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Claims  |
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It is claimed and desired to secure by Letters Patent:
1. A noninvasive, oscillometric blood-pressure measurement method to
determine blood-pressure parameters derived from data acquired relative to
blood-pressure-induced pressure waveforms, said method enabling the
acquisition of such data with improved accuracy and an improved faster
rate, and said method, in operative condition relative to a blood vessel
in a living subject, comprising
establishing in a means for producing a baseline counterpressure adjacent
such vessel a predetermined beginning counterpressure above systolic
pressure, thus to occlude the vessel,
progressively reducing counterpressure in steps from a beginning
counterpressure step to a predetermined ending counterpressure step,
during said reducing, and on a step-by-step basis, monitoring the waveforms
of blood-pressure-induced changes in the pressure in such means,
on the basis of said step-by-step monitoring, developing and storing
waveform-specific partial-area data, and
from such stored data, calculating the desired parameters.
2. The method of claim 1, wherein said monitoring includes sampling, during
the occurrence of each waveform, recurrently, the instantaneous level of
pressure in such means and performing a first artifact-rejection technique
to confirm the validity of each sample, said first technique being
conducted utilizing predetermined criteria established in light of a
previously noted sample, and said developing includes conducting, with
respect to successive validation-confirmed samples, a running
waveform-specific area-data integration for each of such waveforms.
3. The method of claim 2 wherein, with respect to each waveform from which
validated samples result in the storing of waveform-specific area data,
the conducting of integration takes place with respect to validated
samples that extend at least to the point where the waveform reaches a
maximum amplitude.
4. The method of claim 2, wherein said performing employs waveform slope
prediction to establish such predetermined criteria.
5. The method of claim 4, wherein such predetermined criteria include a
first type that are used relative to samples representing a first segment
of such waveforms, and a second type that are used relative to samples
representing a second segment of such waveforms.
6. The method of claims 2, 3, 4 or 5, wherein said developing further
includes performing a second artifact-rejection technique which utilizes
previously stored waveform-specific area data to produce a curve that
predicts subsequent, expected-to-be-stored waveform-specific area data.
7. The method of claim 6, wherein the production of such curve includes the
generation of an associated acceptance window which includes lower and
upper boundaries.
8. The method of claim 6, wherein such second artifact-rejection technique
performed in said developing further includes utilizing subsequent,
developed waveform-specific area data to adjust previously stored
waveform-specific area data.
9. A noninvasive, oscillometric blood-pressure measurement method to
determine blood-pressure parameters derived from data acquired relative to
blood-pressure-induced pressure waveforms, said method promoting the rapid
acquisition and verification of such data, and in operative condition
relative to a blood vessel in a living subject, comprising
establishing in a means for producing a baseline counterpressure adjacent
such vessel a predetermined beginning counterpressure above systolic
pressure, thus to occlude the vessel,
progressively reducing counterpressure in steps from a beginning
counterpressure step to a predetermined ending counterpressure step,
during said reducing, monitoring the waveforms of blood-pressure-induced
changes in the pressure in such means,
on the basis of said monitoring, developing and storing waveform-specific
partial-area data, and
determining from such data:
(a) means arterial pressure to the lowest baseline counterpressure
corresponding in time with occurrence of the blood-pressure waveform
associated with the largest area-data value;
(b) systolic pressure to be the baseline counterpressure corresponding in
time with occurrence of the blood-pressure waveform whose associated
area-data value has a first predetermined fractional relationship with the
value identified above in subparagraph (a); and
(c) diastolic pressure to be the baseline counterpressure corresponding in
time with occurrence of the blood-pressure waveform whose associated
area-data value has a second predetermined fractional relationship with
the value identified above in subparagraph (a).
10. The method of claim 9, wherein said monitoring includes sampling during
the occurrence of each waveform, recurrently, the instantaneous level of
pressure in such means and performing a first artifact-rejection technique
to confirm the validity of each sample, said first technique being
conducted by utilizing predetermined criteria established in light of a
previous noted sample, and said developing includes conducting, with
respect to successive validation-confirmed samples, a running
waveform-specific area-data integration for each of such waveforms.
11. The method of claim 10 wherein, with respect to each waveform from
which validated samples result in the storing of waveform-specific area
data, the conducting of integration takes place with respect to validated
samples that extend at least to the point where the waveform reaches a
maximum amplitude.
12. The method of claim 10, wherein said performing employs waveform slope
prediction to establish such predetermined criteria.
13. The method of claim 12, wherein such predetermined criteria include a
first type that are used relative to samples representing a first segment
of such waveforms, and a second type that are used relative to samples
representing a second segment of such waveforms.
14. The method of claims 10, 11, 12 or 13, wherein said developing further
includes performing a second artifact-rejection technique which utilizes
previously stored waveform-specific area data to produce a curve that
predicts subsequent, expected-to-be-stored waveform-specific area data.
15. The method of claim 14, wherein the production of such curve includes
the generation of an associated acceptance window which includes lower and
upper boundaries.
16. The method of claim 14, wherein such second artifact-rejection
technique performed in said developing further includes utilizing
subsequent, developed waveform-specific area data to adjust previously
stored waveform-specific area data.
17. An artifact-rejection method to be used with noninvasive blood-pressure
measuring apparatus that determines blood-pressure parameters derived from
data acquired relative to blood-pressure-induced pressure waveforms, said
method comprising:
in a means for producing a baseline counterpressure adjacent a blood vessel
in a living subject, progressively reducing such counterpressure in
counterpressure steps from a predetermined beginning, occluding baseline
counterpressure above systolic pressure to a predetermined ending baseline
counterpressure;
during said reducing, and for a predetermined number of such
counterpressure steps, and at each such step, monitoring a plurality of
the waveforms of blood-pressure-induced changes in the pressure of such
means;
on the basis of said monitoring, developing and storing waveform-specific
area data;
at each of such counterpressure steps, choosing a predetermined number of
stored, waveform-specific area-data values as indicative of blood pressure
and computing an average waveform-specific area value from such chosen
values;
thereafter, from such average values, fitting a curve and, from such curve,
predicting an expected-to-be-stored waveform-specific area-data value for
a next baseline counterpressure step, and applying experimentally
determined bounds to such expected-to-be-stored waveform-specific
area-data value;
modifying said monitoring at such next baseline counterpressure step so
that a single waveform-specific area-data value is developed and stored;
and
checking whether such single value is within such bounds from said
predicting as a way of determining the acceptability of the value.
18. The method of claim 17, wherein said checking includes, if such single
value is outside of such bounds, staying at such next counterpressure
step, and repeating said monitoring, integrating, and checking, and if
such single value is within such bounds, accepting such single value, and
repeating said modifying step at a next counterpressure step.
19. An artifact-rejection method to be used with noninvasive blood-pressure
measuring apparatus that processes two streams of data corresponding to
blood-pressure-induced pressure waveforms, said method comprising:
in a means for producing a baseline counterpressure adjacent a blood vessel
in a living subject, progressively reducing such counterpressure in
counterpressure steps from a predetermined beginning,
blood-vessel-occluding, baseline-counterpressure step above systolic
pressure to an ending baseline counterpressure step;
during said reducing, and at each such counterpressure step, monitoring at
least one waveform of blood-pressure-induced changes in the pressure
within such means;
said monitoring including, for each such waveform, acquiring two streams of
data, the first such stream corresponding to baseline counterpressure
data, and the second such stream corresponding to blood-pressure-induced,
pressure-pulsation data,
viewing such data in three different phases of activity; and
from said viewing, verifying that such waveform is blood-pressure-induced
by checking the first such data-phase according to a first, predetermined
verification criteria, by checking the second such phase according to a
second, predetermined verification criteria, and by checking the third
such phase according to a third, predetermined verification criteria.
20. The method of claim 19, wherein such waveform is characterized by the
second stream going from a beginning divergence point, relative to the
first such stream, to a maximum point of divergence, and by then
converging back on the first stream, and the first such phase is
characterized by a first divergence parameter, the second such phase is
characterized by a second divergence parameter, and the third such phase
is characterized by a first convergence parameter. |
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Claims |
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Description |
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BACKGROUND AND SUMMARY OF THE INVENTION
This invention pertains to an improvement of the oscillometric method for
noninvasive blood-pressure measurement, and more particularly to a unique
application for microprocessor-controlled blood-pressure monitoring.
Examples of noninvasive blood-pressure measurement methods in the closest
prior art are disclosed in U.S. Pat. Nos. 4,461,266 to Hood, et al. and
4,638,810 to Ramsey.
In the typical practicing of an oscillometric, noninvasive blood-pressure
measurement method with a person, a counterpressure-producing cuff is
wrapped around the person's upper arm, with the cuff then inflated to a
counterpressure above systolic pressure to occlude a artery (blood vessel)
in the arm. Thereafter cuff counterpressure is progressively reduced in a
stepped fashion from this suprasystolic pressure to an ending
counterpressure where the cuff is substantially deflated. During
progressive reduction of cuff counterpressure, the artery opens
progressively from an occluded state to an unoccluded state.
During the change from the occluded state to the unoccluded state, the
artery begins to pulsate against the cuff, and the waveforms of these
pulsations are monitorable to produce graphic illustrations of
blood-pressure parameters. As is well-known to those skilled in the art in
handling blood-pressure measurements, the pulsations just referred to
increase in amplitude toward a maximum as cuff counterpressure decreases
below systolic pressure, and then decrease in amplitude. By categorizing
these pulsations relative to their occurrences in time and to their
respective amplitudes, desired blood-pressure parameters are determined.
Explaining the significant monitored heart activity with a little more
particularity, during each heart contraction, a force is exerted upon the
blood in the vascular system. During the time that this force is active,
the blood is accelerated, or given momentum. The integral of this force
with respect to time (when the force is active) is called the "impulse" of
the force, with "impulse" bearing the same units as momentum. Accordingly,
if the instantaneous pressure in the cuff is monitored during a
measurement procedure, and integrated over the time during which
measurements are being made, it is possible to develop a data quantity
that is directly proportional to impulse--that characteristic of blood
flow from which, it turns out, the most accurate blood-pressure data can
be derived.
A critical determination is that of mean arterial pressure (MAP). It is
from this determination that systolic and diastolic pressures are
calculated. Typically, MAP has been defined according to the prior art as
the pressure in the cuff where blood-pressure pulsations have the largest
amplitudes. Amplitude data, however, does not relate well to the
characteristic described above as impulse, and, because pulsation impulse
data is, for the sake of ultimate accuracy, the most desirable data, it
does not reliably produce the most accurate ultimate information.
The method of the present invention significantly addresses this issue.
Another consideration is that conventional blood-pressure measuring methods
typically reduce counterpressure, progressively, in steps at a relatively
slow rate. This results in a relatively long time period for an entire
measurement cycle, and often as a consequence, patient discomfort.
Finally, in all methods of acquiring usable blood-pressure data, it is
important to detect, and reject, as faithfully as possible, pressure
"artifacts" which are not induced by blood-pressure pulsations. Artifacts
occur, for example, where a subject moves, changes muscle tension, etc.
An important object of the present invention, accordingly, is to categorize
blood-vessel pulsations in a far more accurate manner by a value that more
closely approximates blood-vessel pulsation impulse.
Another object of the invention is to provide for artifact rejection in a
unique way which ensures that accepted pressure waveforms truly are
blood-pressure induced.
A further object is to decrease the number of pressure waveforms that are
monitored at each cuff counterpressure level, thereby to decrease the
overall time period of a measuring cycle, thus to minimize subject
discomfort. The method of the present invention, which might be thought of
as an "impulse-based method", offers a significant improvement over the
closest prior art because, inter alia, it defines the blood-pressure
pulsation which corresponds to MAP as that pulsation which produces a
waveform having the greatest area, as distinguished from that having the
greatest amplitude--area being a direct indication of impulse. Such
waveform area data is an indicator of MAP which for many reasons is more
accurate than waveform amplitude data.
Another extremely important consideration is that where waveform area
(impulse) data forms the foundation for the determination of MAP, systolic
pressure and diastolic pressure, signal-to-noise problems are greatly
reduced.
For all of the important reasons given above, the desired blood-pressure
parameters of a subject, determined in accordance with the present
invention, have improved accuracy over the same parameters determined in
accordance with the closest prior art.
To deal with the issue of false "artifact" data, the invention employs two
different artifact-rejection techniques, during two different phases of a
blood-pressure measuring cycle, to assure that developed waveform area
data accurately and reliably represents blood-pressure-induced changes in
the occluding cuff.
The first artifact-rejection technique verifies that monitored pressure
signal data corresponding to pressure waveforms is blood-pressure induced.
A second artifact-rejection technique verifies that developed area data
values are also blood-pressure induced. This second technique, after
development of waveform area-data values for a predetermined number of
cuff counterpressure levels at the beginning of a measuring cycle,
predicts a next, expected-to-be-encountered area-data value for the next,
lower cuff counterpressure level. Employing prediction for successive,
next, lower cuff counterpressure levels, provides a simple and accurate
method of artifact rejection that substantially decreases the number of
pressure waveforms required to be monitored at a given cuff
counterpressure level. Therefore, if a next, developed waveform area-data
value for a measured waveform is within experimentally set upper and lower
bounds of its corresponding predicted value, the measured value is
accepted as being blood-pressure induced.
This important feature of area-data acquisition, coupled with on going
next-to-be-expected value prediction, significantly enhances the
likelihood that a false data pulse will be rejected as an artifact.
Using the prediction technique just described for subsequent cuff
counterpressure levels, it will generally be necessary to monitor only one
pressure waveform at a given cuff counterpressure level. As will be
explained, if the first pressure waveform which is monitored does not have
an area-data value that is within the upper and lower bounds of its
corresponding predicted value, subsequent waveforms will be monitored
until one is found which does meet the boundary conditions. This
situation, of "looking" for successive, subsequent "boundary-meeting"
waveforms, continues only for a predetermined ultimate time interval,
after which, if no proper waveform is found, the method of the invention
aborts the measurement cycle.
In addition, and further in accordance with special features of the
invention, the second artifact-rejection technique adjusts previously
encountered (and stored) waveform area-data values based on the difference
between a measured waveform area-data value and a corresponding predicted
waveform area-data value for a given cuff counterpressure level. This is
referred to herein as a "smoothing" technique. By adjusting previously
stored values, this second technique provides further ensurance of the
accuracy of ultimately derived, desired blood-pressure parameters.
These and other objects and advantages which are attained by the invention
will become more fully apparent as the description that now follows is red
in conjunction with the accompanying drawings and computer program flow
charts.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a fragmented graph of cuff counterpressure decreasing over time,
showing two cuff counterpressure levels with blood-pressure-induced
waveforms to illustrate amplitude data being employed, according to the
closest prior art, in the derivation of the usual, desired parameters of
MAP, systolic pressure and diastolic pressure.
FIG. 2 is a graph of waveform-amplitude data, such as that illustrated in
FIG. 1, acquired over an entire measurement cycle of a test subject.
FIG. 3 is like FIG. 1, except that it helps to illustrate the more accurate
derivation of desired parameters from waveform area (impulse) data
acquired according to the method of the present invention.
FIG. 4 is like FIG. 2, except that, as will be explained, what it shows is
based on waveform area (impulse) data noted over an entire measurement
cycle.
FIG. 5 is a schematic/block depiction of apparatus and software suitable
for carrying out the present invention.
FIG. 6 is a fragmented graph of cuff counterpressure levels vs. time,
showing one cuff counterpressure level, and also showing plural,
time-successively-monitored blood-pressure-induced pressure signals that
together form one blood-pressure-induced pressure waveform.
FIGS. 7A-7D, inclusive, are computer-program flow charts illustrating
computer-based implementation of a portion of the method of the present
invention.
FIG. 8A is a fragmented graph of cuff counterpressure decreasing over time,
showing a series of cuff counterpressure levels wherein the waveforms
depicted are monitored by the method of the present invention.
FIG. 8B is a fragmented graph of waveform-specific area data vs. cuff
counterpressure levels that depicts how the second artifact rejection
technique, mentioned earlier, predicts a subsequent, expected-to-be-stored
waveform area-data value for a subsequent cuff counterpressure level.
FIG. 8C is a fragmented graph of waveform area data vs. cuff
counterpressure levels illustrating the "smoothing" technique referred to
earlier.
FIG. 8D is a graph of waveform area data vs. cuff counterpressure levels
for an entire measuring cycle of a living test subject.
FIG. 9 is a computer-program flow chart further illustrating computer-based
implementation of the invention.
DETAILED DESCRIPTION OF THE PREFERRED MANNER OF PRACTICING THE INVENTION
Before describing the present invention in detail, reference is made
selectively to FIGS. 1-4 in order to illustrate generally an important
difference between the method of this invention for noninvasive
blood-pressure monitoring involving waveform area data, and the closest
prior art method which involves waveform amplitude data.
FIGS. 1 and 3 illustrate identically, two successive blood-pressure
waveforms which have been acquired during the performance of a
blood-pressure monitoring cycle. The waveforms illustrated in FIG. 1 have
been acquired by a system which employs the above-mentioned prior art
technique of peak amplitude monitoring. The waveforms illustrated in FIG.
3 have been acquired in a system employing the method of the present
invention, wherein waveform area data is employed in the derivation of the
desired parameters. The waveforms illustrated in FIGS. 1 and 3 are shown
as matching duplicates, in order to illustrate one of the key differences
between the derivation of desired parameters according to the prior art
and that according to the method of the invention.
FIG. 2 illustrates conventional waveform amplitude data forming a
conventional data envelope from which, according to the closest prior art
technique, MAP, systolic pressure and diastolic pressure are determined.
FIG. 4 is similar in appearance except that it illustrates waveform area
data forming a unique data envelope, according to the present invention,
from which these three desired parameters are more accurately derived.
Turning attention specifically to FIG. 1, there are shown two
blood-pressure-induced pressure waveforms 10, 12 occurring at cuff
counterpressuee levels 11, 13, respectively, during a blood-pressure
measurement cycle. FIG. 1 is a fragmented graph specifically showing cuff
counterpressure levels 11, 13 as occurring in the time period of the
measurement cycle when blood-pressure pulsations are at maximum strengths.
According to the method of the closest prior art, and as is depicted in
FIG. 1, amplitudes Amp.sub.1, Amp.sub.2 of waveforms 10, 12, respectively,
are "chosen" to be the acquired significant data relative to these two
waveforms. As a consequence, according to the prior-art amplitude method,
waveform 12 is the larger, important waveform because Amp.sub.2 is greater
than Amp.sub.1.
Looking now at FIG. 2 wherein an overall amplitude data envelope
representing an entire measurement cycle is depicted (by the peaks of the
vertical lines, or spikes), the data acquired for calculation utilization
according to the prior art from pulses 10, 12 in FIG. 1 is shown, at cuff
pressures 11, 13, respectively, with spikes labeled Amp.sub.1 and
Amp.sub.2. Deriving, for example, MAP from the data shown in FIG. 2, MAP
would be defined as the cuff counterpressure (13) where Amp.sub.2
occurred, because Amp.sub.2 turned out to be the largest-amplitude pulse
data acquired and stored. Systolic and diastolic pressures would then be
calculated based upon this selection for determining MAP.
In contrast to the closest prior art as depicted in FIG. 1, FIG. 3 depicts
how the present invention approximates blood-pressure pulsation impulse.
Instead of looking for a waveforms maximum amplitude, the present
invention looks for a waveforms area, at least, as will be explained, up
to the point that the waveform reaches a maximum amplitude. Waveforms 10a,
12a, corresponding to waveforms 10, 12, respectively, are shown occurring
at cuff counterpressure levels 11a, 13a, respectively.
The reason for undertaking waveform area calculation to at least, and
preferably (as will be explained) to slightly beyond, the
slightly-larger-than-half areas Area.sub.1, Area.sub.2 of waveforms 10a,
12a, respectively, is that our experience has shown that such information
most accurately leads, in the shortest time, to calculation of the desired
parameters. Going much beyond the peak amplitude points may be
time-wasteful.
FIG. 4, with an understanding of the importance of looking at waveform area
data, helps to show the increased accuracy in determining MAP that is
possible according to the present invention. In FIG. 4, an entire cycle,
or envelope, of waveform area data is plotted against cuff
counterpressures that occur during the entire envelope. Area.sub.1 and
Area.sub.2 are the waveform area data values of waveforms 10a, 12a of FIG.
3 that directly relate to blood-pressure pulsation impulse. According to
the present invention's waveform area data method of determining MAP, MAP
is the lowest cuff counterpressure having the greatest waveform area data
value. Thus, cuff counterpressure 11a is chosen as the counterpressure
approximating MAP because Area.sub.1 is the greatest area value.
Therefore, because area data most accurately reflects pulsation impulse,
the data of the envelope in FIG. 4 results in a different, and more
accurate, measure of MAP than the measure of MAP obtained from the
waveform amplitude data depicted in FIG. 2.
The improved accuracy offered by the method of the invention vis-a-vis
prior art amplitude-based methods will be seen by those skilled in the art
to be even more significant where very short-duration, large-amplitude,
but low-area pulses enter the monitoring picture.
Turning now to FIG. 5, a schematic/block depiction of blood-pressure
measuring apparatus and software characterizing the present invention is
shown. Generally describing in operational steps and in descriptions of
FIG. 5's contents what is "illustrated" by this figure, cuff 14 is a means
for producing a baseline counterpressure, and is disposed adjacent a blood
vessel 16 (wrapped around the arm) of a living subject 18. To begin a
blood-pressure measuring cycle, a pump 20 inflates cuff 14 to a point
where it exerts a counterpressure against the arm that is above systolic
pressure, thereby occluding vessel 16.
Under the control of a microprocessor (still to be discussed), a valve 22
progressively reduces the cuff counterpressure from the beginning
counterpressure level above systolic pressure to an ending counterpressure
to be described later. Preferably, counterpressure is reduced in a
stepwise fashion in order to monitor pressure changes occurring in cuff 14
at each step, or cuff counterpressure level. In the preferred practice of
the invention cuff counterpressure reduces in steps of 5- to 6-mm Hg.
As the counterpressure in cuff 14 lowers, vessel 16 gradually begins to
pulsate from heart contractions. More specifically, and as previously
noted, these vessel pulsations are caused by blood that has been
accelerated, or given momentum, during the time period of each successive
heart contraction.
At each progressively reduced counterpressure level, a pressure transducer
24 receives, at a predetermined rate, an analog pressure signal composed
of a cuff counterpressure component and a pressure pulsation component
which "may be" blood-pressure-induced.
Transducer 24 converts each pressure signal into an electrical signal which
is amplified by an amplifier 26. The amplified electrical signal is then
sent to two different locations--a band-pass filter 28, and an
analog-to-digital converter 30.
Each signal is sent to filter 28 in order to have the cuff counterpressure
component filtered out. The signal output of filter 28 corresponds to
pressure pulsations. From filter 28, the filtered signal component is fed
to converter 30 from which there emerges a first stream of digitized data
which corresponds to pressure pulsations. The presently preferred
monitoring interval for transducer 24 is about 5.5-msec.
Each signal from amplifier 26 is also sent to converter 30 in order to
provide a second stream of data corresponding to cuff | | |
