
yathered during use of the system described herein. For
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16 Rgc'd PCT/PTO ~ 3 APR 1992
-37-
example, once a red LED, infrared LED, photodiode, and
other hardware items have been con~igur~d to provide the
system described herein, the values obtained for Y~ may be
correlated with the S,Oz value obt~ined using another S~o2
5 determination method, for example, an ~ vitro method.
Alternatively, the subject's S,02 may be altered ~y altering
the composition of the inspired gaseR and monitoring the
composition of the expired gases. Once the YEp look-up
table has been completed, it can be used in the case of any
number of patients if the performance of the apparatus
hardware is maintained within appropxiate parameters
considering the effects of age, temperature, and
variability of mass produced component~.
The S,2 which wa~ determined from the Y~ look-up table
at step 118A is displayed as represented at step 120A in
Figure 5A on the display means 42 represented in Figure 2.
It should be appreciated that the Sl02 value displayed at
step 12OA during the enhancement pressure on interval is
more ac~urate and reliable than S~02 values provided by
previously available pulse oximetry systems due to the
enhancPment of the arterial pulsatile signal output from
the photodiode and the decrease of the capillary oxygen
saturation contribution to the same signal.
Nevertheless, as explained previously, the interval
during which the enhancement pressure is imposed must be
limited due to several considerations including avoiding
pain for the patient and affecting the physiology of the
patient so that the measurementR obtained are altered in
any significant fashion. Thus, the enhancement pressure is
released from the body part for the remainder of the
calibration period and monitoring period as represented at
step 122A as shown in Figure 5~.
As shown in Figure 4, the enhancement pressure-off
interval of the calibration period begins when the
enhancement pressure is released and the pressure on the
body part returns to the ambient pressur~. Again, as
SUB~lPlTlJTE ~HE~T
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, - - . .. , .. , , . ,: ,
"
..
, : ;,
1~ 'd PCI/P,~ O 3 aPR 19~2
.~; ~f ~ n ~ ~ -
-38-
represented ~t step 124A, it i~ necessary to wait at least
two heartbeats before measuring any variable~.
Continuing to refer to Figure 5B and similarly to the
steps taken during the enhancement pressure~on int~xval,
the enhancement pressure-off interval include~ steps to
determine four variables as shown at step 126A. Th~ same
variables previously defined shown at step 126 in Figure 3B
have the same definition in the flow chart of Figures 5A
and 5B when the embodiment operates in a reflective mode.
lo Also, similarly to the steps taken during the
enhancement pressure-on interval, the average of multiple
determinations of the enhancement pressure-off interval
variables (step 128A) is calcuiated until the length of the
enhancement pressure-off interval (t~) is equal to or ~:
greater than the time previously set for the enhancement ~ ;
pressure-off interval (T~) as represented at step 130A in
Figure 5B.
A~ represented in Figuxe 5B, a value for Y~ is then
obtained and stored at step 132A in accordance with
equation (9) provided below.
~VR / VRNP
NP ' ( 9 )
~
VIR' /Vm,
NP NP
Having calculated and stored both Y~ and Y~, ~ may be
calculated according to equation (10). ~
:
(10)
Y~ - 1
Since ~ has been calculated in accordancQ with equation
(10), correction~ may be made to subsequent S,02
mea~urements to account for the effect of S~02 and to reduce
SUB~l~ITUTE SHEET
- IPEA/lJS
.. . - ~ ~ .
.
. . .
., .
'~ ~? ,' ~ 3-;V~ ~ J
39 FeT/~ ~
or eliminate the contribution of Sco2 on the S,0~ level of the
patient to be displayed. Having carried out these steps,
the calibration period is complete.
The first step which takes place during the monitoring
period (tMoN), shown at 136A in Figure 5B, requires that
YMON be calculated according to equation (ll) provided
below.
~V~ / VR
MON MOr,
YMON ( 1
MON MO~
Next, at step 138A, a running average f YMON is
calculated.
Having calculated an average value f YMON~ the S~02 level
may ~e determined by obtaining a valus from the Y~ON look-up
table as represented at step 144A. The YMON look-up table
is derived in an empirical fashion similar to the fashion
described for the Y~ look-up table. Significantly, the
value obtained from the YMON look-up table represents the
. S.02 value since the Sc~ contribution has already been
~Icalibrated out" in previous steps. The value obtained
from the YMO~ look-up table i5 displayed as represented at
step 146A. As shown at step 14~A, the steps of the
monitoring period are repeated until tMON > TMON~
A~ indicated previously, the system represanted in
Figure~ 2 and 2A includes all the hardware necessary to
carry out blood pressure detexminations as described and
claimed in United States Patent Application Serial No.
07/068,107 which was previously incorporated herein by
reference.
As set forth in the aforementioned application, two of
the thxee parameterY (mean arterial pressure and systolic
arterial pressure) may be measured using the widely known
oscillometric method and the third parameter (diastolic
arterial pressure) may be calculated using a recursive
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16 R~c'd P~l/PTO 0 3 APR 199'~ -
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-40-
procedure wherein an estimate of the diastolic pressure is
made and the estimated diastolic pressure, and the other
parameters set forth earlier, are used in Hardy model
calculations. If the eqtimate was correct, the calculated
mean arterial pressure will agree with the measured
arterial pressure. Once all three parameters have been
determined, the Hardy model compliance curve can be used to
continuously calculate a blood pressure waveform using the
VR or the VIR signal. It will be appreciated that the
signal produced by either the red or the infrared LED can
be used to detect volume changes in the arteries being
examinsd. With the relative changes in volume being
available by examining the VR or;the VIR signal, the
! pressure-volume relationship of the artery described by the
Hardy model allows the pressure waveform to be calcula~ed.
As in the case of the enhanced pulsQ oximetry method
described herein, it is necessary to regularly calibrate
the values used in tha blood pressure determinations duQ to
changes in the physiology o~ the patient.
In most cases, it is generally not necessary to conduct
a complete oscillometric determination of both systolic and
mean arterial pressures as often as it is necessary to
begin a calibration period for S~2 determinations. Thus,
the period during which the osciLlometric determination is
carried out i~ referred to as a "super calibration period."
It should b~ understood that the oscillometric method
require~ that the artery be completely occluded and thus
whatever means which i9 used to impose the enhancement
pressure on the body part should bs capable of imposing
such a pressure. Also, because t~e pressure imposed is
greater thzn the systolic pressure, it may require that an
appropriat~ waiting period be provided before S,O2
determinations can be reliably made.
Signi~icantly, the enhancement pressure, which equals
the mean arterial pressure, i~ applied during every
calibration period for S,O2 determinations. This allows the
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1~ R~'d PC~rlPla O 3 APR 1992
-41-
measured mean arterial pre~sure to be compared to the mean
arterial pressure being used in the Hardy model
calculations and, if a significant discrepancy between the
two is found, a cuper calibration period may be begun.
It will thus be appreciated that the pres~nt invention
provides a great advantagQ in allowing both arterial oxygen
and blood pressure determinations to be made using little
more hardware than that which is required for determining
arterial oxygen levels. Also, the present invention is
able to distinguish arterial oxygan saturation levels from
capillary oxygen saturation levels and to provide arterial
oxygen saturation level determinations which are more
acrurate and reliable than those available from previously
known oximetry system~,
The invention may be embodied in other specific forms
without departing from it~ spirit or essential character
istics. The described emhodiment is to be considered in
all respects only as illustrative and not restrictive. The
scope o~ the invention is, therefore, indi~.ated by the
appended claims rather than by the foregoing description.
All changes which come within the meaning and range of
equivalency o~ the claim~ ar~ to ba embraced within their
BCOpe.
SlJB5~TUT?~ SHEET
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