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FIELD OF THE INVENTION
The present invention relates generally to the field of blood pressure
monitoring, and more particularly to automatic monitoring of systolic
blood pressure.
DESCRIPTION OF PRIOR ART
The prior art is replete with devices for measuring systolic pressure of a
living subject. An old and simple device is a pressurizable cuff used in
combination with a mercury manometer which reads pressure in the cuff and
a stethoscope which is used to listen to Korotkoff sounds. More
complicated methods and apparatus based on the same principle of listening
to the Korotkoff sounds replace the mercury manometer with a mechanical or
electromechanical pressure gauge and utilize microphonic detection of the
Korotkoff sounds which are analyzed electrically. In another advanced
method of measuring blood pressure, the distance from a blood pressure
cuff to the wall of an artery is accurately determined by measuring
Doppler shifts of sound waves reflected by the artery. The distance to the
artery varies as a function of pressure within the somewhat pliable walls
thereof. In yet other methods for measuring blood pressure intrusive
devices are often inserted directly into blood vessels.
Oscillometric methods of determining systolic pressure are also well known
in the art. In such methods, the operator observes a representation of the
strength of pulsations of pressure within an artery. This can be done
visually, as by watching the extent of bouncing of the top of a mercury
column in a mercury manometer which is in pressure-communication with a
cuff, or indirectly as by measuring the occlusion which occurs to a blood
vessel in the pinna of the ear as pressure is exerted thereon, as in U.S.
Pat. No. 3,227,155. Oscillometric methods of determining systolic pressure
generally define systolic pressure to be the maximum applied pressure at
which threshold oscillations are observed to occur. With a typical mercury
mamometer and pressurizable cuff, this pressure would then be the highest
pressure at which the operator noted bouncing in the top of the mercury
column as the pressure in the cuff was slowly and relatively uniformly
reduced. However, there are inaccuracies associated with this method for
determining threshold oscillations, since the mercury column does not
noticably respond to narrow-width pressure pulses; i.e., the energy
associated with a narrow-width pulse is insufficient to noticably move the
comparatively high inertia mercury column. In other words, because of
relatively slow response time of a mercury manometer (or the apparently
selectively slow occlusion rate of the pinna of the ear) the quantity
actually being measured is proportional to an integral of the pressure
pulse rather than actual amplitude thereof. Oscillometric methods based on
observing threshold oscillations are thus inherently somewhat inaccurate,
where "threshold" is a parameter or term that generally may be hard to
rigorously and exactly define anyway.
Nevertheless, methods based on listening to Korotkoff sounds are relatively
accurate for measuring systolic pressure, but are burdened with requiring
use of a microphonic detector if they are to be instrumented. The method
based on Doppler shifts is also accurate, but also is burdened with
requiring special measuring apparatus, and has a further shortcoming in
that it is sensitive to positioning of the measuring apparatus relative to
the artery.
The present invention provides a solution to the problems associated with
inaccurate systolic blood pressure measurement and monitoring provided by
simple devices of the prior art. The present invention also provides a
solution to the problems associated with complex and special microphonic
and other apparatus employed in the more accurate prior art devices for
measuring and monitoring systolic blood pressure. The present invention
thus provides apparatus and method for automatically measuring and
monitoring systolic blood pressure, employing a simple cuff and
automatically controlled instrumentation.
A related U.S. patent application, Ser. No. 445,559 filed Feb. 25, 1974,
now U.S. Pat. No. 3,903,872 and assigned to the assignee of the present
invention, entitled "Apparatus and Method for Producing Sphygmometric
Information," is directed to diastolic blood pressure, and is incorporated
herein by reference.
SUMMARY OF THE INVENTION
In one sense, the invention comprises apparatus for determining systolic
pressure, comprising: a pressure cuff attachable to a living test subject
adjacent a blood vessel; means for changing pressure in the cuff and
thereby applying pressure to the subject; means communicating with the
cuff for measuring a quantity proportional to a time-dependent fluctuating
component representative of the pulsatile pressure within the blood vessel
whereby the quantity is proportional to amplitude of the pulsatile
pressure; means for determining the maximum value attained by said
quantity as the applied pressure is changed; means for storing a
representation of the maximum value; means for determining when the
quantity is substantially equal to about a particular fraction of the
maximum value for an applied pressure greater than the pressure applied
when the maximum value occurs or results, the particular fraction
corresponding with that fraction of the total length of the blood vessel
within the cuff which extends from its upstream (proximal) end to the
point at which the cuff applies maximum pressure to the blood vessel wall
between diastolic and systolic pressure and typically being about
one-half; and means for reading out the applied pressure corresponding to
said quantity being substantially equal to about the particular fraction
of the maximum value, said read-out pressure corresponding to the systolic
pressure of said subject.
In another sense, the invention comprises a process or method for
determining systolic pressure, comprising: applying pressure to a living
test subject by changing pressure in a pressure cuff attached to the
subject adjacent a blood vessel; measuring at said cuff a quantity
proportional to a time-dependent fluctuating component representative of
the pulsatile pressure within the blood vessel, said quantity being
proportional to amplitude of the pulsatile pressure; determining the
maximum value attained by said quantity as the applied pressure is
changed; storing a representation of the maximum value; determining when
the quantity is substantially equal to about a particular fraction of the
maximum value for an applied pressure greater than the pressure applied
when the maximum value occurs or results; the particular fraction
corresponding with that fraction of the total length of the blood vessel
within the cuff which extends from its upstream (proximal) end to the
point at which the cuff applies maximum pressure to the blood vessel wall
between diastolic and systolic pressure and typically being about
one-half; and reading out the applied pressure corresponding to the
quantity being substantially equal to about the particular fraction of the
maximum value, the read-out pressure corresponding to the systolic
pressure of the subject.
The advantages of employing the present invention in automatic blood
pressure monitoring thus include at least: simple cuff hookup to the
subject, automatic cuff inflation and pressure measurement, and accurate
systolic pressure monitoring.
It is thus a general object of the present invention to provide an improved
apparatus and process/method for taking blood pressure measurements.
It is another object of the present invention to provide an improved
apparatus and process/method for automatically monitoring systolic blood
pressure, in which the need for a double-cuff and/or an arm-mounted
transducer is eliminated.
It is yet another object of the present invention to provide an apparatus
and process for determining systolic pressure, which apparatus and process
are extremely accurate and compatible with pressure-transducer-based
measurement of diastolic pressure without requiring extra instrumentation.
Other objects and advantages of the present invention will be apparent to
those skilled in the art after referral to the detailed description of the
preferred embodiment in conjunction with the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates in a block diagram the apparatus and process of the
present invention;
FIG. 2 illustrates a typical oscillometric envelope obtainable using the
apparatus and method of the present invention;
FIG. 3 illustrates an oscillometric envelope utilizing measuring means so
that the pulses each represent the integral of the actual pulsatile
pressure fluctuation within a blood vessel;
FIG. 4 illustrates the pressure applied to an artery as a function of
arterial location within a cuff when the applied cuff pressure is between
the systolic and diastolic pressure of the test subject;
FIG. 5 illustrates the pressure applied to an artery as a function of
arterial location within a cuff and the effect upon the artery when the
pressure applied to the cuff is slightly greater than the systolic
pressure of the test subject;
FIG. 6 illustrates an arm and the collapse of the brachial artery at point
L/2 (half cuff length) when systolic pressure equals cuff pressure at L/2,
as shown in FIG. 5; and
FIG. 7 illustrates an arm and the brachial artery without collapse, as
would be obtained from cuff pressure less than systolic pressure, e.g., as
shown in FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to FIG. 1, there is arm 11 of a test subject with artery 13
therein, the arm being surrounded by a typical blood pressure cuff 15.
Typically, the brachial artery located in the upper arm is employed for
this type of blood pressure measurement. Attached to the cuff via conduit
17 is pump 19. Also attached to the cuff via conduit 21 is pressure
transducer 23 having a frequency response adequate to accurately follow
the subject's blood pressure waveform, for instance at least about five
times the pulse rate of the test subject. The pressure transducer 23, in
order to have frequency response of at least about five times the pulse
rate of the subject, will generally have a frequency response of at least
about ten Hertz. The pressure transducer serves to measure pressure within
the cuff, which pressure is the sum of pressure supplied by the pump and a
fraction of pressure produced by blood pressure fluctuation within the
artery. Since the transducer has the required frequency response, the
fluctuating portion of output thereof represents amplitude of pulsatile
pressure rather than the integral thereof. Output of transducer 23
proceeds as represented by line 25 to amplifier 27, wherein the signal is
amplified and passed therefrom, as represented by line 29, to
analog-to-digital (A/D) converter 31. Output of the analog-to-digital
converter is fed, as represented by line 33, to digital peak-to-peak
detector 35, in which a quantity proportional to the time-dependent
fluctuating component representative of pulsatile pressure within the
blood vessel is calculated.
An output comprising said quantity from the digital peak-to-peak detector
35 proceeds, as represented by line 37, to averaging unit 39, wherein an
updated average value for the present and three immediately previous
quantities proportional to the time-dependent fluctuating component
representative of the pulsatile pressure within the artery 13 is
determined. This average value is fed, as represented by line 41, to
comparator 43. The comparator 43, as represented by line 45, controls gate
47. The gate 47 serves to allow the averaging unit 39, as represented by
line 49, to load a selected value of the quantity, as represented by line
51, into storing unit 53. The value of the quantity being stored in
storing unit 53 is supplied to the comparator 43, as represented by line
55. Within comparator 43, stored tentative previous representations of the
maximum value of said quantity are compared with current values of said
quantity introduced into the comparator 43, as represented by line 41.
When a value of said quantity supplied to the comparator 43, as
represented by line 41, is greater than the quantity tentatively stored in
the storage unit 53, as supplied to the comparator 43, as represented by
line 55, then gate 47 is activated by the comparator 43, as represented by
line 45, and the larger value of said quantity replaces the tentative
maximum value in storage unit 53.
The tentative maximum value of said quantity is introduced, as represented
by line 57, into a halving unit (divide-by-two) 59 wherein it is divided
in half. The divided-in-half value is then introduced, as represented by
line 61, to a systolic comparator 63. Also supplied to the systolic
comparator 63 is the (now) current average of four most previous
measurements of said quantity. This is supplied from the averaging unit
39, as represented by line 65. When systolic comparator 63 determines that
the average quantity being supplied thereto, as represented by the line
65, is less than or equal to one-half of the tentative maximum value being
supplied thereto, as represented by line 61, the systolic comparator 63
orders, as represented by line 67, the switching means 69 to stop the pump
19 and bleed the cuff 15 through solenoid control valve 18 and conduit 20,
the stop-and-bleed order being represented by line 71.
The switching means 69, as represented by line 73, and systolic comparator
63 as represented by line 74, also order interpolating unit 75 to
interpolate between values of the applied pressure, that is, the pressure
being applied to cuff 15 by pump 19, so as to determine the precise
applied pressure corresponding to said quantity being about one-half of
said maximum value.
Values of applied pressure are supplied to interpolation unit 75, as
represented by lines 77 and 79. Line 77 represents introduction of the
applied pressure valve for measurement just before the quantity became
less than one-half the maximum value, and line 79 represents applied
pressure when the quantity was equal to or slightly less than one-half the
maximum value. These values of the applied pressure are supplied, as
represented by lines 77 and 79, from storage unit 81 and computing and
averaging unit 83 respectively. The values of the applied pressure are
supplied to averaging unit 83 by gate 85, as represented by line 87. The
value of the just-previous applied pressure is supplied to storage unit 81
from averaging unit 83, as represented by line 89. The value of the
applied pressure is supplied to gate 85 from digital peak-to-peak detector
35, as represented by line 91. The gate 85 is triggered by the output of
an analog-to-digital converter 31, as represented by the line 93. Hence,
one value of applied pressure passes into the average unit 83 for each
pulse which passes into the averaging unit 39. The applied pressure and
pulse pressure values are easily separated from one another because of
their very different frequencies, the applied pressure being usually a
slow ramp function and the pulse pressure having a frequency of about 1
Hertz. In the preferred embodiment of the invention, pump 19 is adjusted
to repetetively apply an increasing ramped pressure to cuff 15.
The apparatus and process of the invention can also, however, be made to
operate with a pump which sequentially applies a decreasing pressure
including decreasing ramped pressure to the cuff. In this case it is
necessary to provide a memory unit wherein successive values of applied
pressure and of corresponding amplitude of the pulsatile quantity measured
are stored for later comparison with one-half of the eventually determined
maximum amplitude. The maximum value of the pulsatile quantity will not be
determined until after the cuff pressure corresponding to systolic
pressure has been passed as the pressure drops. In other words, the half
amplitude is not determined when it occurs, but only after the peak
amplitude is determined, the peak amplitude occurring later in time.
It is understood by those skilled in the art that implementation of the
various functions represented in FIG. 1 is accomplished from commercially
available component parts. The pressure transducer employed converts
pressure to an electrical analog current which is digitized by A/D
converter 31. The remaining functional blocks are constructed primarily
from commercially available microprocessors and other digital circuitry,
excluding those items associated with the pneumatics and pneumatics
controls. Power supplies are not shown, but are to be understood to be
employed as required.
Referring now to FIGS. 2 and 3, one can observe the improved accuracy of
systolic pressure measurements made with apparatus of the present
invention. FIG. 2 is a plot of amplitude of the pulse height obtainable
with apparatus and method of the present invention. The measuring means
has a frequency response of preferably at least five times the subject's
pulse rate. The maximum amplitude is labeled A and the half amplitude
point (at cuff pressure higher than maximum amplitude cuff pressure) is
labeled A/2. Corresponding applied cuff pressure is clearly discernable.
By constrast, FIG. 3 depicts an oscillometric envelope, as may be generated
by a bouncing mercury column of a mercury manometer, or other integrator
device. One observes that the threshold peak corresponding to half
amplitude of FIG. 2 is difficult, at least, to define well.
THEORY
Referring to FIGS. 4 and 5 there is illustrated an explanation of our
discovery as to why this relationship between maximum amplitude and a
particular fraction (herein exemplified as one-half) of maximum amplitude
exists for determining systolic pressure. While it is believed that the
following explanation of this phenomenon is correct, it is to be
understood that the invention is not meant to be limited thereby. The
figures illustrate arm 11 with artery 13 therein surrounded by cuff 15.
The artery within the cuff is of length L. The pressure versus distance
curve is aligned under the artery to illustrate pressure at the artery
wall corresponding to an applied pressure between systolic and diastolic
for FIG. 4 and slightly above systolic for FIG. 5. It will be noted that
pressure at the artery wall, for the illustrated arm and cuff, is highest
opposite the center of the cuff and drops off near edges thereof. This
results because some of the cuff pressure adjacent ends of the cuff leads
to the arm thereat being squeezed out of the cuff.
When pressure in the cuff is between diastolic and systolic pressure of the
test subject, there is no part of the artery which is completely closed
during an entire pulse. The artery is, of course, closed during the period
when the pulse pressure is below the cuff pressure (in FIG. 4 when the
pulse pressure is between 80 torr and 100 torr) but it is also, of course,
open when the blood pressure is between cuff pressure (100 torr) and
systolic pressure (120 torr). In this situation the artery 13 changes
volume along its entire length L as the pressure changes from below to
above 100 torr and vice versa. On the other hand, in the situation shown
in FIG. 5 wherein applied pressure is just very slightly greater than
systolic pressure of the test subject, it will be noted that one-half of
the artery, namely that part of the artery distal from the heart, will for
all practical purposes be constantly closed, since pulsatile pressure
within the artery will never rise high enough to open it. This effect on
one-half of the artery occurs because of the previously mentioned fact
that, in the illustrated embodiment, only at the center of the cuff is the
full applied cuff pressure also applied to the artery. This means that
only one-half of the length of the artery changes volume as the blood
pressure surges from diastolic to systolic during a pulse beat since only
the portion of the artery proximal to the heart is opened against the
pressure exerted at the artery wall by the cuff. Accordingly, the
amplitude of the pressure fluctuation, which is just that quantity
illustrated in FIG. 3, is to a good approximation in the illustrated
embodiment, one-half of the maximum fluctuation thereof which would
comprise a fluctuation of the entire length L of the artery.
Although in the illustrated embodiment, and under most common conditions,
the structure of the cuff and/or the subject's arm (leg. etc.) are such
that the highest pressure at the artery wall between diastolic pressure
and systolic pressure occurs at the center of the cuff and of the length
of blood vessel within the cuff, it will be appreciated that the cuff
might be so designed and/or the radial size gradient of the subject's arm
be so great as to shift this point of highest pressure from the center
toward the proximal or distal end relative to the source of blood supply.
For instance, this point of maximum pressure might be within a range of
fifteen percent or more of the total length L to either side of the center
position, as may be predetermined either empirically and/or through
knowledge of the cuff design and arm geometry.
Accordingly, with a predetermination of the point of maximum pressure on
the blood vessel wall by the cuff between diastolic and systolic pressure,
a particular fraction (X/L) is obtained in which the total length L of the
blood vessel within the cuff comprises the denominator and the numerator x
is comprised of that length of blood vessel within the cuff measured from
its end proximal to the blood supply to said point of maximum pressure by
the cuff. This particular fraction then represents the sensed artery
volume change at applied systolic pressure relative to the maximum sensed
artery volume change which occurs between applied diastolic and systolic
pressures. Accordingly, when the sensed fluctuation value bears this
particular fractional relationship to the maximum sensed fluctuation
value, the cuff pressure is then indicative of the subject's systolic
pressure. The value of x in the fractional expression (X/L) is seen to be
(L/2) in the embodiment illustrated in FIG. 5.
The invention may be embodied in yet other specific forms without departing
from the spirit or essential characteristics thereof. For example, the
applied cuff pressure may variably increase or decrease in any fashion
including linear, nonlinear, and stepped (discontinuous) fashion. The
apparatus for processing the transducer-generated electrical analog signal
can be constructed from analog circuitry, digital circuitry, or both;
specifically, discrete electronic components, discrete digital chips,
microprocessor technology and structure, or a digital computer can be
employed. The pressure cuff may be of the ordinary single cuff variety,
but could also be a double cuff, or guarded cuff, etc. The cuff need not
be an arm cuff, but could function on other limbs, fingers, etc.
Thus, the present embodiments are to be considered in all respects as
illustrative and not restrictive, the scope of the invention being
indicated by the appended claims rather than by the foregoing description,
and all changes which come within the meaning and range of equivalency of
the claims are therefore intended to be embraced therein.
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