A blood-pressure monitoring apparatus including a measuring device which includes an inflatable cuff adapted to apply a pressing pressure to a body portion of a living subject and which measures at least one blood-pressure value of the living subject by changing the pressing pressure of the inflatable cuff, a blood-pressure-relating-value calculating device for iteratively calculating a blood-pressure-relating value relating to the blood pressure of the living subject, a starting device for starting, when a value based on the calculated blood-pressure-relating value satisfies a predetermined first condition, a blood-pressure measurement of the measuring device, a circulatory-system-relating information obtaining device which obtains a circulatory-system-relating information relating to a circulatory system of the living subject, and a condition changing device for changing, when the obtained circulatory-system-relating information satisfies a predetermined second condition, the predetermined first condition to a changed first condition which at least one of respective values based on a plurality of blood-pressure-relating values calculated by the blood-pressure-relating-value calculating device earlier satisfies than satisfying the predetermined first condition so that the starting device earlier starts the blood-pressure measurement of the measuring device.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a blood-pressure monitoring apparatus
 which monitors a blood pressure of a living subject.
 2. Related Art Statement
 As information relating to a pulse wave which propagates through an
 arterial vessel of a living subject, there are known a
 pulse-wave-propagation time DT and a pulse-wave-propagation velocity
 V.sub.M (m/s). The pulse-wave-propagation time DT is a time which is
 needed by a pulse wave to propagate between two different positions of the
 arterial vessel. Additionally, it is known that the above
 pulse-wave-propagation-relating information is, within a predetermined
 range, substantially proportional to the blood pressure ("BP", mmHg) of
 the living subject. Therefore, there has been proposed a BP monitoring
 apparatus which determines, in advance, coefficients a, B in the following
 expression: EBP=.alpha.(DT)+.beta. (where .alpha. is a negative constant
 and .beta. is a positive constant), or EBP=.alpha.(V.sub.M)+.beta. (where
 a and .beta. are positive constants), based on two measured BP values of
 the subject and two measured pulse-wave-propagation time values (DT) or
 two measured pulse-wave-propagation velocity values (V.sub.M), iteratively
 determines an estimated BP value EBP of the subject, based on each set of
 subsequently obtained pulse-wave-propagation-relating information,
 according to the above-indicated first or second expression, and starts a
 BP measurement using an inflatable cuff when an abnormal estimated BP
 value EBP is determined. An example of the BP monitoring apparatus is
 disclosed in U.S. Pat. No. 5,752,920.
 Meanwhile, the elasticity or flexibility of blood vessels of a patient as a
 living subject may be lost due to arteriosclerosis or temporary
 constriction of the blood vessels. In this case, the BP of the patient
 cannot be easily controlled. Therefore, it is needed to quickly measure a
 BP value of the patient and quickly make a decision about whether or not
 any treatments should be given to the patient. This is also the case with
 a patient who has fallen in shock because of excessive expansion of the
 blood vessels upon administration of a hypotensive drug.
 In addition, a relationship between pulse-wave-propagation-relating
 information and estimated BP value EBP changes depending upon a hardness
 of a blood vessel for which the pulse-wave-propagation-relating
 information is obtained. That is, the coefficient .alpha. of each of the
 above-indicated two expressions that defines the relationship between
 pulse-wave-propagation-relating information and estimated BP value EBP
 changes depending upon the hardness of the blood vessel. Therefore, if the
 hardness of the blood vessel largely changes while estimated BP values EBP
 are iteratively determined for monitoring the BP of the patient, the
 accuracy of the estimated BP values EBP decreases, which may lead to
 delaying commencement of a BP measurement using the inflatable cuff.
 SUMMERY OF THE INVENTION
 It is therefore an object of the present invention to provide a
 blood-pressure monitoring apparatus which early starts a blood-pressure
 measurement using an inflatable cuff, when information relating to the
 circulatory system of a living subject satisfies a predetermined
 condition.
 The present invention provides a blood-pressure monitoring apparatus which
 has one or more of the technical features that are described below in
 respective paragraphs given parenthesized sequential numbers (1) to (21).
 Any technical feature which includes another technical feature shall do so
 by referring, at the beginning, to the parenthesized sequential number
 given to that technical feature.
 (1) According to a first feature of the present invention, there is
 provided a blood-pressure monitoring apparatus comprising a measuring
 device which includes an inflatable cuff adapted to apply a pressing
 pressure to a body portion of a living subject and which measures at least
 one blood-pressure value of the living subject by changing the pressing
 pressure of the inflatable cuff; a
 pulse-wave-propagation-relating-information obtaining device which
 iteratively obtains a piece of pulse-wave-propagation-relating information
 relating to propagation of a pulse wave through an arterial vessel of the
 living subject; estimating means for iteratively estimating a
 blood-pressure value of the living subject, based on each piece of
 pulse-wave-propagation-relating information of a plurality of pieces of
 pulse-wave-propagation-relating information obtained by the
 pulse-wave-propagation-relating-information obtaining device, according to
 a predetermined relationship between pulse-wave-propagation-relating
 information and blood pressure; starting means for starting, when a value
 based on the estimated blood-pressure value does not fall within a
 predetermined first range, a blood-pressure measurement of the measuring
 device; index-value calculating means for calculating, based on the at
 least one blood-pressure value of the living subject measured by the
 measuring device, an index value indicative of a hardness of a blood
 vessel of the living subject; and range changing means for changing, when
 the calculated index value does not fall within a predetermined second
 range, the predetermined first range to a changed first range which is
 contained in the predetermined first range and is narrower than the
 predetermined first range. In the present blood-pressure ("BP") monitoring
 apparatus, the range changing means changes, when the calculated index
 value does not fall within the predetermined second range, the
 predetermined first range to the changed first range which is contained in
 the predetermined first range and is narrower than the predetermined first
 range. Thus, the starting means can earlier start the blood-pressure
 measurement of the measuring device. For example, in the case where the
 elasticity or flexibility of blood vessels of the living subject has been
 lost or the blood vessels of the subject are excessively expanded, the
 measuring device can earlier measure a BP value of the subject.
 (2) According to a second feature of the present invention that includes
 the first feature (1), the blood-pressure monitoring apparatus further
 comprises relationship determining means for determining the relationship
 between pulse-wave-propagation-relating information and blood pressure,
 based on at least one blood- pressure value of the living subject measured
 by the measuring device and at least one piece of
 pulse-wave-propagation-relating information obtained by the
 pulse-wave-propagation-relating information obtaining device.
 (3) According to a third feature of the present invention, there is
 provided a blood-pressure monitoring apparatus comprising a measuring
 device which includes an inflatable cuff adapted to apply a pressing
 pressure to a body portion of a living subject and which measures at least
 one blood-pressure value of the living subject by changing the pressing
 pressure of the inflatable cuff; a
 pulse-wave-propagation-relating-information obtaining device which
 iteratively obtains a piece of pulse-wave-propagation-relating information
 relating to propagation of a pulse wave through an arterial vessel of the
 living subject; estimating means for iteratively estimating a
 blood-pressure value of the living subject, based on each piece of
 pulse-wave-propagation-relating information of a plurality of pieces of
 pulse-wave-propagation-relating information obtained by the
 pulse-wave-propagation-relating-information obtaining device, according to
 a predetermined relationship between pulse-wave-propagation-relating
 information and blood pressure; starting means for starting, when a value
 based on to the estimated blood-pressure value does not fall within a
 predetermined first range, a blood-pressure measurement of the measuring
 device; index-value calculating means for calculating, based on the at
 least one blood-pressure value of the living subject measured by the
 measuring device, an index value indicative of a hardness of a blood
 vessel of the living subject; change-value calculating means for
 calculating a change value relating to a change of a first index value
 calculated by the index-value calculating means from a second index value
 calculated prior to the first index value by the index-value calculating
 means; and range changing means for changing, when the calculated change
 value does not fall within a predetermined second range, the predetermined
 first range to a changed first range which is contained in the
 predetermined first range and is narrower than the predetermined first
 range. In the present BP monitoring apparatus, the range changing means
 changes, when the calculated change value does not fall within the
 predetermined second range, the predetermined first range to the changed
 first range which is contained in the predetermined first range and is
 narrower than the predetermined first range. Thus, the starting means can
 earlier start the blood-pressure measurement of the measuring device. For
 example, in the case where the hardness of blood vessels of the living
 subject largely changes and the accuracy of the estimated BP value or
 values decreases during the monitoring of BP of the living subject, the
 measuring device can earlier measure a BP value of the subject.
 (4) According to a fourth feature of the present invention, there is
 provided a blood-pressure monitoring apparatus comprising a measuring
 device which includes an inflatable cuff adapted to apply a pressing
 pressure to a first body portion of a living subject and which measures at
 least one blood-pressure value of the living subject by changing the
 pressing pressure of the inflatable cuff; a
 pulse-wave-propagation-relating-information obtaining device which
 iteratively obtains a piece of pulse-wave-propagation-relating information
 relating to propagation of a pulse wave through an arterial vessel of the
 living subject; estimating means for iteratively estimating a
 blood-pressure value of the living subject, based on each piece of
 pulse-wave-propagation-relating information of a plurality of pieces of
 pulse-wave-propagation-relating information obtained by the
 pulse-wave-propagation-relating-information obtaining device, according to
 a predetermined relationship between pulse-wave-propagation-relating
 information and blood pressure; starting means for starting, when a value
 based on the estimated blood-pressure value does not fall within a
 predetermined range, a blood-pressure measurement of the measuring device;
 a photoelectric-pulse-wave obtaining device which is adapted to be worn on
 a second body portion of the living subject, and which emits, toward the
 second body portion, a first light exhibiting different absorption factors
 with respect to oxygenated hemoglobin and reduced hemoglobin,
 respectively, and a second light exhibiting substantially same absorption
 factors with respect to the oxygenated hemoglobin and the reduced
 hemoglobin, respectively, and obtains a first and a second photoelectric
 pulse wave from the first and second lights received from the second body
 portion, respectively; blood-oxygen-saturation calculating means for
 calculating, based on the obtained first and second photoelectric pulse
 waves, a blood oxygen saturation value of the second body portion of the
 living subject; and range changing means for changing, when the calculated
 blood oxygen saturation value is smaller than a predetermined value, the
 predetermined range to a changed range which is contained in the
 predetermined range and is narrower than the predetermined range, the
 range changing means determining the changed range based on a difference
 between the calculated blood oxygen saturation value and the predetermined
 value. In the present BP monitoring apparatus, the range changing means
 changes, when the calculated blood oxygen saturation value is smaller than
 the predetermined value, the predetermined range to the changed range
 which is contained in the predetermined range and is narrower than the
 predetermined range, and determines the changed range based on the
 difference between the calculated blood oxygen saturation value and the
 predetermined value. Thus, the starting means can earlier start the
 blood-pressure measurement of the measuring device, by a time
 corresponding to the degree of abnormality of the blood oxygen saturation
 of the living subject. The oxygen saturation of blood of a peripheral
 blood vessel of the subject may decrease prior to the change of BP of the
 subject, when the hardness of the blood vessel largely changes. Thus, the
 measuring device can earlier measure a BP value of the subject.
 (5) According to a fifth feature of the present invention, there is
 provided a blood-pressure monitoring apparatus comprising a measuring
 device which includes an inflatable cuff adapted to apply a pressing
 pressure to a first body portion of a living subject and which measures at
 least one blood-pressure value of the living subject by changing the
 pressing pressure of the inflatable cuff; blood-pressure-relating-value
 calculating means for iteratively calculating a blood-pressure-relating
 value relating to the blood pressure of the living subject; starting means
 for starting, when a value based on the calculated blood-pressure-relating
 value satisfies a predetermined first condition, a blood-pressure
 measurement of the measuring device; a circulatory-system-relating
 information obtaining device which obtains a circulatory-system-relating
 information relating to a circulatory system of the living subject; and
 condition changing means for changing, when the obtained
 circulatory-system-relating information satisfies a predetermined second
 condition, the predetermined first condition to a changed first condition
 which at least one of respective values based on a plurality of
 blood-pressure-relating values calculated by the
 blood-pressure-relating-value calculating means earlier satisfies than
 satisfying the predetermined first condition so that the starting means
 earlier starts the blood-pressure measurement of the measuring device. In
 the present BP monitoring apparatus, the range changing means changes,
 when the obtained circulatory-system-relating information satisfies the
 predetermined second condition, the predetermined first condition to the
 changed first condition which at least one of respective values based on a
 plurality of blood-pressure-relating values calculated by the
 blood-pressure-relating-value calculating means earlier satisfies than
 satisfying the predetermined first condition. Thus, the starting means
 earlier starts the blood-pressure measurement of the measuring device, and
 the measuring device can earlier measure a BP value of the subject.
 (6) According to a sixth feature of the present invention that includes the
 fifth feature (5), the starting means comprises means for starting the
 blood-pressure measurement of the measuring device, when the value based
 on the calculated blood-pressure-relating value satisfies the
 predetermined first condition selected from the group consisting of (a)
 the value based on the calculated blood-pressure-relating value does not
 fall within a predetermined first range, (b) the value based on the
 calculated blood-pressure-relating value is greater than a predetermined
 first value, and (c) the value based on the calculated
 blood-pressure-relating value is smaller than a predetermined second
 value.
 (7) According to a seventh feature of the present invention that includes
 the sixth feature (6), the condition changing means comprises means for
 changing the predetermined first condition to the changed first condition
 selected from the group consisting of (d) the at least one of the
 respective values based on the plurality of blood-pressure-relating values
 does not fall within a changed first range which is contained in the
 predetermined first range and is narrower than the predetermined first
 range, (e) the at least one of the respective values based on the
 plurality of blood-pressure-relating values is greater than a changed
 first value smaller than the predetermined first value, and (f) the at
 least one of the respective values based on the plurality of
 blood-pressure-relating values is smaller than a changed second value
 greater than the predetermined second value.
 (8) According to an eighth feature of the present invention that includes
 the seventh feature (7), the circulatory-system-relating-information
 obtaining device comprises index-value calculating means for calculating,
 based on the at least one blood-pressure value of the living subject
 measured by the measuring device, an index value indicative of a hardness
 of a blood vessel of the living subject, and wherein the condition
 changing means comprises means for changing the predetermined first
 condition to the changed first condition, when the calculated index value
 satisfies the predetermined second condition selected from the group
 consisting of (g) the calculated index value does not fall within a
 predetermined second range, (h) the calculated index value is greater than
 a predetermined third value, and (i) the calculated index value is smaller
 than a predetermined fourth value.
 (9) According to a ninth feature of the present invention that includes the
 eighth feature (8), the blood-pressure-relating value calculating means
 comprises a pulse-wave-propagation-relating-information obtaining device
 which iteratively obtains a piece of pulse-wave-propagation-relating
 information relating to propagation of a pulse wave through an arterial
 vessel of the living subject; and estimating means for estimating a
 blood-pressure value of the living subject, based on each piece of
 pulse-wave-propagation-relating information of a plurality of pieces of
 pulse-wave-propagation-relating information obtained by the
 pulse-wave-propagation-relating-information obtaining device, according to
 a predetermined relationship between pulse-wave-propagation-relating
 information and blood pressure, and the index-value calculating means
 comprises means for calculating the index value indicative of the hardness
 of the blood vessel of the living subject, based on the blood-pressure
 value of the subject measured by the measuring device and the piece of
 pulse-wave-propagation-relating information obtained by the
 pulse-wave-propagation-relating-information obtaining device when the
 blood-pressure value of the subject is measured by the measuring device.
 In this case, since the index-value calculating means calculates the index
 value based on the pulse-wave-propagation-relating information that is
 used by the estimating means for estimating the blood-pressure value of
 the living subject, the calculated index value enjoys the accuracy of the
 pulse-wave-propagation-relating information.
 (10) According to a tenth feature of the present invention that includes
 any one of the seventh to ninth features (7) to (9), the
 circulatory-system-relating-information obtaining device comprises
 index-value calculating means for calculating, based on the at least one
 blood-pressure value of the living subject measured by the measuring
 device, an index value indicative of a hardness of a blood vessel of the
 living subject; and change-value calculating means for calculating a
 change value relating to a change of a first index value calculated by the
 index-value calculating means from a second index value calculated prior
 to the first index value by the index-value calculating means, and the
 condition changing means comprises means for changing the predetermined
 first condition to the changed first condition, when the calculated change
 value satisfies the predetermined second condition selected from the group
 consisting of (g) the calculated change value does not fall within a
 predetermined second range, (h) the calculated change value is greater
 than a predetermined third value, and (i) the calculated change value is
 smaller than a predetermined fourth value.
 (11) According to an eleventh feature of the present invention that
 includes any one of the seventh to tenth features (7) to (10), the
 circulatory-system-relating-information obtaining device comprises a
 photoelectric-pulse-wave obtaining device which is adapted to be worn on a
 second body portion of the living subject, and which emits, toward the
 second body portion, a first light exhibiting different absorption factors
 with respect to oxygenated hemoglobin and reduced hemoglobin,
 respectively, and a second light exhibiting substantially same absorption
 factors with respect to the oxygenated hemoglobin and the reduced
 hemoglobin, respectively, and obtains a first and a second photoelectric
 pulse wave from the first and second lights received from the second body
 portion, respectively; and blood-oxygen-saturation calculating means for
 calculating, based on the obtained first and second photoelectric pulse
 waves, a blood oxygen saturation value of the second body portion of the
 living subject; and the condition changing means comprises means for
 changing the predetermined first condition to the changed first condition,
 when the calculated blood oxygen saturation value satisfies the
 predetermined second condition that the calculated blood oxygen saturation
 value is smaller than a predetermined third value.
 (12) According to a twelfth feature of the present invention that includes
 the eleventh feature (11), the condition changing means comprises means
 for changing, when the calculated blood oxygen saturation value is smaller
 than the predetermined third value, the predetermined first condition to
 the changed first condition that the at least one of the respective values
 based on the plurality of blood-pressure-relating values does not fall
 within the changed first range; and means for determining the changed
 first range based on a difference between the calculated blood oxygen
 saturation value and the predetermined third value.
 (13) According to a thirteenth feature of the present invention that
 includes any one of the fifth to twelfth features (5) to (12), the
 blood-pressure-measurement starting means comprises periodically starting
 means for periodically starting a blood-pressure measurement of the
 measuring device at a predetermined period.
 (14) According to a fourteenth feature of the present invention that
 includes the thirteenth feature (13), the blood-pressure monitoring
 apparatus further comprises period changing means for changing, when the
 blood-pressure value of the living subject measured by the measuring
 device is lower than a reference value, the predetermined period to a
 changed period shorter than the predetermined period so that the
 periodically starting means starts a blood-pressure measurement of the
 measuring device at the changed period. In this case, the measuring device
 can earlier measure a BP value of the living subject.
 (15) According to a fifteenth feature of the present invention that
 includes any one of the fifth to fourteenth features (5) to (14), the
 blood-pressure-relating-value calculating means comprises a
 pulse-wave-propagation-relating-information obtaining device which
 iteratively obtains a piece of pulse-wave-propagation-relating information
 relating to propagation of a pulse wave through an arterial vessel of the
 living subject; relationship determining means for determining a
 relationship between pulse-wave-propagation-relating information and blood
 pressure, based on at least one blood-pressure value of the living subject
 measured by the measuring device and at least one piece of
 pulse-wave-propagation-relating information obtained by the
 pulse-wave-propagation-relating-information obtaining device; and
 estimating means for iteratively estimating, as the calculated
 blood-pressure-relating value, a blood-pressure value of the living
 subject, based on each piece of pulse-wave-propagation-relating
 information of a plurality of pieces of pulse-wave-propagation-relating
 information obtained by the pulse-wave-propagation-relating-information
 obtaining device, according to the determined relationship between
 pulse-wave-propagation-relating information and blood pressure.
 (16) According to a sixteenth feature of the present invention that
 includes the fifteenth feature (15), the
 pulse-wave-propagation-relating-information obtaining device comprises at
 least one of pulse-wave-propagation-time calculating means for iteratively
 calculating a pulse-wave propagation time which is needed for each of a
 plurality of heartbeat-synchronous pulses of the pulse wave to propagate
 between two portions of the arterial vessel of the living subject, and
 pulse-wave-propagation-velocity calculating means for iteratively
 calculating a pulse-wave propagation velocity at which each of a plurality
 of heartbeat-synchronous pulses of the pulse wave propagates between two
 portions of the arterial vessel of the living subject.
 (17) According to a seventeenth feature of the present invention that
 includes any one of the fifth to sixteenth features (5) to (16), the
 blood-pressure-relating-value calculating means comprises at least one of
 pulse-period calculating means for iteratively calculating, as the
 calculated blood-pressure-relating value, a pulse period equal to a time
 interval between each pair of successive heartbeat-synchronous pulses of a
 pulse wave obtained from the living subject, and
 pulse-wave-area-relating-value calculating means for iteratively
 calculating, as the calculated blood-pressure-relating value, a
 pulse-wave-area-relating value relating to an area of each of a plurality
 of heartbeat-synchronous pulses of a pulse wave obtained from the living
 subject.
 (18) According to an eighteenth feature of the present invention that
 includes any one of the fifth to seventeenth features (5) to (17), the
 blood-pressure-relating-value calculating means comprises at least one of
 an electrocardiographic-pulse-wave detecting device which includes a
 plurality of electrodes adapted to be put on a plurality of portions of
 the living body and detects an electrocardiographic pulse wave including a
 plurality of heartbeat-synchronous pulses, from the subject via the
 electrodes, and a photoelectric-pulse-wave detecting device which is
 adapted to be worn on a second body portion of the living subject, and
 which emits a light toward the second body portion and obtains a
 photoelectric pulse wave including a plurality of heartbeat-synchronous
 pulses, from the light received from the second body portion.
 (19) According to a nineteenth feature of the present invention that
 includes any one of the fifth to seventeenth features (5) to (18), the
 starting means comprises means for starting, when the calculated
 blood-pressure-relating value satisfies the predetermined first condition,
 the blood-pressure measurement of the measuring device.
 (20) According to a twentieth feature of the present invention that
 includes any one of the fifth to seventeenth features (5) to (18), the
 starting means comprises means for calculating, as the value based on the
 calculated blood-pressure-relating value, a change value relating to a
 change of a first calculated blood-pressure-relating value calculated by
 the blood-pressure-relating-value calculating means from a second
 blood-pressure-relating value calculated prior to the first calculated
 blood-pressure-relating value by the blood-pressure-relating-value
 calculating means; and means for starting, when the calculated change
 value satisfies the predetermined first condition, the blood-pressure
 measurement of the measuring device.
 (21) According to a twenty-first feature of the present invention that
 includes any one of the fifth to twentieth features (5) to (20), the
 blood-pressure monitoring apparatus further comprises an informing device
 which informs, when the value based on the calculated
 blood-pressure-relating value satisfies the predetermined first condition,
 a user of an occurrence of an abnormality to the living subject.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to FIG. 1, there will be described a blood-pressure ("BP")
 monitoring apparatus 8 embodying the present invention.
 In FIG. 1, the BP monitoring apparatus 8 includes an inflatable cuff 10
 which has a belt-like cloth bag and a rubber bag accommodated in the cloth
 bag and which is adapted to be wrapped around, e.g., a right upper arm 12
 of a patient as a living subject, and a pressure sensor 14, a selector
 valve 16 and an air pump 18 each of which is connected to the cuff 10 via
 piping 20. The selector valve 16 is selectively placed in an inflation
 position in which the selector valve 16 permits a pressurized air to be
 supplied from the air pump 18 to the cuff 10, a slow-deflation position in
 which the selector valve 16 permits the pressurized air to be slowly
 discharged from the cuff 10, and a quick-deflation position in which the
 selector valve 16 permits the pressurized air to be quickly discharged
 from the cuff 10.
 The pressure sensor 14 detects an air pressure in the inflatable cuff 10,
 and supplies a pressure signal SP representative of the detected pressure
 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 and extracts, from the pressure signal SP, a static component
 contained in the signal SP, i.e., cuff-pressure signal SK representative
 of the static cuff pressure. The cuff-pressure signal SK is supplied to an
 electronic control device 28 via an analog-to-digital ("A/D") converter
 26. The pulse-wave filter circuit 24 includes a band-pass filter and
 extracts, from the pressure signal SP, an oscillatory component having
 predetermined frequencies, i.e., cuff-pulse-wave signal SM.sub.1. The
 cuff-pulse-wave signal SM.sub.1 is supplied to the control device 28 via
 an A/D converter 30. The cuff-pulse-wave signal SM.sub.1 is representative
 of the cuff pulse wave, i.e., oscillatory pressure wave which is produced
 from a brachial artery (not shown) of the patient in synchronism with the
 heartbeat of the patient and is propagated to the inflatable cuff 10.
 The control device 28 is provided by a so-called 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 signals according, to control programs
 pre-stored in the ROM 31 by utilizing a temporary-storage function of the
 RAM 33, and supplies drive signals to the selector valve 16 and the air
 pump 18 through the I/O port.
 The BP monitoring apparatus 8 further includes an electrocardiographic
 (ECG) pulse wave detecting device 34 which continuously detects an ECG
 pulse wave representative of the action potential of cardiac muscle of the
 patient, through a plurality of electrodes 36 being put on predetermined
 body portions of the patient, and supplies an ECG-pulse-wave signal
 SM.sub.2 representative of the detected ECG pulse wave, to the control
 device 28. The ECG-pulse-wave detecting device 34 is used for detecting a
 Q-wave or an R-wave of each heartbeat-synchronous pulse of the ECG pulse
 wave that corresponds to a time point when the outputting of blood from
 the heart of the patient toward the aorta is started. Thus, the
 ECG-pulse-wave detecting device 34 functions as a first pulse-wave
 detecting device.
 The BP monitoring apparatus 8 further includes a photoelectric-pulse-wave
 detecting probe 38 (hereinafter, referred to as the "probe" 38) which is
 employed as part of a pulse oximeter. The probe 38 functions as a second
 pulse-wave detecting device, or a peripheral-pulse-wave detecting device
 for detecting a peripheral pulse wave propagated to a peripheral artery
 including capillaries. The probe 38 is set on a skin or a body surface 40
 of the patient, e.g., an end portion of a finger of a left hand of the
 patient with the help of a band (not shown), such that the probe 38 is
 held in close contact with the body surface 40. The probe 38 is worn on
 the hand of one arm different from the other arm around which the cuff 10
 is wrapped.
 The probe 38 includes a container-like housing 42 which opens in a certain
 direction, a first and a second group of light emitting elements 44a, 44b,
 such as LEDs (light emitting diodes), which are disposed on an outer
 peripheral portion of an inner bottom surface of the housing 42
 (hereinafter, referred to as the light emitting elements 44 in the case
 where the first and second groups of light emitting elements 44a, 44b need
 not be discriminated from each other), a light receiving element 46, such
 as a photodiode or a phototransister, which is disposed on a central
 portion of the inner bottom surface of the housing 42, a transparent resin
 48 which is integrally disposed in the housing 42 to cover the light
 emitting elements 44 and the light receiving element 46, and an annular
 shading member 50 which is disposed between the light emitting elements 44
 and the light receiving element 46, for preventing the light receiving
 element 46 from receiving the lights emitted toward the body surface 40 by
 the light emitting elements 44 and directly reflected from the body
 surface 40.
 The first group of light emitting elements 44a emit a first light having a
 first wavelength .lambda..sub.1 whose absorbance changes depending on a
 blood oxygen saturation value of the patient. The first elements 44a emit,
 e.g., a red light having about 660 nm wavelength. The second group of
 light emitting elements 44b emit a second light having a second wavelength
 .lambda..sub.2 whose absorbance does not change depending on the blood
 oxygen saturation value of the patient. The second elements 44b emit,
 e.g., an infrared light having about 800 nm wavelength. The first and
 second light emitting elements 44a, 44b alternately emit the red and
 infrared lights, respectively, at a predetermined frequency, e.g., a
 relatively high frequency of several hundred Hz to several thousand Hz.
 The lights emitted toward the body surface 40 by the light emitting
 elements 44 are reflected from a body tissue of the patient where a dense
 capillaries occur, and the reflected lights are received by the common
 light receiving element 46. In place of the 660 nm and 800 nm lights, the
 first and second light emitting elements 44a, 44b may employ various pairs
 of lights each pair of which have different wavelengths, so long as one
 light of each pair exhibits significantly different absorption factors
 with respect to oxygenated hemoglobin and reduced hemoglobin,
 respectively, and the other light exhibits substantially same absorption
 factors with respect to the two sorts of hemoglobin, i.e., has a
 wavelength which is reflected by each of the two sorts of hemoglobin.
 FIG. 2 shows a bottom surface of the probe 38 or the housing 42 that is
 opposed to the body surface 40 of the patient. The light receiving element
 46 is located on the central portion of the housing 42, and the annular
 shading member 50 is fixed to the housing 42 such that the shading member
 50 is concentric with the circular housing 42. The first light emitting
 elements 44a and the second light emitting elements 44a are alternately
 arranged along a circle, indicated by a one-dot chain line, which is
 located outside the shading member 50, has a radius, r, and is concentric
 with the shading member 50 and the housing 42.
 The light receiving element 46 outputs, through a low-pass filter 52, a
 photoelectric-pulse-wave signal SM.sub.3 representative of an amount of
 the first or second light received from the body tissue of the patient.
 The light receiving element 46 is connected to the low-pass filter 52 via
 an amplifier or the like. The low-pass filter 52 removes, from the
 photoelectric-pulse-wave signal SM.sub.3 input thereto, noise having
 frequencies higher than that of a pulse wave, and outputs the noise-free
 signal SM.sub.3, to a demultiplexer 54. The photoelectric-pulse-wave
 signal SM.sub.3 is representative of a photoelectric pulse wave which is
 produced in synchronism with the pulse of the patient.
 The demultiplexer 54 is switched according to signals supplied thereto from
 the control device 28 in synchronism with the alternate light emissions of
 the first and second light emitting elements 44a, 44b. Thus, the
 demultiplexer 54 separates the photoelectric-pulse-wave ("PPW") signal
 SM.sub.3 into two PPW signals which correspond to the first and second
 lights, respectively. More specifically described, the demultiplexer 54
 successively supplies, to the I/O port (not shown) of the control device
 28, a first PPW signal SM.sub.R representative of the red light having the
 first wavelength .lambda..sub.1 through a first sample-and-hold circuit 56
 and an A/D converter 58, and a second PPW signal SM.sub.IR representative
 of the infrared light having the second wavelength .lambda..sub.2 through
 a second sample-and-hold circuit 60 and an A/D converter 62. The first and
 second sample-and-hold circuits 56, 60 hold the first and second PPW
 signals SM.sub.R, SM.sub.IR input thereto, respectively, and do not output
 those current signals to the A/D converters 58, 62, before the prior
 signals SM.sub.R, SM.sub.IR are completely converted by the A/D converters
 58, 62, respectively.
 In the control device 28, the CPU 29 carries out a measuring operation
 according to control programs pre-stored in the ROM 31 by utilizing the
 temporary-storage function of the RAM 33. More specifically described, the
 CPU 29 generates a light emit signal SLV to a drive circuit 64 so that the
 first and second light emitting elements 44a, 44b alternately emit the red
 and infrared lights at a predetermined frequency, respectively, such that
 each light emission lasts for a predetermined duration. In synchronism
 with the alternate light emissions of the first and second light emitting
 elements 44a, 44b, the CPU 29 generates a switch signal SC to the
 demultiplexer 54 to switch the demultiplexer 54 between its first and
 second positions. Thus, the PPW signal SM.sub.3 is separated by the
 demultiplexer 54 such that the first PPW signal SM.sub.R is supplied to
 the first sample-and-hold circuit 56 while the second PPW signal SM.sub.IR
 is supplied to the second sample-and-hold circuit 60.
 FIG. 3 illustrates essential functions of the control device 28 of the
 present BP monitoring apparatus 8. In the figure, a BP measuring means or
 circuit 70 measures a systolic, a mean, and a diastolic BP value
 BP.sub.SYS, BP.sub.MEAN, BP.sub.DIA of the patient, according to a well
 known oscillometric method, based on the variation of respective
 amplitudes of heartbeat-synchronous pulses of the pulse wave represented
 by the cuff-pulse-wave signal SM.sub.1 obtained while the cuff pressure
 which is quickly increased by a cuff-pressure control means or circuit 72
 to a target pressure value PCM (e.g., 180 mmHg), is slowly decreased at
 the rate of about 3 mmHg/sec.
 A pulse wave propagation ("PWP") relating information obtaining means or
 circuit 74 includes a time-difference calculating means or circuit which
 iteratively calculates, as a PWP time DT.sub.RP, a time difference between
 a predetermined point (e.g., R-wave) on the waveform of each of periodic
 pulses of the ECG pulse wave that are successively detected by the
 ECG-pulse-wave detecting device 34 and a predetermined point (e.g., rising
 point, i.e., minimum point) on the waveform of a corresponding one of
 periodic pulses of the photoelectric pulse wave ("PPW") detected by the
 probe 38, as illustrated in FIG. 4. The PPW-relating-information obtaining
 means 74 iteratively calculates a PWP velocity V.sub.M (m/sec) of a pulse
 wave propagated through an artery of the patient, based on the calculated
 PPW time DT.sub.RP, according to the following expression (1) pre-stored
 in the ROM 31:
EQU V.sub.M =L/(DT.sub.RP -T.sub.PEP) (1)
 where L (m) is a length of the artery as measured from the left ventricle
 to the position where the probe 38 is set, via the aorta; and T.sub.PEP
 (sec) is a pre-ejection period between the R-wave of the waveform of each
 pulse of the ECG pulse wave and the minimum point of the waveform of a
 corresponding pulse of an aortic pulse wave.
 The values L, T.sub.PEP are constants, and are experimentally obtained in
 advance.
 A relationship determining means or circuit 76 determines two coefficients
 .alpha., .beta. in the following second or third expression (2) or (3),
 based on two systolic BP values BP.sub.SYS measured by the BP measuring
 means 70, and two PWP time values DT.sub.RP or two PWP velocity values
 V.sub.M calculated by the PPW-relating-information obtaining means 74.
 Each value DT.sub.RP, V.sub.M may be an average of several values
 DT.sub.RP, V.sub.M which are obtained immediately before each BP
 measurement. The above two expressions (2), (3) generally define a
 relationship between PWP time value DT.sub.RP and estimated BP value EBP,
 and a relationship between PWP velocity value V.sub.M and estimated BP
 value EBP, respectively. In place of the above-indicated relationship
 between estimated systolic BP value EBP.sub.SYS and either one of PWP time
 value DT.sub.RP and PWP velocity value V.sub.M, a relationship between
 estimated mean or diastolic BP value EBP.sub.MEAN, EBP.sub.DIA and either
 one of PWP time value DT.sub.RP and PWP velocity value V.sub.M may be
 employed. In short, a relationship between PWP-relating information and
 estimated BP value EBP may be determined depending upon which one of
 systolic, mean, and diastolic BP value is selected as estimated BP value
 EBP, i.e., monitored BP value.
EQU EBP=.alpha.(DT.sub.RP)+.alpha. (2)
 where .alpha. is a negative constant and .beta. is a positive constant.
EQU EBP=.alpha.(V.sub.M)+.beta. (3)
 where .alpha. and .beta. are positive constants.
 An estimated-BP ("EBP") determining means or circuit 78 iteratively
 determines an estimated BP value EBP of the patient, based on either one
 of an actual PWP time value DT.sub.RP and an actual PWP velocity value
 V.sub.M iteratively calculated by the PWP-relating-information obtaining
 means 74, according to the relationship represented by the second or third
 expression (2) or (3).
 The control device 28 controls a display device 32 to concurrently display
 a trend graph of the thus determined estimated BP values EBP, together
 with respective trend graphs of pulse period values RR and pulse-wave area
 values VR (which will be described below), along a common horizontal axis
 indicative of time, as shown in FIG. 5, so that those three trend graphs
 can be compared with one another by a medical person, such as a doctor or
 a nurse, who attend to the patient.
 A pulse-period measuring means or circuit 82 iteratively measures a pulse
 period value RR by measuring or calculating a time difference between
 respective predetermined points (e.g., R-waves) of each pair of successive
 heartbeat-synchronous pulses of the ECG pulse wave detected by the
 ECG-pulse-wave detecting device 34.
 A pulse-wave-area calculating means or circuit 84 calculates a pulse-wave
 area value VR by normalizing an area S defined or enveloped by the
 waveform of each heartbeat-synchronous pulse of the PPW detected by the
 probe 38, based on a period W and an amplitude L of the each pulse. More
 specifically described, as shown in FIG. 6, the waveform of each pulse of
 the PPW is defined by a series of data points indicative of respective
 magnitudes which are input at a predetermined short interval such as
 several milliseconds to several tens of milliseconds. The pulse-wave area
 S is obtained by integrating, over the period W of the each pulse,
 respective magnitudes of the data points of the each pulse, and then the
 normalized pulse-wave area value VR is obtained according to the following
 expression: VR=S/(W.times.L). The normalized pulse-wave area value VR is a
 dimensionless value indicative of the ratio of the pulse-wave area S to a
 rectangular area defined by the period W and the amplitude L of the each
 pulse. For this parameter, the symbol "%MAP" may be used in place of the
 symbol "VR".
 A BP-measurement starting means or circuit 86 starts a BP-measurement of
 the BP measuring means 70, when the absolute value of at least one value
 based on at least one estimated BP value EBP is not smaller than a first
 reference value and simultaneously at least one of the absolute value of
 at least one value based on at least one measured pulse period value RR
 and the absolute value of at least one value based on at least one
 calculated pulse-wave area value VR is not smaller than a corresponding
 one of a second and a third reference value, or periodically at a
 predetermined period T.sub.BP, e.g., 20 minutes. A value based on each
 estimated BP value EBP may be the each value EBP itself, or a change value
 that is an amount of change of the each value EBP from a "control" value
 EBP determined at the time of the last BP measuring operation, or the
 ratio of the amount of change to the "control" value EBP. Similarly, a
 value based on each measured pulse period value RR may be the each value
 RR itself, or a change value that is an amount of change of the each value
 RR from a "control" value RR measured at the time of the last BP measuring
 operation, or the ratio of the amount of change to the "control" value RR,
 and a value based on each calculated pulse-wave area value VR may be the
 each value VR itself, or a change value that is an amount of change of the
 each value VR from a "control" value VR calculated at the time of the last
 BP measuring operation, or the ratio of the amount of change to the
 "control" value VR.
 More specifically described, the BP-measurement starting means 86 includes
 an EBP-abnormality judging means for judging that each estimated BP value
 EBP determined by the EBP determining means 78 is abnormal when at least
 one value based on at least one value EBP including the each value EBP
 does not fall within a first reference range; an RR-abnormality judging
 means for judging that each pulse period value RR measured by the
 pulse-period measuring device 82 is abnormal when at least one value based
 on at least one value RR including the each value RR does not fall within
 a second reference range; a VR-abnormality judging means for judging that
 each pulse-wave area value VR calculated by the pulse-wave area
 calculating means 84 is abnormal when at least one value based on at least
 one value VR including the each value VR does not fall within a third
 reference range; and a period judging means or circuit 90 for judging
 whether time has passed by the predetermined period T.sub.BP. For example,
 when the EBP-abnormality judging means 87 judges that an estimated BP
 value EBP is abnormal and simultaneously at least one of the
 RR-abnormality judging means 88 and the VR-abnormality judging means 89
 judges that a corresponding one of a pulse period value RR and a
 pulse-wave area value VR is abnormal, or when the period judging means 90
 judges that time has passed by the predetermined period T.sub.BP, the
 BP-measurement starting means 86 may start a BP measurement of the BP
 measuring means 70.
 An index-value calculating means or circuit 92 calculates, based on the BP
 values measured in the last or current BP measuring operation of the BP
 measuring means 70, an index value I.sub.a indicative of a hardness of an
 arterial vessel of the patient. More specifically described, the
 index-value calculating means 92 calculates a pulse pressure P.sub.M by
 subtracting the measured diastolic BP value BP.sub.DIA from the measured
 systolic BP value BP.sub.SYS, and calculates an index value I.sub.a by
 dividing the pulse pressure P.sub.M by the measured mean BP value
 BP.sub.MEAN (i.e. I.sub.a =P.sub.M /BP.sub.MEAN). Meanwhile, it is known
 that as the arterial vessel becomes harder, the PWP velocity value V.sub.M
 becomes greater. Therefore, the index-value calculating means 92 may
 calculate an index value I.sub.a by dividing the PWP velocity value
 V.sub.M, or the inverse of the PWP time value DT.sub.RP, by a BP value BP
 measured by the BP measuring means 70 (e.g., measured systolic, mean, or
 diastolic BP value BP.sub.SYS, BP.sub.MEAN, BP.sub.DIA) (i.e. I.sub.a
 =V.sub.M /BP, or I.sub.a =(1/DT.sub.RP)/BP).
 A change-value calculating means or circuit 94 calculates, as a value
 relating to a change of a calculated index value I.sub.a, a change value
 .DELTA.I.sub.a that is an amount of change of the "current" index value
 I.sub.a calculated at the time of the last or current BP measuring
 operation, from a "control" value I.sub.a calculated at the time of the BP
 measuring operation prior to the current BP measuring operation, or the
 ratio of the amount of change to the "control" value I.sub.a
 Alternatively, the change-value calculating means 94 may calculate a
 change value .DELTA.I.sub.a that is an amount of change of the "current"
 index value I.sub.a from a moving average of a predetermined number (e.g.,
 from 3 to 5) of index values I.sub.a calculated at respective times of a
 corresponding number of late BP measuring operations including the last or
 current BP measuring operation.
 A blood-oxygen-saturation calculating means or circuit 96 includes a
 frequency-analysis means or circuit which applies a frequency analysis
 using the method of fast Fourier transform, to each of a plurality of
 predetermined intervals of each of the two PPW signals SM.sub.R SM.sub.IR
 supplied from the demultiplexer 54, and determines a direct-current
 component DC.sub.R and an alternating-current component AC.sub.R of each
 of the intervals of the first PPW signal SM.sub.R and a direct-current
 component DC.sub.IR and an alternating-current component AC.sub.IR of each
 of the intervals of the second PPW signal SM.sub.IR ; and a ratio
 calculating means or circuit which calculates the ratio (i.e., AC.sub.R
 /DC.sub.R) of the alternating-current component AC.sub.R to the
 direct-current component DC.sub.R of each of the intervals of the PPW
 signal SM.sub.R and the ratio (i.e., AC.sub.IR /DC.sub.IR) of the
 alternating-current component AC.sub.IR to the direct-current component
 DC.sub.IR of each of the intervals of the PPW signal SM.sub.IR. The
 blood-oxygen-saturation calculating means 96 calculates a blood oxygen
 saturation value SaO.sub.2 of the patient based on the ratio R (i.e.,
 (AC.sub.R /DC.sub.R)/(AC.sub.IR /DC.sub.IR) of the first ratio AC.sub.R
 /DC.sub.R to the second ratio AC.sub.IR /DC.sub.IR according to the
 following expression (4):
EQU SaO.sub.2 =A.times.R+B (4)
 where A is a negative constant indicative of the slope of a straight line
 represented by the expression (4); and .beta. is a constant indicative of
 the intercept of the straight line.
 Each interval to which the frequency analysis is applied by the
 frequency-analysis means is predetermined at a multiple of a full
 respiration period T.sub.RE, or half the period T.sub.RE, of the patient
 (e.g., a multiple of a time equal to four or two times each measured pulse
 period value RR of the patient), in order to remove respiratory changes
 from the PPW signals SM.sub.R, SM.sub.IR.
 A condition changing means or circuit 98 changes, when the index value
 I.sub.a calculated by the index-value calculating means 92 does not fall
 within a fourth reference range, when the change value .DELTA.I.sub.a
 calculated by the change-value calculating means 94 does not fall within a
 fifth reference range, or when the blood oxygen saturation value SaO.sub.2
 calculated by the blood-oxygen-saturation calculating means 96 is smaller
 than a reference value, the first, second, and/or third reference range to
 a changed first, a changed second, and/or a changed third reference range
 out of which at least one value based on at least one estimated BP value
 EBP, at least one value based on at least one measured pulse period value
 RR, and/or at least one value based on at least one calculated pulse-wave
 area value VR can go, respectively, than can go out of the initial first,
 second, and/or third reference range, respectively. Each of the changed
 first, second, and/or third reference range may be a predetermined range,
 or may be changed stepwise or continuously based on the difference between
 the calculated index value I.sub.a and the upper or lower limit value of
 the fourth reference range, the difference between the calculated change
 value .DELTA.I.sub.a and the upper or lower limit value of the fifth
 reference range, and/or the difference between the calculated blood oxygen
 saturation value .sub.2 and the reference value.
 However, each of the initial first, second, and third reference ranges may
 be replaced with only one of the upper and lower limit values of the each
 range. For example, in the case where the first to third reference ranges
 are replaced with only the respective lower limit values thereof, the
 condition changing means 98 changes the first, second, and/or third lower
 limit values to changed first, second, and/or third lower limit values
 greater than the initial first, second, and/or third lower limit values,
 respectively; and in the case where the first, second, and third reference
 ranges are replaced with only the respective upper limit values thereof,
 the condition changing means 98 changes the first, second, and/or third
 upper limit values to changed first, second, and/or third upper limit
 values smaller than the initial first, second, and/or third lower limit
 values, respectively.
 When the index value I.sub.a does not fall within the fourth reference
 range, it can be speculated that the flexibility or elasticity of the
 arterial vessels of the patient may have been lost because of
 arteriosclerosis or temporary constriction of the arterial vessels and
 accordingly the BP of the patient cannot be easily controlled, or that the
 patient may have fallen in shock because of excessive expansion of the
 arterial vessels. Therefore, it can be judged that the patient needs quick
 treatments. When the change value .DELTA.I.sub.a does not fall within the
 fifth reference range, it can be speculated that the accuracy of the
 estimated BP values EBP may have decreased because the hardness of the
 arterial vessels has largely changed during the monitoring of BP of the
 patient. When the blood oxygen saturation value SaO.sub.2 is smaller than
 the reference value, it can be speculated that the blood oxygen saturation
 SaO.sub.2 measured from the peripheral body portion (e.g., finger) of the
 patient may have largely decreased because the arterial vessels have
 constricted and accordingly the amount of blood flowing through the
 peripheral body portion has decreased. More specifically described, when
 the BP of the patient becomes abnormal because the arterial vessels
 constrict, the blood oxygen saturation SaO.sub.2 may decrease before the
 BP becomes abnormal. That is, when the blood oxygen saturation SaO.sub.2
 decreases, the BP may subsequently become abnormal.
 A period changing means or circuit 100 changes, when a BP value (e.g.,
 systolic BP value BP.sub.SYS) measured by the BP measuring means 70 is
 smaller than a predetermined alarm value AL (e.g., 80 mmHg), the
 predetermined period T.sub.BP (e.g., 20 minutes) to a shorter period
 T.sub.BP ' (e.g., 10 minutes).
 Next, there will be described the operation of the control device 28 of the
 BP monitoring apparatus 8 by reference to the flow charts of FIGS. 7, 10,
 and 12. The flow chart of FIG. 7 represents the BP measuring routine; the
 flow chart of FIG. 10 represents the EBP determining routine; and the flow
 chart of FIG. 12 represents the BP-measurement-start judging routine.
 The control of the CPU 29 begins with Step SA1 of the flow chart of FIG. 7,
 where flags, counters, and registers (not shown) are reset, that is, the
 initialization of the control device 28 is carried out. Step SA1 is
 followed by Step SA2 to calculate, as a PWP time value DT.sub.RP, a time
 difference between a R-wave of the waveform of a heartbeat-synchronous
 pulse of the ECG pulse wave and a rising point of the waveform of a
 corresponding pulse of the PPW which are obtained immediately before the
 increasing of the cuff pressure, and additionally calculate a PWP velocity
 value V.sub.M (m/sec) based on the calculated PWP time value DT.sub.RP
 according to the expression (1). Step SA2 corresponds to the
 PWP-relating-information obtaining means 74. In addition, the CPU 29
 calculates a pulse period value RR based on the time interval between two
 successive pulses of the ECG pulse wave, and calculates a normalized
 pulse-wave area value VR from the waveform of a pulse of the PPW. Thus,
 Step SA2 also corresponds to the pulse-period measuring means 82 and the
 pulse-wave-area calculating means 84.
 The control of the CPU 29 goes to Steps SA3 and SA4 corresponding to the
 cuff-pressure control means 72. At Step SA3, the CPU 29 quickly increases
 the cuff pressure PC for a BP measurement of the BP measuring means 70, by
 switching the selector valve 16 to the inflation position and operating
 the air pump 18. Step SA3 is followed by Step SA4 to judge whether or not
 the cuff pressure PC is equal to or greater than a predetermined target
 pressure value PCM (e.g., 180 mmHg). If a negative judgement is made at
 Step SA4, the control of the CPU 29 goes back to Step SA2 so as to
 continue increasing the cuff pressure P.sub.C.
 If a positive judgement is made at Step SA4, the control of the CPU 29 goes
 to Step SA5 to carry out a BP measuring algorithm. More specifically
 described, the air pump 18 is stopped and the selector value 16 is
 switched to the slow-deflation position where the valve 16 permits the
 pressurized air to be slowly discharged from the cuff 10. A systolic BP
 value BP.sub.SYS, a mean BP value BP.sub.MEAN, and a diastolic BP value
 BP.sub.DIA are determined, according to a well known oscillometric BP
 determining algorithm, based on the variation of respective amplitudes of
 heartbeat-synchronous pulses of the pulse wave represented by the
 cuff-pulse-wave signal SM.sub.1 obtained while the cuff pressure P.sub.C
 is slowly decreased at a predetermined rate of about 3 mmHg/sec, and a
 heart rate HR is determined based on the interval of two successive pulses
 of the pulse wave. The thus measured BP values and heart rate HR are
 displayed on the display device 32, and the selector valve 16 is switched
 to the quick-deflation position where the valve 16 permits the pressurized
 air to be quickly discharged from the cuff 10. Step SA5 corresponds to the
 BP measuring means 70.
 Step SA5 is followed by Step SA6 to determine a relationship between
 PWP-relating information and estimated BP value EBP based on two BP values
 measured at Step SA5 in two control cycles each according to the flow
 chart of FIG. 7, and two PWP time values DT.sub.RP or two PWP velocity
 values V.sub.M calculated at Step SA2 in the two control cycles. More
 specifically described, when the systolic, mean, and diastolic BP values
 BP.sub.SYS, BP.sub.MEAN, BP.sub.DIA are measured at Step SA5, then at Step
 SA6 a relationship between estimated systolic, mean, or diastolic BP value
 EBP.sub.SYS, EBP.sub.MEAN, EBP.sub.DIA and one of PWP time value DT.sub.RP
 and PWP velocity value V.sub.M, represented by the expression (2) or (3),
 is determined based on the two systolic, mean, or diastolic BP values
 BP.sub.SYS, BP.sub.MEAN, BP.sub.DIA measured at Step SA5 in the last two
 control cycles including the last or current control cycle, and the two
 PWP time or velocity values DT.sub.RP, V.sub.M calculated at Step SA2 in
 the last two control cycles. Step SA6 corresponds to the relationship
 determining means 76. In addition, the CPU 29 determines an estimated BP
 value EBP of the patient based on the PWP time or velocity value
 DT.sub.RP, V.sub.M determined at Step SA2, according to the thus
 determined relationship.
 Step SA6 is followed by Step SA7 to judge whether the systolic BP value
 BP.sub.SYS measured at Step SA5 is smaller than a predetermined alarm
 value AL, e.g., 80 mmHg. If a negative judgment is made at Step SA7, the
 control of the CPU 29 skips Step SA8 and directly goes to Step SA9. On the
 other-hand, a positive judgment made at Step SA7 indicates that the BP of
 the patient should be carefully observed. Hence, subsequently the control
 goes to Step SA8 to change the predetermined period T.sub.BP, e.g., 20
 minutes, to a shorter period T.sub.BP, e.g., 10 minutes. Step SA9
 corresponds to the period changing means 100. Step SA8 is followed by Step
 SA9.
 At Step SA9, the CPU 29 calculates an index value I.sub.a indicative of a
 hardness of a blood vessel of the patient, based on the systolic, mean,
 and diastolic BP values BP.sub.SYS, BP.sub.MEAN, BP.sub.DIA measured at
 Step SA5. The CPU 29 calculates a pulse pressure P.sub.M by subtracting
 the measured diastolic BP value BP.sub.DIA from the measured systolic BP
 value BP.sub.SYS, and calculates the index value I.sub.a by dividing the
 pulse pressure P.sub.M by the measured mean BP value BP.sub.MEAN.
 Step SA9 is followed by Step SA10 where the CPU 29 calculates, as the
 change value .DELTA.I.sub.a of the index value I.sub.a, an amount of
 change of the "current" index value I.sub.a calculated in the last or
 current control cycle according to the flow chart of FIG. 7, i.e., at the
 time of the last or current BP measuring operation, from the "control"
 index value I.sub.a calculated in the preceding or prior control cycle
 according to the flow chart of FIG. 7, i.e., at the time of the prior BP
 measuring operation, that is, calculates the absolute value of the
 difference between the two index values I.sub.a. Step SA10 corresponds to
 the change-value calculating means 94.
 At Step SA11, the CPU 29 judges whether the index value I.sub.a calculated
 at Step SA9 in the current control cycle falls outside a predetermined
 normal index-value range, e.g., the range of from 0.4 to 0.6 shown in the
 graph of FIG. 8. The normal index-value range is experimentally determined
 in advance, on the assumption that the index value I.sub.a is calculated
 in the manner employed at Step SA9. The fact that the index value I.sub.a
 is smaller than the lower limit value, 0.4, indicates that the blood
 vessel of the patient is too soft and may have excessively largely
 expanded, and the fact that the index value I.sub.a is greater than the
 upper limit value, 0.6, indicates that the blood vessel of the patient is
 too hard and may have lost its flexibility or elasticity.
 If a negative judgment is made at Step SA11, the control of the CPU 29 goes
 to Step SA12 to judge whether the change value .DELTA.I.sub.a (i.e., the
 absolute value of the amount of change of the index value I.sub.a)
 calculated at Step SA10 is greater than a predetermined reference value,
 e.g., 0.1 as shown in the graph of FIG. 9. This reference value is
 experimentally determined in advance, as a threshold or criterion value
 which indicates that assuming that the change value .DELTA.I.sub.a is
 calculated in the manner employed at Step SA10, the hardness of the blood
 vessel of the patient has significantly largely changed and accordingly
 the accuracy or reliability of the estimated BP value EBP determined at
 Step SA6 has been lost. Since the calculated change value .DELTA.I.sub.a
 is an absolute value, the predetermined reference value (e.g., 0.1)
 defines, in fact, a predetermined reference range (e.g., the range of from
 -0.1 to +0.1).
 If a negative judgment is made at Step SA12, the CPU 29 terminates the BP
 measuring routine of FIG. 7 and proceeds with the EBP determining routine
 of FIG. 10. On the other hand, if a positive judgment is made at Step SA11
 or Step SA12, the control of the CPU 29 goes to Step SA13 corresponding to
 the condition changing means 98, and then terminates the BP measuring
 routine of FIG. 7. At Step SA13, the CPU 29 changes a predetermined
 reference range employed at Step SB11 of FIG. 10 described later, to a
 changed reference range as shown in the graphs of FIGS. 8 and 9. At SB11,
 the CPU 29 judges that the "current" estimated BP value EBP determined at
 Step SB10 in the current control cycle is abnormal, when the absolute
 value of the ratio of the amount of change of the "current" estimated BP
 value EBP from the "control" estimated BP value EBP determined at Step SA6
 at the time of the last BP measuring operation, to the "control" estimated
 BP value EBP, is greater than a predetermined reference value, e.g., 25%,
 after the absolute value of the ratio determined for each of not less than
 nineteen prior values EBP has been found as being greater than the
 reference value. Since the absolute value of the ratio determined for each
 estimated BP value EBP is compared with the reference value (e.g., 25%),
 the reference value defines, in fact, a predetermined reference range
 (e.g., the range of from -25% to +25%). If the index value I.sub.a
 calculated at Step SA9 does not fall within the normal index-value range
 of 0.4 to 0.6, or if the change value .DELTA.I.sub.a calculated at Step
 SA10 is greater than 0.1, the CPU 29 changes the predetermined reference
 range of, e.g., -25% to +25%, to a narrower range of, e.g., -20% to +20%.
 After the BP measuring routine of FIG. 7, the control of the CPU 29 goes to
 the EBP determining routine of FIG. 10. First, at Step SB1, the CPU 29
 judges whether an R-wave of the waveform of a heartbeat-synchronous pulse
 of the ECG pulse wave and a rising point of the waveform of a
 corresponding pulse of the photoelectric pulse wave ("PPW") have been read
 in. If a negative judgment is made at Step SB1, the control of the CPU 29
 waits until a positive judgment is made at Step SB1.
 On the other hand, if a positive judgment is made at Step SB1, the control
 of the CPU 29 goes to Step SB2 to add one to a number counted by a timer
 counter CT. Step SB2 is followed by Step SB3 to judge whether the number
 counted by the timer counter CT is equal to, or greater than, a
 predetermined reference time T.sub.0. This reference time T.sub.0 is equal
 to each predetermined time interval that is subjected to a frequency
 analysis carried out at Step SB4 described below. For example, the
 reference time T.sub.0, i.e., the predetermined time interval may be equal
 to a multiple of a time equal to a full respiration period T.sub.RE, or
 half the period T.sub.RE, of the patient, e.g., a multiple of a time equal
 to four or two times each measured pulse period value RR of the patient.
 Since, initially, a negative judgment is made at Step SB3, the control of
 the CPU 29 skips Steps SB4 to SB8 and goes to Step SB9. Meanwhile, if a
 positive judgment is made at Step SB3, the control goes to Steps SB4 to
 SB6 corresponding to the blood-oxygen-saturation calculating means 96. At
 Step SB4 corresponding to the frequency-analysis means, the CPU 29 applies
 the previously-described frequency analysis to each of the predetermined
 time intervals of each of the two PPW signals SM.sub.R, SM.sub.IR, and
 determines, for each time interval, an alternating-current component
 (signal power) AC.sub.R and a direct-current component DC.sub.R of the
 first PPW signal SM.sub.R, and an alternating-current component AC.sub.IR
 and a direct-current component DC.sub.IR of the second PPW signal
 SM.sub.IR.
 Step SB4 is followed by Step SB5 corresponding to the ratio calculating
 means. At Step SB5, the CPU 29 calculates the ratio of the component
 AC.sub.R to the component DC.sub.R for the first signal SM.sub.R and the
 ratio of the component AC.sub.IR to the component DC.sub.IR for the second
 signal SM.sub.IR, based on the components AC.sub.R, DC.sub.R of the signal
 SM.sub.R and the components AC.sub.IR, DC.sub.IR of the signal SM.sub.IR
 calculated at Step SB4.
 At Step SB6, the CPU 29 calculates a blood oxygen saturation value
 SaO.sub.2 of the patient, based on the ratio of the ratio of AC.sub.R to
 DC.sub.R to the ratio of AC.sub.IR to DC.sub.IR, i.e., R=(AC.sub.R
 /DC.sub.R)/(AC.sub.IR /DC.sub.IR), according to the predetermined
 relationship between ratio R and saturation SaO.sub.2, i.e., the
 expression (4).
 Step SB6 is followed by Step SB7 where the CPU 29 judges whether the blood
 oxygen saturation value SaO.sub.2 calculated at Step SB6 is smaller than a
 predetermined reference value, e.g., 90%. If a negative judgment is made
 at Step SB7, the control of the CPU 29 skips Step SB8 and goes to Step
 SB9. On the other hand, if a positive judgment is made at Step SB7, this
 decrease of the blood oxygen saturation SaO.sub.2 may be followed by a
 decrease of the BP of the patient. Hence, the control goes to Step SB8
 corresponding to the condition changing means 98. At Step SB8, the CPU 29
 changes, based on the difference between the predetermined reference value
 (e.g., 90%) and the oxygen saturation value SaO.sub.2 calculated at Step
 SB5, the predetermined reference range (e.g., from -25% to +25%) employed
 at Step SB11, to a changed reference range represented by a straight line
 shown in the graph of FIG. 11. The straight line defines a relationship
 between the above-indicated difference and the changed reference range.
 The straight line has a negative slope, and accordingly the width of the
 changed reference range linearly decreases from 25% as the oxygen
 saturation value SaO.sub.2 calculated at Step SB6 decreases from 90%. Step
 SB8 is followed by Step SB9.
 Step SB9 corresponding to the PWP-relating-information obtaining means 74.
 At Step SB9, the CPU 29 calculates a PWP time value DT.sub.RP and a PWP
 velocity value V.sub.M based on the R-wave of the waveform of each pulse
 of the ECG pulse wave and the rising point of the waveform of a
 corresponding pulse of the PPW which have been read in at Step SB1, in the
 same manner as that employed at Step SA2.
 Step SB9 is followed by Step SB10 corresponding to the estimated-BP
 determining means 78. At Step SB10, the CPU 29 determines an estimated BP
 value EBP (i.e., an estimated systolic, mean, or diastolic BP value),
 based on the PWP time value DT.sub.RP or the PWP velocity value V.sub.M
 calculated at Step SB9, according to the relationship determined at Step
 SA6 at the time of the last BP measuring operation. Further, the CPU 29
 displays, on the display device 32, a trend graph of the estimated BP
 values EBP which have been determined for successive pulses of the ECG
 pulse wave and the PPW and which include the "current" estimated BP value
 EBP determined in the current control cycle.
 Step SB10 is followed by Step SB11 to start a BP measurement of the BP
 measuring means 70, when the estimated BP value EBP is judged as being
 abnormal and simultaneously at least one of the measured pulse period
 value RR and the calculated pulse-wave area value VR is judged as being
 abnormal, as a result of the execution of the BP-measurement-start judging
 routine of FIG. 12. Step SB11 corresponds to the BP-measurement starting
 means 86.
 At Step SC1 of the flow chart of FIG. 12, the CPU 29 measures a pulse
 period value RR based on the time interval between a pair of successive
 pulses of the ECG pulse wave detected by the ECG-pulse-wave detecting
 device 34. Step SC1 corresponds to the pulse-period measuring means 82.
 Step SC1 is followed by Step SC2 corresponding to the RR-abnormality
 judging means 88. At Step SC2, the CPU 29 judges whether the measured
 pulse period value RR is abnormal. For instance, the CPU 29 judges that
 the pulse period value RR is abnormal when the state in which the pulse
 period value RR measured at Step SC1 in each control cycle is, by not less
 than a predetermined amount or a predetermined ratio (e.g., 5%), greater
 or smaller than the "control" pulse period value RR measured at the time
 of the last BP measuring operation has continued for a time period
 corresponding to not less than a predetermined number of pulses (e.g., 20
 pulses). If a negative judgment is made at Step SC2, the control of the
 CPU 29 skips Step SC3 and goes to Step SC4. On the other hand, if a
 positive judgment is made at Step SC2, the control goes to Step SC3 where
 an RR flag is set "ON" so as to indicate the abnormality of the pulse
 period value RR.
 Step SC3 is followed by Step SC4 to calculate a normalized pulse-wave area
 value VR based on the waveform of a pulse of the PPW detected by the probe
 38. Step SC4 corresponds to the pulse-wave area calculating means 84. Step
 SC4 is followed by Step SC5 to judge whether the PPW signal SM.sub.3
 detected from the peripheral portion (i.e., finger) of the patient is
 normal. At Step SC5, the CPU 29 removes an abnormal waveform from the PPW
 signal SM.sub.3. For example, the CPU 29 removes the waveform of each
 pulse of the PPW, if the inclination of base line of the waveform of each
 pulse is greater than a predetermined reference angle, or if the waveform
 has deformed due to a calibration of the monitoring apparatus 8. If a
 negative judgment is made at Step SC5, the control of the CPU 29 goes to
 Step SC10. On the other hand, if a positive judgement is made at Step
 SACS, the control of the CPU 29 goes to Step SC6.
 At Step SC6 corresponding to the VR-abnormality judging means 89, the CPU
 29 judges whether the normalized pulse-wave area value VR calculated at
 Step SC4 is abnormal. For instance, the CPU 29 judges the pulse-wave area
 value VR is abnormal when the state in which the pulse-wave area value VR
 calculated at Step SC4 in each control cycle is, by not less than a
 predetermined amount or a predetermined ratio (e.g., 3%), greater or
 smaller than the "control" pulse-wave area value VR calculated at the time
 of the last BP measuring operation has continued for a time period
 corresponding to not less than a predetermined number of pulses (e.g., 20
 pulses). If a negative judgment is made at Step SC6, the control of the
 CPU 29 goes to Step SC8. On the other hand, if a positive judgment is made
 at Step SC6, the control of the CPU 29 goes to Step SC7 where a VR flag is
 set "ON" so as to indicate the abnormality of the pulse-wave area value
 VR.
 Next, Step SC7 is followed by Step SC8 corresponding to the EBP-abnormality
 judging means 87. At Step SC8, the CPU 29 judges whether the estimated BP
 value EBP determined at Step SB10 is abnormal. For instance, the CPU 29
 judges that the estimated BP value EBP determined at Step SB10 is abnormal
 when the state in which the estimated BP value EBP in each control cycle
 is, by not less than a predetermined amount or a predetermined ratio
 (e.g., 25%), greater or smaller than the "control" estimated BP value EBP
 determined at the time of the last BP measuring operation has continued
 for a time period corresponding to not less than a predetermined number of
 pulses (e.g., 20 pulses). If a negative judgment is made at Step SC8, the
 control of the CPU 29 goes to Step SC10. On the other hand, if a positive
 judgment is made at Step SC8, the control of the CPU 29 goes to Step SC9
 where an EBP flag is set "ON" so as to indicate the abnormality of the
 estimated BP value EBP.
 Step SC9 is followed by Step SC10 to judge whether the EBP flag is "ON" and
 simultaneously at least one of the RR flag and the VR flag is "ON". If a
 negative judgment is made at Step SC10, the control of the CPU 29 goes to
 Step SB12 corresponding to the period judging means 90. At Step SB12, the
 CPU 29 judges whether the predetermined period T.sub.BP (e.g., 20
 minutes), that is, the calibration period, has passed after the last BP
 measuring operation was carried out at Step SA5 of FIG. 7. If a negative
 judgment is made at Step SB12, the control of the CPU 29 goes back to Step
 SB1 and the following steps so as to carry out the EBP determining
 routine, that is, determine an estimated BP value EBP for each of
 successive heartbeat-synchronous pulses, and display, on the display
 device 32, the trend graph of the determined estimated BP values EBP. On
 the other hand, if a positive judgment is made at Step SB12, the control
 of the CPU 29 goes back to the BP measuring routine of FIG. 7 so as to
 determine a new relationship between PWP-relating information and
 estimated BP value EBP.
 Meanwhile, if a positive judgment is made at Step SC10, the control of the
 CPU 29 goes to Step SB13 of FIG. 10. At Step SB13, the CPU 29 displays the
 abnormality of the estimated-BP value EBP on the display device 32. Then,
 the control of the CPU 29 goes back to the BP measuring routine of FIG. 7
 so as to start a BP measurement using the inflatable cuff 10 and determine
 a new relationship between PWP-relating information and estimated BP value
 EBP.
 In the present embodiment, the index-value calculating means 92 calculates
 the index value I.sub.a indicative of the hardness of the blood vessel of
 the patient based on the BP values measured by the BP measuring means 70,
 and the condition changing means 98 changes, when the index value I.sub.a
 does not fall within the predetermined normal index-value range, the
 predetermined reference range (e.g., from -25% to +25%) employed by the
 EBP-abnormality judging means 87 at Step SC8, to the changed reference
 range (e.g., from -20% to +20%) which is contained in the predetermined
 reference range and is narrower than the same. Since the changed reference
 range is narrower than the predetermined reference range, the
 BP-measurement starting means 86 can earlier start a BP measurement of the
 BP measuring means 70. Therefore, in the case where the blood vessel of
 the patient has lost its flexibility or elasticity and accordingly becomes
 too hard, or has excessively largely expanded and accordingly becomes too
 soft, the present BP monitoring apparatus 8 can earlier measure an
 accurate and reliable BP value of the patient using the inflatable cuff
 10.
 In addition, the index-value calculating means 92 calculates the index
 value I.sub.a indicative of the hardness of the blood vessel of the
 patient based on the BP values measured by the BP measuring means 70, the
 change-value calculating means 94 calculates the change value
 .DELTA.I.sub.a of the index value I.sub.a, and the condition changing
 means 98 changes, when the change value .DELTA.I.sub.a (absolute value) is
 greater than the predetermined reference value, the predetermined
 reference range employed by the EBP-abnormality judging means 87 at Step
 SC8, to the changed reference range narrower than the predetermined
 reference range. Since the changed reference range is narrower than the
 predetermined reference range, the BP-measurement starting means 86 can
 earlier start a BP measurement of the BP measuring means 70. Therefore, in
 the case where the hardness of the blood vessel of the patient has
 significantly changed and accordingly the estimated BP value EBP has lost
 its accuracy, the present BP monitoring apparatus 8 can earlier measure an
 accurate and reliable BP value of the patient using the cuff 10.
 In the present embodiment, the blood-oxygen-saturation calculating means 96
 calculates the blood oxygen saturation value SaO.sub.2 of the peripheral
 body portion (e.g., finger) of the patient, based on the PPW, i.e.,
 peripheral pulse wave detected by the PPW detecting probe 38, and the
 condition changing means 98 changes, when the calculated blood oxygen
 saturation value SaO.sub.2 is smaller than the predetermined reference
 value (e.g., 90%), the predetermined reference range employed at Step SC8,
 to the changed reference range which is narrower than the predetermined
 reference range and is determined based on the difference between the
 calculated blood oxygen saturation value SaO.sub.2 and the predetermined
 reference value. Accordingly, the BP-measurement starting means 86 can
 earlier start a BP measurement of the BP measuring means 70, by a time
 corresponding to the degree of abnormality of the calculated blood oxygen
 saturation value SaO.sub.2, when the blood oxygen saturation value
 SaO.sub.2 of the peripheral body portion of the patient has decreased
 prior to the change of BP of the patient because the hardness of the blood
 vessel of the patient has largely changed. Thus, the BP measuring means 70
 can earlier measure an accurate and reliable BP value of the patient using
 the cuff 10.
 The present BP monitoring apparatus 8 includes the BP-measurement starting
 means 86 which periodically starts a BP-measurement of the BP measuring
 means 70 at the predetermined period T.sub.BP, and the period changing
 means 100 which changes, when the systolic BP value BP.sub.SYS measured by
 the BP measuring means 70 is smaller than the predetermined alarm value AL
 (e.g., 80 mmHg), the predetermined period T.sub.BP to the shorter period
 T.sub.BP ', so that the starting means 86 can start a BP-measurement of
 the BP measuring means 70 at the changed, short period T.sub.BP '.
 Therefore, the BP measuring means 70 can earlier measure an accurate and
 reliable BP value of the patient using the cuff 10.
 While the present invention has been described in its preferred embodiment
 by reference to the drawings, it is to be understood that the invention
 may otherwise be embodied.
 While in the illustrated embodiment the BP-measurement starting 86 starts a
 BP measurement of the BP measuring means 70, when it is judged at Step
 SC10 that the EBP flag is ON and at least one of the RR flag and the VR
 flag is ON. However, the BP-measurement starting 86 may be adapted to
 start a BP measurement of the BP measuring means 70, when it is judged at
 Step SC10 that at least one of the EBP flag, the RR flag, and the VR flag
 is ON.
 At Step SC9, the EBP flag may be set ON only if it is judged at Step SC8
 that a single estimated BP value EBP determined in each control cycle is,
 by not less than the predetermined ratio (e.g., 25%), greater or smaller
 than the "control" estimated BP value EBP determined at the time of the
 last BP measuring operation. This may apply to Steps SC2 and SC3, and
 Steps SC6 and SC7.
 At Steps SC8 and SC9, the parameter DT.sub.RP, V.sub.M may be employed in
 place of the parameter EBP, because each value DT.sub.RP, V.sub.M
 corresponds to each value EBP, one by one, as defined by the second or
 third expression (2), (3).
 While in the illustrated embodiment the index-value calculating means 92
 calculates, at Step SA9 of the BP measuring routine of FIG. 7, the index
 value I.sub.a by dividing the pulse pressure P.sub.M by the measured mean
 BP value BP.sub.MEAN, the index-value calculating means 92 may calculate
 an index value I.sub.a by dividing the PWP velocity value V.sub.M, or the
 inverse, 1/DT.sub.RP, of the PWP time value DT.sub.RP, by the systolic,
 mean, or diastolic BP value BP.sub.SYS, BP.sub.MEAN, BP.sub.DIA measured
 at Step SA5. In the latter case, since the index value I.sub.a is
 calculated based on the PWP-relating information V.sub.M, DT.sub.RP that
 is used for determining the estimated BP values EBP of the patient, the
 index value I.sub.a and the change value .DELTA.I.sub.a enjoy the accuracy
 of the PWP-relating information V.sub.M, DT.sub.RP.
 Although in the illustrated embodiment the period changing means 100
 changes the predetermined period T.sub.BP to the short period T.sub.BP '
 when the systolic BP value BP.sub.SYS measured by the BP measuring means
 70 is lower than the predetermined alarm value AL.sub.SYS, the period
 changing means 100 may change the predetermined period T.sub.BP to the
 short period T.sub.BP ' when the mean or diastolic BP value BP.sub.MEAN,
 BP.sub.DIA measured by the BP measuring means 70 is lower than a
 corresponding predetermined alarm value AL.sub.MEAN, AL.sub.DIA.
 In the illustrated embodiment, the relationship determining means 76
 determines the relationship represented by the second or third expression
 (2) or (3). However, since each of the pulse period value RR and the
 pulse-wave area value VR relates to the BP of the patient, the
 relationship determining means 76 may determine a relationship represented
 by the following fifth expression (5):
EQU EDP=.alpha.VM+.beta.RR+.gamma.VR+.delta. (5)
 where .alpha., .beta., .gamma., and .delta. are constants.
 In the illustrated embodiment, the condition changing means 98 changes, at
 Step SB8 of the flow chart of FIG. 10, the predetermined reference range
 employed at Step SB11, to the changed reference range whose width
 decreases as the difference between the calculated blood oxygen saturation
 value SaO.sub.2 and the predetermined reference value increases. However,
 the condition changing means 98 may change the predetermined reference
 range employed at Step SB11, to a changed reference range which is
 inverse-proportional to the difference between the calculated blood oxygen
 saturation value SaO.sub.2 and the predetermined reference value.
 In the illustrated embodiment, the pulse period RR (sec) may be replaced
 with heart rate HR (1/min), because the heart rate HR corresponds to the
 pulse period RR, one to one, according to the following expression:
 HR=60/RR.
 It is to be understood that the present invention may be embodied with
 other changes and modifications that may occur to those skilled in the art
 without departing from the scope of the invention.