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Claims  |
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We claim:
1. A method of determining arterial blood pressures, comprising:
attaching a photoplethysmograph sensor to a patient so that light from said
sensor passes through an artery of said patient;
connecting said sensor to a photoplethysmograph, said photoplethysmograph
generating an electrical output signal having a predetermined relationship
to the volume of blood in said artery;
calibrating said photoplethysmograph during a calibration period by
determining the patient's actual arterial blood pressure by means other
than said photoplethysmograph, and then determining the value of a first
arterial characteristic in a predetermined relationship between said first
arterial characteristic, arterial blood volume as indicated by said
photoplethysmograph output signal, a conversion constant corresponding to
arterial blood volume at infinite pressure, and said actual arterial blood
pressure during said calibration period; and
analyzing said photoplethysmograph output signal during a measurement
period to determine an arterial blood pressure corresponding to said
output signal in accordance with said predetermined relationship.
2. The method of claim 1 wherein said conversion constant corresponding to
arterial blood volume at infinite pressure is determined by examining the
relationship between arterial blood volume and arterial blood pressure,
and then determining said arterial blood volume at infinite pressure as
the asymptotic value of arterial blood volume in said relationship.
3. The method of claim 1 wherein said predetermined relationship is defined
by the formula:
.psi.=.psi..sub.inf (1-Kexp(kP))
where .psi. corresponds to the arterial blood volume as indicated by said
photoplethysmograph output signal, .psi..sub.inf is the conversion
constant corresponding to said arterial blood volume at infinite pressure,
k corresponds to said first arterial characteristic, K corresponds to a
second arterial characteristic, and P corresponds to said actual arterial
pressure during said calibration period.
4. The method of claim 1, further including the steps of determining from
said photoplethysmograph output signal a value t.sub.d corresponding to
the duration of the cardiac cycle during said measurement period, a value
S corresponding to systolic pressure during said measurement period, a
value D corresponding to diastolic pressure during said measurement
period, and a value ARC corresponding to the integral with respect to time
of the difference between the photoplethysmograph output signal during
said measurement period and a value of said photoplethysmograph output
signal corresponding to diastolic pressure.
5. The method of claim 1, wherein said step of calibrating said
photoplethysmograph during a calibration period includes the steps of:
determining the actual arterial systolic, diastolic, and mean blood
pressures, P.sub.s, P.sub.d, and P.sub.m, respectively, during said
calibration period by means other than said photoplethysmograph;
determining values V.sub.d and V.sub.S of said photoplethysmograph output
signal corresponding to respective diastolic and systolic arterial
pressures during said calibration period;
determining the value V.sub.m corresponding to the mean of said
photoplethysmograph output signal during said calibration period; and
calculating said first arterial characteristic k from the relationship:
##EQU7##
where V.sub.d and V.sub.s are respective values corresponding to said
photoplethysmograph output signal corresponding to diastolic and systolic
pressures during said calibration period, P.sub.d, P.sub.s, and P.sub.m
are respective values corresponding to said actual arterial diastolic,
systolic and mean arterial pressures during said calibration period, and k
is the value of said first arterial characteristic determined during said
calibration period.
6. The method of claim 5, wherein said step of determining a value V.sub.m
corresponding to the mean of said photoplethysmograph output signal during
said calibration period is accomplished by calculating V.sub.m from the
relationship:
V.sub.m =V.sub.d -(ARC/t.sub.d)
where V.sub.d, ARC, and t.sub.d are their respective values corresponding
to said photoplethysmograph output signal during said calibration period.
7. The method of claim 1 further including the step of determining from
said photoplethysmograph output signal a value X corresponding to arterial
pulse pressure during said measurement period by the steps of:
determining values V.sub.d and V.sub.S of said photoplethysmograph output
signal corresponding to respective diastolic and systolic arterial
pressures during said calibration period;
determining a value V.sub.m corresponding to the mean of said
photoplethysmograph output signal during said calibration period; and
calculating the value X corresponding to arterial pulse pressure during
said measurement period from the relationship:
##EQU8##
where V.sub.d, V.sub.s, and V.sub.m are respective values corresponding to
said photoplethysmograph output signal corresponding to diastolic,
systolic, and mean arterial pressures during said calibration period, and
k is the value of said first arterial characteristic determined during
said calibration period.
8. The method of claim 7, wherein said step of calibrating said
photoplethysmograph during said calibration period further includes the
steps of:
determining the actual arterial systolic and diastolic blood pressures,
P.sub.s and P.sub.d, respectively, during said calibration period by means
other than said photoplethysmograph;
calculating the ratio V.sub.0 /V.sub.inf during said calibration period
from the relationship:
##EQU9##
where V.sub.d and V.sub.s are respective values corresponding to said
photoplethysmograph output signal corresponding to diastolic and systolic
arterial pressures during said calibration period, P.sub.d and P.sub.s are
respective values corresponding to said actual arterial diastolic and
systolic arterial pressures during said calibration period, Vo is a value
corresponding to arterial blood volume at zero pressure, Vinf is a value
corresponding to arterial blood volume at infinite pressure, and k is the
value of said first arterial characteristic determined during said
calibration period.
9. The method of claim 8 further including the step of determining from
said photoplethysmograph output signal a value D corresponding to
diastolic pressure during said measurement period by calculating D from
the relationship:
##EQU10##
where V.sub.d and V.sub.s are the respective values corresponding to said
photoplethysmograph output signal during said calibration period
corresponding to diastolic and systolic pressure, X is a value
corresponding to said arterial pulse pressure during said measurement
period determined in accordance with the method of claim 7, the ratio
V.sub.0 /V.sub.inf was determined in accordance with the method of claim
8, and k is the value of said first arterial characteristic determined
during said calibration period.
10. The method of claim 7, wherein said step of calibrating said
photoplethysmograph during said calibration period further includes the
step of:
determining V.sub.inf at said calibration period from the relationship:
V.sub.inf =exp{[ln(V.sub.s)-(lnV.sub.d)exp(-kX)]/[1-exp(-kX)]};
determining V.sub.0 from the relationship:
##EQU11##
where V.sub.d and V.sub.s are the respective values of said
photoplethysmograph output signal corresponding to diastolic and systolic
pressure during said calibration period, P.sub.d and P.sub.s are
respective values corresponding to said actual arterial diastolic and
systolic pressures during said calibration period, X is a value
corresponding to said arterial pulse pressure during said measurement
period determined in accordance with the method of claim 7, Vo is a value
corresponding to arterial blood volume at zero pressure, Vinf is a value
corresponding to arterial blood volume at infinite pressure, and k is the
value of said first arterial characteristic determined during said
calibration period.
11. The method of claim 10 further including the step of determining from
said photoplethysmograph output signal a value D corresponding to
diastolic pressure during said measurement period by calculating D from
the relationship:
##EQU12##
where V.sub.d and V.sub.s are the respective values of said
photoplethysmograph output signal during said calibration period
corresponding to diastolic and systolic pressure, X is a value
corresponding to said arterial pulse pressure during said measurement
period determined in accordance with the method of claim 7, V.sub.o and
V.sub.inf were determined in accordance with the method of claim 10, and k
is the value of said first arterial characteristic determined during said
calibration period. |
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Claims |
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Description |
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TECHNICAL FIELD
This invention relates generally to blood pressure measurements. More
particularly, it relates to a method of non-invasively determining blood
pressure using a photoplethysmograph.
BACKGROUND OF THE INVENTION
Arterial blood pressure measurements provide valuable information about a
patient's condition. The heart's cyclical action produces a blood pressure
maximum at systole, called systolic pressure, and a minimum pressure at
diastole, called diastolic pressure. While the systolic and diastolic
pressures are themselves important in gauging the patient's condition,
other useful parameters are the mean (average) blood pressure during a
heart cycle, and the pulse pressure, which is the arithmetic difference
between the systolic and diastolic pressures.
The importance of arterial blood pressure has spurred the development of
numerous methods of determining it. The most widely used method is
probably the familiar blood pressure cuff, which consists of an expandable
ring (1) inflated to stop arterial blood flow and (2) then gradually
contracted. Using a stethoscope, medical personnel listen to the artery to
determine at what pressure blood flow begins, establishing the systolic
pressure, and at what pressure flow is unrestricted, establishing the
diastolic pressure. More advanced blood pressure monitoring systems plot
the arterial blood pressure through a complete heart cycle. Typically,
these systems use catheters having piezoelectric pressure transducers that
produce output signals dependent upon the instantaneous blood pressure.
The output signals are monitored and used to determine the arterial blood
pressures over a complete heart cycle. These systems are advantageous in
that the blood pressure is continuously measured and displayed.
While prior art methods are useful, they have disadvantages. Cuff-type
systems require restricting arterial blood flow and are not suitable for
continuous use. The piezoelectric-type systems generally require
undesirable invasive techniques, costly disposable materials, and time and
skill to set-up. However, during certain critical periods, such as
surgery, continuous arterial blood pressure monitoring is highly
desirable. Therefore, it would be beneficial to have a method of
continuously and non-invasively measuring a patient's blood pressure.
Photoplethysmographs are well-known instruments which use light for
determining and registering variations in a patient's blood volume. They
can instantaneously track arterial blood volume changes during the cardiac
cycle. Since photoplethysmographs operate non-invasively, much work has
gone into using them to determine blood pressure. In 1983, inventor Warner
was issued U.S. Pat. No. 4,418,700 on a method of determining circulatory
parameters, wherein signals from a photoplethysmograph were used to
determine arterial blood pressure.
Significant problems were found when investigating the Warner method.
Therefore, it is clear that the need for a practical method of
continuously and non-invasively monitoring arterial blood pressure has
remained.
SUMMARY OF THE INVENTION
It is an object of this invention to provide an improved method for
continuously and non-invasively measuring arterial blood pressure.
It is another object of the present invention to provide an improved method
and system for non-invasively determining arterial systolic and diastolic
blood pressures with a photoplethysmograph.
These and other objects, which will become apparent as the invention is
more fully described below, are obtained by providing a method and
apparatus for determining arterial blood pressures using a
photoplethysmograph. The inventive method comprises the steps of
calibrating the photoplethysmograph output with a patient's arterial blood
pressure to determine an arterial constant k in the formula,
.psi.=.psi..sub.inf (1-Kexp(-kP))
where .psi. is the arterial blood volume, .psi..sub.inf is a conversion
constant corresponding to arterial blood volume at infinite pressure, K
and k are arterial constants for the patient, and P is the instantaneous
arterial blood pressure; gathering data from the photoplethysmograph
output during a measurement period; and computing the arterial systolic
and diastolic pressures at the measurement period using the evaluated
arterial constant k and the data gathered during the measurement period.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partial cutaway view, partial application depiction, and
partial block diagram illustrating a preferred method in operation.
FIG. 2 is a sketch of the output waveform from a photoplethysmograph
receiver over two cardiac cycles.
FIG. 3 is a block diagram illustrating the basic procedural steps of the
preferred method of FIG. 1.
FIG. 4 is a flow diagram of the preferred procedure for calibrating the
photoplethysmograph output to a patient according to the inventive method.
FIG. 5 is a flow diagram of the output monitoring and data acquisition
steps according of the inventive method.
FIG. 6 is a flow diagram outlining the preferred procedural steps for
arterial blood pressure determination according to the inventive method.
FIG. 7 is a flow diagram of an alternative procedure for calibrating the
photoplethysmograph output to a patient according to the inventive method.
FIG. 8 is a flow diagram outlining alternative procedural steps for
arterial blood pressure determination according to the inventive method.
DETAILED DESCRIPTION OF THE INVENTION
A preferred embodiment of the present invention, shown in FIG. 1, uses a
transmitter 2 portion of a photoplethysmograph 4 to cause monochromatic
light 6, preferably in the red and IR ranges, to be emitted from a
photodiode light source 8. The emitted monochromatic light 6 travels
through a patient 9, along a light path which includes blood 10 in an
artery 12, to a photodiode light detector 14. While artery 12 has been
described, and is shown in FIG. 1, as a single artery, in all practical
cases the light path actually passes through many arteries. These arteries
can be lumped together and treated as if only one artery 12 existed.
Therefore, for simplicity, the remainder of this application will only
discuss one artery 12, but it is to be understood that it represents the
composite effects of many individual arteries. The light path is also
through background tissue 16. The transmitter 2 controls the amount of
monochromatic light 6 emitted by varying the amount of current through the
light source 8. In the preferred embodiment, the transmitter 2 regulates
the monochromatic light 6 at a fixed level.
As the monochromatic light 6 travels along its light path it is partially
absorbed by the background tissue 16 and the blood 10. A portion of the
monochromatic light 6 is not absorbed and impinges on the light detector
14, creating electrical signals which are applied to a receiver 18 of the
photoplethysmograph 4. The magnitudes of these electrical signals depend
upon the amount of monochromatic light emitted by the light source 8, the
path lengths through the background tissue 16 and the blood 10, the amount
of light absorbed per unit length by the blood 10 and tissue 16, the
conversion efficiency of the light detector 14, and various lumped losses
such as poor focusing of the monochromatic light 6.
Since the artery 12 is pliant, as blood pressure increase so does the
volume of blood 10 within the artery 12. As the heart beats, its cyclical
action causes the arterial blood pressure to change. This causes the
electrical signals to change since the path length through the blood 10
changes, causing the amount of monochromatic light 6 absorbed by the blood
10 to change. Therefore, the electrical signals from the light detector 14
applied to the receiver 18 is a function of the arterial blood pressure.
The receiver 18 amplifies the electrical signals to a usable level and
applies them as analog signals, via a receiver line 22, to an
analog-to-digital converter A/D 23. The A/D 23 converts the outputs of the
receiver 18 to time sampled digital signals which are applied to the
computer 24 via a computer bus 25.
The signals on the receiver line 22 can be represented by the
photoplethysmograph output waveform 26, shown in FIG. 2 for two cardiac
cycles. The horizontal axis designates time and, in the present apparatus,
the vertical axis designates volts, but current levels would also be
suitable. Times t0 and t1, denoting the beginning of each cardiac cycle,
are clearly marked. The waveform 26 can be described mathematically as a
function of time, with the description being f(t). The voltage waveform is
inverted from the common pressure waveform because the voltage corresponds
to transmitted light. The highest voltage obtained over a cardiac cycle,
V.sub.d, coincides with the diastolic pressure and the lowest voltage,
V.sub.s, coincides with the systolic pressure. Between V.sub.s and V.sub.d
is a mean pressure voltage V.sub.m, which corresponds to the mean, or
average, arterial pressure over a full cardiac cycle. The duration of the
cardiac cycle, t.sub.d is the time between reoccurrences of the diastolic
or systolic voltages. The area between the waveform function f(t) and the
diastolic voltage line, shown in crosshatch in FIG. 2, is called the
"ARC." The particular values for V.sub.s, V.sub.m, V.sub.d, as well as the
waveform function f(t) and the area ARC, change with different patients,
photoplethysmographs, sensor locations, and photoplethysmograph settings.
However, these parameters are functions of the arterial blood pressure.
In a preferred method of the present invention, three major steps are used
to determine arterial blood pressure, shown in FIG. 3. The first, shown in
block 310, is the calibration of the photoplethysmograph output to the
patient. Referring now to FIG. 1, the calibration is accomplished by
matching the photoplethysmograph output on the computer bus 25 at the time
of calibration with the systolic, P.sub.s, and diastolic P.sub.d, blood
pressures from the auxiliary blood pressure instrument 20. In the
preferred embodiment, these blood pressure measurements are entered via a
keyboard to the computer 24. However, preferably this information would be
entered directly via an instrument bus 28. The photoplethysmograph output
is compared with the systolic and diastolic pressures, P.sub.s and
P.sub.d, from the auxiliary blood pressure instrument 20 and several
constants are determined, as is subsequently discussed.
As is shown in FIG. 3, block 320, the next step is the measurement of the
photoplethysmograph outputs during a measurement period to determine
various information. This information includes the systolic, mean, and
diastolic photoplethysmograph voltages V.sub.s, V.sub.m, and V.sub.d,
respectively, the cardiac duration t.sub.d, and the ARC. The final steps,
shown in FIG. 3, blocks 330 and 340, are the calculations of the systolic
and diastolic blood pressures, P.sub.s and P.sub.d, respectively, using
the determined photoplethysmograph information and the constants
determined in blocks 320 and 310. After the systolic and diastolic blood
pressures are determined, the information is output to medical personnel
on a display 30. If more measurements are desired, decision block 350
causes blocks 320, 330, and 340 to be repeated. However, only one
calibration phase 310 is required. These major steps are expanded upon
below.
DERIVATION OF THE MATHEMATICAL MODEL
The principle of the inventive method is derived from the Beer-Lambert law
of analytical chemistry. The Beer-Lambert law gives the relationship
between the absorption of monochromatic light by a concentration of a
material in a solution as a function of the path length through the
solution. Mathematically, the Beer-Lambert law is expressed as:
I=I.sub.o exp.sup.-cex
where I is the intensity of transmitted light, I.sub.o is the intensity of
incident light, c is the concentration of material, e is the extinction
coefficient of monochromatic light at a wavelength .lambda., and x is the
light path length through the medium.
The present invention analogizes blood 10 and tissue 16 density to
concentration, modifies the Beer-Lambert law so that the light intensity
terms are given in terms of receiver 18 output voltages, and breaks the
light path into individual lengths containing the background tissues 16
and the arterial blood 10. Therefore, the modified version of the
Beer-Lambert law is:
V=ZI.sub.o exp.sup.(-c.sub.t.sup.e.sub.t.sup.x.sub.t.sup.-
c.sub.a.sup.e.sub.a.sup.x.sub.a.sup.)
where the .sub.t refers to the background tissues 16, .sub.a refers to the
blood 10 in the artery 12, V is an equivalent transmission voltage
corresponding to the transmitted light, and Z is a constant relating light
intensity to the receiver 18 output voltage.
This can be simplified to:
V=A.sub.o exp.sup.(-c.sub.t.sup.e.sub.t.sup.x.sub.t.sup.)
exp.sup.(-c.sub.a.sup.e.sub.a.sup.x.sub.a.sup.)
where A.sub.o =ZI.sub.o.
This version has separable components, A.sub.o
exp.sup.(-c.sub.t.sup.e.sub.t.sup.x.sub.t.sup.) which relates to the
conversion constant and the background tissues 16, and
exp(.sup.-c.sub.a.sup.e.sub.a.sup.x.sub.a), which relates to the arterial
blood 10. For simplicity, the first component can be given as V.sub.o
=A.sub.0 exp.sup.(-c.sub.t.sup.e.sub.t.sup.x.sub.t.sup.), the background
transmission voltage. Therefore, the equivalent transmission voltage can
be calculated as:
V=V.sub.o exp(.sup.-c.sub.a.sup.e.sub.a.sup.x.sub.a)
It is convenient to express the above formula in terms of arterial blood
volume rather than light path length. Therefore, letting .psi. be the
arterial blood volume, and substituting for the light path .sup.x.sub.a :
V=V.sub.o exp(-b.psi..sup.1/2),
where b is equal to c.sub.a e.sub.a (4/.psi.L).sup.1/2, and L is the light
path width through the artery 12. Taking the natural logarithm results in:
lnV=-b.psi..sup.1/2 +lnV.sub.o
This version becomes more useful after incorporation of the arterial
volume-pressure relationship:
.psi.=.psi..sub.inf (1-Kexp(-kP))
where .psi. is still the arterial blood volume, .psi..sub.inf is a
conversion constant corresponding to the blood volume at infinite blood
pressure, and K and k are constants for the artery 12, and P is the
instantaneous arterial blood pressure. This arterial volume-pressure
relationship is a good approximation at the pressures of interest,
Substituting this formula for .psi. in the logarithmic version:
lnV=-b(.psi..sub.inf).sup.1/2 (1-Kexp(-kP)).sup.1/2 +lnV.sub.o
This can be expanded using a Taylor series. Expanding and eliminating
higher terms results in:
lnV=f+(n)exp(-kP)
f is equal to lnV.sub.o -b(.psi..sub.inf).sup.1/2, and n is equal to
(Kb(.psi..sub.inf).sup.1/2)/2. This can be converted to:
V=(u)exp((n)exp(-kP))
where u is equal to exp(f). In terms of systolic, diastolic, and mean
pressures:
V.sub.s =(u)exp((n)exp(-kP.sub.s)) for systolic Pressure
V.sub.d =(u)exp((n)exp(-kP.sub.d)) for diastolic Pressure
V.sub.m =(u)exp((n)exp(-kP.sub.m)) for mean Pressure
V.sub.inf =u
V.sub.0 =(u)exp(n)
V.sub.0 /V.sub.inf =exp(n)
Where V.sub.inf is the equivalent receiver voltage at infinite pressure and
V.sub.0 is the equivalent receiver voltage at zero pressure.
Establishing various ratios:
V.sub.d /V.sub.s =exp((n)(exp(-kP.sub.d)-exp(-kP.sub.s)))
V.sub.d /V.sub.m =exp((n)(exp(-kP.sub.d)-exp(-kP.sub.m)))
ln(V.sub.d /V.sub.s)=(n)(exp(-kP.sub.d)-exp(-kP.sub.s))
ln(V.sub.d /V.sub.m)=(n)(exp(-kP.sub.d)-exp(-kP.sub.m))
and
ln(V.sub.0 /V.sub.inf)=n
leads to useful ratios:
##EQU1##
where P.sub.p is termed "pulse pressure" and is equal to P.sub.s -
P.sub.d.
DETAILS OF THE PREFERRED METHOD
The previous section derived various relationships useful in the preferred
method as outlined in FIG. 3. The step of calibrating the
photoplethysmograph outputs to the patient 9, shown in FIG. 3, block 310
is shown in expanded detail in FIG. 4. The first two steps, shown in block
410 and block 420 are the determination and entering of the systolic and
diastolic blood pressures, P.sub.s and P.sub.d, respectively, at
calibration into the computer 24. As previously indicated and as shown in
FIG. 1, these blood pressures are determined by an auxiliary blood
pressure instrument 20, preferably an accurate blood pressure cuff having
direct inputs to the computer 24 via the instrument bus 28.
The next two steps, shown in blocks 430 and 440 of FIG. 4 are the
determination of the photoplethysmograph voltages, V.sub.s and V.sub.d,
from the receiver 18 output at the calibration systolic and diastolic
blood pressures, respectively. These photoplethysmograph voltages are
readily determined since they are the minimum and maximum output signals,
respectively, from the A/D converter 23. Next, as shown in block 450, the
duration of the cardiac cycle, t.sub.d is determined from the output of
the A/D converter 23. This is also readily accomplished by using a counter
to determine the time between the diastolic voltages, times t.sub.0 and
t.sub.1 of FIG. 2.
To determine various patient arterial constants, the preferred method
requires that the area between the diastolic voltage V.sub.d and waveform
function f(t), or ARC, be determined. This step is shown in block 460 and
is preferably accomplished by determining the integral of the
photoplethysmograph voltages over the cardiac cycle using:
##EQU2##
where ARC is the area between the waveform f(t) and the diastolic voltage
line V.sub.d, time t.sub.0 is the time at the start of a cardiac cycle,
t.sub.1 is the time at the start of the next cardiac cycle and (t.sub.1 -
t.sub.0) is the cardiac cycle duration t.sub.d. The calculation of ARC is
easily performed using a digital computer since the output of the A/D
converter 23 is a series of digital representations of the
photoplethysmograph signals over time. Using the Simpson approximation to
determine the integral is particularly expedient because the digital
magnitudes can be multiplied by the sampling time between readings, then
summed, to arrive at ARC. While ARC is preferably determined using
integral equations, other methods of determining it are also acceptable.
Next, as shown in block 470, the photoplethysmograph voltage, V.sub.m
corresponding to the mean pressure is determined from the formula
V.sub.m =V.sub.d -(ARC/t.sub.d);
where all terms are as previously given.
With V.sub.m known, the next steps, shown in block 480 and 490, are to
determine the patient's arterial constant k, solved numerically, and the
ratio V.sub.0 /V.sub.inf solved using either algebraic or numeric methods:
##EQU3##
With the above patient arterial constant k and V.sub.0 /V.sub.inf in
memory, the patient's arterial blood pressures can be determined only from
the photoplethysmograph output. This requires that various information be
determined during a measurement period, as shown in block 320 of FIG. 3
and with expanded detail in FIG. 5. Referring to FIG. 5, when arterial
blood pressures are to be determined, the computer 24 monitors the
photoplethysmograph outputs to determine, at the time of measurement, the
systolic voltage V.sub.s, the diastolic voltage V.sub.d, the duration of
the cardiac cycle t.sub.d and the ARC, as shown in blocks 510, 520, 530
and 540, of FIG. 5 respectively. With the information V.sub.d, t.sub.d and
ARC determined, the computer 24 then determines, as shown in block 550,
the equivalent photoplethysmograph voltage V.sub.m using the formula:
V.sub.m =V.sub.d -(ARC/t.sub.d)
With the arterial constant k and the ratio V.sub.0 /V.sub.inf determined
according to the flow chart of FIG. 4, and the photoplethysmograph
information determined according to the flow chart of FIG. 5, the computer
24 determines the patient's systolic and diastolic blood pressures as
shown in the flow chart of FIG. 6, which is a more detailed description of
blocks 330 and 340 of FIG. 3. The most efficient method of determining
systolic and diastolic blood pressures appears to be, as shown in block
610, to first calculate the pulse pressure P.sub.p, using numerical
methods, from the formula:
##EQU4##
Next, the diastolic blood pressure P.sub.d is determined, as shown in
block 620, using the formula
##EQU5##
The determination of the systolic blood pressure P.sub.s, is then readily
accomplished, as shown in block 630, using the equation P.sub.s =P.sub.d
+P.sub.p. While the above is the preferred method of calculating arterial
systolic and diastolic blood pressures from the photoplethysmograph
outputs, other schemes are possible.
The systolic and diastolic blood pressures are then available for output to
medical personnel as shown in block 640, in a variety of way such as by
digital or analog readouts, chart recorders, voice synthesis, or as in the
present embodiment on a display monitor 30. If another set of measurements
is desired then decision block 650 causes the flow shown in FIGS. 5 and 6
to be repeated.
The preferred embodiment described above is useful, can be readily
implemented on a digital computer, and provides accurate and rapid
measurements of arterial blood pressures non-invasively and in a manner
suitable for continuous measurements. However, in some patients and under
some conditions, the preferred method leads to inaccuracies because of
time variations in V.sub.inf, the equivalent receiver voltage at infinite
pressure. V.sub.inf, in the preferred method was part of the ratio V.sub.0
/V.sub.inf determined during calibration and presumed constant. The
preferred embodiment can be modified to compensate for changes in
V.sub.inf but at the expense of additional computation difficulty and
time.
The alternative embodiment follows the same three major steps as shown in
FIG. 3 for the preferred embodiment. However, the calibration procedure of
FIG. 4 is modified to that shown in FIG. 7. These calibration procedures,
shown in FIG. 7 blocks 710 through 780, are identical until V.sub.inf is
determined in block 790. It can be shown that V.sub.inf is determinable by
the following formula:
V.sub.inf =exp{[ln(V.sub.s) -(lnV.sub.d)exp(-kP.sub.p)]/[1-exp(-kP.sub.p)]}
With V.sub.inf thus determined in block 790, V.sub.0, the equivalent
receiver voltage at zero pressure, is determined, as shown in block 799,
from the formula:
##EQU6##
After the photoplethysmograph output is calibrated according to the
alternative embodiment, as shown in FIG. 7, the patient constants k and
V.sub.0 are known.
According to the alternative embodiment, the data gathering steps depicted
in FIG. 5 remain the same. However, during blood pressure determination,
the flow diagram of FIG. 6 is modified to the procedural steps shown in
FIG. 8. Referring now to FIG. 8, after determination of the pulse pressure
P.sub.p in block 810, in the same manner as it was determined in block
610, the V.sub.inf at the time of measurement is determined, as shown in
block 820, from equation:
V.sub.inf =exp{ln(V.sub.s)-[exp(-kP.sub.p)]lnV.sub.d ]/[1-exp(-kP.sub.p)]}
where V.sub.s and V.sub.d are also the values at the time of measurement.
This new V.sub.inf is then used in the equation of block 830, along with
the previously stored value of V.sub.0, to determine the diastolic
pressure P.sub.d. This alternative embodiment reduces the effects of
changes in V.sub.inf. The calculation of the systolic pressure P.sub.s,
shown in Block 840, and the output of the systolic and diastolic
pressures, P.sub.d and P.sub.s, respectively, as shown in block 850 are
performed in the same manner as they were in blocks 630 and 640,
respectively, of FIG. 6. Likewise, the decision block 860 operates in the
same manner as the decision block 650 in FIG. 6.
The apparatus for practicing the present invention uses a modified pulse
oximeter-type photoplethysmograph 4 having numerous user controls, such as
receiver 18 gain and light source 8 current settings. It outputs an analog
voltage representation of the photodiode output to an analog-to-digital
converter A/D 23 which digitizes the receiver 18 output and applies it to
an IBM-AT type personal computer 24 under the control of software stored
in a hard-disk drive. The display 30 output is on a computer monitor. The
required auxiliary blood pressure instrument 20 readings are input by
keyboard when directed by software programmed prompts. In future
applications, the separate photoplethysmograph 4, A/D converter 23, and
computer 24 will probably be replace by similar structures within a single
chassis and calibration data will be automatically inputted by an
automatic blood pressure cuff.
From the foregoing, it will be appreciated that the invention, as described
herein for purposes of illustration, provides an advancement in
non-invasive blood pressure instruments. Although alternative embodiments
have been described herein, various modifications may be made without
departing from the spirit and scope of the present invention. Accordingly,
the scope of the invention extends to the broad general meaning of the
appended claims.
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