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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a pulse wave detecting apparatus which
presses a pulse wave sensor against an arterial vessel of a living body or
subject via body surface above the artery and detects a pressure pulse
wave produced from the artery.
2. Related Art Statement
There is known a pulse wave detecting device including (a) a pulse wave
sensor having a press surface and including a plurality of pressure
sensing (PS) elements provided in one or more arrays in the press surface,
the pulse wave sensor being adapted to be pressed against an arterial
vessel of a living subject via a body surface of the subject such that a
direction of the array or arrays of PS elements intersects a direction of
extension of the artery, so that each of the pressure sensing elements
detects a pressure pulse wave produced from the artery and generates a
pulse wave signal representing the detected pulse wave, (b) a pressing
device which produces a pressing force to press the press surface of the
pulse wave sensor against the artery via the body surface, and (c)
regulating means for continuously changing the pressing force of the
pressing device applied to the pulse wave sensor, determining an optimum
value of the pressing force based on one or more of the pulse wave signals
generated from the PS elements, and holding the pressing force of the
pressing device at the thus determined optimum value. The prior apparatus
reads in the pulse wave signal or signals supplied from one or more of the
PS elements pressed with the optimum pressing force, and obtains the
pressure pulse wave of the subject based on the pulse wave signal or
signals. The prior apparatus is disclosed in, for example, Japanese Patent
Application laid open for inspection purpose under Publication No.
1(1989)-285244.
However, in the prior pulse wave detecting device, the condition of
pressing of the pulse wave sensor against the body surface (e.g., manner
of contact of the former with the latter) may change due to, for example,
motion of the subject (e.g., motion of his or her wrist on which the
sensor is being worn). In this event, the reading or detected magnitude of
the pulse wave is adversely affected. The detected pulse wave may contain
both change due to natural change of blood pressure of the subject and
change due to artificial change of the pressing condition of pulse wave
sensor. Thus, the accuracy of detection of the prior device is not
satisfactory.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a pulse wave
detecting apparatus which continuously detects a pressure pulse wave of a
living subject with high accuracy.
The Applicants have intensively studied for achieving the above object, and
have found that the variation (i.e., distribution or pattern) of the
respective lower peaks of the pulse waves detected through the pressure
sensing elements with respect to the array of pressure sensing elements,
with the pulse wave sensor being pressed with the optimum pressing force,
is intimately related to the condition of pressing of the pulse wave
sensor against the body surface of the subject. The present invention has
been developed based on this discovery.
The above object has been achieved by the present invention, which provides
a pulse wave detecting apparatus for detecting a pulse wave from a living
subject, the pulse wave comprising a plurality of pulses produced from an
arterial vessel of the subject in synchronism with heartbeat of the
subject, comprising (a) a pulse wave sensor having a press surface and
including at least one array of pressure sensing elements provided in the
press surface, the press surface of the pulse wave sensor being adapted to
be pressed against the arterial vessel of the living subject via a body
surface of the subject such that a direction of the array of pressure
sensing elements intersects a direction of extension of the arterial
vessel, so that each of the pressure sensing elements detects the pulse
wave produced from the arterial vessel and generates a pulse wave signal
representing the detected pulse wave, (b) a pressing device which produces
a pressing force to press the press surface of the pulse wave sensor
against the arterial vessel via the body surface, (c) regulating means for
changing the pressing force of the pressing device applied to the pulse
wave sensor, determining an optimum value of the pressing force based on
at least one of the pulse wave signals generated from the pressure sensing
elements, and holding the pressing force of the pressing device at the
thus determined optimum value, (d) lower-peak variation determining means
for determining a lower peak of at least one pulse of each of the pulse
waves represented by the respective pulse wave signals from the pressure
sensing elements, the lower-peak variation determining means iteratively
determining a variation of the respective lower peaks of the pulse waves
with respect to the array of pressure sensing elements after the pressing
force of the pressing device is held at the optimum value, and (e) judging
means for judging whether a pressing condition of the pulse wave sensor on
the body surface is stable, based on change of the lower-peak variations
determined by the lower-peak variation determining means.
In the pulse wave detecting apparatus constructed as described above, the
lower-peak variation determining means determines iteratively determines a
variation of the respective lower peaks of the pulse waves with respect to
the array of pressure sensing elements after the pressing force of the
pressing device is held at the optimum value, and the judging means judges
whether a pressing condition of the pulse wave sensor on the body surface
is stable, based on change of the lower-peak variations determined by the
lower-peak variation determining means. When the judging mans provides a
negative judgment that the pressing condition of the pulse wave sensor on
the body surface is not stable, i.e., has changed, the regulating means
may be operated for updating the optimum pressing force of the pressing
device, thereby changing the pressing condition of the pulse wave sensor.
Alternatively, the present apparatus may further comprise an informing
device which informs an operator or user of the negative judgment, so that
the operator or user can recognize that the pressing condition of the
pulse wave sensor has changed. Thus, the accuracy of detection of the
present apparatus is improved.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and optional objects, features, and advantages of the present
invention will be better understood by reading the following detailed
description of the preferred embodiments of the invention when considered
in conjunction with the accompanying drawings in which:
FIG. 1 is a diagrammatic view of a pulse wave detecting apparatus embodying
the present invention;
FIG. 2 is an enlarged view of a pulse wave sensor incorporated in the
apparatus of FIG. 1, as seen by a person facing the press surface of the
sensor in which surface an array of pressure sensing elements are
provided;
FIGS. 3(a) and 3(b) are flow charts representing respective parts of the
control program used by the apparatus of FIG. 1;
FIGS. 4(a) and 4(b) are flow charts representing other parts of the control
program used by the apparatus of FIG. 1;
FIG. 5 is a view of a reference minimum tonogram curve, MTC.sub.s, and a
subsequent minimum tonogram curve, MTC, each determined at Step S9 of the
flow chart of FIG. 3;
FIG. 6 is a view of (a) a reference curve MTC.sub.s and (b) a subsequent
curve MTC translated so that the two curves MTC.sub.s, MTC take an
identical value with respect to an optimum pressure sensing element of the
pulse wave sensor of FIG. 2;
FIGS. 7(a) to 7(j) are views of various patterns of change of a subsequent
curve MTC from a reference curve MTC.sub.s, each pattern belonging to a
first group of patterns, I, which indicate that the pressing condition of
the pulse wave sensor is stable;
FIGS. 8(a) to 8(l) are views of various patterns of change of a subsequent
curve MTC from a reference curve MTC.sub.s, each pattern belonging to a
second group of patterns, II, which indicate that the pressing condition
of the pulse wave sensor is not stable; and
FIGS. 9(a) to 9(d) are views of various patterns of change of a subsequent
curve MTC from a reference curve MTC.sub.s, each pattern belonging to a
third group of patterns, II, which indicate that the pressing condition of
the pulse wave sensor is not stable.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to FIG. 1, there is shown a pulse wave detecting apparatus
embodying the present invention. In the figure, reference numeral 10
designates a container-like housing. The housing 10 has a wall-free end
closed by a pulse wave (PW) sensor 20 and a diaphragm 18. The present
apparatus is detachably set on a wrist 16 of a living subject with the
help of setting bands 14, 14, such that the wall-free end of the housing
10 is held in contact with body surface 12 of the subject 16. The
diaphragm 18 is flexible enough to permit the PW sensor 20 to displace
relative to the housing 10 and advance out of the wall-free end of the
housing 10. The housing 10, diaphragm 18, and PW sensor 20 cooperate with
each other to define a pressure chamber 22, which is supplied with
pressurized fluid such as pressurized air from a fluid supply device 24
via a pressure regulator valve 26. A pressure sensor 42 detects the fluid
pressure in the pressure chamber 22 (hereinafter, referred to as the
"chamber pressure P"), and generates an electric signal representing the
detected chamber pressure P. The PW sensor 20 is pressed on the body
surface 12 with a pressing force corresponding to the chamber pressure P.
In the present embodiment, the housing 10, diaphragm 18, fluid supply 24,
and pressure regulator 26 cooperate with each other to serve as a pressing
device for pressing the PW sensor 20 against the body surface 12.
As shown in FIG. 2, the PW sensor 20 has a press surface 28 defined by a
semiconductor chip of, for example, monocrystalline silicon. A single
array of pressure sensing (PS) elements 31 (e.g., thirty PS diodes) are
provided straightly in the press surface 28. However, the PS elements 31
may be arranged in two or more arrays. The PW sensor 20 is pressed against
a radial artery 30 via the body surface 12 such that the direction of the
array of PS elements 31 generally perpendicularly intersects the direction
of extension of the radial artery 30, so that each of the PS elements 31
detects an oscillatory pressure wave or pressure pulse wave that is
produced from the radial artery 30 and is propagated to the body surface
12. A pressure pulse wave contains a plurality of pulses lack of which is
produced from an arterial vessel of a living subject in synchronism with a
heartbeat of the subject. The PS elements 31 are provided equidistantly
from each other in the press surface 28, and the distance between the PS
elements 31 is pre-selected at a sufficiently small value which enables a
sufficiently great number of PS elements 31 to ride directly above the
radial artery 30. Additionally, the overall length of the array of PS
elements 31 is pre-selected at a greater value than the lumen or diameter
of the radial artery 30. Each of the PS elements 31 generates an electric
signal, i.e., pulse wave signal, SM, representing the pressure pulse wave
detected thereby from the radial artery 30, and the pulse wave signals SM
generated from all the PS elements 31 are supplied to a control device 32.
The control device 32 also receives from the pressure sensor 42 the
pressure signal representing the chamber pressure P.
The control device 32 is essentially constituted by a microcomputer
including a central processing unit (CPU) 34, a read only memory (ROM) 36,
and a random access memory (RAM). The CPU 34 processes input signals
according to control programs pre-stored in the ROM 36 by utilizing
temporary-data-storage function of the RAM 38. Specifically, according to
well-known control programs, the CPU 34 determines an optimum pressing
force (in the present embodiment, optimum chamber pressure, Pa) to be
applied to the PW sensor 20, and selects an optimum PS element 31a from
the array of PS elements 31, based on the pulse wave signals SM supplied
from the PS elements 31 while the chamber pressure P is changed. The CPU
34 supplies control signal, SD, to the pressure regulator 26 to hold the
chamber pressure P at the thus determined optimum value Pa. After the
chamber pressure P is held at the optimum value Pa, the CPU 34 reads in,
and stores in the RAM 38, the pressure pulse wave in the form of the pulse
wave signal SM supplied from the optimum PS element 31a, and supplies
control signal, SI, to a display and record device 40 to display and
record the thus obtained pulse wave that contains heartbeat-synchronous
pulses. Meanwhile, the CPU 34 determines a lower peak of one or more
pulses of the pulse wave represented by the pulse wave signal SM supplied
from each of the PS elements 31. The CPU 34 determines a variation, MTC
(FIG. 5), of the lower peaks of the pulse waves with respect to the array
of PS elements 31, in each of periodic cycles after the chamber pressure P
is held at the optimum value Pa. The CPU 34 judges whether or not the
condition of pressing of the PW sensor 20 on the body surface 12 is
stable, based on time-wise change of the pulse-wave lower-peak variations
determined in the periodic cycles. In the event that the CPU 34 judges
that the pressing condition of the PW sensor 20 is not stable, i.e., has
changed due to, e.g., motion of the wrist 16, the CPU 34 updates the
optimum chamber pressure Pa and holds the chamber pressure P at the
updated optimum pressure Pa. Thus, the present apparatus continues to
press the PW sensor 20 against the body surface 12 under the stable
condition, so that the optimum PS element 31a continues to detect the
pressure pulse wave (i.e., pulse wave signal SM) with high accuracy. In
the following description, the pulse-wave lower-peak variation determined
by the CPU 34 in each determination cycle is referred to as the "minimum
tonogram curve MTC". In the present embodiment, the CPU 34 does not
determine a "literal" curve MTC as illustrated in FIG. 5. However, as the
number of the PS elements 31 employed increases, the pulse-wave lower-peak
variation determined approaches the curve MTC.
Hereinafter there will be described the operation of the pulse wave
detecting apparatus constructed as described above, by reference to the
flow charts of FIGS. 3(a), 3(b), 4(a), and 4 (b).
Upon application of electric power to the present apparatus, initialization
of the apparatus is carried out in a step (not shown) in which flag, F,
and counters, C.sub.1 and C.sub.2, (described later) each are reset to
zero. Subsequently, when a start and stop switch (not shown) is operated
to its "ON" state, the control of the CPU 34 starts with Step S1 to
operate the pressure regulator 26 so as to allow fluid to discharge from
the pressure chamber 22 and subsequently supply the chamber 22 with the
pressurized fluid fed from the supply device 24. Specifically, as the
pressurized fluid is fed, the chamber pressure P is slowly and gradually
increased up to a predetermined level, e.g., about 250 mmHg. During this
chamber pressure P increasing operation, the CPU 34 receives the pulse
wave signal SM from each of the PS elements 31 and the pressure signal
from the pressure sensor 42 which signal represents the chamber pressure P
currently being increased. The CPU 34 determines the amplitude (i.e.,
difference between the upper-peak and lower-peak magnitudes or between the
maximum and minimum magnitudes) of each pulse of the pulse wave signal SM
from each of the PS elements 31, and selects as the optimum PS element 31a
one of the PS elements 31 which has supplied a maximum pulse having a
maximum amplitude. Additionally, the CPU 34 determines as the optimum
pressure Pa the chamber pressure P at the time of detection (or reception)
of the maximum pulse from the optimum PS element 31a.
Step S1 is followed by Step S2 to hold the chamber pressure P at the
optimum pressure Pa determined at Step S1. In this situation, the wall of
the radial artery 30 is partially flattened under the press surface 28 of
the PW sensor 20, as shown in FIG. 1. In the present embodiment, the
pressure sensor 42, Steps S1 and S2 of FIG. 3(a), and a portion of the
control device 32 to carry out those steps cooperate with each other to
serve as regulating means for regulating the pressing force of the
pressing device 18, 22, 24, 26 so as to press the PW sensor 20 with the
optimum pressing force Pa.
At the following step, Step S3, the CPU 34 reads in signal SM corresponding
to one pulse, from each of the PS elements 31, and stores the one-pulse
signals SM from all the PS elements 31 in an appropriate area of the RAM
38. Step S3 is followed by Step S4 to operate the display/record device 40
to display and record the one-pulse signal SM supplied from the optimum PS
element 31a and stored in the RAM 38. At the following step, Step S5, the
CPU 34 judges whether the content of flag F is 1 (i.e., F=1). Since flag F
is reset to 0 (i.e., F=0) at the time of initialization, a negative
judgment is made at Step S5 in the current control cycle. Therefore, the
control of the CPU 34 proceeds with Step S6.
At Step S6, the CPU 34 judges whether the CPU 34 has read in a total of
eight pulses from each PS element 31 after the chamber pressure P is held
at the optimum pressure Pa which has been determined for the first time
after the start/stop switch is operated to the "ON" state. So long as a
negative judgment is made at Step S6, Steps S3 to S6 are repeated.
Meanwhile, if a positive judgment is made at Step S6, the control of the
CPU 34 goes to Step S7 to determine the lower-peak (i.e., minimum)
magnitude of each of the last eight pulses supplied from each PS element
31 and stored in the RAM 38. Step S7 is followed by Step S8 to determine
an average of the thus determined eight lower-peak magnitudes with respect
to each of the PS elements 31.
Subsequently, at Step S9, the CPU 34 determines a current minimum tonogram
curve MTC as indicated in solid line in the two-dimensional coordinate
system of FIG. 5. This coordinate system has an axis of abscissa 50
indicative of the serial number (No.) assigned to each PS element 31 of
the PW sensor 20, and an axis of ordinate 52 indicative of the lower-peak
magnitude of the pulse wave (i.e., pulse wave signal SM) in terms of mmHg.
The current curve MTC is obtained by plotting in the graph a point
representing the average lower-peak magnitude of each PS element 31, and
connecting the plotted points with a line. Serial numbers (Nos.) are
assigned to the respective PS elements 31 in the order of location thereof
in the array provided in the press surface 28 of the PW sensor 20. Step S9
is followed by Step S10 to set flag F to F=1, indicating that a curve MTC
has been determined at Step S9. Subsequently, at Step S11, the CPU 34
judges whether the curve MTC determined at Step S9 in the current control
cycle is the first one determined after the chamber pressure P is last
held at the optimum pressure Pa. Immediately after the beginning of
operation of the apparatus, a positive judgment is made at Step S11, so
that the control of the CPU 34 goes to Step S12. Hereinafter, the curve
MTC determined immediately after the chamber pressure P is last held at
the optimum pressure Pa is referred to as the "reference curve MTC.sub.s
".
At Step S12, the CPU 34 judges whether fifteen seconds have passed after a
curve MTC (for the current control cycle, reference curve MTC.sub.s) is
determined at Step S9. If a negative judgment is made at Step S12, Steps
S3-S5 and S12 are repeated to detect, store, display, and record
respective pulses of the pulse wave from the optimum PS element 31a.
Meanwhile, if a positive judgment is made at Step S12, the control of the
CPU 34 goes to Step S13 to reset flag F to F=0, and subsequently the
control of the CPU 34 returns to Step S5. Since flag F has just been reset
to F=0 at Step S13, a negative judgment is made at Step S5. Since, at Step
S6, the CPU 34 always obtains a positive result after having once obtained
a positive judgment at this step, the control of the CPU 34 goes to Steps
S7-S9 to determine a current minimum tonogram curve MTC as indicated in
broken line in FIG. 5. Then, at Step S10, the CPU 34 sets flag F to F=1,
and subsequently the control of the CPU 34 goes to Step S11. At this time,
a negative judgment is made at Step S11, therefore the control of the CPU
34 goes to Step S14 and the following steps. Thus, after the determination
of the reference curve MTCs, a current curve MTC is determined at regular
intervals of 15 seconds.
At Step S14, the CPU 34 calculates a first-end area, SL, an optimum-portion
area, SM, and a second-end area, SR, (FIG. 5) for the purpose of
determining a pattern of change of the current curve MTC from the
reference curve MTC.sub.s. In the present embodiment, the first end area
SL is calculated or approximated by summing the values obtained by
subtracting the respective values on the reference curve MTC.sub.s from
the corresponding values on the current curve MTC, with respect to each of
the three PS elements 31 located at one (i.e., left-hand end in FIG. 5) of
opposite ends of the array of PS elements 31 of the PW sensor 20.
Similarly, the second-end area SR is approximated by summing the values
obtained by subtracting the respective values on the reference curve
MTC.sub.s from the corresponding values on the current curve MTC, with
respect to each of the three PS elements 31 located at the other end
(i.e., right-hand end in FIG. 5) of the array of PS elements 31. The
optimum-portion area SM is approximated by summing the values obtained by
subtracting the respective values on the reference curve MTC.sub.s from
the corresponding values on the current curve MTC, with respect to each of
the three PS elements 31 consisting of the optimum PS element 31a and two
adjacent PS elements 31 located on both sides of the element 31a in the
array of PS elements 31.
Step S14 is followed by Step S15 to calculate an overall amount of change,
S, of the current curve MTC from the reference curve MTC.sub.s, according
to the following expression (1):
##EQU1##
where i: the serial number (No.) assigned to each PS element 31;
MTC(i): the value on the curve MTC with respect to the element 31 numbered
"i";
MTC.sub.s (i): the value on the curve MTC.sub.s with respect to the element
31 numbered "i"; and
diff: the value obtained by subtracting the value on the curve MTC.sub.s
from the corresponding value on the curve MTC with respect to the optimum
PS element 31a.
The change amount S approximates the change area enveloped by (a) the
reference curve MTC.sub.s and (b) the current curve MTC translated along
the axis of ordinate 52 by subtracting the value, diff, from the
respective values on the curve MTC with respect to all the PS elements
31a, as shown in FIG. 6. In the coordinate system of FIG. 6, the two
curves MTC.sub.s, MTC take an identical value with respect to the optimum
PS element.
At the following step, S16, in FIG. 4 (a), the CPU 34 judges whether the
change area S is not greater than a first reference value, e.g., 80. A
positive judgment at Step S16 that the change area S is not greater than
80 indicates that the change of the pulse waves (or pulse wave signals SM)
represented by the change area S resulted from natural change of blood
pressure of the subject 16 and therefore that the pressing condition of
the PW sensor 20 against the body surface 12 is stable. Thus, the control
of the CPU 34 goes to Step S17 to reset first and second counters C.sub.1,
C.sub.2 each to zero (C.sub.1 =0, C.sub.2 =0). Then, the control of the
CPU 34 returns to Step S3 to continue to detect the pressure pulse wave
through the optimum PS element 31a. On the other hand, if a negative
judgment is made at Step S16, the control of the CPU 34 goes to Step S18
to judge whether the change area S is greater than a second reference
value, e.g., 400, greater than the first reference value employed at Step
S16. A positive judgement at Step S18 that the change area S is greater
than 400 indicates that the change of the pressure pulse wave represented
by the change area S did not result from artificial change of blood
pressure of the subject and therefore that the pressing condition of the
PW sensor 20 against the body surface 12 is not stable. Thus, the control
of the CPU 34 goes to Step S19 to reset first and second counters C.sub.1,
C.sub.2 to C.sub.1 =0 and C.sub.2 =0, respectively. Then, the control of
the CPU 34 returns to Step S1 to re-determine or update the optimum
chamber pressure Pa, re-select or update the optimum PS element 31a, hold
the chamber pressure P at the updated optimum pressure Pa, and resume
detecting the pressure pulse wave of the subject through the updated
optimum PS element 31a.
On the other hand, if a negative judgment is made at Step S18, i.e., if the
change area S falls within the range, 80<S.ltoreq.400, the control of the
CPU 34 goes to Step S20 to judge whether the first-end area SL is equal to
the optimum-portion area SM and simultaneously the optimum-portion area SM
is equal to the second-end area SR. If a positive judgment is made at Step
S20, the control goes to Step S21 to conclude that the pattern of change
of the current curve MTC from the reference curve MTC.sub.s corresponds to
a first group of patterns, I, more specifically, one of patterns shown in
FIGS. 7(a) and 7(b). This result indicates that the pressing condition of
the PW sensor 20 on the body surface 12 is stable. Therefore, the control
of the CPU 34 goes to Step S17 to reset counters C.sub.1, C.sub.2 to
C.sub.1 =0 and C.sub.2 =0, respectively, and subsequently returns to Step
S3 to continue to detect the pressure pulse wave of the subject 16.
On the other hand, if a negative judgment is made at Step S20, the control
of the CPU 34 goes to Step S22 to judge whether the absolute value of the
first-end area SL is smaller than the absolute value of the
optimum-portion area SM and simultaneously the absolute value of the
optimum-portion area SM is greater than the absolute value of the
second-end area SR. If a positive judgment is made at Step S22, the
control goes to Step S21 to conclude that the pattern of change of the
current curve MTC from the reference curve MTC.sub.s corresponds to the
first pattern I, more specifically, one of patterns shown in FIGS. 7(c) to
7(j) wherein both of the end portions of the current curve MTC did not
change from the corresponding portions of the reference curve MTC.sub.s.
This result indicates that the change of the pressure pulse wave
represented by the change area S resulted from the change of blood
pressure of the subject and that the pressing condition of the PW sensor
20 on the body surface 12 is stable. Therefore, the control of the CPU 34
goes to Step S17 to reset counters C.sub.1, C.sub.2 to C.sub.1 =0 and
C.sub.2 =0, respectively, and subsequently returns to Step S3 to continue
to detect the pressure pulse wave of the subject 16.
On the other hand, if a negative judgment is made at Step S22, the control
of the CPU 34 goes to Step S23 to judge whether the absolute value of the
first-end area SL is greater than the absolute value of the
optimum-portion area SM and simultaneously the absolute value of the
optimum-portion area SM is smaller than the absolute value of the
second-end area SR. If a positive judgment is made at Step S23, the
control goes to Step S24 to judge whether the product of the first-end
area SL and the second-end area SR is negative, i.e., judge whether one of
the two areas SL, SR is negative and the other area SL, SR is positive.
If a positive judgment is made at Step S24, the control goes to Step S25 to
conclude that the pattern of change of the current curve MTC from the
reference curve MTC.sub.s corresponds to a second group of patterns, II,
more specifically, one of patterns shown in FIGS. 8(i) to 8(l) wherein
both of the end portions of the current curve MTC largely changed from the
corresponding portions of the reference curve MTC.sub.s. This result
suggests that the change of the pressure pulse wave represented by the
change area S did not result from the change of blood pressure of the
subject and that the pressing condition of the PW sensor 20 is not stable,
i.e., has changed. Subsequently, the control of the CPU 34 goes to Step
S26 to judge whether the change area S is greater than a third reference
value, e.g., 150, greater than the first reference value used at Step S16
and smaller than the second reference value used at Step S18. A positive
judgement at Step S26 that the change area S is greater than 150,
indicates that the pressing condition of the PW sensor 20 against the body
surface 12 has not changed, i.e., remains stable. Thus, the control of the
CPU 34 goes to Step S27 to reset counter C.sub.2 to C.sub.2 =0, and
respectively. Then, the control of the CPU 34 returns to Step S3 to
continue to detect the pressure pulse wave of the subject 16.
On the other hand, if a positive judgment is made at Step S26, the control
of the CPU 34 goes to Step S28 to add one to the content of second counter
C.sub.2. The content of counter C.sub.2 indicates the number of positive
judgment or judgements made at Step S26. Subsequently the control goes to
Step S29 to judge whether the content of counter C.sub.2 is two (i.e.,
C.sub.2 =2). If a negative judgment is made at Step S29, i.e., if the
content of counter C.sub.2 is C.sub.2 =1, it cannot readily be concluded
that the pressing condition of the PW sensor 20 against the body surface
12 has changed, i.e., the pressing condition may remain stable. Thus, the
control returns to Step S3. On the other hand, if a positive judgment is
made at S29, it can be concluded that the pressing condition of the PW
sensor 20 has changed. Thus, the control goes to Step S30 to reset the
content of counter C.sub.2 to C.sub.2 =0, and subsequently returns to Step
S1 to update the optimum chamber pressure Pa and the optimum PS element
31a, hold the chamber pressure P at the updated optimum pressure Pa, and
resume detecting the pressure pulse wave of the subject 16 through the
updated optimum PS element 31 a.
Meanwhile, if a negative judgment is made at Step S23, the control of the
CPU 34 goes to Step S25 to conclude that the pattern of change of the
curve MTC from the curve MTC.sub.s corresponds to the second pattern, II,
more specifically, one, of patterns shown in FIGS. 8(a) to 8(h) wherein
one of the end portions of the current curve MTC largely changed from the
corresponding portion of the reference curve MTC.sub.s. This result
suggests that the change of the pressure pulse wave represented by the
change area S did not result from the change of blood pressure of the
subject and that the pressing condition of the PW sensor 20 has changed.
If a negative judgment is made at Step S24, i.e., if the first-end area SL
and the second-end area SR are both positive or both negative, the control
of the CPU 34 goes to Step S31 to conclude that the pattern of change of
the current curve MTC from the reference curve MTC.sub.s corresponds to a
third pattern, III, i.e., one of patterns shown in FIGS. 9(a) to 9(d)
wherein both of the end portions of the current curve MTC largely changed
from the corresponding portions of the reference curve MTC.sub.s. This
result suggests that the change of the pressure pulse wave represented by
the change area S did not result from the change of blood pressure of the
subject and that the pressing condition of the PW sensor 20 has changed.
Step S31 is followed by Step S32 to judge whether the change area S is
greater than, e.g., 80. A negative judgment at Step S32 indicates that the
pressing condition of the PW sensor 20 has not changed, i.e., remains
stable. Therefore, the control of the CPU 34 goes to Step S33 to reset the
content of first counter C.sub.1 to C.sub.1 =0, and subsequently returns
to Step S3 to continue to detect the pressure pulse wave of the subject
16.
On the other hand, if a positive judgment is made at Step S32, the control
of the CPU 34 goes to Step S34 to add one to the content of first counter
C.sub.1, and subsequently to Step S35 to judge whether the content of
counter C.sub.1 is two (i.e., C.sub.1 =2). The counter C.sub.1 counts the
number of positive judgment or judgments made at Step S32. If a negative
judgment is made at Step S35, i.e., if the content of counter C.sub.1 is
C.sub.1 =1, it cannot readily be concluded that the pressing condition of
the PW sensor 20 against the body surface 12 has changed, i.e., it may
remain stable. Thus, the control returns to Step S3. On the other hand, if
a positive judgment is made at S35, it can be concluded that the pressing
condition of the PW sensor 20 against the body surface 12 has changed.
Thus, the control goes to Step S36 to reset the content of first counter
C.sub.1 to C.sub.1 =0 and subsequently returns to Step S1 and following
steps to update the optimum chamber pressure Pa and the optimum PS element
31a, hold the chamber pressure P at the updated optimum pressure Pa, and
resume detecting the pressure pulse wave of the subject 16 through the
updated optimum PS element 31a. In the present embodiment, Steps S14-S36
and a portion of the control device 32 to carry out those steps cooperate
with each other to serve as judging means for judging whether the pressing
condition of the PW sensor 20 on the body surface 12 is stable, based on
the change amount S of the current curve MTC from the reference curve
MTC.sub.s.
As is apparent from the foregoing description, after the beginning of the
pulse wave detection with the PW sensor 20 (i.e., optimum PS element 31a)
pressed at the optimum pressing force Pa, the present apparatus
periodically judges whether the pressing condition of the PW sensor 20
against the body surface 12 remains stable, based on the amount of change
S and/or pattern of change SL, SM, SR of each subsequent | | |
