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Description  |
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FIELD OF THE INVENTION
The present invention relates generally to the field of continuous
noninvasive measurement of blood pressure. More specifically, the present
invention provides a method and apparatus for monitoring a transducer
array of individual pressure or force sensitive elements and for selecting
the element within the array which most tracks the actual pulse waveform
in an underlying artery, thus providing the most accurate mesurement of
the patient's blood pressure.
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, 3,219,035 to G. L.
Pressman, P. M. Newgard and John J. Eige, 3,880,145 to E. F. Blick,
4,269,193 to Eckerle, and 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 arragnements the elements
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 latter method is
shown in the above-mentioned J. S. Eckerle Patent No. 4,269,193. the
selection method disclosed in Patent No. 4,269,193 generally identifies
the correct transducer element to be used. However, pressure readings
provided by individual elements of a transducer array may not be perfectly
accurate due to any number of factors. Even small errors in the pressure
reading may result in the selection of an incorrect transducer element
using the system disclosed in Patent No. 4,269,193, in which case the
blodd pressure measurements are inaccurate. A method for determining the
correct transducer element for measuring blood pressure is disclosed in
copending application Ser. No. 927,843 assigned to SRI International. The
method disclosed in present invention represents an improvement on the
method disclosed in the above-referenced patent application.
SUMMARY OF THE INVENTION
The present invention relates to a blood pressure monitoring system
employing a transducer which comprises an array of individual pressure
sensitive elements, each of which elements have at least one dimension
smaller than the lumen of the underlying artery in which blood pressure is
measured. The elements are of sufficiently small size such that the array
positioned so as to extend across the artery a plurality of elements are
located over the artery. The outputs of all of the transducer elements are
employed in locating the particular element which is centrally located
over the artery. A limited number of elements exhibiting local minima of
either diastolic or systolic pressure is first chosen. Then, pulse
amplitude outputs from the limited number of transducer elements are
employed in selecting that element within the limited-number group which
is to be used for obtaining blood pressure measurements. The difference
between the systolic and diastolic pressure values is defined herein as
the pulse amplitude of the blood pressure waveform.
Theoretically, the graph of diastolic or systolic pressure versus
transducer element exhibits two peaks corresponding to the transducer
elements which overlie in two edges of the artery, since those elements
are subject to bending forces produced by the natural stiffness of the
artery. The centered element, on the other hand, is not subjected to these
forces and only measures the fluid pressure within the artery. The
centered element, therefore, always exhibits a local minimum in the
diastolic or systolic pressure graph. Physical theory also dictates that a
graph of pulse amplitude versus transducer element would exhibit a
symmetrical distribution of values centered about, and having the greatest
magnitude at, those elements directly overlying the flattened artery. The
method provided by the present invention selects from the limited-number
group of elements having local diastolic minima that element about which
is centered the greatest spatially weighted average of a predetermined
number of pulse amplitude values.
The method of the present invention also satisfactorily handles pressure
versus transducer element curves which are not smooth due to noise or the
physical configuration of the elements. For example, if adjacent elements
are also separated longitudinally, and the plane in which the elements lie
is not perfectly perpendicular to the force vectors exerted at the
flattened artery wall, adjacent elements would sense slightly different
pressure values. These artifical variations can produce local minima in
the diastolic pressure versus transducer element curve which may not be
distinguishable from the "true" minimum produced by the centered element.
This problem is solved by choosing that element about which is centered
the greatest spatially averaged pulse amplitude.
The method of the present invention also compensates for failure of one or
more of the transducer elements. In particular, the element selection
method of the present invention is capable of ignoring those elements
which are detected to unusable and can still provide accurate pressure
readings.
Finally, the method of the present invention incorporates pulse amplitude
detection citeria which result in an error condition being reported unless
the pulse amplitude exceeds a certain predetermined value. The pulse
amplitude will be less than that value when the hold-down pressure is
insufficient to flatten the artery.
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 and 5B together show a flow chart for use in explaining overall
operation of this invention.
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 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
stresses 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
model 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. An improved method for locating the rider which best
represents the actual waveform in the underlying artery is described in
greater detail below. 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 T
ransducer" 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=.infin.). 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, and a pressurizable chamber
40. 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 38 is
supplied to data display and recorder means 50 which may include a
recorder, cathode ray tube montor, a solid state display, or an 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 aray 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 and 5b 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 microprocess 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 at which time system
powder is turned on or a reset operation is performed by means not shown,
and counters, registers, and timers in microprocessor 48 are initialized.
The transducer is attached to the subject at step 102 at a location
wherein at least one transducer element, such as element 22-E of
transducer 22 should overlie the center of the artery 24. Next, 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 transducer piston 16 outwardly a short
distance from the bo ttom of the transducer case.
With the transducer attached to the subject, step 106 is entered wherein
the transducer element to be used in monitoring blood pressure is
selected. Novel algorithms which may be used in selecting the proper
transducer element are described in detail hereinbelow. At step 108, the
location of the selected element is displayed and in step 110 a decision
is made to determine whether the selected element is near the center of
the transducer array 22. If the selected element is not near the center of
the array, step 112 is entered wherein the transducer is repositioned on
the subject and step 1-4 through 110 are reentered. If the determination
of step 110 indicates that the selected element is near the center of the
transducer array, then step 114 is entered wherein the optimal hold down
pressure is computed. Novel means for determining the optimum holddown
pressure are described in a patent application entitled "Pressure Control
System for Continuous Blood Pressure Monitor Transducer," filed on even
date herewith. In step 116, the computed hold-down pressure is set by
control of pressure controler 54 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.
At step 118, the set of usable transducer elements U is determined by the
system. This is accomplished from either operator input or by the system
receiving an out-of-range output from one of the elements. From the set U
of usable elements, the differences in diastolic pressure between each
element and its immediately adjacent neighbor are calculated as follows:
##EQU1##
where d.sub.n is the diastolic pressure measured by the transducer element
n. Next those transducer elements exhibiting a local minimum of diastolic
pressure are grouped into a set L. Set L is generated by selecting those
elements n for which diff.sub.n-1 is negative and diff.sub.n is positive.
These elements then correspond to regions on the diastolic pressure versus
transducer element waveform negative-to-positive slope changes, referred
to herein as local minima. An odd-integer k, corresponding to the minimum
number of transducer elements which spans one artery diameter, is then
generated. For each element E.sub.n in L, a spatially weighted average of
k pulse amplitude values from k transducer elements centered about element
n is calculated, using Wavg.sub.n. Wavg.sub.n is calculated at step 120 by
first multiplying each pulse amplitude p.sub.i from element E.sub.i by a
weighting factor W.sub.i. W.sub.i is calculated a (k+1)/2 when i=n, where
E.sub.n is the element about the average is to be centered, and decrements
by one sequentially for each element on either side of E.sub. n. For
example, if K=5 and n=7:
##EQU2##
Next a sum is calculated of the products of each pressure p.sub.i times it
weighting factor W.sub.i, and this sum is divided by the sum of the
weighting factors, i.e., W.sub.n-[(k-1)/2] +. . . +W.sub.n +. . .
+W.sub.n+[(k -1)/2]. Thus,
##EQU3##
where i varies from n-[k-1)/2 to n+](k-1)/2] over both summations.
When elements E.sub.i are either unusable or do not physically exist,
W.sub.i in the formula above is assigned a value of zero. for example,
assuming k=5 and n=7, and that element E.sub.5 does not exist:
##EQU4##
If, on the other hand, E.sub.6 was unusable, the resulting value of Wavg7
would be calculated as:
##EQU5##
From the set L, the element E.sub.i having the largest spatially weighted
pulse amplitude average centered about it is selected as the measuring
transducer element at step 122. With the correct pressure transducer
element selected, step 124 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 126. Also, pulse rate is readily
calculated by determining the time between successive diastolic or
systolic pressures. At step 126, values calculated and determined in step
124 are displayed and/or recorder along with the actual waveform.
After the values identified in step 126, such as systolic and/or diastolic
pressure, are displayed, decision step 126 is entered wherein the system
determines whether there has been a change in the selected element. Such a
change can be related to a number factors, including detection of motion
in the patient's wrist or repositioning of the transducer on the patient's
wrist. If no such change has been indicated, step 124 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 122, as
shown in FIG. 5b.
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
appended claims.
* * * * *
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