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Description  |
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FIELD OF THE INVENTION
The present invention relates generally to a method an apparatus for
continuous noninvasive measurement of blood pressure. More specifically,
the present invention provides a method and apparatus for ascertaining the
correct transducer hold-down pressure required for obtaining accurate
blood pressure measurements.
BACKGROUND
There has been considerable interest in recent years in the development of
a monitoring system for obtaining a continuous measurement of a patient's
blood pressure. One of the most promising techniques for obtaining such a
continuous measurement involves the use of an arterial tonometer
comprising an array of small pressure sensing elements fabricated in a
silicon "chip." The use of such an array of sensor elements for blood
pressure measurements is disclosed generally in the following U.S.
Patents: U.S. Pat. No. 3,123,068 to R. P. Bigliano, U.S. Pat. No.
3,219,035 to G. L. Pressman, P. M. Newgard and John J. Eige, U.S. Pat. No.
3,880,145 to E. F. Blick, U.S. Pat. No. 4,269,193 to Eckerle, and U.S.
Pat. No. 4,423,738 to P. M. Newgard, and in an article by G. L. Pressman
and P. M. Newgard entitled "A Transducer for the Continuous External
Measurement of Arterial Blood Pressure" (IEEE Trans. Bio-Med. Elec., April
1963, pp. 73-81).
In a typical tonometric technique for monitoring blood pressure, a
transducer which includes an array of pressure sensitive elements is
positioned over a superficial artery, and a hold-down force is applied to
the transducer so as to flatten the wall of the underlying artery without
occluding the artery. The pressure sensitive elements in the array have at
least one dimension smaller than the lumen of the underlying artery in
which blood pressure is measured, and the transducer is positioned such
that more than one of the individual pressure-sensitive elements is over
at least a portion of the underlying artery. The output from one of the
pressure sensitive elements is selected for monitoring blood pressure. The
element that is substantially centered over the artery has a signal output
that provides an accurate measure of intraarterial blood pressure.
However, for the other transducer elements the signal outputs generally do
not provide as accurate a measure of intraarterial blood pressure as the
output from the centered element. Generally, the offset upon which
systolic and diastolic pressures depend will not be measured accurately
using transducer elements that are not centered over the artery. In some
prior art arrangements the pressure sensitive element having the maximum
pulse amplitude output is selected, and in other arrangements the element
having a local minimum of diastolic or systolic pressure which element is
within substantially one artery diameter of the element which generates
the waveform of maximum pulse amplitude is selected.
The pressure measured by the selected pressure sensitive element, i.e., the
element centered over the artery, will depend upon the hold-down pressure
used to press the transducer against the skin of the subject. Although
fairly accurate blood pressure measurements are obtained when a hold-down
pressure within a rather wide pressure range is employed, it has been
found that there exists a substantially unique value of hold-down pressure
within said range for which tonometric measurements are most accurate.
This so-called "correct" hold-down pressure varies among subjects. With
prior art tonometric type transducers, the correct hold-down pressure
often is not determined, thereby leading to inaccuracies in the blood
pressure measurements. A method for determining optimal hold down pressure
is disclosed in application Ser. No. 007,038 assigned to SRI
International. The method disclosed in the present invention represents an
improvement on the method disclosed in the aforementioned patent
application.
SUMMARY OF THE INVENTION
The present invention includes a transducer array for generation of
electrical waveforms indicative of blood pressure in an artery. Using the
selected pressure sensing element that is determined to be positioned
substantially over the center of the underlying artery, a set of data
corresponding to the diastolic pressure and the pulse amplitude pressure
is collected and stored. The diastolic pressures and pulse amplitude
pressures are taken as a function of hold down pressure over a range of
hold down pressures between the pressure at which the artery is
unflattened and the pressure at which the artery is occluded. First and
second polynomials are fitted to the diastolic pressure data set and the
pulse amplitude data set, respectively. The hold-down pressure at the
point of minimum slope of the first polynomial fitted to the diastolic
versus hold-down pressures values provides one estimate of the correct
hold-down pressure. Another estimate of the correct hold-down pressure
using the pulse amplitude measurements is provided by locating the point
where the slope of the second polynomial is zero. In another embodiment of
the method of the present invention, the two above described estimates are
combined into a single estimate.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view of the continuous blood pressure monitoring transducer of
the present invention attached to a patient's wrist at a position
overlying the radial artery.
FIG. 2 is a schematic diagram illustrating the force balance between the
artery and the multiple transducer elements (arterial riders), with the
artery wall properly depressed to give accurate blood pressure readings.
FIG. 3 is a simplified block diagram of the transducer assembly and
associated system components for the continuous blood pressure monitoring
system of the present invention.
FIG. 4 is a waveform of human blood pressure versus time of the type which
may be obtained using the present invention for illustrating systolic and
diastolic pressures and pulse amplitude of the blood pressure wave.
FIGS. 5A, 5B and 5C together show a flow chart for use in explaining
overall operation of this invention.
FIG. 6 shows plots of diastolic pressure and pulse amplitude versus
hold-down pressure for a typical subject.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Reference is now made to FIG. 1 wherein a continuous blood pressure monitor
transducer 10 is shown attached to a patient's wrist at a point overlying
the radial artery. The transducer is attached by means of a strap 12 in a
manner similar to a conventional wristwatch. A cable assembly 14 connected
to the transducer contains electrical cables for carrying electrical
signals to and from the transducer. The cable assembly 12 also contains a
pneumatic tube for providing pressurized air to a pressurizable bladder in
the interior of the transducer in order to bring a sensor into contact
with the patient's skin in a manner described in greater detail
hereinbelow.
For the transducer to properly measure blood pressure it is important that
the underlying artery be partially compressed. Specifically, it is
important that the artery be flattened by a plane surface so that the
stress developed in the arterial wall perpendicular to the face of the
sensor are negligible. This generally requires that the blood pressure
measurement be taken on a superficial artery which runs over bone, against
which the artery can be flattened.
Reference now is made to FIG. 2 wherein a diagrammatic mechanical model is
shown which is representative of physical factors to be considered in
blood pressure measurements using tonometry techniques. The illustrated
models is adapted from that shown in the above-mentioned U.S. Pat. No.
4,269,193, issued to J. S. Eckerle, which by this reference is
incorporated for all purposes. An array 22 of individual pressure
sensitive elements or transducers 22-A through 22-E, which constitute the
arterial riders, is positioned so that one or more of the riders are
entirely over an artery 24. The individual riders 22-A through 22-E are
small relative to the diameter of the artery 24, thus assuring that a
plurality of the riders overlie the artery. The skin surface 26 and artery
underlying the transducer must be flattened by application of a hold-down
pressure to the transducer. One rider overlying the center of the artery
is identified as the "centered" rider, from which rider pressure readings
for monitoring blood pressure are obtained. Means for selecting the
centered rider are discussed generally in the above mentioned U.S. Pat.
No. 4,269,193. In addition, an improved means for selecting the best
pressure sensing element for measuring blood pressure is disclosed in a
patent application entitled "Active Element Selection for Continuous Blood
Pressure Monitor Transducer" filed on even date herewith. For present
purposes it will be understood that one of the riders, such as rider 22-E,
may be selected as the "centered" rider, in which case the remainder of
the riders, here riders 22-A through 22-D and 22-F through 22-J, comprise
"side plates" which serve to flatten the underlying skin and artery.
Superficial arteries, such as the radial artery, are supported from below
by bone which, in FIG. 2, is illustrated by ground symbol 28 under the
artery. The wall of artery 24 behaves substantially like a membrane in
that it transmits tension forces but not bending moments. The artery wall
responds to the loading force of the transducer array, and during blood
pressure measurements acts as if it is resting on the firm base 28. With
the illustrated system, the transducer assembly 10 and mounting strap 12,
together with air pressure applied to a pressurizable bladder in the
transducer assembly, supply the required compression force and hold the
riders 22-A through 22-J in such a manner that arterial pressure changes
are transferred to the riders which overlie the artery 24. This is
illustrated schematically in FIG. 2 by showing the individual riders 22-A
through 22-J backed by rider spring members 30-A through 30-J,
respectively, a rigid spring backing plate 32, and hold-down force
generator 36 between the backing plate 32 and the mounting strap system
38.
If, without force generator 36, the coupling between the mounting strap
system 38 and spring backing plate 32 were infinitely stiff to restrain
the riders 22-A through 22-J rigidly with respect to the bone structure
28, the riders would be maintained in a fixed position relative to the
artery. In practice, however, such a system is not practical, and
hold-down force generator 36, comprising (in the present example) a
pneumatic loading system, is included to keep constant the force applied
by the mounting strap system 38 to riders 22-A through 22-J. In the
mechanical model the spring constant, k (force per unit of deflection) of
the force generator, 36, is nearly zero. Pneumatic loading systems are
shown and described in the above-referenced U.S. Pat. Nos. 3,219,035 and
4,269,193, and the Pressman and Newgard IEEE article. In addition, an
improved pneumatic loading system is disclosed in a patent application
entitled "Pressurization System for Continuous Blood Pressure Monitor
Transducer" filed on even date herewith.
In order to insure that the riders 22-A through 22-J flatten the artery and
provide a true blood pressure measurement, they must be rigidly mounted to
the backing plate 32. Hence, the rider springs 30-A through 30-J of the
device ideally are infinitely rigid (spring constant k=.alpha.). It is
found that as long as the system operates in such a manner that it can be
simulated by rider springs 30-A through 30-J having a spring constant on
the order of about ten times the corresponding constant for the
artery-skin system, so that the deflection of riders 22-A through 22-J is
small, a true blood pressure measurement may be obtained when the correct
hold-down pressure is employed.
Referring to FIG. 3, a simplified illustration of the transducer assembly
10 is shown to include a transducer piston 16, a pressurizable chamber 40
and a position controller 60. The output of the individual pressure
sensors (not shown) on the sensor 20 are connected by appropriate
electrical wiring 42 to the input of a multiplexer 44. From the
multiplexer, the signals are digitized by an analog-to-digital (A-D)
converter 46, and the digitized signals are supplied to a microprocessor
48. Output from the microprocessor 48 is supplied to data display and
recorder means 50 which may include a recorder, cathode ray tube monitor,
a solid state display, or any other suitable display device. Also, the
output from the microprocessor is provided to the pressure controller 52
which controls a pressure source 54 to maintain the appropriate hold down
pressure for the transducer piston 16. Operation of the microprocessor can
be controlled by a program contained in program storage 56 or by user
input from the user input device, which can be in the form of a keyboard
or other interface device.
Reference is now made to FIG. 4 which illustrates the signal waveform of
the output from one of the pressure sensitive elements 22-A through 22-J
which overlies artery 24. Other elements of the transducer array which
overlie the artery will have waveforms of similar shape. With a correct
hold-down pressure and correct selection of the "centered" arterial rider
(i.e., the rider substantially centered over the artery) the waveform is
representative of the blood pressure within the underlying artery.
Systolic, diastolic and pulse amplitude pressures are indicated on the
waveform, wherein pulse amplitude is the difference between the systolic
and diastolic pressures for a given heartbeat.
FIGS. 5A, 5B, and 5C together show a flow chart of an algorithm for general
overall operation of the blood pressure monitoring system. Some of the
operations indicated therein are under control of the microprocessor 48
responsive to programming instructions contained in program storage 56.
Obviously, several program steps may be involved in the actual
implementation of the indicated operations. Since the programming of such
steps is well within the skill of the average programmer, a complete
program listing is not required and is not included herein.
Preparation for monitoring is begun at START step 100, shown in FIG. 5A, at
which time system power is turned on or a reset operation is performed by
means not shown, and counters, registers, timers in the microprocessor 48
are initialized. Next, ate step 104, the transducer is attached to the
patient at a location wherein a centrally located transducer element, such
as element 22-E of tranducer array 22 overlies the center of the artery
24. Of course, the exact position of the transducer array relative to the
subject, or operator, and repositioning of the transducer may be required
to properly position the same. At step 104 a nominal hold down pressure
(H.D.P.) is applied wherein air under pressure from source 54 is supplied
to the transducer. For example, a hold-down pressure of 40 mmHg may be
supplied to the transducer, which pressure serves to extend the
pressurizable chamber 40 whereby the transducer piston extends outwardly a
short distance from the bottom of the transducer case.
With the transducer attached to the subject, step 106 is entered and the
location of the currently selected element is identified and displayed. At
step 108, a decision is made about whether the currently selected element
is in the center of the array of pressure sensing elements. If it is
determined that the selected element is not near the center of the array,
then step 110 is entered wherein the transducer is repositioned on the
subject and step 106 is reentered. The process is repeated until the
transducer is properly located on the subject. However, if the decision of
step 108 is affirmative, then a series of data collection steps, shown in
the flowchart of FIG. 5B, are entered beginning with step 112.
In step 112, upper and lower hold down pressure limits are set. These
limits can be predetermined values stored in the memory of the computer
62, or can be entered by the operator. In step 114, the rate of increase
is entered for varying the hold down pressure from the lower limit to the
upper limit entered in step 112. Again, this value can be a predetermined
value stored in the computer or can be a value entered by the operator. In
step 116, the initial hold down pressure is initialized to the lower limit
and in step 118 the hold down pressure is increased in a continuous linear
manner until the detection of a heartbeat in step 120. Once a heartbeat
has been detected, step 122 is entered wherein the systolic and diastolic
pressures are recorded. Processes which may be employed in step 122,
including identifying systolic and diastolic pressures, pulse amplitude,
maxima, local minima, from the transducer outputs are readily implemented
using the microprocessor 48. In step 124, the current hold down pressure
is recorded and in step 126 the decision is made as to whether the current
hold down pressure exceeds the upper hold down pressure limit. If hold
down pressure does not exceed the upper limit, then step 120 is reentered.
However, if it is determined that the upper limit has been exceeded, then
a series of computational steps 128 through 134 is entered for the
determination of optimal hold down pressure. Novel algorithms which may be
used in computing the correct hold-down pressure will be described in
greater detail below. For present purposes, it will be understood that a
correct hold-down pressure for accurate blood pressure monitoring is
computed and set at step 132, following which, at step 134, the computed
hold-down pressure is set by control of pressure controller 52 by the
microprocessor 48. With the transducer properly positioned on the subject
and the correct hold-down pressure supplied thereto, the system is in
condition for obtaining accurate blood pressure readings.
With the correct hold down pressure set, the step 136 is entered wherein
the system waits for the next heartbeat. From the output of the selected
transducer element, systolic and diastolic pressure values together with
pulse amplitude values are readily determined in step 138. Also, pulse
rate is readily calculated by determining the time between successive
diastolic or systolic pressures. At step 140, values calculated and
determined in step 138 are displayed and/or recorded along with the actual
waveform.
After the values identified in step 122, such as systolic and/or diastolic
pressure, are displayed, decision step 142 is entered wherein the system
determines whether there has been a request from the operator for a
recomputation of hold down pressure. If no such request has been received,
step 136 is reentered wherein the system waits for the next heartbeat.
However, if such a request for recomputation has been received, then the
system returns to step 104, as shown in FIG. 5A.
DETERMINATION OF HOLD-DOWN PRESSURE
1. Diastolic Pressure vs. Hold-Down Pressure
Reference now is made to FIG. 6 wherein plots of diastolic pressure and
pulse amplitude versus hold-down pressure are shown which will facilitate
an understanding of novel means for determining correct hold-down pressure
for accurate blood pressure measurements. A third-order polynomial is
fitted using, for example, least squares techniques to the FIG. 6 series
of diastolic pressure points to provide a curve 140 which has the typical
shape shown regardless of physical characterstics of the subject.
A third-order polynomial fitted to the measured data may be written as
follows:
D(h)=a.sub.0 +a.sub.1 h+a.sub.2 h.sup.2 +a.sub.3 h.sup.3 (1)
wherein:
D(h)=measured diastolic pressure at hold down pressure h,
h=hold-down pressure, and
a.sub.0, a.sub.1, a.sub.2, and a.sub.3 are coefficients of the polynomial.
The derivative of the function D(h) may be written as D'(h)=a.sub.1
+2a.sub.2 h+3a.sub.3 h.sup.2. This derivative will be used in the final
selection of optimum hold down pressure, as discussed in greater detail
below. For hold-down pressures between zero and P1, the underlying artery
remains unflattened, and the measured pressure is primarily dependent upon
the hold-down pressure and secondarily upon the intraarterial pressure,
P.sub.a. The graph of the polynomial is a relatively straight line over
this range. Up to pressure P1, the effective spring constant of the
artery, using the mechanical model of the system shown in FIG. 2, is
large.
Between hold-down pressures P1 and P2, the hold-down pressure is great
enough to partially flatten the underlying artery, but not great enough to
occlude it. Experiments have shown that most accurate blood pressure
measurements are obtained when a hold-down pressure that is substantially
midway between pressures P1 and P2. Between pressures P1 and P2, the
effective spring constant of the artery, using the mechanical model of
FIG. 2, is relatively small.
At hold-down pressures greater than P2, the underlying artery is completely
occluded, and the effective spring constant of the underlying artery is
again relatively large. Consequently, the measured pressure is again
substantially independent of the intraarterial pressure, P.sub.a, and the
curve is substantially a straight line above pressure P2. As seen in FIG.
6, the slope of curve 140 is lowest between pressures P1 and P2 where the
underlying artery is flattened but not occluded. In theory, the correct
hold down pressure could be determined from this curve alone by locating
the minimum slope point, which is point P3 on FIG. 6.
2. Pulse Pressure vs. Hold Down Pressure
Reference is again made to FIG. 6 wherein plots of diastolic pressure and
pulse amplitude versus hold-down pressure are shown. A third-order
polynomial is again fitted using least squares techniques to the FIG. 6
series of pulse pressure points to provide a curve 142 which has the
typical shape shown regardless of physical characteristics of the subject.
For the determination of the hold down pressure using the pulse amplitude
points, it is highly desirable to use a mathematical relationship which
provides a high correlation coefficient. It has been found that a third
order polynomial provides this desired relationship. The third-layer
polynomial fitted to the measured data may be written as follows:
P(h)=b.sub.0 +b.sub.1 h+b.sub.2 h.sup.2 +b.sub.3 h.sup.3 (2)
wherein:
P(h)=measured pulse pressure at hold down pressure h,
h=hold-down pressure, and
b.sub.0, b.sub.1, b.sub.2, and b.sub.3 are coefficients of the polynomial.
In theory, the correct hold down pressure could be determined from this
curve alone by locating the maximum point which is point P3' in FIG. 6.
Furthermore, as discussed in greater detail below, it is expected that the
maximum point P3' on curve 142 will coincide with the minimum slope point
P3 for curve 140.
3. Combination Method
As discussed above, an optimum hold-down pressure can be estimated by
finding the h value for which D(h) has a maximum slope in equation (1), or
alternatively, by finding the h value for which P(h) is a maximum in
equation (2). In order to best combine these two estimates, a new function
G(h) is defined as follows:
##EQU1##
where D'(h) and P(h) are as defined hereinabove. The optimum hold-down
pressure h is then computed to be that h for which G(h) is a maximum. This
is best done by the microprocessor 48 sequentially computing G(h) for each
discrete value of h while simultaneously searching for the maximum.
The basis for the method described above is related to the fact that G(h)
will be maximum at some h if P(h) is maximum and D'(h) is minimum at h. At
the minimum slope point of D(h), D'(h) is approximately zero. The term
D'(h)+1 was chosen to prevent a division by zero which could result in
G(h) going to infinity. Such a result would have the effect of causing the
D(h) curve to dominate the selection of hold down pressure. The method
thus gives an estimate of the optimal hold-down pressure pulse amplitude
and diastolic pressure methods described earlier when the latter two
methods each produce the same result.
When the pulse amplitude and diastolic pressure methods differ, however,
the present method produces an estimate of the optimum hold-down pressure
which is a weighted average of the two different hold-down pressure
estimates. For example, if there is a very small difference between the
maximum and minimum value of the curve 142, then the function P(h) can be
considered to be approximately a constat value, P. For this case:
##EQU2##
The maximum of G(h) then occurs at the h for which D'(h) is a minimum. On
the other hand, if the variation in the slope at the various points on the
curve 140 is very small, then the function D'(h) can be considered to be a
constant value D'. For this case:
##EQU3##
The maximum G(h) occurs in this case at the same h for which P(h) is a
maximum.
Although the method and apparatus of the present invention has been
described in connection with the preferred embodiment, it is not intended
to be limited to the specific form set forth herein, but on the contrary,
it is intended to cover alternatives and equivalents as may reasonable be
included within the spirit and scope of the invention as defined by the
appended claims.
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