Electronic blood pressure meter

Abstract: A electronic blood pressure meter capable of performing a proper automatic pressuring operation according to a blood pressure of a subject and measuring the blood pressure rapidly with accuracy. An electronic blood pressure meter comprises a cuff (1), a pressure controller (2) for controlling an inside fluid pressure of the cuff (1), a pressure sensor (7) for sensing the inside fluid pressure of the cuff (1) to produce a cuff pressure detecting signal, an extracting unit for extracting a pulse wave component superposed on the cuff pressure detecting signal during pressure control operations performed by the pressure controller, a calculating unit for calculating an amplitude of the extracted pulse wave every pressure, a unit for determining a blood pressure during evacuating operation of the pressure controller based on output signals of the calculating unit and the pressure sensor (7) and maximum blood pressure estimating unit for obtaining an estimated value of a maximum blood pressure in response to output signals of the extracting unit and pressure sensor (7) during pressuring operation of the control means (2), the pressure controller (2) stopping the pressuring operation when the inside fluid pressure of the cuff (1) exceeds a total pressure of the estimated value and a predetermined value.

Description: BACKGROUND OF THE INVENTION The present invention relates to an electronic blood pressure meter capable of performing a proper automatic pressuring operation according to a blood pressure of a subject and measuring the blood pressure rapidly with accuracy. A electronic blood pressure meter includes a cuff, a pump for pressuring the cuff, an evacuation valve for evacuating the cuff, a pressure sensor for detecting a cuff pressure, and a microcomputer (MPU). The MPU includes the functions of calculating a pulse wave amplitude value from the pulse wave component detected from an output signal of the pressure sensor, and determining a maximum blood pressure value (SYS) and a minimum blood pressure value (DIA) from the cuff pressure and the pulse wave amplitude value. The electronic blood pressure meter generally adopts a threshold technique in determining the blood pressure value. In making measurements, the cuff is pressured to check the flow of blood in the artery, and then pulse wave amplitude values incorporated in the cuff pressure are detected during the evacuation process. The pulse wave is a volumetric variation of the artery caused when the blood starts flowing during the process of evacuating the cuff at a slow speed, and it is this volumetric variation that is transmitted to the cuff and thus detected. The pulse wave amplitude value depicts a curve (an envelope) that gradually increases as the cuff pressure decreases, reaches a maximum, and then tends to decrease temporarily. Here, the maximum pulse wave amplitude value is detected, and in the above-mentioned phase in which the pulse wave amplitude increases, the pulse wave amplitude value closest to a threshold that is a predetermined percentage of the maximum pulse wave amplitude value (e.g., 50% of the maximum pulse wave amplitude value) is detected, and the cuff pressure at this point is determined as the maximum blood pressure value. Further, in the phase in which the pulse wave amplitude decreases, the pulse wave amplitude value closest to another threshold that is a predetermined percentage of the maximum pulse wave amplitude value (e.g., 70% of the maximum pulse wave amplitude value) is detected, and the cuff pressure at this point is determined as the minimum blood pressure value. The electronic blood pressure meter pressures the cuff to a target pressuring value (a pressuring value above the maximum blood pressure value) to stop the flow of blood in the artery, obtains thereafter a cuff pressure and pulse wave information during the process of evacuating the cuff at a predetermined low speed (4 mmHg/s), and determines a blood pressure based on the cuff pressure and the pulse wave information. Oscillometric electronic blood pressure meters track a pulse wave superposed on a cuff pressure and calculate a blood pressure based on a change in the amplitude of the pulse wave. The amplitude is, as shown in Figure 1, small when the cuff pressure is sufficiently large with respect to a systolic pressure (a maximum blood pressure), increases as the cuff is being evacuated thereafter, and maximizes immediately before the cuff pressure becomes equal to a diastolic pressure (a minimum blood pressure). It is assumed that the cuff pressure gets substantially equal to the average blood pressure. It maximizes at a maximum pulse wave point P. The amplitude decreases thereafter. The blood pressure is calculated based on this change in amplitude. In an oscillometric method in which the blood pressure is calculated based on this amplitude variation, an entire amplitude change pattern (an envelope) is required. For example, according to a technique, a systolic pressure (the maximum blood pressure) and a diastolic pressure (the minimum blood pressure) are calculated as points S and D at which the pulse waves become equal to certain values on the higher pressure side and on the lower pressure side than the maximum points (points P), respectively, the certain values being obtained by multiplying the maximum values by certain percentages. More specifically, the blood pressure cannot be calculated unless all the points S, P, D on the envelope are tracked. To track all these three points means that it is necessary to pressure the cuff to a value larger than the point S before starting a measuring operation. If a cuff pressure larger than the point S is not obtained, the absence of the point S is found out when the point P has detected in the blood pressure measurement operation, which makes the measurement unsuccessful due to failure to calculate the systolic pressure (the minimum blood pressure). Therefore, the user must repeat the pressuring operation with a point higher than in the last pressuring operation and make a measurement again. Since the point S (systolic pressure) is unknown to the user and it is very difficult to set the pressuring operation properly by estimating this point when a subject is a hypertensive whose blood pressure undergoes a noticeable variation. Further, if a measurement has been unsuccessful, it takes time to complete the measurement. Hence, a conventional practice is that the pressuring value is set to a point higher than necessary, which often keeps a subject under pain disadvantageously. For these reasons, a function of automatically setting the pressuring value has been called for. In the above-mentioned conventional electronic blood pressure meter, the pulse wave amplitude is tracked during the cuff pressuring process in which the cuff is being pressured for a predetermined time and at a predetermined pressuring speed as shown in Figure 2. If such a plurality of pulse wave amplitudes as to allow a maximum blood pressure to be estimated can be obtained during the pressuring process, a maximum blood pressure can be estimated by applying a predetermined algorithm, and by stopping the cuff pressuring operation when the cuff pressure reaches a value defined by adding a predetermined pressure to the estimated maximum blood pressure, an ideal pressuring target can be obtained. However, as shown in Figure 3, due to an arm being thin or for a like reason, the cuff pressuring speed may become too fast. In such a case, if the pressuring speed is not variable as in the conventional system, a part of the pulse wave is missed no matter how many times the cuff pressuring operations are repeated. As a result, such pulse wave information as to permit a maximum blood pressure to be determined cannot be obtained, which allows no maximum blood pressure to be estimated. Thus, there has existed the disadvantage that no proper pressuring target can be determined. Since the predetermined slow evacuation is effected during a measurement in this blood pressure measuring system, it takes time to complete the measurement. As a result, the subject must endure pressure from the cuff for a long time, and the measurement thus brings some pain to the subject. Particularly, a hypertensive must be subjected to a slow evacuation process from the state in which a large pressure is applied to the cuff, and this demands so long a measurement time that the subject feels numb in his arm or gets some congestion at the artery, etc. An electronic blood pressure meter according to the preamble of claim 1 is known from EP-A-0 332 701. SUMMARY OF THE INVENTION In view of the above, an object of the invention is to overcome the above problems and thus to provide an electronic blood pressure meter capable of reducing the measurement time and entailing less pain to a subject. The electronic blood pressure meter is also capable of tracking a pulse wave surely during the cuff pressuring process, calculating a maximum blood pressure easily as well as quickly, and surely performing the automatic setting of a pressuring operation. The above and other objects of the present invention are achieved by an electronic blood pressure meter as defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings, Figure 1 is a diagram illustrative of a blood pressure determining process in an oscillometric blood pressure meter; Figure 2 is a diagram illustrative of a case in which a maximum blood pressure can be estimated because the pressuring speed is appropriate; Figure 3 is a diagram illustrative of a case in which a maximum blood pressure cannot be estimated because the pressuring speed is too fast; Figure 4 is a block diagram showing an exemplary circuit configuration of the electronic blood pressure meter; Figure 5 is a main flowchart showing a specific operation of an electronic blood pressure meter, which is a first embodiment of the invention; Figure 6 is a flowchart showing a blood pressure estimating process during pressuring in the electronic blood pressure meter; Figure 7 is a flowchart showing a blood pressure calculating process during evacuating in the electronic blood pressure meter; Figure 8 is a flowchart showing an operation of an electronic blood pressure meter, which is a second embodiment of the invention; Figure 9 is a diagram illustrative of a state in which a pressuring target is determined by the electronic blood pressure meter; Figures 10 (A) and 10 (B) are flowcharts showing specific operations of an electronic blood pressure meter, which is a third embodiment of the invention; and Figure 11 is a diagram illustrative of the relationship between a cuff pressure and a pulse wave amplitude to indicate the state of measuring a blood pressure with the electronic blood pressure meter. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 4 is a circuit block diagram showing an electronic blood pressure meter. The electronic blood pressure meter includes a cuff 1, a pressuring pump 2 for pressuring the cuff 1, a rapid evacuation valve 3 for decreasing the internal pressure of the cuff, and a slow evacuation valve 4, with these components being connected to each other through an air tube 5 to form an air system. The rapid evacuation valve 3 and the pressuring pump 2 are electrically connected to an MPU 6 to be described later, and controlled by the MPU 6 independently. Further, a pressure sensor 7 is connected to the air tube 5. For the pressure sensor 7, e.g., a diaphragm converter using a strain gauge, or a semiconductor pressure conversion element is used. An output signal (an analog quantity) of the pressure sensor 7 is sent to an A/D converter 9 through a lowpass filter 8 to be converted into a digital signal by the A/D converter 9. The lowpass filter 8 is arranged to remove pressure noise at the pressuring pump 2 that may undesirably be introduced on a cuff pressure signal when detecting a pulse wave during the pressuring process. A cut-off frequency of the lowpass filter 8 is set within a range of 10 to 30 Hz. The MPU 6 periodically receives a digitally converted, noiseless output signal of the pressure sensor 7. This MPU 6 includes a function of extracting pulse waves (the function of tracking a pulse wave from cuff pressure data), a function of calculating a pulse wave amplitude (the function of calculating a pulse wave amplitude by recognizing the start and end points of a pulse wave every pressure), and a function of calculating a blood pressure (the function of calculating maximum and minimum blood pressure values from a plurality of pulse wave amplitudes, i.e., an envelope, obtained). Further, the MPU 6 has a function of calculating a threshold for determining a systolic pressure based on a maximum pulse wave during a cuff pressuring operation. The threshold is a value that is 50% of a maximum pulse wave Amax. The MPU 6 also has a function of determining an estimated maximum blood pressure by judging whether or not the pressured pulse wave is smaller than the threshold and determining as an estimated maximum blood pressure a cuff pressure at the time the pressured pulse wave is smaller than the threshold, and a function of setting a pressuring operation by calculating an appropriate pressuring value based on the estimated maximum blood pressure and stopping the cuff pressuring operation at the time the cuff pressure gets equal to the calculated pressure value. The MPU 6 has a further function of displaying the determined maximum and minimum blood pressure values on a display 10. Figure 5 is a flowchart showing a specific operation of the thus embodied electronic blood pressure meter. When making a measurement, as both a power switch and a pressuring switch have been turned on, a pressuring operation of the cuff 1 is started (Step 1 (the word "Step" will hereinafter be referred to simply as "ST"). During this cuff pressuring operation, pulse waves are read based on an output signal of the pressure sensor 7 and a pulse wave amplitude is calculated by recognizing the start and end points of each pulse wave every pressure. Further, a maximum blood pressure is estimated from the obtained pulse wave envelope (ST 2). In ST 3, it is judged whether the estimated maximum blood pressure SP has been determined. If the cuff pressure does not increase adequately and the systolic pressure (SP) has not yet been determined, then the judgment result in ST 3 becomes "NO," causing the MPU 6 to return to ST 2. If it is assumed that the systolic pressure (ST) is now determined, then "YES" is the judgment result in ST 3, thus causing the MPU 6 to proceed to ST 4. In ST 4, it is judged whether or not the cuff pressure has reached an appropriate pressuring value. That is, whether or nor the current pressure of the cuff 1 that is being pressured has reached an appropriate pressuring value matching the determined estimated systolic pressure. Here, the term "appropriate pressuring value" means a value that is neither more nor less than the estimated systolic pressure. More specifically, the appropriate pressuring value is set by adding a predetermined value, e.g., 30 mmHg, to the systolic pressure, while considering systolic pressure estimating errors and a pressure drop between a pressuring stop to a measurement start. If it is assumed now that the cuff pressure has not increased to the appropriate pressuring value, then the judgment result in ST 4 becomes "NO," causing the MPU to continue pressuring so that the appropriate pressuring value can be obtained. And as the cuff pressure reaches the appropriate pressuring value, the judgment result in ST 4 becomes "YES," causing the MPU 6 to stop the driving of the pressuring pump 2 (ST 5), and the operation of slowly evacuating the cuff 1 is started and a measurement is then made (ST 6). During this slow evacuation process, the processing of extracting a pulse wave, calculating both a pulse wave amplitude and a blood pressure is similarly executed as in the pressuring operation (ST 7). In ST 8, it is judged whether or not a minimum blood pressure DP (including the maximum blood pressure) has been calculated. This processing is continued until the maximum and minimum blood pressure values are calculated in the slow cuff evacuation process, and when the diastolic pressure (the minimum blood pressure) has been determined, the judgment result in ST 8 becomes "YES," causing the MPU to rapidly evacuate the cuff 1 and display the measured blood pressure on the display 10 (ST 9). Figure 6 is a flowchart showing a detailed operation of the blood pressure estimating processing (ST 2) performed during the pressuring operation. Here, reference character (n) in a pulse wave amplitude AMP (n) designates a pulse wave number. It is assumed that the AMP (n) and a cuff pressure PC (n) have already been calculated in the pulse wave amplitude calculation processing that has been performed before this blood pressure estimating processing. Further, the number (n) of a pulse wave and a variable Amax for storing the maximum of a pulse wave amplitude is initialized to "0" at a measurement start. First, when the number n of a pulse wave is updated (ST 21), the pulse wave amplitude AMP (n) is compared with the maximum pulse wave amplitude Amax (ST 22). If AMP (n) is larger than Amax at this point, while judging that the pulse wave envelope is on the increase (i.e., the pulse wave has not yet reached its maximum point), the judgment result becomes "YES," which causes the MPU to substitute the AMP (n) for the Amax (ST 23) and return thereafter. As AMP (n) becomes smaller than Amax later, the judgment result in ST 22 becomes "NO." That is, the pulse wave envelope has already passed over the maximum point, and this causes the MPU 6 to judge the envelope to be on the decrease, causing the MPU to move to ST 24. In ST 24, it is judged whether or not AMP (n) has decreased below the threshold for determining the systolic pressure. It is set to 0.5 x Amax in the embodiment). If AMP (n) is not decreased below the threshold, then the judgment result in ST 24 becomes "NO," causing the MPU to return (to ST 21). However, if AMP (n) is decreased below the threshold, the judgment result in ST 24 becomes "YES," causing the MPU to calculate an estimated systolic pressure (an estimated maximum blood pressure) Esp (ST 25). During the pressuring operation, the cuff pressure changes greatly and the pressure interval between pulse waves is large. Thus, if the cuff pressure at the time the pulse wave amplitude is decreased below the threshold is taken as an estimated value, its accuracy cannot be guaranteed. To overcome this problem, Esp is calculated through direct extrapolation using the following equation. Esp = Pc(n) + THs - AMP(n) AMP(n-1) - AMP(n) x [Pc(n) - Pc(n-1)] where THs is the threshold. After having calculated the estimated pressuring value in this way, the MPU 6 returns. Figure 7 is a flowchart showing a detailed operation of the blood pressure calculation processing (ST 7) performed during the evacuation process. Here, it is assumed that required data such as AMP (n) and Pc (n) have already been calculated in the pulse wave extraction processing and pulse wave amplitude calculation processing performed immediately before this blood pressure calculation processing. It is also assumed that the number n of a pulse wave and a systolic pressure SP as well as a diastolic pressure DP have already been initialized to "0." First, the number n of a pulse wave is incremented by 1 (ST 71). Then in ST 72, AMP (n) as immediately calculated is compared with Amax. If AMP (n) is smaller than Amax, supposing that the pulse wave envelope has not reached its maximum point, the judgment result in ST 72 becomes "YES," which causes the MPU to substitute AMP (n) for Amax (ST 73) and return to ST 71. If, on the other hand, AMP (n) is larger than Amax, the judgment result in ST 72 becomes "NO," causing the MPU to advance to ST 74. That AMP (n) is larger than Amax means that the envelope has already passed over the maximum point and is on the decrease. In ST 74, it is judged whether or not the systolic pressure (the maximum blood pressure) SP is "0." At this point if SP is "0," it is judged that SP has not yet been determined. In this case, the judgment result in ST 74 becomes "YES," and the MPU thus performs the SP calculation processing in ST 75 to ST 78. Conversely, if SP has been determined, the judgment result in ST 74 becomes "NO," and the MPU moves to ST 79 to execute the DP calculation processing. If it is assumed now that SP is "0" and thus not has been determined, then a pulse wave counter j is set to the current pulse wave number in ST 75 (ST 75). Then, the counter j is incremented by 1 (St 76), and a pulse wave amplitude AMP (j) specified by j is compared with a maximum Amax x 0.5 (ST 77). Here, if AMP (j) is larger than Amax x 0.5, then the judgment result in ST 77 becomes "NO," causing the MPU to return to ST 76, while, on the other hand, if AMP (j) is smaller than Amax x 0.5, then Pc (j) is considered as the systolic pressure (the maximum blood pressure) SP (ST 78). Then, the MPU advances to the diastolic pressure DP calculation processing. First, it is judged whether or not AMP (n) has decreased below a threshold (AMP x 0.7) in ST 79. If AMP (n) has decreased below Amax x 0.7, the judgment result in ST 79 becomes "YES," causing the MPU to recognize Pc (n) as a DP (ST 80) and then return. As described in the foregoing, the invention detects the maximum of pulse wave amplitudes extracted during a cuff pressuring operation to calculate a threshold based on the maximum. Then, judging whether or not the pulse wave amplitude during the pressuring operation is smaller than the threshold, a cuff pressure at the time the pulse wave amplitude during the pressuring operation has become smaller than the threshold is taken as an estimated maximum blood pressure, and the cuff pressuring operation is brought to an end when the cuff pressure is further increased by a predetermined pressure value with respect to the estimated maximum blood pressure. Thus, by extracting a pulse wave component during the cuff pressuring operation and estimating the systolic pressure based on the extracted pulse wave signal, the cuff pressuring operation can automatically be stopped at a cuff pressure that is neither more nor less to match the blood pressure. As a result, the first embodiment of the invention can provide excellent advantages of improving the operability as well as implementing quick measurement and reduction in pain during measurement and cost. The second embodiment of the invention will be described with reference to Figures 4, 8 and 9. As is similar to the first embodiment, the second embodiment of the electronic blood pressure meter also includes a cuff 1, a pressuring pump 2 for pressuring the cuff, a rapid evacuation valve 3 for evacuating the cuff, and a slow evacuation valve 4, with these components being connected to each other through an air tube 5 to form an air system. This rapid evacuation valve 3 and the pressuring pump 2 are electrically connected to an MPU 6 to be described later, and controlled by the MPU 6 independently. Further, a pressure sensor 7 for detecting the pressure inside the cuff is connected to the air tube 5. The pressure sensor 7 detects the air pressure inside the cuff 1, sends the detection signal to an A/D converter 9 through a lowpass filter 8, and the A/D converter 9 converts this analog signal into a digital signal and transmits the digital signal to the MPU 6. The thus embodied electronic blood pressure meter adopts an oscillometric system as a blood pressure determining means. That is, the arterial wall pressured by the cuff 1 vibrates in accordance with pulsation. The amplitude of this vibration varies in accordance with the magnitude of a pressure. And this vibration causes an air pressure within the cuff 1 to vibrate through the cuff 1. The vibration of the air pressure within the cuff 1 is detected as a pulse wave. That is, a pulse wave that can be carried on the cuff pressure signal detected by the pressure sensor 7 is extracted to calculate a pulse wave amplitude value. The MPU 6 determines a maximum blood pressure value and a minimum pressure value by applying a predetermined known algorithm to the tracked pulse wave information. The MPU 6 also performs the function of executing a maximum blood pressure estimating process and determining as a pressuring target a cuff pressure defined by adding a predetermined pressure value (e.g., 15 mmHg) to the estimated maximum blood pressure. Further, the MPU 6 has not only the function of judging whether or not pulse wave information required for estimating the maximum blood pressure at the time the cuff 1 is being pressured for a predetermined time and at a predetermined pressuring speed has been obtained, but also the function of stopping the cuff 1 pressuring operation to rapidly evacuate the cuff 1 to such an extent that the cuff pressure is decreased to, e.g., 30 mmHg, and estimating a maximum blood pressure and determining the pressuring target by repressuring the cuff after the cuff pressuring speed is decreased to 70% of a predetermined pressuring speed in the case where it is judged that no pulse wave information required for estimating the maximum blood pressure has been obtained even if the cuff pressure has already reached, e.g., a level of 180 mmHg as shown in Figure 9. The MPU 6 also performs the function of displaying the maximum blood pressure value and the minimum blood pressure value determined during the cuff evacuation process on a display 10. Figure 8 is a flowchart showing a specific operation of the thus embodied electronic blood pressure meter. When a measurement switch has been pressed with the cuff 1 being wrapped around, e.g., a brachium, the pressuring pump 2 is driven to start pressuring the cuff 1. At this point a repressuring operation identifying flag P is set to "0" at the same time Step ST91. And during the cuff 1 pressuring process, the pulse wave information is detected (ST 92). In ST 93, whether the repressuring operation is necessary or not is judged. What is being judged here is whether or not the pulse wave information for estimating the maximum blood pressure has been detected; i.e., whether or not the pulse wave is not properly tracked due to the pressure speed being too fast when the cuff 1 is being pressured for a predetermined time and at a predetermined speed. If the pulse wave for estimating a maximum blood pressure is detected, no repressuring operation is necessary. Thus, "NO" is the judgment result in ST 93, which causes the MPU to proceed to ST 94. In ST 94, whether or not an estimated maximum blood pressure value (SP) has been determined is judged. The MPU repeats ST 92 to ST 94 until the estimated maximum blood pressure value can be obtained. If it is assumed that the maximum blood pressure value is estimated now, then the judgment result in ST 94 becomes "YES," and the MPU proceeds to ST 95. In ST 95, whether or not the cuff 1 has reached the pressuring target. In other words, whether or not the cuff 1 has been pressured to a value defined by adding, e.g., 15 mmHg to the estimated maximum blood pressure is judged. Supposing that the cuff 1 is pressured to reach the pressuring target, the judgment result in ST 95 becomes "YES," which then terminates the pressuring operation (ST 96). And the MPU proceeds to the cuff 1 evacuation process, and then a blood pressure calculating process for calculating maximum and minimum blood pressure values based on the pulse wave information obtained during this evacuation process is executed (ST 98). In ST 99, whether or not the maximum blood pressure value (DP) has been calculated. That is, whether the maximum blood pressure value and the minimum blood pressure value have been determined is judged. When the maximum blood pressure value and the minimum blood pressure value have been determined, the judgment result in ST 99 becomes "YES," causing the MPU to rapidly evacuate the cuff 1 and display the determined maximum and minimum blood pressure values on the display 9 (ST 100). On the other hand, if it is judged that the repressuring operation is necessary in ST 93, i.e., because the arm of a subject is thin, etc., the cuff 1 pressuring speed is too fast and, as a result, the cuff pressure has already reached, e.g., a level of 180 mmHg, it is judged that pulse wave information required for estimating a maximum blood pressure has not been obtained, then the judgment result in ST 93 becomes "YES," which causes the MPU to proceed to ST 101. In ST 101, whether or not the repressuring operation identifying flag P is set to "1." In other words, whether or not the repressuring operation has once been performed is judged. In this case, since it is a first repressuring operation and P is thus set to "0," the judgment result in ST 101 becomes "NO," which not only causes the MPU to stop the cuff 1 pressuring operation but also to proceed to the rapid evacuation operation in ST 102. And in ST 103, whether or not the cuff pressure is decreased to 30 mmHg. That is, whether or not the cuff pressure is decreased to such an extent as to allow a pulse wave signal to be adequately detected. If the cuff 1 has been evacuated to a level of 30 mmHg, the judgment result in ST 103 becomes "YES," which causes the MPU to start a repressuring operation with the cuff 1 pressuring speed successively set to 70% of the predetermined speed (ST 104). In other words, the pressuring speed is adjusted to such an extent as to allow a pulse wave signal to be tracked without fail even if the arm is too thin (see Figure 9). At this point the repressuring operation identifying flag P is set to "1" (ST 105), and the MPU returns to ST 92 to perform the blood pressure estimating process at this pressuring speed, and the maximum blood pressure estimating process and the pressuring target determining process are similarly executed thereafter. The repressuring operation is performed only once in this embodiment, and if it is judged that the repressuring operation must be repeated (ST 93) after the first repressuring operation, then the MPU, judging, e.g., that the cuff 1 wrapping condition is not satisfactory, returns from ST 101 to ST 100 to suspend the blood measurement. While the example in which pulse wave information required for estimating a maximum blood pressure has been obtained under the pressuring operation for a predetermined time and at a predetermined pressuring speed has been described as a criterion of whether a repressuring operation is necessary or not in the above embodiment, the application of the invention is not limited thereto. For example, it may be judged that a repressuring operation is required in the case where not a single pulse wave is detected even if the internal pressure of the cuff 1 has reached a level of 140 mmHg, or in the case where no falling line of a pulse wave can be found even if the cuff 1 is pressured to 220 mmHg, or in a like case. As described in the foregoing description, the invention evacuates the cuff to a predetermined pressure and repressures it after reducing the cuff pressuring speed to a predetermined speed when a part of the pulse waves cannot be picked up and thus the estimated maximum blood pressure cannot be determined during the cuff pressuring operation. As a result, disadvantages such as not being able to determine a proper pressuring target due to the arm of a subject being thin, etc., which does not allow a maximum blood pressure to be estimated, can be eliminated. This contributes to determining the pressuring target quickly, easily, as well as surely. These are the excellent advantages that the invention can provide while achieving its object. With the above described embodiments, the cuff evacuation in the blood pressure measurement is effected in a fixed, standardized manner by the predetermined slow speed evacuation (e.g., a 4 mmHg/s evacuation). However, in the third embodiment, the interval not essential for the blood pressure measurement (the interval between the timing after which the maximum blood pressure has been found and the timing at which the maximum pulse wave has been found) is substantially skipped by rapidly evacuating the cuff, so that the measurement time can be reduced, and the subject can thereby be relieved from pain caused by the pressure during the measurement. The third embodiment of the present invention has circuit construction substantially same as that shown in Figure 4. In the third embodiment, the MPU 5 has the function of calculating a pulse wave amplitude value and determining a minimum blood pressure value and a maximum blood pressure value from the obtained pulse wave amplitude and cuff pressure values. The MPU 5 also has the function of estimating a cuff pressure and a maximum blood pressure at the time the extracted maximum pulse wave is present based on the pulse wave information obtained during the cuff pressuring process and storing the estimated pressure values in a memory. The MPU 5 further has the function of rapidly evacuating the cuff from a cuff pressure point defined by subtracting a predetermined value (15 mmHg) from the estimated maximum blood pressure value to a cuff pressure point defined by adding a predetermined value (15 mmHg) to the cuff pressure at the time the maximum pulse wave is present. Further, the MPU 5 has the function of displaying the maximum and minimum blood pressure values on a display 10. Figures 10 (A) and 10 (B) are flowcharts showing a specific operation of the thus embodied electronic blood pressure meter. When a pressuring switch of the blood pressure meter is turned on (Step ST 111), the pressuring pump 2 is driven (ST 112). Simultaneously therewith, the control valve 3 is brought into a complete "closed" position (ST 113), and the cuff 1 gets rapidly pressured. While the cuff 1 is being thus pressured, not only the cuff pressure is read but also the pulse wave is extracted (ST 114 and ST 115). In ST 116, whether the maximum pulse wave has been extracted or not is judged. As shown in Figure 11, when the maximum pulse wave (the pulse wave peak value) has been extracted at the rapid cuff pressuring stage, "YES" is the result of the judgment in ST 116, which causes the CPU to store a cuff pressure P M ' at the time the maximum pulse wave is present (see Figure 11). The cuff pressure is continuously read and the pulse wave is extracted (ST 118 and ST 119). In ST 120, it is judged whether a maximum blood pressure value has been estimated or not. The maximum blood pressure is estimated as a pressure corresponding to a cuff pressure at the time 50% of the maximum pulse wave is present in its increasing phase. When the maximum blood pressure value P S ' has been estimated, the judgment result in ST 120 becomes "YES," which causes the MPU to store the estimated maximum blood pressure value P S ' in the memory (ST 121). When the cuff 1 is pressured to reach a pressuring target, the operation of slowly evacuating the cuff 1 is started (ST 123) after the driving of the pressuring pump 2 has been stopped (ST 122). The slow evacuation of the cuff 1 is controlled by the control valve so as to be executed at a speed of about 4 mmHg/s. With the cuff being slowly evacuated, the cuff pressure is read (ST 124). In ST 125, it is judged whether or not the current cuff pressure is smaller than a pressure (P S ' + α) defined by adding a predetermined value α (15 mmHg) to the estimated maximum blood pressure. As shown in Figure 11, when the cuff 1 is evacuated to cause the cuff pressure to be below this point, the judgment result in ST 125 becomes "YES." This causes the CPU to read the cuff pressure and extract the pulse wave with the cuff being slowly evacuated (ST 126 and ST 127). And, in ST 128, it is judged whether or not the current cuff pressure is smaller than a value (P S ' - α) defined by subtracting a predetermined value α from the estimated maximum blood pressure. If the cuff 1 has been evacuated to have a value below this point, then the judgment result in ST 128 becomes "YES," which causes the CPU to rapidly evacuate the cuff 1 (ST 129). In short, the cuff pressure and the pulse wave amplitude in a range (±15 mmHg) around the maximum blood pressure value P S ' that is roughly estimated during the process of pressuring the cuff 1 are detected to find a genuine P S (a pulse wave that is a value 50% of the maximum pulse wave and a cuff pressure corresponding to such pulse wave), and the cuff 1 is rapidly evacuated thereafter. And as shown in Figure 10 (B), during the process of rapidly evacuating the cuff 1, a cuff pressure is being read (ST 130). In ST 131, it is judged whether or not the cuff pressure has been below a value (P M ' + α) defined by adding a predetermined value α to the cuff pressure P M ' at the time the maximum pulse wave is present. In other words, it is judged whether or not the cuff pressure has neared at a point where the maximum pulse wave is present. As shown in Figure 11, when the pressure of the cuff 1 has dropped to the point (P M ' + α), the judgment result in ST 131 becomes "YES." which causes the CPU to suspend the rapid evacuation by the control value 3 and the cuff 1 is thereby subjected to a slow evacuation again (ST 132). While the cuff is being slowly evacuated, the cuff pressure is being read and the pulse wave is being extracted (ST 133 and ST 134) to detect a maximum pulse wave A M (ST 135). A value A S (a pulse wave amplitude at the time of SYS) is calculated from A M by a predetermined operation, and the cuff pressure (larger than a cuff pressure at the time A M is present) at the time A S is present is calculated as a maximum blood pressure (ST 136). Further, A M is subjected to predetermined processing to obtain A D (a pulse wave amplitude at the time of DIA) (ST 137). Then, the cuff is being read and the pulse wave is being extracted (ST 138 and ST 139). In ST 140, it is judged whether or not the cuff pressure P D at the time A D that has been calculated in ST 137 is present (see Figure 11), i.e., the minimum blood pressure, has been stored is judged. If the minimum blood pressure has been stored, and if the minimum blood pressure has been stored, then the judgment result in ST 140 becomes "YES." This causes the CPU to display the maximum blood pressure and the minimum blood pressure on the display 9 (ST 141), and the cuff 1 is thereafter subjected to rapid evacuation (ST 142). As described in the foregoing, the invention estimates both the cuff pressure and the maximum blood pressure at the time the maximum pulse wave is present based on the pulse wave information obtained during the cuff pressuring process, and then rapidly evacuates the cuff from the cuff pressure point defined by subtracting a predetermined value from the estimated maximum blood pressure to the cuff pressure point defined by adding a predetermined value to the cuff pressure at the time the maximum pulse wave is present during the cuff evacuating process. Therefore, the interval that is not essential to the measurement, i.e., an interval between the timing after which the maximum blood pressure is present to the timing at which the maximum wave pulse is present, can be skipped by rapidly evacuating the cuff. Hence, the wasteful interval can be eliminated from the measurement, which contributes not only to achieving a reduction in measurement time, but also to making a measurement without keeping the subject under pain for a long time. Further, since the reduction in measurement time leads to a reduction in the rate of mixture of noise such as body movements, thereby allowing the reliability in measurement accuracy to be improved. Such excellent advantages are provided by the invention by successfully achieving its object.

Claims: An electronic blood pressure meter comprising: a cuff (1), means (2, 3, 4) for controlling an inside fluid pressure of said cuff, means (7) for detecting the inside fluid pressure of said cuff to produce a cuff pressure detecting signal, means for detecting the amplitude of a pulse wave superposed on said inside fluid pressure, maximum blood pressure estimating means for obtaining an estimated value of a maximum blood pressure in response to output signals of said pulse wave amplitude detecting means, said maximum blood pressure estimating means detecting the maximum of the pulse wave amplitude during a pressurizing operation of said controlling means, and means for determining a blood pressure based on output signals of said pulse wave amplitude detecting means and said pressure detecting means, characterized in that said maximum blood pressure estimating means determines a threshold value from the maximum of the pulse wave amplitude and determines as the estimated value of the maximum blood pressure the inside pressure of said cuff when the detected amplitude of the pulse wave becomes smaller than the threshold value, that said controlling means stops the pressurizing operation when the inside fluid pressure of said cuff exceeds a total pressure of the estimated value and a predetermined value, and that said controlling means depressurizes said cuff after stopping the pressurizing operation so that said determining means determines a blood pressure. An electronic blood pressure meter as defined in claim 1, wherein the threshold value is one-half of the maximum in the pulse wave amplitude. An electronic blood pressure meter as defined in claim 1 or 2, wherein the predetermined value is 30 mmHg. An electronic blood pressure meter as defined in claim 1 or 2, wherein the predetermined value is 15 mmHg. An electronic blood pressure meter as defined in any of the preceding claims, wherein in case of no determination of the estimated value, said control means evacuates the cuff (1) to a predetermined pressure, and then repressurizes said cuff with a cuff pressurizing speed reduced to a predetermined speed. An electronic blood pressure meter as defined in any of the preceding claims, further comprising means for obtaining an estimated value of a maximum blood pressure and an estimated value of the inside fluid pressure of said cuff upon occurrence of the maximum of the pulse wave amplitude during the pressurizing operation of said controlling means in response to output signals of said pulse wave amplitude detecting means and pressure detecting means, said controlling means decreasing the inside fluid pressure of said cuff rapidly from a pressure defined by subtracting a predetermined value from the estimated value of the maximum blood pressure to a pressure defined by adding a predetermined value to the estimated value of the inside fluid pressure of said cuff upon the occurrence of the maximum amplitude.

Inventors: FUKURA MASASHI, MIYAWAKI YOSHINORI, SASABATA AKIHIRO, SHIRASAKI OSAMU, FUKURA, MASASHI, MIYAWAKI, YOSHINORI, SASABATA, AKIHIRO, SHIRASAKI, OSAMU, Fukura, Masashi, c/o Intellectual Property Center, Miyawaki, Yoshinori, c/o Intellectual Prop. Center, Sasabata, Akihiro, c/o Intellectual Prop. Center, Shirasaki, Osamu, c/o Intellectual Property Center
Applicants: OMRON TATEISI ELECTRONICS CO, OMRON CORPORATION
Application year: 1992
Main classification: A61B5
IPC codes: A61B5