Note: Descriptions are shown in the official language in which they were submitted.
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sackyround of the Invention
1. 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.
2. 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 pre-
ssurizable 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 appar-
atus based on the same principle of listening to the Korotkoff
sounds replace the mercury manometer with a mechanical or electro-
mec~anical 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 deter-
mined by measuring Doppler shifts of sound waves reflected by the
artery. The distance to the artery varies as a function of pre-
ssure within the somewhat pliable walls thereof. In yet othermethods for measuring blood pressure intrus~ve 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-co~munication with a cuff, or indirectly as by
29 measuring the occlusion which occurs to a blood vessel in the pinna
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of the ear as pressure is exerted thereon, as in U. S. Patent
No. 3,227,155. Oscil]ometric methods of determining systolic
pressure generally define systolic pressure to be the maximum
applied pressure at which thresllold oscillations are observed
to occur, With a typical mercury manometer and pressurizable
cuff, this pressure would then be the highest pressure at which
tlle operator noted bouncing in the top of the mercury column
as the pressure in the cuff was slowly and relatively uniformly
reduced. ~-Iowever, there are inaccuracies associated with this
method for determining threshold oscillations, since the mercury
column does not noticeablyrespond to narrow-width pressure pulses;
i.e., the energy associated with a narrow-width pulse is insuffi-
cient to no~iceablymove the comparatively high inertia mercury
column. In other words, because of relatively slow response
time of a mercury manometer (or the apparantly 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 para-
meter 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 w~th 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
29 that it is sensitive to positioning of the measuring apparatus
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relative to the artery.
The present invention provides a solution to the prob-
lems associated with inaccurate systolic blood pressure measure-
ment and monitoring provided by sa~ple 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.
Applicant's U. S. Patent No. 3,903,872, issued September
9, 1975, entitled "Apparatus and Method for Producing Sphygmo-
metric Information", is directed to related subject matter but
involving diastolic blood pressure.
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;
~eans for storing a representatlon of the maximum value, means
for determining when the quantity is substantially equal to about
29 one-half of the maximum value for an applied pressure greater
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than the pressure applied when tlle maximum value occurs or results;
and means for reading out the applied pressure corresponding to
said quantity being substantially equal to about one-l~alf of the
maximum value, said read-out pressure correspondin~ to the systolic
pressure of said subject.
In another sense, the invention comprises a process for
method for determining systolic pressure, comprising: applying
pressure to a living tes~ subject by changing pressure in a pre-
ssure cuff attached to the subject adjacent a blood vessel; measur-
ing 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 one-half of the
ma~imum value for an applied pressure greater than the pressure
applied ~Ihen the maximum value occurs or result and reading out
the applied pressure corresponding to the quantity being sub-
stantially equal to about one-half 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 monitor-
ing.
It is thus a general object of the present invention to
provide an improved apparatus and process/method for taking blood
29 pressure measurements.
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It is anotller object o~ the present invention to provide
an improved apparatus and process/method for automatically monitor-
ing 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 pro-
vide an apparatus and process for determining systolic pressure,
which apparatus and process are extremely accurate and compatible
witll pressure-transducer-based measurements of diastoljc pressure
witllout 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 conjunc-
tion with the appended drawings.
~rief Description of the Drawings
Figure 1 illustrates in a block diagram the apparatus
and process of the present invention;
Figure 2 illustrates a typical oscillometric envelope
obtainable using the apparatus and me~hod of the present inven-
tion;
Figure 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;
Figure 4 illustrates the pressure applied to an artery
as a function of arterial location witnin a cuff when the applied
cuff pressure is between the systolic and diastolic pressure
of the test subject;
Figure 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
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greater than the systolic pressure of the test subject.
Fi~ure 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 Figure 5; and
Figure 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 Figure 4.
Detailed Description of the Preferred Embodimen_
~ eferring first to Figure 1, there is arm 11 of a test
subject with artery 13 therein, the arm be;ng 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 of at least about five
times the pulse rate of the test subject, which is adequate to
accurately follow the subject's blood pressure waveform. The
pressure transducer 23, in order to satisfy the criterion of
having a frequency response of at least about five times the
pulse rate of the subject, will generally have a frequency res-
ponse 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 pro-
duced by blood pressure fluctuation within the artery. Since the
transducer has the required frequency response, the fluctuating
portion of output thereof represents amplltude of pulsatile
pressure rather than the integral thereof. O~tput of transducer
23 proceeds as represented by line 25 to amplifier 27, wherein
29 the signal is amplified and passed therefrom, as represented by
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line 29, to analog-to-digital (~/D) converter 31. Output of the
analog-to-digital converter is fed, as represented by line 3
to digital peak-to-peak detector 35, in which a quantity pro-
portional to the time-dependent fluctuating component represent-
ative 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 rep-
resented 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 com-
parator 43, as represented by line 45, and the larger value of
said quantity replaces the tentative maximum value in storage
unit 53.
29 The tentative maximum value of said quantity is intro-
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duced, as represented by line 57, into a halving unit (divide-
by-two) 59 wherein it is divi~ed in half. The divided-in-half
value is then introduced, as represented by line 61, to a systo-
lic comparator 63. ~lso 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 rep-
resented 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 corres-
ponding 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 value 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
28 of the applied pressure are supplied as represented by lines 77
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and 79, ~rom storaye unit 8] 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 stor-
age 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 8~ is triggered ~y the output of the analog-to-digital
converter 31, as represented by the line 93. Hence, one value
of applied pressure passes into the average unit 83 or each
pulse which passes into the averaging unit 39. The applied pre-
ssure 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 E~ert~. In the preferred embodiment -
of the invention, pump 19 is adjusted to repetitively apply an
increasing ramped pressure to cuff 15.
The apparatus and process of the invention can also, how-
ever, 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 successiye 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
29 only after the peak amplitude is determined, the peak amplitude
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occurring later in time.
It is understood by those skilled in the art that imple-
mentation of the various functions represented in Figure 1 is
accomplished from commercially available component parts. The
pressure transducer employed conyerts 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 cir-
cuitry, 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 Figures 2 and 3, one can observe the
improved accuracy of systolic pressure measurements made with
apparatus of the present invention. Figure 2 is a plot of ampli-
tude 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 contrast, Figure 3 depicts an oscillometric envelope,
as may be generated by a bouncing mercury column of a mercury
manome~er, or other integrator device. One observes that the
threshold peak corresponding to half amplitude of Figure 2 is
difficult, at least, to define well.
Theory
Referring to Figures 4 and 5 there is illustrated an
29 explanation of our discovery as to why this relationship between
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maximum amplitude and one half maximu~ amplitude e~ists for
determining systolic pressure. While it is belieyed that the
following explanation of this phenomenon is correct, it is to
be understood that the invention is not meant to be limited
thereby. ~he ~igures illustrate arm 11 with artery 13 therein
~- surrounded by cuff 15. The artery within th~ cuff is of length
L. The pressure versus distance curve i5 aligned under the
artery to illustrate pressure at the artery wall correspending
~; to an applied pressure between systolic and diastolic for Figure
4 and slightly above systolic for Figure 5. It will be noted
r ~ 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 systo-
:
lic 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 Figure 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 systo-
lic pressure a20 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 sho~n in Figure 5 wherein applied pressure is just
yery slightly greater than systolic pressure of the test subject,
it will be noted that one-half of the artery, namely that part
29 of the artery distal from the heart, will for all practical pur-
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poses 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 mention-
ed 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 exert-
ed at the artery wall by the cuff. Accordingly, the ~mplitudeof the pressure fluctuation, ~hich is just that quantity illustrat-
ed in Figure 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.
:. ,
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 process-
ing 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 ordin-
ary single cuff variety, but could also be a double cuff, or'guard-
ed 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
29 all respects as illustrative and not restrictive, the scope of
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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
4 therefore intended to be embraced therein. - :
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Supplementary Disclosure
The preferred embodiment uti.lizes the relationship
between maximum amplitude and one-halE maximum amplitude of
measured pressure for determining systolic pressure in tle
situation where the pressure cuff applies maximum pressure
midway of its total length. It has been determined that in
some circumstances systolic pressure may correspond with a
pulsatile pressure other than one-half of said maximum value
if the patient s arm is particularly obese and/or the pressure
cuff is structured such as to apply maximum pressure other than
at its midpoint.
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 xL is
obtained in which the total length L of the blood vessel within
0 the cuff comprises the denomir.ator and the numerator x is
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comprised of that ]ength of blood vessel within the cuffmeasured 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 press~lre 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 L is seen to be 2 in the embodiment illustrated in
Fig. 5.
Thus 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 repre-
sentative 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 w-lich the cuff applies maximum pressure to the
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b]ood vessel wal.l betweell 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 determini.ng 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; deter-
mining 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
28 pressure corresponding to the systolic pressure of the subject.
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