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Claims  |
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Having described the invention, what is claimed as new and secured by
Letters Patent is:
1. A method for measuring diastolic pressure of a subject comprising the
steps of
A. sensing arterial pressure signals with a pressure transducer applied
non-intrusively to the subject's body,
B. applying arterially-constricting pressure to the artery being sensed and
changing the applied pressure value over a range of at least sub-diastolic
values,
C. determining the minimum sub-diastolic value of applied pressure at which
the rate of change of the portion of sensed signal corresponding to left
ventricle ejection undergoes a progressive change in response to
monodirectionally-changing applied pressure, and
D. producing a value indicative of diastolic pressure for the subject in
response to said determined value of sub-diastolic applied pressure for
that subject.
2. A method according to claim 1 in which said value-producing step
includes the step of adding a subject-independent pressure value to said
determined value of applied pressure.
3. A method according to claim 1 in which said determining step comprises
the steps of
A. producing a measure of the rate of change of the portion of sensed
signal corresponding to left ventricle ejection, for each of a succession
of arterial pressure pulses sensed at different applied pressures,
B. producing a representation of the graph of said measures for said
successive signals as a function of applied pressure,
C. identifying a progressive change in the slope of said graphical
representation, and
D. identifying the applied pressure at which said progressive change
occurs.
4. A method according to claim 3 in which said step of producing said
measure of rate of change includes the step of filtering each sensed
pressure signal for removing selected low frequency components thereof.
5. A method according to claim 3 in which said step of producing said
measure of rate of change includes differentiating each sensed pressure
signal as a function of time and thereby providing a derivative signal
providing said measure of rate of change.
6. A method according to claim 3 including the further steps of storing
information identifying each measure of rate of change and storing
information identifying the corresponding value of applied pressure.
7. A method according to claim 3 in which said step of producing said
measure of rate of change includes verifying each measure of rate of
change signal by testing the duration thereof.
8. A method according to claim 1 in which said pressure-applying step
includes
A. changing the applied pressure monodirectionally over said range of
sub-diastolic values, and
B. extending said range above and below a pressure value below the expected
diastolic value by an offset amount of substantially 26 millimeters of
mercury for monodirectionally-increasing applied pressure, and of
substantially 16 millimeters of mercury for monodirectionally-decreasing
applied pressure.
9. A method according to claim 1 in which said determining step includes
A. sensing plural pressure signals at different sub-diastolic values of
applied pressure,
B. producing for each sensed pressure signal a signal having a parameter
which is a measure of the rate of change of the portion of the sensed
signal corresponding to left ventrical ejection, and
C. determining said progressive change as a progressive change in said
signal parameter as a function of applied pressure.
10. A method according to claim 1 in which said determining step includes
A. sensing plural pressure signals at different sub-diastolic values of
applied pressure,
B. producing, for each sensed arterial pressure signal, a measure of the
rate of change of the portion of sensed signal corresponding to left
ventricle ejection,
C. producing a graphical representation of the slope of said rate of change
measures as a function of applied pressure,
D. determining said progressive change as a progressive change in the slope
of said graphical representation, and
E. identifying the value of applied pressure at which said change in slope
occurs.
11. A method for measuring diastolic pressure of a subject with the steps
of
(i) sensing arterial pressure signals with a pressure transducer applied
non-intrusively to the subject's body, and
(ii) applying arterially-constricting pressure to the artery being sensed
and changing the applied pressure monodirectionally over a selected range,
said method comprising the further steps of
A. identifying the occurrence of a progressive change in the rate of change
of the portion of sensed signal corresponding to left ventricle ejection
at a value of applied pressure between 20 millimeters of mercury and 125
millimeters of mercury, and
B. determining the value of applied pressure at which said progressive
change occurs, thereby to determine, at said value of applied pressure, a
sub-diastolic pressure which is offset below the diastolic value of the
subject by a known amount.
12. In apparatus for measuring diastolic pressure of a subject in response
to arterial pressure signals sensed for each of a succession of arterial
pulses under different values of applied arterially-constricting pressure,
the combination comprising
A. means for changing the applied pressure over a range of sub-diastolic
values,
B. means responsive to said sensed pressure signals and to said applied
pressure for determining the minimum subdiastolic value of applied
pressure at which the rate of change of the portion of sensed signal
corresponding to left ventricle ejection undergoes a progressive change as
a function of monodirectionally-changing applied pressure, and
C. means responsive to said determining means for producing a value
indicative of diastolic pressure in response to said determined value of
sub-diastolic applied pressure for that subject.
13. In apparatus according to claim 12, the further combination in which
said value-producing means includes means for adding a known
subject-independent pressure value to said determined applied pressure.
14. In apparatus according to claim 12, the further combination in which
said means for determining comprises
A. means for producing a measure of rate of change of the portion of sensed
signal corresponding to left ventricle ejection for each of a succession
of arterial pressure signals sensed at different applied pressures,
B. means for producing a representation of a graph of said measures as a
function of applied pressure,
C. means for identifying a progressive change in slope of the graphical
representation in response to a monodirectionally-changing sub-diastolic
applied pressure, and
D. means for identifying the value of applied pressure at which said
progressive change occurred.
15. In apparatus according to claim 14, the further combination in which
said means for producing a measure includes means for filtering each
sensed pressure signal for removing selected low frequency components
thereof.
16. In apparatus according to claim 14, the further combination in which
said means for producing a measure includes means for differentiating each
sensed pressure signal as a function of time and thereby producing a
derivative signal providing said measure of rate of change.
17. In apparatus according to claim 14, the further combination of means
for storing information identifying each measure of rate of change and
means for storing information identifying the value of applied pressure
corresponding with each such measure.
18. In apparatus according to claim 14, the further combination in which
said means for producing a measure includes timing verification means for
verifying each measure of rate of change signal by testing the duration
thereof.
19. In apparatus according to claim 12, the further combination in which
said pressure-changing means includes means for changing the applied
pressure monodirectionally over a range of sub-diastolic pressure values
extending above and below a pressure value below the expected diastolic
value of the subject by an offset amount of approximately 26 millimeters
of mercury for a monodirectionally increasing applied pressure and of
substantially 16 millimeters of mercury for monodirectionally decreasing
applied pressure.
20. In apparatus according to claim 12 the further combination in which
said means responsive to signals includes
A. means for sensing plural pressure signals at different sub-diastolic
values of applied pressure,
B. means for producing for each sensed pressure signal a signal having a
parameter which is a measure of the rate of change of the portion of the
sensed signal corresponding to left ventricle ejection, and
C. means for determining said progressive change as a progressive change in
said signal parameter as a function of applied pressure.
21. In apparatus according to claim 12, the further combination in which
said means responsive to sensed signals comprises
A. means for sensing plural pressure signals at different sub-diastolic
values of applied pressure,
B. means for producing, for each sensed arterial pressure signal, a measure
of the rate of change of the portion of sensed signal corresponding to
left ventricle ejection,
C. means for producing a graphical representation of the slope of said rate
of change measures as a function of applied pressure,
D. means for determining said progressive change as a progressive change in
the slope of said graphical representation, and
E. means for identifying the value of applied pressure at which said change
in slope occurs.
22. In apparatus for measuring diastolic pressure of a subject in response
to arterial pressure signals sensed with a pressure transducer applied
non-intrusively to the subject's body for each of a succession of arterial
pulses produced under different values of applied arterially-constricting
pressure and having means for applying arterially-constricting pressure to
the artery being sensed and for changing the applied pressure
monodirectionally over a selected range, the further combination
comprising
A. means for identifying the occurrence of a progressive change in the rate
of change of the portion of sensed signal corresponding to left ventricle
ejection at a value of applied pressure between 20 millimeters of mercury
and 125 millimeters of mercury, and
B. means for determining the applied pressure at which said identified
progressive change occurs, thereby for determining a sub-diastolic
pressure which is offset below the diastolic value of the subject by a
known amount. |
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Claims |
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Description |
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BACKGROUND
This invention relates to a new measure of diastolic pressure. It provides
a method and apparatus for diastolic pressure measurement characterized by
accuracy, operator ease, and little patient discomfort. Further, the
invention can be practiced with a variety of instrument configurations.
The accurate measurement of blood pressure is an important tool in
preventive and recuperative cardiac care. The common sphygmomanometer
employing a manually inflated pressure cuff and mercury manometer is
accordingly well known. It is used with a stethoscope, with which the
medical professional listens for different acoustic pulses, termed
Korotkoff sounds.
The prior art regarding the accurate measurement of blood pressure also
includes the disclosures in the following U.S. Pat. Nos. 3,527,197;
3,552,381; 3,903,872; 3,905,354; 3,939,824; 4,009,709; 4,058,117; and
4,074,711. Problems with these and other prior sphygmomanometers include
operator difficulty in perceiving important changes in Korotkoff sounds
and in distinguishing Korotkoff sounds from motion artifacts and other
background noise. Also, the common sphygmomanometer first occludes the
subject's artery to measure systolic pressure, which can cause sufficient
discomfort so that the subsequent measurement of diastolic pressure is not
accurate. Electronic techniques for measuring blood pressure also have
shortcomings stemming from the difficulty in detecting and identifying
pressure pulses properly. These problems are particularly persistent in
measuring diastolic pressure, rather than in measuring more readily
discernible systolic value.
It is accordingly an object of this invention to provide an improved method
and apparatus for measuring diastolic pressure. More particularly, it is
an object to provide a diastolic pressure measuring method and apparatus
free of problems heretofore encountered in distinguishing Korotkoff sounds
of interest from other signals. Accordingly, the objects of the invention
include providing such a method and apparatus which are accurate and easy
to use without error.
It is a further object of the invention to provide a method and apparatus
for the non-intrusive measurement of diastolic pressure which subjects the
patient to little discomfort, and which can readily be automated.
It is also an object of the invention to provide a diastolic pressure
measuring method and apparatus which can readily be practiced without
requiring sound perception by the operator. This both avoids a potential
source of error and overcomes one possible source of concern for the
patient, as contrasted with prior devices which require the operator to
listen carefully for significant sound characteristics.
Other objects of the invention will in part be obvious and will in part
appear hereinafter.
SUMMARY OF THE INVENTION
The arterial pressure pulse monitored with a conventional sphygmomanometer
is known to have a leading side portion correlated to left ventricle
ejection and which manifests the fastest rate of pressure change of any
portion of the pulse. The rate of change in this fast pulse portion
changes with the constricting pressure which a cuff applies, and it has
been found that the pattern of the change undergoes a progressive
transition at a value of applied cuff pressure that is offset by a fixed
amount below the true diastolic value.
The measurement of diastolic pressure according to the invention thus
involves determining the applied cuff pessure at which a selected
time-dependent function of the pressure pulse monitored in a conventional
sphygmomanometer undergoes a unique change in progressive character when
examined as a function of the applied cuff pressure. The change is denoted
as being progressive to distinguish from a reversal in character. The
function of the pressure pulse that manifests the transition of interest
is one which measures the rate of change of the rapidly-changed pressure
pulse portion which correlates to left ventricle ejection. The desired
signal information for practice of the invention accordingly can be
produced, by way of example, by separating it from the pressure pulse with
a high pass filter, or by differentiating the pressure pulse as a function
of time.
The invention thus stems from the finding that the fast portion of the
conventionally-monitored pressure pulse manifests a unique progressive
change as the applied constricting pressure changes over a sub-diastolic
range. Further, the value of the applied pressure at which the change
occurs is a substantially constant amount below the true diastolic
pressure of the subject.
One practice of the invention thus involves applying a conventional
pressure cuff, fitted with a pressure sensor to the patient, and
progressively increasing the cuff pressure. The fast signal portion which
correlates to left ventricle ejection is separated from the sensed
pressure pulse by means of a high pass filter, and the amplitude of the
resultant rate-of-change signal is monitored. The rate-of-change signal,
which repeats with each cardiac pulse, exhibits little if any increase in
amplitude as the cuff pressure is initially increased. However, with
continued increase in cuff pressure below the diastolic value of the
patient, the monitored signal begins to increase significantly more
rapidly. The cuff pressure at which this transition occurs has a
subject-independent and substantially constant value below the true
diastolic pressure for the patient. Adding this constant offset pressure
value to the measured value this yields the desired diastolic measurement.
The signal transition of interest can be determined with any of several
pattern recognition techniques. In one illustrative instance, peak
detecting equipment senses when the monitored waveform of interest changes
relative to preceding pulses by more than a selected threshold amount. The
peak detecting equipment preferably stores and compares a number of
rate-of-change pulses to increase the accuracy of the determination.
In another instance, the pattern recognition is carried out with a recorder
that records both the repeating pulses and the applied cuff pressure, as
functions of time. The peaks of successive pulses define a curve which has
a significant and readily apparent change in slope at the transition of
interest. In particular, at cuff pressures below the value which coincides
with the selected transition, the pulse peaks lie along a relatively
straight line of small slope, and at pressures above the transition the
pulse peaks define a second line of significantly greater slope. The
applied cuff pressure of interest is the value which coincides with the
intersection of the two slopes. The diastolic pressure of the subject is
greater than this identified value by the known pressure offset.
Alternative to operating with an increasing applied pressure, the invention
can be practiced by determining the transition of the rate-of-change
pulses with progressively decreasing constricting pressure, from a value
typically above the expected diastolic value to well below it. In this
practice of the invention, the monitored pulses again manifest a
progressive transition at a value of cuff pressure which is offset by a
fixed value below the diastolic value, but the offset is smaller from that
encountered with an increasing constricting pressure. In particular, the
offset for measurements made with increasing cuff pressure is in the order
of 26 or 27 millimeters of mercury, whereas the offset with decreasing
cuff pressure is in the order of 16 to 17 millimeters of mercury. The
values of offset pressure discussed herein are in reference to the value
of diastolic pressure as determined with a stethoscope.
An instrument employing the invention can provide many advantages, some
inherent and other optional but readily provided. The instrument operates
without the operator listening to Korotkoff sounds and accordingly is free
of acoustic preception problems, both by the operator and by the
equipment. The instrument operates readily with patients that do not
exhibit the classical Phase IV Korotkoff sounds, or no discernible Phase
V. It is readily arranged to provide accurate measurements and analysis of
physical conditions, such as pulses alternans, which complicate the
conventional occluding cuff/stethoscope sphygmomanometer technique. The
instrument also has a relatively high degree of freedom from errors due to
background noise, including motion artifacts. Another advantage is a high
degree of freedom from variability in pulse waveform frequencies and
pressure pulse amplitudes between different subjects, i.e. in variations
from subject to subject. Moreover, the instrument readily provides a high
and reliable degree of accuracy that stems from analysis of a series of
cardiac pressure pulses, rather than on single pulse transitions. Another
advantage is that the instrument provides an accurate diastolic
measurement quickly, and without first occluding the artery; this
minimizes patient discomfort and concern. The instrument moreover can
readily be provided in a form suitable for continuous pressure measuring,
or monitoring, without occluding the artery and hence with minimal
discomfort and with diminished danger of impairing mobility.
The invention accordingly comprises the several steps and the relation of
one or more of such steps with respect to each of the others, and the
apparatus embodying features of construction, combinations and arrangement
of parts adapted to effect such steps, all as exemplified in the following
detailed disclosure, the scope of the invention is indicated in the claims
.
For a fuller understanding of the nature, objects and advantages of the
invention, reference should be made to the following detailed description
taken in connection with the accompanying drawings, in which:
FIG. 1 is a plot of a sensed cardiac pressure pulse, and of a selected rate
of change waveform produced from the pressure pulse for practice of the
invention;
FIGS. 2 and 3 are graphs of waveforms depicting the practice of the
invention;
FIG. 4 is a functional schematic representation of an instrument according
to the invention;
FIGS. 5 and 6 are block schematic representations of instruments for
practicing the invention; and
FIG. 7 is a block schematic representation of a further instrument for
practicing the invention.
DESCRIPTION OF ILLUSTRATED EMBODIMENTS
A pressure pulse detected from an unconstricted systemic artery with a
conventional arrangement of an acoustic sensor applied non-intrusively, as
under a pressure cuff, has a waveform 10 of the type which FIG. 1 shows.
The portion 10a of the pulse having the fastest rise time is the leading
edge portion, denoted between the times A and B, which has been correlated
to the left ventricle ejection time.
The rate of pressure change in the fast pulse portion 10a changes with
constriction of the artery being monitored and accordingly changes with
the pressure being applied to the artery by a typical sphygmomanometer
cuff. Moreover, it has been found that the pattern of the change undergoes
a progressive transition at a value of applied cuff pressure that is
offset by an essentially fixed amount below the true diastolic value of
the patient.
The invention contemplates measuring the applied cuff pressure at which the
transition occurs. To this end, the fast pulse portion 10a is essentially
separated from the overall pressure pulse, as can be done with any of
numerous rate-of-change detectors. For example, FIG. 1 shows the separated
pulse 12 that results from applying the waveform 10 to a high pass filter.
The duration of the leading edge of the separated pulse 12 corresponds
with the duration of the pulse portion 10a, as the time markers A and B
designate. The amplitude of the rate-of-change pulse 12 is a function of
the rate of change which occurs in the waveform portion 10a between the
times A and B.
The FIG. 1 wavefom 10 and rate-of-change pulse 12 repeat with every cardiac
beat. As one increases the pressure in a constricting cuff at a uniform
rate at sub-diastolic values, the resultant series of rate-of-change
pulses, after separation from the applied cuff pressure on which it is
superposed, has a waveform of the type which FIG. 2 shows. The pulses 12
are seen to have essentially the same or only slightly increasing
amplitudes for increasing pressure at low values, as denoted with an
initial series of pulses P1, P2, P3, P4 and P5, which respectively have
relative values V1, V2 . . . V5. A curve 14 defined by the peaks of these
initial pulses appears essentially as a straight line having a small
slope, of value S1. However, at higher pressure values, the rate-of-change
pulses 12 increase in amplitude significantly more rapidly, as shown for
the succeeding series of pulses P6, P7 etc., which have progressively
increasing values V6, V7 . . . . A curve 16 defined by the peaks of the
latter pulses is again substantially a straight line, under at least
idealized conditions, but with a slope S2 significantly greater than slope
S1.
The transition between the two series of pulses is the point x (pt x)
denoted by the intersection of the two slope curves 14 and 16. It occurs
at a cuff pressure designated Px, and it has been found that this pressure
has a relatively predictable constant value below the true diastolic
pressure of the subject. The amount of the offset below the diastolic
value has been found statistically to have a value of 26 or 27 millimeters
mercury. This offset is consistent not only for any given subject but also
from subject to subject. Accordingly, adding the value of this offset to
the measured pressure value Px yields the true diastolic pressure of the
subject. It should, however, be understood that the diastolic pressure
measured with the conventional stethoscope technique evades precise
measurement due to human perception factors, variations in procedure, and
instrument imprecision.
FIG. 3 is a waveform of rate-of-change pulses 12 produced in the same
manner as the waveform of FIG. 2, but for progressively decreasing
pressure, again over a sub-diastolic range. The waveform of FIG. 3 is thus
in many ways a mirror image, about the y-axis, of the FIG. 2 waveform,
except that the offset pressure is different. The pulses 12 produced with
progressively decreasing pressure initially decrease in amplitude to
define a curve 18 of slope S3. There is a transition in the monitored
pulses, however, and at pressures below it the pulses decrease in value at
a significantly slower rate to define a peak-following curve 20 of slope
S4. The point y (pt y) defined by the intersection of the curves 18 and 20
occurs at a value of applied cuff pressure designated Py, and this
pressure again has a predictable and constant value below the true
diastolic pressure of the subject. However, in this instance of decreasing
cuff pressure, the statistically determined offset is 16 or 17 millimeters
mercury. Hence adding this offset to the measured pressure Py yields an
accurate measure of the patient's diastolic pressure.
With further refernce to FIGS. 2 and 3, the depicted diastolic measuring
technique can for example be practiced with a controlled rate of cuff
pressure change of two millimeters mercury per second. In this instance a
nominal pulse rate of one beat per second readily yields within a
half-minute a series of pulses for calculating the true diastolic pressure
with significant accuracy and low error. Further, these procedures enable
the pressure measurement to be made without the discomfort of occluding
the systemic artery, as occurs with prior art techniques. Further, an
instrument for practicing the measuring technique of FIGS. 2 and 3 can be
initialized to an initial cuff pressure, and thereby further reduce
patient discomfort and measuring time. For example the slope-intersection
determination in FIG. 2 can reliably be made with ten or fewer pressure
pulses to define the curve 14 of slope S1. Hence the instrument can
initialize the pressure cuff to a starting pressure level of roughly 20
millimeters mercury below the point x which, combined with the offset
pressure 26-27 millimeters means that the instrument can safely be preset
to a starting pressure of some 45 millimeters mercury below the nominal
value of the diastolic pressure being measured.
It should further be noted that an instrument operating in accordance with
the invention can detect the condition known as the aortic regurgitation,
or the lack of Phase V Korotkoff sounds, a condition which is difficult to
detect with prior techniques. The diastolic pressure-measuring waveforms
which FIGS. 2 and 3 show, however, can readily be obtained from subjects
having this condition.
FIG. 4 shows that a diastolic pressure-measuring instrument 21 embodying
the invention has a pressure cuff 22 which a pressure source 24
selectively inflates. The pressure cuff and the pressure source can employ
known constructions. A pressure meter 26 is connected with the pressure
cuff to display or otherwise manifest the constricting pressure which the
cuff applies to the subject's arm or other limb at which the pressure is
being measured. Also connected with the pressure cuff is a transducer 28
for receiving the sensed arterial pressure waveforms 10 shown in FIG. 1
and for converting them to electrical signals.
A rate of change detector 30 receives the resultant signals and, in
response, produces signals, e.g. the FIG. 1 pulse 12, responsive to the
rate of change of the FIG. 1 waveform portion 10a. More particularly, the
detector 30 produces, for each cardiac beat, a pulse or other signal set
having a parameter which is a measure of the time rate of change of the
cardiac beat fast portion. The rate of change detector accordingly can, by
way of illustrative examples, be a filter or a differentiating circuit. As
a further alternative, it can be a metering circuit triggered by the onset
of the rapid waveform portion and disabled by the termination thereof.
A pattern recognition element 32 receives the rate-of-change signals from
the detector 30, and determines the occurrence of the transition point for
those signals, e.g. the slope transition designated in FIG. 2 as point x
and in FIG. 3 as point y. The element 32 typically is connected with the
pressure meter 26, as designated with the broken line, to display, record
or otherwise manifest the value of applied cuff pressure at the time when
the element 32 determines the transition has occurred. This value is the
FIG. 3 pressure Px, or the FIG. 4 pressure Py, whichever is applicable.
The appropriate offset pressure is then added to the determined value of
transition pressure, preferably automatically as by the display unit, to
yield the true diastolic pressure of the subject.
FIG. 5 shows one specific preferred form of the FIG. 4 instrument 21.
Elements of FIG. 5 which correspond to those in FIG. 4 have the same
reference numeral with an additional superscript prime. In addition to a
pressure cuff 22' connected with a pressure source 24' and a
pneumatic-to-electrical transducer 28', the FIG. 5 instrument has a
display unit 36 connected with the pressure meter 26' for selectively
displaying the measured value of applied cuff pressure; the display
typically is in digital format and calibrated in millimeters of mercury. A
high pass filter 38 receives the electrical signals from the transducer
28', and in turn is connected to a peak detector 40.
The high pass filter 38 provides the function of the FIG. 4 rate-of-change
detector 30, and the peak detector 40 provides the function of FIG. 4
pattern recognition element 32. More particularly, the diastolic pressure
measuring instrument of FIG. 5 develops, with the high pass filter 38, a
pulse 12 of the type described above with reference to FIG. 1 in response
to each sensed arterial pressure waveform 10 as shown in FIG. 1, and
accordingly applies to the peak detector 40 a series of pulses as shown
for example in FIG. 2. The filter 38 and the detector 40, as well as the
display unit 36, can employ any of numerous constructions within the skill
of those practiced in the art. The filter blocks low frequency signal
components while passing those that identify the rate of change of the
FIG. 1 waveform portion 10a. Typically the high-pass filter characteristic
has a three db point at about 12 to 14 Hertz. Where desired, a series low
pass filter can be added, with a three db point at approximately 50 Hertz
to block noise at frequencies above the range of interest.
The illustrated peak detector 40 determines the transition point by
comparing rate-of-change pulses from the filter. In an elementary
construction with a pressure source 24' of increasing applied pressure,
the detector identifies the transition point simply in response to a
rate-of-change pulse with an amplitude that exceeds the preceding pulse by
a threshold amount. It is preferable that the comparison be made against
the average of several prior pulses, for example the average value of the
pulses P1 to P2 of FIG. 2. It is also preferable that the detector
identify the transition point in response to a determination that not only
one but several successive pulses exceed a stored pre-transition pulse
value. Thus, in this embodiment the peak detector 40 initially produces an
average pre-transition pulse value and updates that value upon sensing
each successive pulse having an amplitude within a selected range relative
to the average value. Upon sensing a pulse that differs from the average
value by a selected relative amount, the detector disables the updating
function, holds the average value determined at that point and compares
succeeding pulses with the stored average value. Upon determining that
each of a selected number of successive pulses, for example three or more,
exceeds the average pulse value by a threshold amount, the peak detector
produces a signal indicating that the transition point has indeed been
detected. This signal actuates the display unit 36 to manifest the value
of applied cuff pressure at the time when the peak detector detected the
first of the succession of pulses exceeding the average value by the
selected amount. In one preferred embodiment of this type, the peak
detector 40 rejects potentially erroneous pulses by updating the average
pre-transition pulse value only with pulses that do not depart from the
value by a relatively small proportional amount, and identifying as
post-transition pulses only those which exceed the stored average value by
a larger proportional amount.
The threshold level(s) for the peak detector 40 are selected according to
the nature of the detector. For example, a detector that simply compares
two successive signals will generally have a different--usually
larger--threshold than one which compares each new signal with an average
of pre-transition signals. Similarly, a detector that responds only to
positive results from repeated comparisons can be more sensitive and hence
employ a smaller threshold than one which responds to a single positive
result. Illustrative threshold values are typically between 10% and 20% of
pre-transition signal, but these values are not exclusive of others.
The illustrated FIG. 5 instrument also includes, as part of the FIG. 4
pulse recognition sensor 32, a timing circuit 42 that disables the peak
detector from responding to signals having a duration outside of at least
one selected limit. The timing circuit 42 thus limits response of the
instrument to pulses having a duration within a selected time window. More
particularly, the time interval between the times | | |
