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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a blood pressure monitor system which
continuously measures intra-arterial blood pressure of a living subject
and particularly relates to the art of improving the accuracy of blood
pressure measurement.
2. Related Art Statement
There has been proposed a blood pressure monitor system including a
pressure sensor having a press surface and including one or more pressure
sensing elements provided in the press surface; a pressing device which
presses the pressure sensor against an arterial vessel of a living subject
such as a patient via a body surface of the subject so that each pressure
sensing element of the pressure sensor measures pressures at the body
surface of the subject; pressing-force determining means for determining
an optimum pressing force of the pressing device at which a portion of a
wall of the artery is flattened under the pressure sensor pressed by the
pressing device; and blood-pressure determining means for operating the
pressing device to maintain the determined optimum pressing force and
press the pressure sensor against the artery via the body surface or skin,
and continuously determining intra-arterial blood pressure values of the
artery, based on the pressure magnitudes or values measured by the
pressure sensor at the body surface. An example of this monitor system is
disclosed in U.S. Pat. No. 5,119,822 or U.S. Pat. No. 5,179,956.
In the above-indicated prior monitor system, the pressure sensor is pressed
against the artery via the body surface or skin, such that the wall of the
artery is partly flattened under the pressure sensor. Since the pressure
values measured by the pressure sensor through the flattened wall of the
artery are free from adverse influences of the tensile forces produced in
the arterial wall, they well reflect intra-arterial blood pressure values
of the artery. According to this blood pressure measurement principle, the
prior monitor system continuously measures the blood pressure of the
subject by using the pressure sensor pressed at the optimum pressing
force.
Meanwhile, the experiments the present inventors conducted have elucidated
that the blood pressure values continuously measured by the
above-indicated prior monitor system tend to be higher than the blood
pressure values measured using an inflatable cuff, and do not enjoy
sufficiently high measurement accuracy. In this background, the present
inventors have made various studies and experiments, and found that the
soft and elastic subcutaneous tissue exists between the arterial vessel
and the pressure sensor and that a "provisional" blood pressure measured
by the pressure sensor, i.e., pressure sensing element positioned directly
above the artery and pressed at the optimum pressing force contains both a
"true" intra-arterial blood pressure of the artery and an "additional"
pressure added thereto because of the elastic force of the subcutaneous
tissue under the pressure sensor.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a blood
pressure monitor system which continuously measures intra-arterial blood
pressure of a living subject with high accuracy.
The above object has been achieved by the present invention, which provides
a blood pressure monitor system comprising: (a) a pressure sensor having a
press surface and including at least one pressure sensing element provided
in the press surface; (b) a pressing device which presses the pressure
sensor against an arterial vessel of a living subject via a body surface
of the subject so that the pressure sensing element of the pressure sensor
measures a pressure value at the body surface of the subject; (c)
pressing-force determining means for determining an optimum pressing force
of the pressing device at which a portion of a wall of the arterial vessel
of the subject is flattened under the pressure sensor pressed by the
pressing device; (d) inflection-point determining means for changing
pressing forces of the pressing device applied to the pressure sensor, and
determining a point of inflection of a curve representing a relationship
between the changed pressing forces of the pressing device and pressure
values measured by the pressure sensor at the body surface of the subject;
(e) correction-value determining means for determining a correction value
based on the pressure value of the determined point of inflection; and (f)
blood-pressure determining means for operating the pressing device to
maintain the determined optimum pressing force and press the pressure
sensor against the arterial vessel of the subject via the body surface of
the subject, and continuously determining intra-arterial blood pressure
values of the arterial vessel of the subject by subtracting the correction
value from the pressure values measured by the pressure sensor at the body
surface of the subject.
In the blood pressure monitor system constructed as described above, the
correction-value determining means determines a correction value based on
the pressure value of the determined inflection point, and the
blood-pressure determining means operates the pressing device to maintain
the determined optimum pressing force and press the pressure sensor
against the artery via the body surface or skin, and continuously
determines intra-arterial blood pressure values of the artery by
subtracting the correction value from the pressure values measured by the
pressure sensor at the body surface. The correction value corresponds to
the above-explained "additional" pressure added to the "true"
intra-arterial blood pressure of the artery because of the elasticity of
the subcutaneous tissue occurring between the artery and the pressure
sensor. Since the additional pressure is removed by subtracting the
correction value from the provisional blood pressure values measured by
the pressure sensor, the present monitor system enjoys the sufficiently
high accuracy of blood pressure measurement.
In a preferred embodiment of the present invention, the correction-value
determining means comprises: a memory which stores a plurality of pressure
correcting curves each of which represents a relationship between
correction values and pressing forces of the pressing device; selecting
means for selecting one of the pressure correcting curves which provides a
same difference between a first correction value corresponding to the
pressing force of the determined point of inflection and a second
correction value corresponding to the determined optimum pressing force of
the pressing device, as an actual difference between the pressure value of
the point of inflection and the pressure value corresponding to the
optimum pressing force of the pressing device; and determining means for
determining, as the correction value, the second correction value
corresponding to the optimum pressing force of the pressing device,
according to the selected one pressure correcting curve. Each of the
pressure correcting curves represents a relationship between the pressing
forces of the pressing device and the above-explained "additional"
pressure values that increase because of the elasticity of the
subcutaneous tissue as the pressing forces of the pressing device increase
.
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 blood pressure monitor system embodying
the present invention;
FIG. 2 is a bottom view of a pulse wave sensor of the monitor system of
FIG. 1;
FIG. 3 is a flow chart representing a control program according to which a
control device of the monitor system of FIG. 1 operates;
FIG. 4 is a flow chart representing the correction value determine routine
carried out at Step S4 of FIG. 3;
FIG. 5 is a graph showing a curve, T.sub.DIA, representing a relationship
between chamber pressure values, HDP, and provisional diastolic blood
pressure values, P.sub.DIA, which curve is obtained at Step S4-1 of FIG.
4; and
FIG. 6 is a graph showing a pressure correcting curve, f.sub.k, selected at
Step S4-5 of FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIGS. 1 and 2, there is shown a blood pressure monitor system
8 embodying the present invention. In FIG. 1, reference numeral 10
designates a container-like housing which is open at one end thereof. The
housing 10 is detachably set on a wrist 16 of a living subject such as a
patient, with a pair of bands 14, 14, such that the open end of the
housing 10 is held in contact with a body surface or skin 12 of the
subject. A pulse wave sensor 20 is secured to a flexible diaphragm 18
which is supported by inner surfaces of side walls of the housing 10 and
which closes the open end of the housing 10, such that the pulse wave
sensor 20 is displaceable relative to the housing 10 and is advanceable
out of the open end of the housing 10. The housing 10, diaphragm 18, and
pulse wave sensor 20 cooperate with each other to define a pressure
chamber 22 which is supplied with a pressurized fluid such as a
pressurized air from an air-supplying device 24 via a pressure regulator
valve 26. Thus, the pulse wave sensor 20 is pressed against the skin 12
with a pressing force corresponding to an air pressure in the pressure
chamber 22 (hereinafter, referred to as the "chamber pressure HDP"). In
the present embodiment, the housing 10, diaphragm 18, air-supplying device
24, pressure regulator valve 26, and others cooperate with each other to
provide a pressing device for pressing the pulse wave sensor 20 against
the skin 12. An air-pressure sensor 27 is provided to measure the chamber
pressure HDP. The air-pressure sensor 27 supplies a pressure signal, SP,
representing the measured chamber pressure HDP, to a control device 32.
The pulse wave sensor 20 shown in FIG. 2 includes a chip of a semiconductor
material such as a monocrystalline silicon. A predetermined number of
pressure sensing (PS) elements (e.g., thirty elements) 31 such as pressure
sensing diodes are provided in an array in a press surface 28 of the
semiconductor chip. With the pulse wave sensor 20 being pressed against
the skin 12 in the above-described manner, the array of PS elements 31
substantially perpendicularly intersects a radial artery 30 of the wrist
16, so that each of the PS elements 31 detects an oscillatory pressure
wave, i.e., pressure pulse wave that is produced from the radial artery 30
in synchronism with heartbeats of the subject and is transmitted to the
skin or body surface 12. The individual PS elements 31 are spaced by
sufficiently small distances from each other in the array thereof, so that
a sufficiently large number of PS elements 31 are positioned directly
above the artery 30. The overall length of the array of PS elements 31 is
greater than the diameter or lumen of the artery 30.
The semiconductor chip of the pulse wave sensor 20 has a thickness of about
300 microns (.mu.m). An elongate recess (not shown) is formed in a back
surface of the chip opposite to the press surface 28, so that the chip has
an elongate thin portion having a thickness of about several to ten and
several microns (.mu.m). In this elongate thin portion, the thirty PS
elements 31 are provided at regular intervals of distances, e.g.,
intervals of about 0.2 mm. Each PS element 31 is constituted by a
resistance bridge including four strain-resisting elements produced by a
well-known semiconductor manufacturing process such as diffusion or
injection of impurities. The PS element or resistance bridge is disclosed
in U.S. Pat. No. 5,101,829 assigned to the Assignee of the present
application. Each PS element 31 generates an electric signal whose
magnitudes correspond to pressure magnitudes input thereto from the radial
artery 30 via the skin 12, i.e., generates a pulse wave signal, SM,
representing the pressure pulse wave produced from the artery 30. The
pulse wave signal SM is supplied to the control device 32.
The control device 32 includes a microcomputer comprised of a central
processing unit (CPU) 34, a read only memory (ROM) 36, and a random access
memory (RAM) 38. The CPU 34 processes input signals according to control
programs pre-stored in the ROM 36 by utilizing a temporary-storage
function of the RAM 38. Specifically described, the CPU 34 determines an
optimum chamber pressure, HDPS, as an optimum pressing force to be applied
to the pulse wave sensor 20, and selects an optimum PS element 31a from
the thirty PS elements 31, each based on the pulse wave signals SM
supplied from the thirty PS elements 31 to the control device 32 while the
chamber pressure HDP is continuously increased. The CPU 34 controls the
pressure regulator valve 26 to hold the chamber pressure HDP at the thus
determined optimum value HDP.sub.S and thereby obtain a pressure pulse
wave of the subject as the pulse wave signal SM supplied from the thus
selected optimum PS element 31a pressed with the optimum pressure
HDP.sub.S. The CPU 34 controls an output device 40 to display a waveform
representing intra-arterial blood pressure values, P.sub.BP, of the radial
artery 30, and record the same on a record sheet (not shown), each based
on the pulse wave signal SM supplied from the optimum PS element 31a. An
upper peak and a lower peak of each of successive pulses of the waveform
displayed and recorded by the output device 40 correspond to a systolic
blood pressure, P.sub.SYS, and a diastolic blood pressure, P.sub.DIA, in
the artery 30. The output device 40 displays, in digits, the systolic and
diastolic blood pressure values P.sub.SYS, P.sub.DIA for each one pulse,
and additionally displays, using points or other symbols, respective
time-wise changes of the systolic and diastolic blood pressure values
P.sub.SYS, P.sub.DIA for the successive pulses.
When the radial artery 30 under the skin 20 is pressed by the pulse wave
sensor 20 with the optimum chamber pressure HDP.sub.S, a portion of the
wall of the artery 30 is flattened as shown in FIG. 1. Pressure magnitudes
or values, P, measured by the pulse wave sensor 20 through the flattened
wall of the artery 30 are free from adverse influences of the tensile
forces produced in the wall of the artery 30, and accordingly they reflect
the intra-arterial blood pressure values of the artery 30. According to
this blood pressure measurement principle, the control device 32 controls
the present monitor system 8 to continuously measure the blood pressure
values P.sub.BP in the artery 30 of the subject.
The various functions of the present blood pressure monitor system 8 for
carrying out the continuous blood pressure measurement of a living subject
are summarized as follows: The pulse wave sensor 20 functioning as a
pressure sensor is pressed, at the optimum chamber pressure HDP.sub.S,
i.e., optimum pressing force of the pressing device 10, 18, 24, 26
determined by the control device 32 functioning as pressing-force
determining means, against the radial artery 30 under the body surface or
skin 12 of the subject. The control device 32 also functions as
inflection-point determining means for determining a point of inflection,
H, of a curve, T.sub.DIA, representing a relationship between
"provisional" diastolic blood pressure values P.sub.DIA measured by the
pulse wave sensor 20 and chamber pressure values HDP measured by the
air-pressure sensor 27 while the chamber pressure HDP is continuously
changed by the control device 32 as the pressing-force determining means.
The control device 32 also functions as correction-value determining means
for determining a correction value, K.sub.S, based on a provisional
diastolic blood pressure value, P.sub.H, of the determined point of
inflection H. The control device 32 further functions as blood-pressure
determining means for determining a "true" intra-arterial blood pressure
value P.sub.BP of the artery 30 by subtracting the determined correction
value K.sub.S from each provisional blood pressure value, Pa, measured by
the pulse wave sensor, i.e. pressure sensor 20 pressed at the optimum
chamber pressure HDP.sub.S, i.e., optimum pressing force of the pressing
device 10, 18, 24, 26.
Hereinafter, there will be described the blood pressure measuring operation
of the present monitor system 8, by reference to the flow charts of FIGS.
3 and 4.
Upon application of electric power to the present monitor system 8, an
initialization step (not shown) is carried out. Then, if a start/stop
button (not shown) is operated, the CPU 34 of the control device 32 starts
with Step S1 to judge whether a flag, F, is set at "1" i e F=1. That the
flag F is set at F=1 means that the optimum chamber pressure HDP.sub.S and
the optimum PS element 31a have been determined and selected.
Assuming that a negative judgment is made at Step S1, the control of the
CPU 34 goes to Step S2 to determine the optimum chamber pressure HDP.sub.S
and subsequently to Step S3 to select the optimum PS element 31a. For
example, these operations are carried out in the following manner: After
the chamber pressure HDP has been decreased down to a sufficiently low
level by controlling the pressure regulator valve 26 and thereby
discharging the air from the pressure chamber 22, the chamber pressure HDP
is slowly increased up to a predetermined level at a suitable rate of
change, so that the pulse wave sensor 20 is pressed with the increasing
pressing forces against the radial artery 30 via the skin 12. During this
pressing force increasing operation, the CPU 34 reads in the respective
pulse wave signals SM supplied from the individual PS elements 31 of the
pulse wave sensor 20, together with the pressure signal SP supplied from
the air-pressure sensor 27. As described above, the pressure signal SP
represents the slow and monotonous increasing of the chamber pressure HDP
of the pressure chamber 22. The CPU 34 calculates, from each of the thus
obtained pulse wave signals SM, the amplitude of each of successive pulses
corresponding to heartbeats of the subject and selects, as the optimum PS
element 31a, one of the thirty PS elements 30 which has detected a maximum
pulse having the greatest amplitude of all the calculated amplitudes. The
amplitude of each pulse is calculated by subtracting the magnitude of the
lower peak of each pulse from the magnitude of the upper peak of the same
pulse. The CPU 34 additionally determines, as the optimum chamber pressure
HDP.sub.S, a chamber pressure HDP at the time when the maximum pulse has
been detected by the optimum PS element 31a. The thus determined optimum
chamber pressure HDP.sub.S is stored in the RAM 38. In the graph of FIG.
5, the optimum chamber pressure HDP.sub.S corresponds to an upper peak,
PTP.sub.max, of a curve, PTP, representing the change of the respective
amplitudes of the successive pulses of the pressure pulse wave represented
by the pulse wave signal SM supplied from the optimum PS element 31a.
Step S3 is followed by Step S4, i.e., correction value determine routine
shown in FIG. 4. First, at Step S4-1 of the flow chart of FIG. 4, the CPU
34 determines a curve T.sub.DIA, indicated at solid line in FIG. 5, which
represents a relationship between the provisional diastolic blood pressure
values P.sub.DIA measured by the pulse wave sensor 20 and the chamber
pressure values HDP measured by the air-pressure sensor 27 while the
chamber pressure HDP is continuously increased at Step S2 under the
control of the CPU 34. The curve T.sub.DIA is obtained by smoothly
connecting the respective lower-peak points of the successive pulses of
the pressure pulse wave represented by the pulse wave signal SM supplied
from the optimum PS element 31a.
The curve T.sub.DIA includes an increasing portion 50, and a level portion
52 called "plateau" which appears following the increasing portion 50
during the chamber pressure increasing operation carried out at Step S2.
Subsequently, at Step S4-2, the CPU 34 determines an inflection point H
connecting the increasing portion 50 and the level portion 52 of the curve
T.sub.DIA. For example, the inflection point H is determined by
identifying a point where the slopes (i.e., differential values) of the
curve T.sub.DIA significantly largely decreases, i.e., identifying an
upper-peak point of a curve representing the change of slopes of the curve
T.sub.DIA, according to an algorithm prestored in the ROM 36. At the
following Step S4-3, the CPU 34 stores, in the RAM 38, a provisional
diastolic blood pressure value P.sub.H and a chamber pressure value
HDP.sub.H of the thus determined inflection point H. Step S4-3 is followed
by Step S4-4 to determine a point, S, of the curve T.sub.DIA corresponding
to the stored optimum chamber pressure HDP.sub.S and store, in the RAM 38,
a provisional blood pressure value P.sub.S corresponding to the thus
determined point S.
Subsequently, the control of the CPU 34 goes to Step S4-5 to select one of
a plurality of relationships, K=f.sub.n (HDP) where n=1, 2, 3, . . . , m),
pre-stored in the ROM 36, in such a manner that the selected one
relationship, K=f.sub.k (HDP), indicated at solid line in FIG. 6, provides
the same difference, K.sub.S -K.sub.H, between a correction value,
K.sub.S, corresponding to the stored optimum chamber pressure value
HDP.sub.S and a correction value, K.sub.H, corresponding to the stored
chamber pressure value HDP.sub.H, as the difference, P.sub.S -P.sub.H,
between the stored pressure values P.sub.S, P.sub.H corresponding to the
points S, H, respectively. Step S4-5 is followed by Step S4-6 to determine
the correction value K.sub.S corresponding to the stored optimum chamber
pressure value HDP.sub.S, according to the thus selected relationship,
i.e., pressure correcting curve K=f.sub.k (HDP). The thus determined
correction value KS corresponds to the distance or difference between the
point S and a one-dot chain line shown in the graph of FIG. 5. The one-dot
chain line represents an ideal or theoretical curve T.sub.DIA, which would
be obtained by directly applying the pulse wave sensor 20 to the radial
artery 30 with the skin tissue 12 being removed.
The pressure correcting curve K=f.sub.n (HDP) represents a relationship
between chamber pressure values HDP, and "additional" pressure values
added to "true" intra-arterial blood pressure values because of the
elasticity of the subcutaneous tissue 12 located between the radial artery
12 and the pulse wave sensor 20 (i.e., each PS element 31). The additional
pressure values increase as the chamber pressure values HDP increase, as
shown in FIG. 6. This relationship K=f.sub.n (HDP) varies depending upon
the elastic characteristic of the subcutaneous tissue of an individual
living subject. The various relationships or curves K=f.sub.n (HDP) are
obtained by experiments. Since the additional pressure values, i.e.,
correction values K are a non-linear function of the chamber pressure
values HDP, each curve K=f.sub.n (HDP) may be approximated by, e.g., a
quadratic function, K=a.multidot.(HDP).sup.2 +b.multidot.(HDP)+c, where a,
b, and c are constants. The curves K=f.sub.n (HDP) represented by the
corresponding quadratic functions are indicated at one-dot chain line in
FIG. 6.
The pressure value determine routine of Step S4 is followed by Step S5 of
FIG. 3. At this step, the CPU 34 sets the flag F to F=1. Subsequently, the
control of the CPU 34 goes to Step S6 to control the pressure regulator
valve 26 so as to press the pulse wave sensor 20 at the optimum chamber
pressure HDP.sub.S. Thus, the optimum chamber pressure HDP.sub.S is
maintained at the optimum value HDP.sub.S. Step S6 is followed by Step S7
to judge whether the CPU 34 receives, from the optimum PS element 31a, a
length or amount of the pulse wave signal SM representing one pulse
corresponding to one heartbeat of the subject. Steps S8 and S9 are not
carried out so long as a negative judgment is made at Step S7. If a
positive judgment is made at this step, the control of the CPU 34 goes to
Step S8 to calculate a "true" blood pressure value P.sub.BP of the
subject, according to the following, pressure correcting expression (1):
P.sub.BP =P.sub.a -K.sub.S (1)
The CPU 34 determines the "true" or intra-arterial blood pressure value
P.sub.BP of the artery 30 by subtracting the determined correction value
K.sub.S (=f.sub.k (HDP.sub.S)) from the provisional blood pressure value
Pa measured by the pulse wave sensor 20 or optimum PS element 31a pressed
at the optimum chamber pressure HDP.sub.S. The thus determined blood
pressure value P.sub.BP is stored in the RAM 38. The true blood pressure
values P.sub.BP are continuously determined and stored in very short
sampling cycles, so as to provide a corrected pulse wave which changes in
synchronism with the heartbeats of the subject. The CPU 34 determines, as
systolic and diastolic blood pressure values, P.sub.SYS and P.sub.DIA, the
respective blood pressure values of the upper and lower peaks of each of
successive pulses of the corrected pulse wave, according to a well-known
algorithm pre-stored in the ROM 36. The thus determined systolic and
diastolic blood pressure values P.sub.SYS and P.sub.DIA are stored in the
RAM 38.
Subsequently, the control of the CPU 34 goes to Step S9 to control the
output device 40 to display the waveform of the corrected pulse wave,
i.e., "true" blood pressure values P.sub.BP obtained at Step S8 of the
current control cycle, following the waveform which had been obtained at
Step S8 in the control cycles prior to the current control cycle. In
addition, the output device 40 is operated to display, in digits, the
systolic and diastolic blood pressure values P.sub.SYS, P.sub.DIA
determined at Step S8, and add a point (or symbol) representing each of
the values P.sub.SYS, P.sub.DIA to a corresponding one of respective
time-wise changes of the points (or symbols) P.sub.SYS, P.sub.DIA.
It emerges from the foregoing description that, in the present embodiment,
Step S4-2 and a portion of the control device 32 for carrying out Step
S4-2 function as the inflection-point determining means for determining
the inflection point H of the curve T.sub.DIA representing the
relationship between the pressing forces HDP of the pressing device 10,
18, 24, 26 and the provisional blood pressure values Pa measured by the
pulse wave sensor 20 as the pressure sensor while the pressing forces or
chamber pressure values HDP are continuously changed, that Step S4-6 and a
portion of the control device 32 for carrying out Step S4-6 function as
the correction-value determining means for determining the correction
value K.sub.S based on the provisional blood pressure value P.sub.H
measured by the pulse wave sensor 20 and corresponding to the determined
inflection point H, and that Step S8 and a portion of the control device
32 for carrying out Step S8 function as the blood-pressure determining
means for operating the pressing device 10, 18, 24, 26 to maintain the
optimum pressing force or chamber pressure HDP.sub.S and thereby press the
pulse wave sensor 20 against the radial artery 30 and continuously
determining the true intra-arterial blood pressure values P.sub.BP of the
artery 30 by subtracting the correction value K.sub.S from the provisional
blood pressure values P.sub.a measured by the pulse wave sensor 20 at the
body surface or skin 12 of the subject.
Since the soft and elastic subcutaneous tissue 12 exists between the radial
artery 30 and the pulse wave sensor 20, the provisional blood pressure
value Pa measured by the optimum PS element 31a positioned directly above
the artery 30 and pressed at the optimum pressing force HDP.sub.S contains
both the true intra-arterial blood pressure value P.sub.BP of the artery
30 and the additional pressure value increased by the elastic force of the
subcutaneous tissue 12. However, in the present blood pressure monitor
system 8, the true intra-arterial blood pressure value P.sub.BP of the
artery 30 is determined with high accuracy by subtracting, from the
provisional blood pressure value P.sub.a measured by the pulse wave sensor
20, the correction value K.sub.S approximating the additional pressure
resulting from the elastic force of the subcutaneous tissue 12.
While the present invention has been described in its preferred embodiment,
the present invention may otherwise be embodied.
For example, although in the illustrated embodiment the inflection point H
of the curve T.sub.DIA is determined at Step S4-2 and the correction value
K.sub.S is determined based on the pressure value P.sub.H of the
inflection point H at S4-6, it is possible to determine, as an inflection
point of the curve T.sub.DIA, a point, G, connecting the level portion 52
and a second increasing portion 54 at Step S4-2, and determine a
correction value K.sub.S based on a pressure value, P.sub.G, of the
inflection point G at Step S4-6. In the latter case, at Step S4-5, the CPU
34 selects one of the pressure correcting curves K=f.sub.n (HDP) in such a
manner that the selected one curve K=f.sub.k (HDP) provides the same
difference, K.sub.G -K.sub.S, between a correction value, K.sub.G,
corresponding to the pressure value P.sub.G and the correction value
K.sub.S corresponding to the optimum chamber pressure value HDP.sub.S, as
the difference, P.sub.G -P.sub.S, between the pressure values P.sub.G,
P.sub.S corresponding to the points G, S, respectively.
While both in the illustrated embodiment and the above-indicated modified
embodiment the curve T.sub.DIA is obtained to determine the inflection
point H or G at Step S4-1, it is possible to obtain, at Step S4-1, a curve
T.sub.SYS by smoothly connecting the respective upper-peak points of
successive pulses of the pressure pulse wave represented by the pulse wave
signal SM supplied from the optimum PS element 31a. In the latter case, an
inflection point corresponding to the point H or G is determined on the
curve T.sub.SYS at Step S4-2, and a correction value K.sub.S is determined
based on a provisional blood pressure value P of the determined inflection
point.
In the illustrated embodiment, at Step S2, the optimum pressing force or
chamber pressure HDP.sub.S is determined by identifying the pressing force
or chamber pressure HDP at the time of detection of the maximum pulse by
the optimum PS element 31a of the pulse wave sensor 20. It is known that
the pressing force HDP at the time of detection of the maximum pulse well
corresponds to the middle point of the level portion 52 of the curve
T.sub.DIA. Therefore, at Step S2, it is possible to identify the middle
point of the level portion 52 of the curve T.sub.DIA and determine a
pressing force or chamber pressure HDP of the identified middle point, as
the optimum pressing force or chamber pressure HDP.sub.S.
In the illustrated embodiment, at Step S4-2, the inflection point H of the
curve T.sub.DIA is determined by identifying a point where the slopes of
the curve T.sub.DIA significantly largely changes. However, it is possible
to determine the inflection point H of the curve T.sub.DIA by identifying
a point, PTP.sub.H, of the pulse-amplitude curve PTP, as shown in FIG. 5.
The point PTP.sub.H has an amplitude smaller by a predetermined proportion
(e.g., about 10%) than the maximum amplitude PTP.sub.max of the curve PTP,
and corresponds to a pressing force HDP smaller than the optimum pressing
force HDP.sub.S. In this case, the inflection point H of the curve
T.sub.DIA corresponds to the pressing force HDP of the point PTP.sub.H.
In the illustrated embodiment, at Step S4, the pressure correcting curves
K=f.sub.n (HDP) represented by the quadratic functions are used. However,
in place of the quadratic functions, logarithmic or exponential functions
may be used to represent the curves K=f.sub.n (HDP). Since, actually, only
particular portions of the curves K=f.sub.n (HDP) corresponding to low
chamber pressure values HDP in the graph of FIG. 6 are used, the curves
K=f.sub.n (HDP) may be approximated by linear functions.
While in the illustrated embodiment Step S7 is provided, before Steps S8
and S9, to wait for supplying of each one pulse of the pulse wave signal
SM, i.e., pressure pulse wave, Step S7 may be omitted.
It is to be understood that the present invention may be embodied with
other changes, improvements, and modifications that may occur to those
skilled in the art without departing from the spirit and scope of the
present invention defined in the appended claims.
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