Abstrict
An oscillometric, noninvasive blood pressure monitor comprising
an inflatable cuff adapted for placement around a body member, a
pump for cuff inflation, a pressure transducer connected to the
cuff, means for detecting oscillations in arterial pressure occurring
during a transition in cuff pressure between a pressure greater
than normal systolic pressure and a pressure less than normal diastolic
pressure, and a blood pressure measurement circuit which is capable
of determining the maximum amplitude (A.sub.m) of the oscillations,
identifying mean cuff pressure (P.sub.m) as the coincident value
of the cuff-pressure signal from the pressure transducer, and determining
systolic pressure as a function of both A.sub.m and P.sub.m. In
accordance with one aspect of the invention, the blood pressure
monitor has an optical sensor including a light source and photodetector
optically coupled to the body member proximate to the cuff. Oscillations
in the output signal of the photodetector are detected, and the
blood pressure measurement circuit determines the oscillation amplitude
corresponding to systolic pressure (A.sub.s) as a function of both
A.sub.m and P.sub.m.
Claims
We claim:
1. An oscillometric, noninvasive blood pressure monitor, comprising:
an inflatable cuff; a pump connected to said cuff; a pressure transducer
connected to said cuff, said pressure transducer producing a cuff-pressure
signal; means for detecting oscillations in arterial pressure occurring
during a transition in cuff pressure between a pressure greater
than normal systolic pressure and a pressure less than normal diastolic
pressure; and a blood pressure measurement circuit responsive to
said oscillations, said circuit determining the maximum amplitude
A.sub.m of said oscillations, identifying mean cuff pressure P.sub.m
as the coincident value of said cuff-pressure signal, and determining
systolic pressure as a function of both A.sub.m and P.sub.m.
2. The blood pressure monitor of claim 1, further comprising an
optical sensor including a light source and photodetector, wherein
said detecting means detects said arterial pressure oscillations
as oscillations in the output signal of said photodetector, and
wherein said blood pressure measurement circuit determines the oscillation
amplitude A.sub.s corresponding to systolic pressure as a function
of both A.sub.m and P.sub.m.
3. The blood pressure monitor of claim 2, wherein said blood pressure
measurement circuit determines the amplitude A.sub.s corresponding
to systolic pressure based on an equation of the form A.sub.s=A.sub.m(a-b
P.sub.m) where a and b are constants.
4. The blood pressure monitor of claim 3, wherein the value a is
greater than 0.7, and the value b is greater than 0.001 for P.sub.m
in units of mm Hg.
5. The blood pressure monitor of claim 4, wherein the value a is
approximately 0.84 and the value b is approximately 0.004.
6. The blood pressure monitor of claim 1, wherein said detecting
means is coupled to said pressure transducer and detects said arterial
pressure oscillations as oscillations in said cuff-pressure signal,
and wherein said blood pressure measurement circuit determines the
oscillation amplitude A.sub.s corresponding to systolic pressure
as a function of both A.sub.m and P.sub.m.
7. The blood pressure monitor of claim 6, wherein said blood pressure
measurement circuit determines the amplitude A.sub.s corresponding
to systolic pressure based on an equation of the form A.sub.s=A.sub.m(a-b
P.sub.m) where a and b are constants.
8. The blood pressure monitor of claim 7, wherein the value a is
greater than 0.7, and the value b is greater than 0.001 for P.sub.m
in units of mm Hg.
9. The blood pressure monitor of claim 8, wherein the value a is
approximately 0.84 and the value b is approximately 0.004.
10. An oscillometric, noninvasive method of measuring blood pressure,
comprising: inflating a cuff, producing a cuff-pressure signal with
a pressure transducer connected to said cuff; detecting oscillations
in arterial pressure occurring during a transition in cuff pressure
between a pressure greater than normal systolic pressure and a pressure
less than normal diastolic pressure; determining the maximum amplitude
A.sub.m of said oscillations; identifying mean cuff pressure P.sub.m
as the value of said cuff-pressure signal coinciding in time with
A.sub.m; and determining systolic pressure as a function of both
A.sub.m and P.sub.m.
11. The method of claim 10, wherein said arterial pressure oscillations
are detected as oscillations in the output signal of an optical
sensor, said method further comprising: determining the oscillation
amplitude A.sub.s corresponding to systolic pressure as a function
of both A.sub.m and P.sub.m.
12. The method of claim 11, wherein the amplitude A, corresponding
to systolic pressure is determined based on an equation of the form
A.sub.s=A.sub.m(a-b P.sub.m) where a and b are constants.
13. The method of claim 12, wherein the value a is greater than
0.7, and the value b is greater than 0.001 for P.sub.m in units
of mm Hg.
14. The method of claim 13, wherein the value a is approximately
0.84 and the value b is approximately 0.004.
15. The method of claim 10, wherein said arterial pressure oscillations
are determined as oscillations in said cuff-pressure signal, said
method further comprising: determining the oscillation amplitude
A.sub.s corresponding to systolic pressure as a function of both
A.sub.m and P.sub.m.
16. The method of claim 15, wherein the amplitude A.sub.s corresponding
to systolic pressure is determined based on an equation of the form
A.sub.s=A.sub.m(a-b P.sub.m) where a and b are constants.
17. The method of claim 16, wherein the value a is greater than
0.7, and the value b is greater than 0.001 for P.sub.m in units
of mm Hg.
18. The method of claim 17, wherein the value a is approximately
0.84 and the value b is approximately 0.004.
Description BACKGROUND OF THE INVENTION
This invention relates to the noninvasive measurement of blood
pressure, and more particularly to the noninvasive measurement of
blood pressure by the oscillometric method.
A number of noninvasive methods of measuring blood parameters are
known. For example, blood pressure has been measured by the auscultatory
method which uses a cuff and a stethoscope, and by the oscillometric
method which only requires a cuff applied to a body member. The
conventional oscillometric method relies on the small-amplitude
pulsatile pressure oscillations communicated to the cuff by the
underlying artery in the body member during cuff deflation from
above systolic pressure to zero pressure. Such arterial pressure
oscillations cause corresponding small oscillations in cuff pressure
which can be amplified and used to identify systolic, mean and diastolic
pressure. For example, it has been established by Posey et al. that
the cuff pressure for maximal amplitude oscillations corresponds
to mean arterial pressure. See Posey et al., "The Meaning of
the Point of Maximum Oscillations in Cuff Pressure in the Direct
Measurement of Blood Pressure," Part 1, Cardiovascular Res.
Ctr. Bull. 8(1):15 25, 1969. See also Ramsey, "Noninvasive
Automatic Determination of Mean Arterial Pressure," Med. Biol.
Eng. Comput. 17:17 18, 1979; and Geddes et al., "Characterization
of the Oscillometric Method for Measuring Indirect Blood Pressure,"
Annals of Biomedical Engineering, Vol. 10, pp. 271 280, 1982. All
such references are incorporated herein by reference.
Commercially available oscillometric devices are useful for noninvasive
blood pressure measurement, but a need remains for improvement in
accuracy, particularly with respect to identification of systolic
and diastolic pressure.
SUMMARY OF THE INVENTION
The present invention meets the above-stated need and others by
providing an oscillometric, noninvasive blood pressure monitor comprising
an inflatable cuff adapted for placement around a body member, a
pump for cuff inflation, a pressure transducer connected to the
cuff, means for detecting oscillations in arterial pressure occurring
during a transition in cuff pressure between a pressure greater
than normal systolic pressure and a pressure less than normal diastolic
pressure, and a blood pressure measurement circuit which is capable
of determining the maximum amplitude (A.sub.m) of the oscillations,
identifying mean cuff pressure (P.sub.m) as the coincident value
of the cuff-pressure signal from the pressure transducer, and determining
systolic pressure as a function of both A.sub.m and P.sub.m. An
inflatable cuff as that term is used herein is an inflatable bladder,
capsule or other member suitable for occluding a blood vessel, and
may cover a small area on a subject's skin or may surround a finger,
limb or other body part.
In accordance with one aspect of the invention, the blood pressure
monitor has an optical sensor including a light source and photodetector
optically coupled to the body member through at least one surface
of the cuff. The oscillations in arterial pressure are detected
as oscillations in the output signal of the photodetector, and the
blood pressure measurement circuit determines the oscillation amplitude
corresponding to systolic pressure (A.sub.s) as a function of both
A.sub.m and P.sub.m. In a preferred embodiment, the amplitude A.sub.s
corresponding to systolic pressure is determined based on an equation
of the form A.sub.s=A.sub.m(a-b P.sub.m)
The invention provides more accurate blood pressure measurement
by determining systolic pressure according to an algorithm which
includes mean cuff pressure as a factor. The principles of the invention
are particularly suited for use with the optical oscillometric method
but are equally applicable to blood pressure measurement by the
conventional pneumatic oscillometric method.
The objects and advantages of the present invention will be more
apparent upon reading the following detailed description in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side view of a cylindrical embodiment of a transilluminating
cuff for use in a blood pressure monitor according to the present
invention.
FIG. 2 is a perspective view of a hinged embodiment of a transilluminating
pressure cuff.
FIG. 3 is a transverse cross-section of the cuff of FIG. 2.
FIG. 4 is a block diagram of one embodiment of a blood pressure
monitor according to the present invention.
FIG. 5 is a set of sample waveforms obtained with a blood pressure
monitor according to the present invention, with a cuff on the little
finger of a human subject.
FIG. 6 is another set of sample waveforms.
FIG. 7 is a graph of blood pressure measured using an algorithm
according to the present invention against blood pressure measured
directly.
FIG. 8 is an example of a calibration curve for use in oxygen saturation
measurement according to another embodiment the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
For the purpose of promoting an understanding of the principles
of the invention, reference will now be made to the embodiment illustrated
in the drawings and specific language will be used to describe the
same. It will nevertheless be understood that no limitation of the
scope of the invention is thereby intended, such alterations and
further modifications in the illustrated device, and such further
applications of the principles of the invention as illustrated therein
being contemplated as would normally occur to one of ordinary skill
in the art to which the invention relates.
FIG. 1 illustrates one embodiment of a member-transilluminating,
transparent pressurizable cuff 10 for use in a blood pressure monitor
according to the present invention. A rigid tube 12 contains an
elastic sleeve 14 which may be provided with an inlet 16 for connection
to a pressure source, e.g., an air supply, and an outlet 18 for
connection to, e.g., a manometer. Alternatively, inlet 16 may be
the only pressure line, as in the embodiment of FIG. 2 described
below. Pressure applied between the elastic sleeve and the rigid
tube causes the sleeve to compress a body member therein such as
a finger 20 placed therein. This embodiment is also useful on a
small animal tail or tongue, for example, among other applications.
The rigid tube includes a light source 22 and a photodetector 24
which may be diametrically opposed as illustrated in the drawing.
Alternatively, two light sources may be provided as described below.
In another alternative embodiment, the light source and photodetector
are mounted side-by-side on the cuff housing, and blood pressure
and oxygen saturation are measured based on reflection of light
by tissue in the body member.
Referring to FIGS. 2 and 3, another embodiment of a cuff 30 for
use with a blood pressure monitor according to the present invention
includes a hinged cuff housing 32 having first and second semicylindrical
sections 34 and 36 and a hinge 38 parallel to the longitudinal axis
of the semicylindrical housing sections. The axes of the two housing
sections are parallel to each other and coincide to form a common
axis when the hinged housing is closed. In order to facilitate use
of the cuff on a bone-containing body member, i.e., to avoid bone
shadow, two light sources 40 and 42 are circumferentially spaced
on one housing section in opposition to a photodetector 44 mounted
on the other housing section. This configuration increases the transmission
of light through the tissue bed around the bone 45 in the member
in which blood pressure is measured noninvasively. The angular spacing
of the LEDs and the photodetector may be as shown in FIG. 3, or,
alternatively, the LEDs and photodetector may be spaced approximately
120.degree. apart. An optically transparent, inflatable cuff 46,
which may be provided as a disposable item with an inflation tube
56, is adapted to fit within cuff housing 32 and around the body
member, and is held in place by means of a plurality of clips 48
which are provided in the housing for this purpose. Cuff 30 is further
described in U.S. Pat. No. 6,801,798, entitled Body-Member-Illuminating
Pressure Cuff For Use In Optical Noninvasive Measurement Of Blood
Parameters, issued Oct. 5, 2004 and hereby incorporated by reference.
Blood pressure, including systolic, mean and diastolic pressures,
can be obtained with the optical sensor unit from the amplitude
spectrum of the pulses obtained during deflation of the cuff from
a suprasystolic pressure to zero pressure, as described below. Monochromatic
LEDs are suitable for monitoring blood pressure. For example, the
transducer may employ infrared LEDs such as PDI-E801 or PDI-E804
880 nm LEDs available from Photonic Detectors, Inc. The LEDs and
photodetector are preferably matched to operate at a desired wavelength.
One example of a suitable photodetector is a Fairchild Semiconductor
QSD723 phototransistor, with a peak sensitivity at 880 nm. Another
suitable operating wavelength for the LEDs and photodetector is
805 nm, at which wavelength the blood pressure pickup has no oxygen-saturation
error, as will be appreciated from the discussion of pulse oximetry
below. An advantage of either of the example wavelengths is that
there are few environmental light sources in this infrared region.
Referring to FIG. 4, the cuff is connected by inflation tube 56
to a pump 58 which is controlled by a microprocessor 60. Pressure
in the line to the cuff is measured by means of a pressure transducer
62 having a signal output connected to the microprocessor. Suitable
transducers are available from Cobe Labs, Littleton, Colo. In embodiments
such as that of FIG. 1 in which the cuff has an inlet and an outlet,
the pressure transducer is connected to the outlet. A/D conversion
may be provided in the microprocessor or in the transducer or with
a separate A/D converter provided between the two. The microprocessor
controls the LEDs and, during blood pressure measurement, energizes
both LEDs continually. The photodetector produces an output signal
which is supplied to the microprocessor through an amplifier 64.
The amplified photodetector output signal is converted to digital
form in the microprocessor itself if the microprocessor has an internal
A/D converter, or in a separate A/D converter provided between the
amplifier and the microprocessor.
The microprocessor is suitably programmed to identify, based on
the digitized output signal of the photodetector, the points in
the cuff pressure signal which correspond to systolic, mean and
diastolic pressure, and displays the corresponding values on a display
65 which may comprise separate indicators as shown in FIG. 4, or
may provide an output for distant recording.
One suitable embodiment of amplifier 64 is a variable-gain amplifier.
With such an amplifier, and with a feedback circuit 66 connected
to its gain-control input, as shown in FIG. 4, the sensitivity of
the measuring system may be adjusted automatically to a proper level
for measurement of blood pressure. It has been found useful to set
the sensitivity based on the amplitude of the photodetector output
pulses before inflation of the cuff. Such pulses may have a peak-to-peak
amplitude on the order of one-third to one-half the maximum peak-to-peak
amplitude of the pulses obtained during blood pressure measurement.
Such pulses are identified by the reference numeral 68 in FIG. 6,
which shows a sample waveform for the photodetector output signal
both prior to and during blood pressure measurement. Sensitivity
adjustment is inhibited when blood pressure is measured. That is,
the gain of amplifier 64 and thus the system sensitivity is fixed
at that time. It should be noted that pulse rate can be determined
from the optical pulses occurring before cuff inflation and after
cuff deflation and may be displayed along with blood pressure values
as indicated in FIG. 4.
Blood pressure is measured during a transition in cuff pressure
between a suprasystolic pressure and zero pressure. The transition
may be an upward or downward transition but is described below in
terms of a gradual downward transition such as shown in FIG. 5,
which shows a sample optical pulse waveform 69 obtained during a
cuff pressure cycle represented by curve 70, which is marked to
indicate the points corresponding to systolic (S), mean (M), and
diastolic (D) pressure. When cuff pressure is raised above systolic
pressure, all oscillations are extremely small. As pressure in the
cuff falls below systolic pressure, the pulses increase, and as
the pressure is reduced further, the optical pulse amplitude increases
and reaches a maximum, labeled A.sub.m in FIG. 5, at which point
the cuff pressure is equal to mean arterial pressure, labeled M
in FIG. 5. With a continued decrease in cuff pressure, the oscillation
amplitude decreases and returns to a uniform level.
The peak-to-peak amplitudes of the optical pulse waveform at the
points coinciding with the occurrence of systolic and diastolic
pressure are designated respectively as A.sub.s and A.sub.d in FIG.
5. In the system described in the above-referenced U.S. Pat. No.
6,801,798, those amplitudes are calculated as fixed percentages
of A.sub.m, and the corresponding points in time are identified
on the optical pulse waveform, by interpolation if necessary between
adjacent pulses, after which the values of cuff pressure at those
points in time are identified as systolic (S) and diastolic (D)
pressure, respectively. Appropriate ratios have been determined
experimentally. With a conventional cuff applied to the upper arm
of a human subject, and with the cuff width (axial length) nominally
equal to 40% of the member circumference, systolic pressure is typically
identified as the value of cuff pressure at the point when the amplitude
ratio A.sub.s/A.sub.m is 0.5; diastolic pressure is typically identified
as the value of cuff pressure at the point when the ratio of A.sub.d/A.sub.m
equals 0.8. With the optical oscillometric method, a ratio of 0.7
has been found more suitable for identifying diastolic pressure.
Systolic pressure, however, is preferably not identified on the
basis of a fixed percentage of A.sub.m. The amplitude of the optical
pulse waveform corresponding to systolic pressure has been found
to depend on mean pressure (P.sub.m), unlike the fixed-value systolic
pressure algorithm. More accurate measurements can be obtained by
calculating A.sub.s, the optical pulse amplitude corresponding in
time with systolic pressure, according to an algorithm which includes
mean cuff pressure as a factor. The following equation represents
one form of such an algorithm: A.sub.s=A.sub.m(a-b P.sub.m) where
a and b are experimentally determined constants.
The improvement in predicting systolic pressure using this algorithm
can be appreciated from FIG. 7, in which line A, corresponding to
results using this algorithm with the values a and b set equal to
0.84 and 0.004, respectively, is virtually coincident with line
B, the line of equal values in the graph. That is, systolic pressure
predicted with the above algorithm is virtually the same as directly
measured systolic pressure throughout the range of interest.
FIG. 6 illustrates sample waveforms for an embodiment of the invention
in which cuff pressure is increased linearly and then decreased
linearly, as illustrated respectively by segments 72 and 74 of the
cuff pressure signal, and two sets of optical pulsatile data 76
and 78 are acquired. As shown in the drawings, the first set of
pulses 76 includes indications of the points in time during the
cuff pressure rise 72 at which diastolic, mean and systolic pressure
occur, in that order. Conversely, the second set of pulses 78 includes
indications of the points in time during the cuff pressure fall
74 at which systolic, mean and diastolic pressure occur, in that
order. In this way, two values for each pressure may be acquired
and averaged and the average value may be displayed.
The system may have LEDs which operate at different wavelengths
for oxygen saturation measurement. Blood oxygen saturation is defined
as the ratio of oxygenated hemoglobin (HbO.sub.2) to the total hemoglobin
(Hb+Hb0.sub.2), and is typically expressed as a percentage. The
oximeter determines oxygen saturation (SaO.sub.2) by measuring the
optical transmission at two wavelengths of light passing through
a tissue bed. Although other wavelengths are contemplated, it is
presently preferred to operate at wavelengths of approximately 650
nm and 805 nm for oxygen saturation measurement. As shown in the
above-referenced U.S. Pat. No. 6,801,798, hemoglobin (Hb) has negligible
transmission at 650 nm, and hemoglobin (Hb) and oxygenated hemoglobin
(HbO.sub.2) transmit equally well at 805 nm; the latter wavelength
is known as the isobestic point. That is, the transmission at 805
nm is independent of oxygen saturation. The optical sensor may have
separate narrowband LEDs, e.g., a red LED emitting at approximately
650 nm and an infrared LED preferably emitting at approximately
805 nm, and a broadband photodetector. As an alternative to separate
narrowband LEDs, a red LED and infrared LED may be combined in one
multi-wavelength LED such as type Epitex L660/805/975-40D00, available
from Epitex, Kyoto, Japan, and each light source 40 and 42 may comprise
such a multi-wavelength LED.
The red and infrared LEDs are preferably energized alternately
in rapid succession, e.g., at a rate of 200 pulses per second. This
technique permits the use of high-intensity short-duration pulses.
Synchronous detection is used to achieve the highest signal-to-noise
ratio. Two benefits result: 1) a low average power and minimum heating,
and 2) the system is less sensitive to stray ambient illumination.
The red and infrared signals are sampled and processed to obtain
SaO.sub.2, which may then be displayed on display 65 of FIG. 4.
The automatic sensitivity adjustment is disabled during measurement
of oxygen saturation.
A base line for measurement may be established by first inflating
the cuff to a high pressure sufficient to squeeze all of the blood
out of the member in the cuff and thus out of the optical path.
For example, the cuff pressure may be held at a maximum pressure
as indicated by the plateau 73 in FIG. 6 for a desired time period
to obtain the bloodless transmission reading, which can be assigned
a value of 100% transmission. When the cuff pressure is released,
blood enters the optical path and the red and infrared transmissions
are measured. The optical density is computed for each of the transmitted
signals, and the ratio of red to infrared optical density is calculated
and scaled to provide an output value corresponding to the percentage
of oxygen saturation.
Beer's law relates the optical density (D) to the concentration
of a dissolved substance. Optical density (D) is equal to log 1/T,
where T is the transmittance. Therefore the oxygen saturation (SaO.sub.2)
is given by: .times..times. ##EQU00001## where A and B are experimentally
determined constants for a given application. This equation predicts
a linear relationship based on Beer's law. However, Beer's law applies
to solutions in which the absorbing substance is dissolved. Blood
is a suspension, and, consequently, the relationship between SaO.sub.2
and the ratio of the optical density for red and infrared radiation
is nonlinear, as shown in FIG. 8. Between 30% and 60% saturation,
the relationship is almost linear; above this range the relationship
is nonlinear. The curve in FIG. 8 is an example of a suitable calibration
curve which may be programmed into the microprocessor, e.g., in
the form of a lookup table, for calculation of SaO.sub.2. Further
information regarding methods of measuring blood oxygen saturation
may be found in the following references which are hereby incorporated
by reference: Geddes, "Heritage of the Tissue-Bed Oximeter,"
IEEE Engineering in Medicine and Biology, 87 91, March/April 1997;
Geddes and Baker, Principles of Applied Biomedical Instrumentation,
3.sup.rd ed., Wiley, New York, 1989.
Calibration of the oximeter also involves balancing the outputs
for the red and infrared channels to obtain the same optical sensitivity
for both, and ensuring that both channels have a linear response
to the red and infrared radiation.
While the invention has been illustrated and described in detail
in the drawings and foregoing description, the same is to be considered
as illustrative and not restrictive in character, it being understood
that only the preferred embodiments have been shown and described
and that all changes and modifications that come within the spirit
of the invention are desired to be protected. For example, although
the embodiment of FIG. 2 is described above as having two emitters
and one detector, it may instead be provided with one emitter and
two detectors or with a combination of multiple emitters and multiple
detectors. The emitter(s) and detector(s) may be mounted inside
the cuff, on the inside surface of the cuff's inner wall, that is,
the wall which contacts the subject's skin during use, and may be
affixed thereto with an optically clear adhesive, e.g., Superglue
or other adhesive suitable for the particular material used for
the cuff. The emitter(s) and detector(s) may be affixed to the cuff
wall before the cuff is completely formed or sealed, and the cuff
may then be sealed so as to enclose the emitter(s) and detector(s).
The system may also be provided with an alarm which is triggered
when the transducer is off the body member desired to be monitored;
an alarm circuit may be designed to respond, for example, to an
optical sensor output signal level that is beyond a predetermined
threshold, indicative of the absence of absorbing material in the
optical path, or to such a condition combined with the additional
condition of an absence of optical pulses.
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