Peripheral blood-flow condition monitor

An apparatus for monitoring a peripheral blood-flow condition of a living subject by detecting a peripheral blood-flow resistance of the subject, including a first and a second pulse-wave sensor which are adapted to be worn on a first and a second portion of the subject, respectively, to detect a first and a second pulse wave, respectively, each of which is produced in synchronism with a heartbeat of the subject, a phase-difference determining device for determining a difference of respective phases of the first and second pulse waves detected by the first and second pulse-wave sensors, and a peripheral blood-flow resistance determining device for determining the peripheral blood-flow resistance of the subject, based on the phase difference determined by the phase-difference determining device, according to a predetermined relationship between peripheral blood-flow resistance and phase difference.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a monitor which monitors peripheral 
blood-flow condition of a living subject by determining peripheral 
blood-flow resistance values of the subject. 
2. Related Art Statement 
A blood pressure (BP) monitor may be used to monitor BP values of a patient 
in an operation room or an intensive care unit (ICU). Even when the BP 
monitor reads accurate BP values of the patient, it is, however, not 
determinable whether the blood appropriately circulates or flows through 
the patient. Hence, a medical person such as a doctor or nurse may need to 
monitor peripheral blood-flow condition of the patient by touching a hand 
or a foot of the patient and judging whether the peripheral portion or 
tissue of the patient has an extremely low temperature. 
Meanwhile, it has been proposed to measure BP values of a patient from each 
of his or her trunk and periphery portion and display respective time-wise 
trends of the two series of BP values, side by side, along a common time 
axis, so that a medical worker can quickly notice a possible significant 
change of the peripheral blood-flow condition of the patient. This 
technique is employed by a BP monitor disclosed in Japanese Patent 
Application filed by the Assignee of the present U.S. application and laid 
open for inspection purposes under Publication No. 61(1986)-119239. 
The above-identified BP monitor requires the medical person to compare two 
curves representing the two time-wise BP trends with each other and 
qualitatively judge whether the peripheral blood-flow condition of the 
patient has significantly largely changed. However, only persons who are 
well familiar with the monitor device can make accurate judgments. In 
addition, the prior BP monitor does not provide any quantitative reading 
of the peripheral blood-flow condition of the patient. In particular, in 
the case where a patient under general anesthesia is monitored, his or her 
peripheral blood-flow condition may largely change due to the excitation 
of his or her nervous system, the administration of BP controlling agents, 
and/or his or her current body temperature. Therefore, the reading of 
quantitative values of the peripheral blood-flow condition of a patient is 
very important. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a monitor 
which monitors, with accuracy, the peripheral blood-flow condition of a 
living subject by determining peripheral blood-flow resistance values of 
the subject. 
The above object has been achieved by the present invention, which provides 
an apparatus for monitoring a peripheral blood-flow condition of a living 
subject by detecting a peripheral blood-flow resistance of the subject, 
comprising a first and a second pulse-wave sensor which are adapted to be 
worn on a first and a second portion of the subject, respectively, to 
detect a first and a second pulse wave, respectively, each of which is 
produced in synchronism with a heartbeat of the subject, phase-difference 
determining means for determining a difference of respective phases of the 
first and second pulse waves detected by the first and second pulse-wave 
sensors, and peripheral blood-flow resistance determining means for 
determining the peripheral blood-flow resistance of the subject, based on 
the phase difference determined by the phase-difference determining means, 
according to a predetermined relationship between peripheral blood-flow 
resistance and phase difference. 
In the peripheral blood-flow condition monitor apparatus in accordance with 
the present invention, the peripheral blood-flow resistance determining 
means determines the peripheral blood-flow resistance of the subject, 
based on the phase difference determined by the phase-difference 
determining means, according to a predetermined relationship between 
peripheral blood-flow resistance and phase difference. The relationship 
may be predetermined based on experimental data, e.g., peripheral 
blood-flow resistance values and phase-difference values obtained from 
many people. The peripheral blood-flow resistance well reflects the 
peripheral blood-flow condition, such that higher peripheral blood-flow 
resistance values indicate worse peripheral blood-flow conditions and 
lower resistance values indicate better conditions. Thus, the present 
monitor apparatus monitors, with accuracy, the peripheral blood-flow 
condition of a living subject by determining the peripheral blood-flow 
resistance values of the subject. 
According to a preferred feature of the present invention, the monitor 
apparatus further comprises a blood pressure measuring device which 
measures a blood pressure of the subject, and the peripheral blood-flow 
resistance determining means determines the peripheral blood-flow 
resistance of the subject, based on the phase difference determined by the 
phase-difference determining means and the blood pressure measured by the 
blood pressure measuring device, according to the predetermined 
relationship defined by a function of phase difference and blood pressure 
as variables. Since a mathematical function of phase difference and blood 
pressure as variables is used as the relationship for determining the 
peripheral blood-flow resistance values of the subject, the present 
monitor apparatus monitors, with higher accuracy, the peripheral 
blood-flow condition of the subject. 
According to another feature of the present invention, the blood pressure 
measuring device comprises an inflatable cuff adapted to be wound around a 
body portion of the subject, and measuring means for measuring the blood 
pressure of the subject by changing a pressure of the cuff applied to the 
body portion of the subject, and the first pulse-wave sensor comprises the 
cuff, a pressure sensor which detects the pressure of the cuff, and a 
pulse-wave filter circuit which extracts, as the first pulse wave, an 
oscillatory pressure wave including a plurality of pulses produced in the 
cuff in synchronism with the heartbeat of the subject, from the cuff 
pressure detected by the pressure sensor. Since the cuff is used as not 
only a part of the blood pressure measuring means but also a part of the 
first pulse-wave sensor, the present monitor apparatus enjoys a simple 
construction. 
According to another feature of the present invention, the monitor 
apparatus further comprises a cuff-pressure regulating device which 
increases the pressure of the cuff up to a predetermined value lower than 
a diastolic blood pressure of the subject and holds the cuff pressure at 
the predetermined value, and wherein the pulse-wave filter circuit 
extracts, as the first pulse wave, a plurality of heartbeat-synchronous 
pulses produced in the cuff held at the predetermined value, from the cuff 
pressure detected by the pressure sensor. 
According to another feature of the present invention, the second 
pulse-wave sensor comprises a pressure pulse wave sensor which is adapted 
to be pressed against an artery of the subject via a skin of the subject, 
the pressure pulse wave sensor detecting, as the second pulse wave, a 
pressure pulse wave including a plurality of pulses produced from the 
artery of the subject in synchronism with the heartbeat of the subject. 
Each of the first and second pulse-wave sensor may otherwise be selected 
from the group consisting of an impedance-pulse-wave sensor which includes 
electrodes adapted to be held in contact with the surface of a body 
portion of a living subject and detects an impedance pulse wave as the 
change of impedance of the body portion; a supersonic-pulse-wave sensor 
which is held in contact with the surface of a body portion of a subject, 
emits supersonic wave toward an artery of the body portion via the 
surface, and detects, as a supersonic pulse wave, the displacement or 
vibration of the wall of the artery; or a photoelectric-pulse-wave sensor 
which is adapted to be worn on the surface of a body portion of a subject, 
emits light toward the body portion, and detects, as a photoelectric pulse 
wave, the light reflected from, or transmitted through, the body portion 
of the subject. 
According to another feature of the present invention, the monitor 
apparatus further comprises index-value determining means for determining 
an index value indicative of a characteristic of a waveform of a 
decreasing portion of a heartbeat-synchronous pulse of the second pulse 
wave detected by the second pulse-wave sensor, wherein the peripheral 
blood-flow resistance determining means comprises correcting means for 
correcting, based on the index value determined by the index-value 
determining means, the peripheral blood-flow resistance determined based 
on the phase difference determined by the phase-difference determining 
means. Thus, the present monitor apparatus monitors, with higher accuracy, 
the peripheral blood-flow resistance condition of the subject. 
According to another feature of the present invention, the monitor 
apparatus further comprises a display device which displays, along a time 
axis, a time-wise trend of respective values of the peripheral blood-flow 
resistance determined by the peripheral blood-flow resistance determining 
means. A medical person such as a doctor or nurse can easily notice a 
significant change of the peripheral blood-flow condition of the subject 
by viewing the screen image of the display device. 
According to another feature of the present invention, the phase-difference 
determining means comprises means for determining a phase difference of 
each of heartbeat-synchronous pulses of the first pulse wave and a 
corresponding one of heartbeat-synchronous pulses of the second pulse 
wave, and wherein the peripheral blood-flow resistance determining means 
successively determines, according to the predetermined relationship, a 
peripheral blood-flow resistance of the subject, based on the phase 
difference of the each heartbeat-synchronous pulse of the first pulse wave 
and the corresponding heartbeat-synchronous pulse of the second pulse wave 
.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1 to 11, there will be described a blood pressure (BP) 
monitor 8 to which the present invention is applied. The BP monitor 8 may 
be used to monitor BP values of a patient who is undergoing, or has 
undergone, a surgical operation. The BP monitor also functions as a 
peripheral blood-flow condition monitor as described below. 
In FIG. 1, the BP monitor 8 includes an inflatable cuff 10 including a 
rubber bag and a band-like cloth bag in which the rubber bag is 
accommodated. The cuff 10 is wound around, e.g., an upper arm 12 of a 
patient. The cuff 10 is connected via piping 20 to a pressure sensor 14, a 
selector valve 16, and a first air pump 18. The selector valve 16 is 
selectively placed, under control of an electronic control device 28, in a 
first state in which the valve 16 permits pressurized air to be supplied 
from the air pump 18 to the cuff 10 to increase quickly the air pressure 
of the cuff 10 (hereinafter, referred to as the "cuff pressure"), a second 
state in which the valve 16 causes the cuff 10 to be deflated slowly, and 
a third state in which the valve 16 causes the cuff 10 to be deflated 
quickly. 
The pressure sensor 14 detects the cuff pressure (i.e., the air pressure of 
the cuff 10), and generates a pressure signal, SP, representing the 
detected cuff pressure. The pressure signal SP is supplied to each of a 
static-pressure filter circuit 22 and a pulse-wave filter circuit 24. The 
static-pressure filter circuit 22 includes a low-pass filter which 
extracts, from the pressure signal SP, a cuff-pressure signal, SK, 
representative of a static or direct-current component of the pressure 
signal SP. The cuff-pressure signal SK is supplied via a first 
analog-to-digital (A/D) converter 26 to the control device 28. 
The pulse-wave filter circuit 24 includes a band-pass filter which 
extracts, from the pressure signal SP, a pulse-wave signal, SM.sub.1, 
representative of an oscillating or alternating-current component of the 
pressure signal SP, based on a frequency characteristic of the signal 
SM.sub.1. The pulse-wave signal SM.sub.1 is supplied via a second A/D 
converter 30 to the control device 28. The alternating-current component 
represented by the pulse-wave signal SM.sub.1 corresponds to an 
oscillatory pressure wave, i.e., pulse wave which is produced from a 
brachial artery (not shown) of the patient in synchronism with the 
heartbeat of the patient and is propagated via skin tissue to the cuff 10. 
This pulse wave is referred to as the "cuff pulse wave" to be 
distinguished from a "pressure pulse wave" which will be described later. 
An example of the cuff pulse wave is shown in an upper portion of the 
graph of FIG. 2. In the present embodiment, the cuff 10, the pressure 
sensor 14, and the pulse-wave filter circuit 24 cooperate with one another 
to provide a first pulse wave sensor 25 (FIG. 4). 
The control device 28 is provided by a microcomputer including a central 
processing unit (CPU) 29, a read only memory (ROM) 31, a random access 
memory (RAM) 33, and an input and output (I/O) port (not shown). The CPU 
29 processes input signals, including the signals SK, SM.sub.1, by 
utilizing the temporary-storage function of the RAM 33, according to 
control programs pre-stored in the ROM 31. In addition, the CPU 29 
supplies drive signals via the I/O port to drive circuits (not shown) 
associated with the selector valve 16 and the air pump 18, respectively. 
Thus, the CPU 29 controls respective operations of the valve 16 and the 
pump 18. For example, when an oscillometric BP measurement using the cuff 
10 is carried out to calibrate the present BP monitor 8, the CPU 29 
controls the valve 16 and the pump 18 to increase quickly the cuff 
pressure up to a predetermined target value and subsequently decrease the 
cuff pressure at a low rate of 2 to 3 mmHg/sec. Based on the variation of 
the cuff pulse wave represented by the pulse-wave signal SM.sub.1 provided 
by the pulse-wave filter circuit 24 during the low-rate decreasing of the 
cuff pressure, the CPU 29 determines a systolic and a diastolic BP value 
of the patient, according to a known oscillometric BP measuring method. In 
addition, the CPU 29 controls a display 32 to display the thus determined 
BP values. 
A pressure-pulse-wave (PPW) detecting probe 34 includes a container-like 
sensor housing 36, and a fastening band 40 connected to the sensor housing 
36. With the help of the fastening band 40, the PPW detecting probe 34 is 
detachably attached to a wrist 42 of the same arm 12 of the patient on 
which the cuff 10 is worn, or the other arm of the patient, such that an 
opening of the sensor housing 36 is opposed to a body surface 38 of the 
patient. A PPW sensor 46 is secured via an elastic diaphragm 44 to inner 
surfaces of the sensor housing 36 such that the PPW sensor 46 is movable 
relative to the housing 36 and is advanceable through the opening of the 
housing 36 toward the body surface 38 of the patient. The sensor housing 
36 and the diaphragm 44 cooperate with each other to define a pressure 
chamber 48, which is supplied with pressurized air from a second air pump 
50 via a pressure regulator valve 52. Thus, the PPW sensor 46 is pressed 
on the body surface 38 with a pressing force, P.sub.HD, corresponding to 
the air pressure of the pressure chamber 48. In the present embodiment, 
the pressing forces of the PPW sensor 46 applied to the body surface 38 or 
the radial artery 56 are indicated in terms of the pressure values (mmHg) 
of the pressure chamber 48. The sensor housing 36, the diaphragm 44, the 
pressure chamber 48, the second air pump 50, the pressure regulator valve 
52, etc. cooperate with one another to provide a pressing device which 
presses the PPW sensor 46 against the radial artery 56 via the body 
surface or skin tissue 38. 
The PPW sensor 46 includes a semiconductor chip formed of a monocrystalline 
silicon which has a press surface 54, and a number of pressure-sensing 
semiconductor elements (not shown) which are arranged, in the press 
surface 54, in an array at a regular interval of distance (about 0.2 mm), 
such that the array of pressure-sensing elements extends in the direction 
of width of the radial artery 56. When the PPW sensor 46 is pressed 
against the radial artery 56 via the body surface 38 of the wrist 42, the 
PPW sensor 46 detects an oscillatory pressure wave, i.e., pressure pulse 
wave (PPW) which is produced from the radial artery 56 in synchronism with 
the heartbeat of the patient and is propagated via the body surface 38 to 
the PPW sensor 46. The PPW sensor 46 generates a PPW signal, SM.sub.2, 
representing the detected PPW, and supplies the PPW signal SM.sub.2 to the 
control device 28 via a third A/D converter 58. An example of the pressure 
pulse wave (PPW) detected by the PPW sensor 46 is shown in a lower portion 
of the graph of FIG. 2, along the same time axis as that of the cuff pulse 
wave detected by the first pulse wave sensor 25 and shown in the upper 
portion of the graph. The PPW sensor 46 provides a second pulse wave 
sensor. 
The CPU 29 of the control device 28 processes the input signals, including 
the PPW signal SM.sub.2, by utilizing the temporary-storage function of 
the RAM 33, according to the control programs pre-stored in the ROM 31, 
and supplies drive signals to drive circuits (not shown) associated with 
the second air pump 50 and the pressure regulator valve 52, respectively. 
Thus, the CPU 29 controls respective operations of the pump 50 and the 
valve 52 to regulate the pressure of the pressure chamber 48 applied to 
the PPW sensor 46, i.e., the pressing force of the PPW sensor 46 applied 
to the radial artery 56 via the body surface or skin tissue 38. 
When a continuous BP monitoring operation is carried out, the CPU 29 
determines an optimum pressing force, P.sub.HDP, of the PPW sensor 46 
applied to the radial artery 56, based on the PPW detected by the PPW 
sensor 46 while the pressure of the pressure chamber 48 is slowly changed, 
and controls the pressure regulator valve 52 to maintain the pressure of 
the chamber 48 at the determined optimum pressing force P.sub.HDP. In 
addition, the CPU 29 determines a relationship between BP values and PPW 
magnitudes P.sub.M (i.e., voltage values of the PPW signal SM.sub.2), 
based on a systolic and a diastolic BP value, SAP, DAP, measured using the 
cuff 10 according the oscillometric BP measuring method, and a maximum and 
a minimum magnitude, P.sub.Mmax, P.sub.Mmin, of one heartbeat-synchronous 
pulse of the PPW detected by the PPW sensor 46 being pressed on the body 
surface 38 with the optimum pressing force P.sub.HDP. According to the 
thus determined relationship, the CPU 29 determines a systolic and a 
diastolic BP value (i.e., monitor BP values), MBP.sub.SYS, MBP.sub.DIA, of 
the patient, based on a maximum magnitude (i.e., upper-peak magnitude) 
P.sub.Mmax and a minimum magnitude (i.e., lower-peak magnitude), 
P.sub.Mmin, of each of successive heartbeat-synchronous pulses of the PPW 
detected by the PPW sensor 46 being pressed with the optimum pressing 
force P.sub.HDP. Subsequently, the CPU 29 controls the display 32 to 
successively display, for each heartbeat-synchronous pulse, the thus 
determined monitor BP values MBP.sub.SYS, MBP.sub.DIA, in digits, and 
continuously display the waveform of the PPW detected by the PPW sensor 
46. This waveform represents the instantaneous monitor BP values MBP of 
the patient. 
FIG. 3 shows an example of a relationship between BP values (monitor BP 
values MBP) and PPW magnitudes, P.sub.M, that is determined by the CPU 29. 
This relationship is expressed by the following linear function (1): 
EQU MBP=A.multidot.P.sub.M +B (1) 
where A is a constant corresponding to the slope of the linear function (1) 
and B is a constant corresponding to the intercept of the axis of ordinate 
indicative of the monitor BP values MBP. 
FIG. 4 illustrates various functions of the electronic control device 28 of 
the present BP monitor 8. The static-pressure filter circuit 22 cooperates 
with the control device 28 to provide a BP measuring device 72 which 
measures, according to the oscillometric BP measuring method (JIS T 1115; 
JIS is Japanese Industrial Standard), a systolic BP value SAP and a 
diastolic BP value DAP of a living subject based on the variation of 
respective amplitudes of heartbeat-synchronous pulses of the cuff pulse 
wave detected by the first pulse wave sensor 25 while the pressure of the 
cuff 10 is slowly increased or decreased at the rate of 2 to 3 mmHg/sec. 
The cuff pulse wave is represented by the pulse-wave signal SM.sub.1 
provided by the pulse-wave filter circuit 24. The PPW sensor 46 is 
preferably worn on the wrist of the other arm of the patient different 
from the arm 12 on which the cuff 10 is worn, and detects the PPW produced 
from the radial artery of the other arm. The PPW sensor 46 provides a 
second pulse wave sensor. The control device 28 functions as relationship 
determining means 74 for determining a MBP-P.sub.M relationship between 
monitor BP values MBP and PPW magnitudes P.sub.M that is expressed by the 
linear function (1) and is shown in FIG. 3, based on the PPW detected by 
the PPW sensor 46 and the BP values measured by the BP measuring device 
72. The control device 28 also functions as monitor-BP-value (MBP) 
determining means 76 for successively determining, according to the 
MBP-P.sub.M relationship, a monitor BP value MBP of the subject based on a 
magnitude of each of heartbeat-synchronous pulses of the PPW detected by 
the PPW sensor 46. The selector valve 16 and the first air pump 18 
cooperate with the control device 28 to provide a cuff-pressure regulating 
device 78 which regulates the air pressure of the cuff 10 (i.e., cuff 
pressure), that is detected by the pressure sensor 14 when each 
oscillometric BP measurement using the cuff 10 is carried out. The 
cuff-pressure regulating device 78 changes the cuff pressure according to 
a well-known procedure, so that the BP measuring device 72 can measure BP 
values of the patient using the cuff 10 at a regular interval of time and 
the relationship determining means 74 calibrates or updates the 
MBP-P.sub.M relationship based on the BP values measured using the cuff 
10. For example, the regulating device 78 increases the cuff pressure up 
to a target value, e.g., 180 mmHg, which is higher than an estimated 
systolic BP value of the patient and subsequently decreases the cuff 
pressure slowly at the rate of 2 to 3 mmHg/sec, during a measurement 
period in which BP values of the patient are determined by the BP 
measuring device 72 according to a well-known oscillometric BP determining 
algorithm. After the BP measuring operation, the regulating device 78 
quickly deflates the cuff 10. In addition, during a continuous BP 
monitoring operation, the regulating device 78 maintains the cuff pressure 
at a predetermined hold pressure sufficiently lower than a diastolic BP 
value DAP of the patient, so that the first pulse wave sensor 25 detects 
the cuff pulse wave (i.e., first pulse wave) from the cuff 10 being held 
at the predetermined hold pressure. 
Moreover, the control device 28 functions as phase-difference determining 
means 80. During a continuous BP monitoring operation in which monitor BP 
values MBP of the patient are successively determined by the MBP 
determining means 76, the phase-difference determining means 80 
successively determines a phase difference, D.sub.CP (msec), of each of 
heartbeat-synchronous pulses of the cuff pulse wave (first pulse wave) 
obtained from the cuff 10 being held at the above-described hold pressure 
and a corresponding one of heartbeat-synchronous pulses of the pressure 
pulse wave (second pulse wave) detected by the PPW sensor 46 from the 
radial artery 56. The phase differences D.sub.CP determined by the 
phase-difference determining means 80 are shown in the graph of FIG. 2. 
The first and second pulse wave sensors 25, 46 are worn on the different 
arms of the patient, respectively, or the two different portions of the 
same arm 12 of the patient, respectively. 
The control device 28 also functions as peripheral blood-flow resistance 
determining means 82 for successively determining a peripheral blood-flow 
resistance, R.sub.BF, of the patient, based on each phase difference 
determined by the phase-difference determining means 80, according to a 
predetermined relationship between peripheral blood-flow resistance 
R.sub.BF and phase difference D.sub.CP. The peripheral blood-flow 
resistance R.sub.BF determined by the peripheral blood-flow resistance 
determining means 82 is defined as an index, MBF/MAP, where MAP is a mean 
BP value (mmHg) of the patient and MBF is a mean blood flow rate (cm.sup.3 
/sec) at a peripheral portion or tissue of the patient. 
In the present embodiment, the peripheral blood-flow resistance determining 
means 82 determines the peripheral blood-flow resistance R.sub.BF based on 
a phase difference and a BP value of the patient, according to a function 
of phase difference D.sub.CP and blood pressure BP as variables shown in 
the graph of FIG. 5. FIG. 6 shows a basic relationship between peripheral 
blood-flow resistance R.sub.BF and phase difference D.sub.CP, and FIG. 7 
shows a relationship between blood pressure and phase difference D.sub.CP. 
The relationship shown in FIG. 5 is derived from the two relationships 
shown in FIGS. 6 and 7. 
The control device 28 further functions as index-value determining means 84 
for determining an index value indicative of a characteristic of the 
waveform of a decreasing portion of each of heartbeat-synchronous pulses 
of the PPW detected by the PPW sensor 46, so that the peripheral 
blood-flow resistance determining means 82 corrects, based on the index 
value determined by the index-value determining means 84, the peripheral 
blood-flow resistance R.sub.BF determined based on the phase difference 
determined by the phase-difference determining means 80. For example, the 
index value determined by the index-value determining means 84 may be a 
curvature of the waveform of a specific range of the decreasing portion of 
each pulse of the PPW, a time constant of the decreasing portion, or a 
value, %MAP. The waveform of the decreasing (or diastolic-period) portion 
of each PPW pulse reflects a flexibility or softness of the radial artery 
56. In particular, the index %MAP is defined as a/b.times.100% as 
indicated in the graph of FIG. 8, where b is an amplitude obtained by 
subtracting a minimum magnitude corresponding to a diastolic BP value DAP 
from a maximum magnitude corresponding to a systolic BP value SAP and a is 
a height of a magnitude corresponding to a mean BP value MAP, obtained by 
subtracting the minimum magnitude from a magnitude corresponding to the 
mean BP value MAP. The systolic, diastolic, and mean BP values SAP, DAP, 
MAP are measured by the BP measuring device 72. Otherwise, the value a may 
be defined as a height of the center of gravity of an area defined by the 
waveform of each pulse of the PPW and the base line corresponding to the 
diastolic BP value DAP. Since peripheral blood-flow resistance may be 
defined by a function of index %MAP as a variable, the peripheral 
blood-flow resistance determining means 82 may determine, according to 
that function, another or second peripheral blood-flow resistance of the 
subject based on an index value % MAP determined with respect to each 
pulse of the PPW, and may correct, based on each second peripheral 
blood-flow resistance, a corresponding first peripheral blood-flow 
resistance R.sub.BF determined based on a corresponding phase difference 
D.sub.CP. For example, each pair of first and second peripheral blood-flow 
resistance values are multiplied by a first weighed coefficient, .alpha. 
(0&lt;.alpha.&lt;1), and a second weighed coefficient, .beta. (0&lt;.beta.&lt;1, 
.alpha.+.beta.=1), respectively, to obtain two products the sum of which 
provides a corrected peripheral blood-flow resistance R.sub.BF. The 
control device 28 functions as R.sub.BF -trend displaying means 86 for 
controlling the display 32 to display, along a time axis in a screen image 
thereof, a time-wise trend of respective values of peripheral blood-flow 
resistance R.sub.BF successively determined and corrected by the 
peripheral blood-flow resistance determining means 82. FIG. 9 shows an 
example of time-wise trend of the peripheral blood-flow resistance values 
R.sub.BF. 
Next, there will be described the operation of the BP monitor 8 constructed 
as described above, by reference to the flow charts of FIGS. 10 and 11 
representing the control programs pre-stored in the ROM 31. 
First, at Step SA1, the CPU 29 of the control device 28 controls the second 
air pump 50 and the pressure regulator valve 52 to increase slowly the 
pressure of the pressure chamber 48, and determines, as an optimum 
pressing force P.sub.HDP, a pressure P.sub.HD of the chamber 48 when the 
PPW sensor 46 detects a maximum pulse having the greatest amplitude of 
respective amplitudes of all the pulses detected thereby during the slow 
increasing of the pressure of the chamber 48. Subsequently, the CPU 29 
maintains or holds the pressure of the chamber 48 at the thus determined 
optimum pressing force P.sub.HDP. Thus, the optimum pressing force 
P.sub.HDP is applied to the PPW sensor 46 to press the radial artery 56 
via the body surface 38. 
Next, the control of the CPU 29 proceeds with Step SA2 to start increasing 
the pressure of the cuff 10 for measuring actual BP values of the patient. 
Step SA2 corresponds to the cuff-pressure regulating device 78. Step SA2 
is followed by Step SA3 to carry out a known oscillometric BP determining 
algorithm. Specifically described, the selector valve 16 is switched to 
the first state and the first air pump 18 is operated, so the cuff 
pressure continues to increase up to a target pressure (e.g., 180 mmHg) 
higher than an estimated systolic BP value of the patient. Subsequently, 
the air pump 18 is stopped and the selector valve 16 is switched to the 
second state, so that the cuff pressure decreases at a predetermined low 
rate (e.g., about 3 mmHg/sec). Based on the variation of respective 
amplitudes of heartbeat-synchronous pulses of the cuff-pulse-wave (CPW) 
signal SM.sub.1 obtained during this slow decreasing of the cuff pressure, 
the CPU 29 determines a systolic, a mean, and a diastolic BP value SAP, 
MAP, DAP of the patient according to the oscillometric BP determining 
algorithm. More specifically, the CPU 29 determines, as the systolic BP 
value SAP, a cuff pressure at the time when the pulse amplitudes 
significantly largely increase, determines, as the diastolic BP value DAP, 
a cuff pressure at the time when the pulse amplitudes significantly 
largely decrease, and determines, as the mean BP value MAP, a cuff 
pressure at the time when the pulse amplitudes become maximum. In 
addition, the CPU 29 determines a pulse rate of the patient based on the 
interval of time between respective upper peaks of two successive 
heartbeat-synchronous pulses of the CPW signal SM.sub.1. The thus measured 
BP values and pulse rate are stored in the RAM 33 and displayed by the 
display device 32. Then, the selector valve 16 is switched to the third 
state and then to the first state, so that the cuff pressure is first 
quickly decreased and then is held at a hold pressure which is 
pre-determined to be sufficiently lower than the measured diastolic BP 
value DAP. Step SA3 corresponds to the BP measuring means 72. 
Subsequently, the control of the CPU 29 goes to Step SA4 to determine a 
relationship between monitor BP value MBP and magnitude P.sub.M of 
pressure pulse wave (i.e., voltage of the pressure-pulse-wave (PPW) signal 
SM.sub.2) as shown in FIG. 3. More specifically, the CPU 29 newly reads in 
one heartbeat-synchronous pulse of the PPW signal SM.sub.2 supplied from 
the PPW sensor 46, determines a maximum and a minimum magnitude P.sub.Max, 
P.sub.Min , of the one pulse, and determines the previously-indicated 
linear function (1) based on the systolic and diastolic BP values SAP, DAP 
of the patient measured at Step SA3 and the thus determined maximum and 
minimum magnitudes P.sub.Mmax, P.sub.Mmin , of the one pulse of the PPW 
signal SM.sub.2. Step SA4 corresponds to the relationship determining 
means 74. 
After the MBP-P.sub.M relationship shown in FIG. 3 is determined at Step 
SA4, the control of the CPU 29 goes to Step S5 to judge whether the CPU 29 
has read in one heartbeat-synchronous pulse of the PPW signal SM.sub.2 
supplied from the PPW sensor 46 being pressed at the optimum pressing 
force P.sub.HDP and has read in a corresponding heartbeat-synchronous 
pulse of the CPW signal SM.sub.1 obtained from the cuff 10 being held at 
the low hold pressure. If a negative judgment is made at Step SA5, the CPU 
29 waits for detecting one pulse of each of the PPW and CPW signals 
SM.sub.1, SM.sub.2. Meanwhile, if a positive judgment is made at Step SA5, 
the control of the CPU 29 goes to Step SA6 to determine a maximum 
(upper-peak) magnitude P.sub.Max and a minimum (lower-peak) magnitude 
P.sub.Mmin , of the one pulse of the PPW signal SM.sub.2. Step SA6 is 
followed by Step SA7 to determine a systolic and a diastolic BP value 
MBP.sub.SYS, MBP.sub.DIA (monitor BP values) of the patient, based on the 
maximum and minimum magnitudes P.sub.Mmax, P.sub.Mmin , of the one pulse 
of the PPW signal SM.sub.2 determined at Step SA6, according to the 
MBP-P.sub.M relationship determined at Step SA4. The CPU 29 controls the 
display device 32 to display, on its image screen, not only the thus 
determined monitor BP values MBP but also the waveform of the one pulse 
that is continuous with the waveforms of the previous pulses. Steps SA6 
and SA7 correspond to the MBP determining means 76. 
Subsequently, the control of the CPU 29 goes to Step SA8, i.e., peripheral 
blood-flow resistance determining routine represented by the flow chart of 
FIG. 11. At Step SA8-1, the CPU 29 determines a phase difference D.sub.CP 
(msec) of the one pulse of the CPW signal SM.sub.1 and the corresponding 
pulse of the PPW signal SM.sub.2, based on the signals SM1, SM2 read in at 
Step SA5. For example, the CPU 29 determines, as the phase difference 
D.sub.CP, a time interval between the respective Upper peaks of the one 
pulse of the CPW signal SM.sub.1 and the corresponding pulse of the PPW 
signal SM.sub.2, as shown in FIG. 2. Step SA8-1 corresponds to the 
phase-difference determining means 80. 
Step SA8-1 is followed by Step SA8-2 to determine an index value %MAP 
indicative of a characteristic of the waveform of a decreasing portion of 
the one pulse of the PPW signal SM.sub.2 read in at Step SA5. The CPU 29 
determines, as the index value %MAP, a ratio a/b (.times.100%) of a height 
or magnitude a of the center of gravity of the area of the one pulse to a 
pulse amplitude b of the one pulse, as shown in FIG. 8. The height or 
magnitude a of the center of gravity of the area of the one pulse 
substantially corresponds to the mean BP value MAP of the subject measured 
at Step SA3. Step SA8-2 corresponds to the index-value determining means 
84. Step SA8-2 is followed by Step SA8-3 to determine a peripheral 
blood-flow resistance R.sub.BF of the patient based on the actual phase 
difference D.sub.CP determined at Step SA8-1 and the BP value of the 
patient (the monitor or estimated BP value MBP determined at Step SA7, or 
the actual BP value AP measured at Step SA3), according to the 
relationship, shown in FIG. 5, which is pre-determined and is pre-stored 
in the ROM 31. In the case where the relationship is pre-determined based 
on systolic blood pressure values of human beings, the CPU 29 determines 
the resistance R.sub.BF based on the monitor systolic BP value MBP.sub.SYS 
or the actual systolic BP value SAP; and, in the case where the 
relationship is pre-determined based on diastolic blood pressure values of 
human beings, the CPU 29 determines the resistance R.sub.BF based on the 
monitor diastolic BP value MBP.sub.DIA or the actual diastolic BP value 
DAP. 
At Step SA8-3, the CPU 29 corrects, based on the index value %MAP 
determined at Step SA8-2, the resistance R.sub.BF determined according to 
the relationship shown in FIG. 5. The resistance R.sub.BF that relates to 
the flexibility or softness of the walls of arteries of the patient is 
also a function of index value %MAP and BP value of the patient. 
Accordingly, the CPU 29 determines a second peripheral blood-flow 
resistance R.sub.BF2 based on the index value %MAP solely, or the index 
value %MAP and BP value in combination. The CPU 29 determines a weighed 
average of the first and second resistances R.sub.BF, R.sub.BF2 by 
multiplying the two resistances R.sub.BF, R.sub.BF2 by a first and a 
second weighing coefficient .alpha., .beta. (0&lt;.alpha., .beta.&lt;1; 
.alpha.+.beta.=1), respectively, and summing the thus obtained two 
products. Step SA8-3 corresponds to the peripheral blood-flow resistance 
determining means 82. However, this correction may be omitted. 
Step SA8-3 is followed by Step SA8-4 to control the display device 32 to 
display, on the screen thereof, a time-wise trend of the corrected 
peripheral blood-flow resistance values R.sub.BF along an axis indicative 
of time as shown in FIG. 9. Step SA8-4 corresponds to the R.sub.BF -trend 
displaying means. 
Step SA8 is followed by Step SA9 to judge whether a predetermined period 
(i.e., calibration period) of about 10 to 20 minutes has elapsed after a 
BP measurement using the cuff 10 is carried out at Step SA3 in the current 
control cycle. If a negative judgment is made at Step SA9, the CPU 29 
repeats Step SA5 and the following steps including Step SA8, so that a 
monitor systolic BP value MBP.sub.SYS and a monitor diastolic BP value 
MBP.sub.DIA of the patient are determined for each heartbeat-synchronous 
pulse of the PPW signal SM.sub.2 and displayed by the display device 32. 
Meanwhile, if a positive judgment is made at Step SA9, the control of the 
CPU 29 goes back to Step SA2 and the following steps to update the 
MBP-P.sub.M relationship shown in FIG. 3. 
As is apparent from the foregoing description, in the present BP monitor 8, 
the phase difference D.sub.CP is determined based on the cuff pulse wave 
(first pulse wave) detected by the first pulse wave sensor 25 and the 
pressure pulse wave (second pulse wave) detected by the PPW sensor (second 
pulse wave sensor) 46, at Step SA8-1, and the peripheral blood-flow 
resistance R.sub.BF of the patient is determined based on the phase 
difference R.sub.BF according to the relationship shown in FIG. 5, at Step 
SA8-3. The first and second pulse wave sensors 25, 46 are worn on 
different portions of the patient. A higher resistance R.sub.BF indicates 
a worse peripheral blood-flow condition or state of the patient; and a 
lower resistance R.sub.BF indicates a better peripheral blood-flow 
condition of the patient. Thus, the resistance R.sub.BF well reflects the 
peripheral blood-flow condition of the patient. Thus, the present BP 
monitor can monitor, with high accuracy, the peripheral blood-flow 
condition of the patient. 
When the present BP monitor 8 determines a peripheral blood-flow resistance 
R.sub.BF of the patient at Step SA8-3, the CPU 29 utilizes the function of 
phase difference D.sub.CP and BP value as variables, shown in FIG. 5. That 
is, the CPU 29 determines, according to this function, a resistance 
R.sub.BF of the patient based on an actual BP value AP measured at Step 
SA3 or a monitor BP value MBP determined at Step SA7, in addition to the 
phase difference D.sub.CP determined at Step SA8-1. Thus, the BP monitor 8 
can determine, with higher accuracy, the peripheral blood-flow resistance 
values R.sub.BF of the subject. Resistance R.sub.BF is a function of not 
only phase difference D.sub.CP but also BP value. 
In addition, at Step SA8-2, the CPU 29 determines an index value %MAP 
indicative of a characteristic of the waveform of a decreasing portion of 
each heartbeat-synchronous pulse of the PPW signal SM.sub.2. At Step 
SA8-3, the CPU 29 corrects, based on the index value %MAP, the peripheral 
blood-flow resistance value R.sub.BF determined based on the phase 
difference D.sub.CP and the BP value of the subject according to the 
relationship shown in FIG. 5. Thus, the BP monitor 8 can determine, with 
still higher accuracy, the peripheral blood-flow resistance values 
R.sub.BF of the subject. 
Moreover, at Step SA8-4, the present BP monitor 8 controls the display 
device 32 to display a time-wise trend of the peripheral blood-flow 
resistance values R.sub.BF of the subject which have been determined for 
the respective heartbeat-synchronous pulses of the CPW or PPW signal 
SM.sub.1 or SM.sub.2 obtained at Step SA5. Thus, a medical person such as 
a doctor or nurse can easily read the time-wise change of the resistance 
R.sub.BF of the patient by just observing the screen image of the display 
device 32. 
The BP monitor 8 measures actual BP values AP of the patient by using the 
cuff 10 being worn on the patient, the pressure sensor 14 for detecting 
the cuff pressure, and the pulse-wave filter circuit 24 for extracting; 
from the cuff pressure detected by the sensor 14, the cuff pulse wave that 
is an oscillatory pressure wave produced in the cuff 10 in synchronism 
with the heartbeat of the patient. The cuff 10, the sensor 14, and the 
filter circuit 24 also function as the first pulse wave sensor 25 for 
detecting the cuff pulse wave as the first one of the two sorts of pulse 
waves whose phase difference is utilized to determine the peripheral 
blood-flow resistance R.sub.BF of the patient. Thus, no exclusive first 
pulse wave sensor is needed for detecting the first pulse wave and 
accordingly the BP monitor 8 enjoys a simple construction. 
Similarly, the PPW sensor 46 that is pressed against the artery 56 of the 
patient via the body surface 38 to detect the pressure pulse wave produced 
from the artery 56, also functions as the second pulse wave sensor for 
detecting the second pulse wave. Thus, no exclusive second pulse wave 
sensor is needed for detecting the second pulse wave and eventually 
determining the peripheral blood-flow resistance R.sub.BF of the patient, 
and accordingly the BP monitor 8 enjoys a simpler construction. 
While the present invention has been described in its preferred 
embodiments, the present invention may otherwise be embodied. 
For example, although in the illustrated embodiment the CPU 29 determines 
the peripheral blood-flow resistance R.sub.BF based on the phase 
difference D.sub.CP and the actual or monitor BP value AP or MBP according 
to the relationship (i.e., function of two variables D.sub.CP, AP (or MBP) 
shown in FIG. 5, it is possible to determine a peripheral blood-flow 
resistance of a patient based on a phase difference D.sub.CP according to 
the relationship (i.e., function of phase difference D.sub.CP) shown in 
FIG. 6. In addition, the CPU 29 may determine a peripheral blood-flow 
resistance based on an actual index value %MAP according to a function of 
index value %MAP and BP value AP (or MBP) or a function of index value 
%MAP. In the last case, the first pulse wave sensor 25 may be omitted. 
In the illustrated embodiment, the first pulse wave sensor 25 is provided 
by the cuff 10, the pressure sensor 14, and the pulse-wave filter circuit 
24 all of which are employed for measuring actual BP values of a patient, 
and the second pulse wave sensor is provided by the PPW sensor 46 which is 
employed for detecting the pressure pulse wave and continuously monitoring 
the blood pressure of the patient. One or each of the first and second 
pulse wave sensors employed for determining the phase difference D.sub.CP 
of the first and second pulse waves may be provided by an exclusive pulse 
wave sensor which is independent of the function of measuring the BP 
values of the patient or the function of detecting the pressure pulse wave 
of the patient. The exclusive sensor may be, for example, an 
impedance-pulse-wave sensor which includes electrodes adapted to be held 
in contact with the surface of a body portion of a patient and detects an 
impedance pulse wave as the change of impedance of the body portion; a 
supersonic-pulse-wave sensor which is held in contact with the surface of 
a body portion of a patient, emits supersonic wave toward an artery of the 
body portion via the surface, and detects, as a supersonic pulse wave, the 
displacement or vibration of the wall of the artery; or a 
photoelectric-pulse-wave sensor which is adapted to be worn on the surface 
of a body portion of a patient, emits light toward the body portion, and 
detects, as a photoelectric pulse wave, the light reflected from, or 
transmitted through, the body portion of the subject. The 
photoelectric-pulse-wave sensor may be one which is employed by a pulse 
oximeter or a pulse-rate meter. 
While in the illustrated embodiment the. CPU 29 determines, as the phase 
difference D.sub.CP, the difference of respective times of detection of 
respective upper peaks of each CPW pulse and each PPW possible to den in 
FIG. 2, it is possible to determine, as the phase difference D.sub.CP, the 
difference of respective times of detection of respective lower peaks of 
the two sorts of pulse waves. 
Although the BP monitor 8 measures actual BP values of a patient according 
to an oscillometric BP measuring method, it is possible to employ a 
microphone for detecting Korotkoff sounds produced from the arteries 
underlying the cuff 10 and measures actual BP values of a patient 
according to a Korotkoff-sound BP measuring method in which a systolic 
and/or a diastolic BP value of the patient are/is determined based on the 
appearing and/or disappearing of the Korotkoff sounds detected by the 
microphone while the cuff pressure is changed. 
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 
invention defined in the appended claims.