Abstrict
A blood pressure monitor system including a pressure sensor having
a press surface and including pressure sensing elements provided
in the press surface; a pressing device which presses the pressure
sensor against an artery via a body surface so that each pressure
sensing element measures a pressure value at the body surface; a
first device 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; a second device for changing pressing forces
of the pressing device, and determining a point of inflection of
a curve representing a relationship between the changed pressing
forces and pressure values measured by the pressure sensor at the
body surface; a third device for determining a correction value
based on the pressure value of the determined point of inflection;
and a blood-pressure determining device for operating the pressing
device to maintain the determined optimum pressing force and press
the pressure sensor against the artery via the body surface, and
continuously determining 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.
Claims
What is claimed is:
1. A blood pressure monitor system comprising:
a pressure sensor having a press surface and including at least
one pressure sensing element provided in said press surface;
a pressing device which presses said pressure sensor against an
arterial vessel of a living subject via a body surface of the subject
so that said pressure sensing element of the pressure sensor measures
a pressure value at said body surface of the subject;
pressing-force determining means for determining an optimum pressing
force of said pressing device at which a portion of a wall of said
arterial vessel of the subject is flattened under said pressure
sensor pressed by the pressing device;
inflection-point determining means for changing pressing forces
of said pressing device applied to said 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 said body surface of the
subject;
correction-value determining means for determining a correction
value based on the pressure value of the determined point of inflection;
and
blood-pressure determining means for operating said pressing device
to maintain the determined optimum pressing force and press said
pressure sensor against said arterial vessel of the subject via
said body surface of the subject, and continuously determining intra-arterial
blood pressure values of the arterial vessel of the subject by subtracting
said correction value from the pressure values measured by the pressure
sensor at the body surface of the subject.
2. A monitor system according to claim 1, wherein said 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 said pressing device;
selecting means for selecting one of said pressure correcting curves
which provides a same difference between a first correction value
corresponding to the pressing force of said determined point of
inflection and a second correction value corresponding to said determined
optimum pressing force of said pressing device, as an actual difference
between the pressure value of said point of inflection and the pressure
value corresponding to said optimum pressing force of the pressing
device; and
determining means for determining, as said correction value, said
second correction value corresponding to said optimum pressing force
of the pressing device, according to the selected one pressure correcting
curve.
3. A monitor system according to claim 2, wherein said memory of
said correction-value determining means stores said pressure correcting
curves each of which is defined by a quadratic function.
4. A monitor system according to claim 2, wherein said memory of
said correction-value determining means stores said pressure correcting
curves each of which is defined by a linear function.
5. A monitor system according to claim 1, wherein said pressure
sensor comprises a pulse wave sensor which has said press surface
and which detects a pressure pulse wave including a plurality of
successive pulses produced from said arterial vessel of said subject
in synchronism with heartbeats of the subject and transmitted to
said press surface of said pulse wave sensor via said body surface
of the subject.
6. A monitor system according to claim 5, wherein said pulse wave
sensor comprises a semiconductor chip having said press surface,
and a plurality of pressure sensing elements as said at least one
pressure sensing element, said pressure sensing elements being provided
in an array in said press surface of said semiconductor chip, said
array of pressure sensing elements being adapted to intersect said
arterial vessel of the subject with said pulse wave sensor being
pressed against the arterial vessel of the subject via said body
surface of the subject.
7. A monitor system according to claim 5, wherein said pressing
device comprises:
a housing having an open end;
a flexible diaphragm closing said open end of said housing and
thereby defining a fluid-tight chamber in the housing, said pressure
sensor being secured to an outer surface of said diaphragm;
a fluid supply which supplies a pressurized fluid to said fluid-tight
chamber of said housing to press said pulse wave sensor against
said arterial vessel of said subject via said body surface of the
subject; and
a pressure regulator which regulates a fluid pressure in said fluid-tight
chamber of said housing.
8. A monitor system according to claim 7, wherein said pressing-force
determining means comprises:
a fluid-pressure sensor which measures said fluid pressure in said
fluid-tight chamber of said housing; and
means for determining, as said optimum pressing force of said pressing
device, the fluid pressure measured by said fluid-pressure sensor
at a time when said pulse wave sensor has detected a maximum pulse
having a greatest amplitude of the successive pulses of said pressure
pulse wave detected by the pulse wave sensor while said fluid pressure
of said fluid-tight chamber is continuously changed.
9. A monitor system according to claim 5, wherein said inflection-point
determining means comprises means for determining, as said curve,
a curve representing a relationship between said changed pressing
forces of said pressing device and provisional diastolic blood pressure
values measured by said pulse wave sensor at said body surface of
said subject, said provisional diastolic blood pressure values corresponding
to respective lower peaks of the successive pulses of said pressure
pulse wave detected by the pulse wave sensor while the pressing
force of the pressing device is continuously changed.
10. A monitor system according to claim 5, wherein said inflection-point
determining means comprises means for determining, as said curve,
a curve representing a relationship between said changed pressing
forces of said pressing device and provisional systolic blood pressure
values measured by said pulse wave sensor at said body surface of
said subject, said provisional systolic blood pressure values corresponding
to respective upper peaks of the successive pulses of said pressure
pulse wave detected by the pulse wave sensor while the pressing
force of the pressing device is continuously changed.
11. A monitor system according to claim 5, wherein said blood-pressure
determining means comprises means for successively determining,
as said intra-arterial blood pressure values of said arterial vessel
of said subject, at least one of a systolic and a diastolic blood
pressure value by subtracting said correction value from at least
one of provisional systolic and diastolic blood pressure values
corresponding to the upper and lower peaks of each of the successive
pulses of said pressure pulse wave detected by the pulse wave sensor.
12. A monitor system according to claim 11, further comprising
an output device including at least one of (a) a display which displays
at least one of (i) digits representing each of the successively
determined values of said at least one of said systolic and diastolic
blood pressure, (ii) symbols representing a time-wise change of
said successively determined values of said at least one of said
systolic and diastolic blood pressure, (iii) a waveform of the continuously
determined intra-arterial blood pressure, and (b) a recorder which
records, on a recording medium, at least one of (i) digits representing
each of the successively determined values of said at least one
of said systolic and diastolic blood pressure, (ii) symbols representing
a time-wise change of said successively determined values of said
at least one of said systolic and diastolic blood pressure, (iii)
a waveform of the continuously determined intra-arterial blood pressure.
13. A monitor system according to claim 1, wherein said inflection-point
determining means comprises means for determining, as said point
of inflection, a point of said curve between a plateau region of
said curve and an increasing portion of said curve, where the slopes
of the plateau region and the increasing portion significantly differ.
Description 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):
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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