AUTOMATIC TONOMETER

Abstract: The cuff is pressurised at a relatively high rate before the estimated maximum blood pressure value is almost attained. The rate is then reduced down to one required by that measurement so as to measure the maximum blood pressure value. To measure the minimum blood pressure value, the pressure is reduced quickly to a level a predetermined value above the estimated minimum blood pressure value immediately after completion of measurement of the maximum blood pressure value, and the pressure reducing speed is then reduced to actually measure the minimum blood pressure value.

Description: Technical Field The present invention relates to an automatic sphygmomanometer and, more particularly, to an automatic sphygmomanometer arranged to automatically process a sequence of blood pressure measurement operations. Background Art Conventional sphygmomanometers of this type are commonly arranged first to increase the air pressure in the air back incorporated in an arm band (hereinafter referred to simply as the "arm band") to a pressure which is approximately 30 mmHg higher than the expected maximal blood pressure of a subject (the maximal blood pressure expected by a person who conducts the measurement of blood pressure) and then to measure the blood pressure while reducing that air pressure at a rate as slow as 3-4 mmHg/second. As is known, it is customary to preset the pressure corresponding to the expected maximal blood pressure plus 30 mmHg by means of a switch having, for example, five settings. Blood pressure measurement utilizing such a sphygmomanometer will be described in detail below. First, the instant that Korotkoff sounds initially appear while the pressure in the arm band is being reduced from its fully increased state or the time instant that small variations in the pressure in the arm band increase is detected, and the air pressure in the arm band at that instant is measured to obtain the maximal blood pressure. Thereafter, the instant that the Korotkoff sounds disappear while the pressure in the arm band is being gradually reduced or the instant that small variations in the pressure in the arm band abruptly decreases is detected, and the air pressure in the arm band at that instant is measured to obtain the minimal blood pressure. However, since the pressure in the arm band is slowly reduced from a pressure higher than the expected maximal blood pressure to the minimal blood pressure, it has been impossible to avoid the problem that the discomfort inflicted upon an arm of the subject becomes serious. Moreover, it has been necessary to involve the awkward operation of setting a pressure which is approximately 30 mmHg higher than the pressure expected as the maximum blood pressure. Prior art document US-A-4 461 266 discloses an adaptive incremental blood pressure monitor for rapidly determining blood pressure. This monitor selects an initial cuff pressure to be applied to an artery of a test subject and then measures the amplitude of pressure pulses caused by the pumping of blood by the heart of the subject. The cuff pressure is incrementally increased while the pulse amplitudes are monitored in order to obtain blood pressure readings by an oscillometric method. If the pulse amplitudes decrease for increases in pressure above the initial value, it is taken as an indication that mean arterial pressure is below the initial cuff pressure. Thus, the cuff pressure is substantially decreased to a new initial value, and the process is restarted. If it should be found upon a determination of the mean arterial pressure and the systolic pressure, that the initial or new cuff pressure was not low enough to determine diastolic pressure, the cuff pressure is automatically decreased in one large step to a level below the initial pressure and is then decreased in steps until diastolic pressure is located. Disclosure of the Invention In view of the foregoing difficulties with the prior art, it is an object of the present invention to provide an automatic sphygmomanometer which is designed to predict the maximal blood pressure of a subject from the pulse-wave signal thereof, then to increase cuff pressure to a level immediately below the predicted maximal blood pressure, and subsequently to decrease the rate of pressure increase to a rate which allows measurement to be conducted to thereby measure the maximal blood pressure, whereby it is not necessary to increase the pressure in the arm band to a level substantially higher than the maximal blood pressure. To solve this object the present invention provides an automatic sphygmomanometer as specified in claim 1. Preferred embodiments of this sphygmomanometer are described in claims 2 to 7. It is another object of the present invention to provide an automatic sphygmomanometer arranged to predict the maximal and minimal blood pressures of a subject by utilizing a pulse-wave signal thereof, increase the pressure in an arm band to a level immediately below the predicted maximal blood pressure, reduce the rate of pressure increase to a rate which allows measurement of the maximal blood pressure to be conducted, rapidly reduce the pressure in the arm band to a predetermined level immediately above the predicted minimal blood pressure after completion of the measurement of the maximal blood pressure, and reduce the rate of pressure reduction to a rate which allows measurement of the minimal blood pressure to be conducted to thereby measure the minimal blood pressure. To solve this object the present invention provides an automatic sphygmomanometer as specified in claim 8. Preferred embodiments of this sphygmomanometer are described in claims 9 to 18. Brief Description of the Drawings Figs. 1(a) and 1(b) are block diagrams showing an embodiment of an automatic sphygmomanometer according to the present invention; Fig. 2 is a graphic representation showing the relationship between time and the amplitude of a pulse-wave signal when the rate of pressure increase is constant; Fig. 3 is a graphic representation showing variations in the pressure in the cuff of the embodiment during measurement of blood pressure; and Figs. 4(a) to 4(c) are flow charts which serve to illustrate a process procedure for measuring blood pressure in the present embodiment. Best Mode for Carrying Out the Invention An embodiment of the present invention will be described in detail below with reference to the accompanying drawings. Figs. 1(a) and 1(b) are block diagrams which show the basic construction of an embodiment of an automatic sphygmomanometer according to the present invention. In these figures, a microprocessor (hereinafter referred to simply as "CPU") for controlling the whole apparatus is denoted at 100, and CPU 100 is arranged to operate in accordance with the process procedure (program) shown in the flowchart of Figs. 4(a) to 4(c), which will be explained later. This process procedure is stored as a program in ROM 101a incorporated in memory 101. Memory 101 further includes RAM 101b which is used as a work area for CPU 100 or for storage of various types of data. A cuff (arm band) is denoted at 102, and a constant-rate pressure reducing valve 103 is provided in this cuff 102 so that air is exhausted at a constant rate. Reference numeral 104 denotes a pressure sensor for detecting the air pressure within the cuff 102 to output the detection value as an analog value. This output is amplified by a pressure gauge amplifier 114, while the pulse-wave component contained in the output is amplified by a pulse-wave amplifier 115. The analog outputs of the respective amplifiers 114 and 115 are converted into digital values by an A/D converter 105. A microphone 106 is provided in the cuff 102, and serves to detect Korotkoff sounds (hereinafter referred to as the "K sounds") with the cuff 102 placed around an arm of the subject. The K sounds detected by the microphone 106 are amplified by a K sound amplifier 116 and fed to the A/D converter 105 described above. A pump for pumping air into the cuff 102 is denoted by 107, and is driven by a motor driver 108. Reference numeral 109 denotes a pulse-wave filter for shaping the flow of the air discharged from the pump 107 when the air is being charged into the cuff 102. Reference numerals 111 and 113 denote valve drivers for opening and closing a bypass filter 113 and an exhaust valve 112, respectively. A pressure limit value setting switch is denoted by 117, and the pressure limit value set by this switch 117 is fed to CPU 100. This arrangement makes it possible to prevent pressure above this limit value from being applied to the subject. After a pulse-wave peak point has been detected, a predicted value plus 30 mmHg is employed as the pressure limit value. This predicted value is obtained by predicting a maximal blood pressure by using the following equation on the basis of a minimal blood pressure derived from a pulse wave detected during a pressure increasing process or a minimal blood pressure derived from the K sounds and an average blood pressure corresponding to the pulse-wave peak point. Alternatively, it is also possible to employ, as the pressure limit value, the blood pressure existing when the amplitude of a pulse wave reaches 5-30 % of the pulse-wave amplitude of the average blood pressure corresponding to the pulse-wave peak point. In other words, while CPU 100 is increasing the pressure in the cuff 102 during measurement of blood pressure, if CPU 100 determines that the cuff pressure has reached the expected maximal blood pressure, then CPU 100 causes air to be exhausted from the cuff 102 in order to immediately stop the measurement. The basic operation of the present sphygmomanometer having the above-described arrangement will be explained below. As air is being fed into the cuff 102 by driving the pump 107 with the cuff (arm band) 102 placed around an arm of the subject, a pulse-wave signal is detected by the pressure sensor 104. This pulse wave is shown in electrical-signal form in Fig. 2. For the sake of simplifying the explanation, it is assumed that the cuff pressure is increased at a constant ratio. As the cuff pressure is increased, the amplitude of the pulse-wave signal gradually increases, and after this amplitude passes a peak point (P₁ · P₂), it gradually decays. In general, as shown in Fig. 2, if the peak point of the amplitude of the pulse wave and that amplitude can be detected, it is possible to predict the maximal and minimal blood pressures of the subject. In the present embodiment, it is predicted that the minimal blood pressure is present in the neighborhood of an amplitude (P₃ · P₄) which corresponds to 80% of the amplitude of the peak position, that is, the peak point (P₁ · P₂) and that the maximal blood pressure is present in the neighborhood of an amplitude (P₅ · P₆) which corresponds to 50% of the amplitude of the peak point (P₁ · P₂). Incidentally, the above noted numerical values, such as 80 % and 50 %, are not construed as restrictive values since they may vary, depending upon the construction of the hardware. It will be understood from the foregoing that, when the maximal blood pressure is to be measured, the pressure within the cuff 102 may be increased at a relatively high rate up to the instant of time slightly beyond the time corresponding to the peak position of the pulse-wave signal. In the present embodiment, the maximal amplitude is utilized as a limit up to which the cuff pressure can be increased at this relatively high rate, and after the amplitude of the pulse-wave signal has reached the limit, the rate of pressure increase is slowed down to initiate measurement of the maximal blood pressure. That is to say, the maximal blood pressure is measured while the cuff pressure is being increased. Accordingly, it is possible to measure the maximal blood pressure without the need to impose upon the arm of the subject a burden which is 30 mmHg greater than the expected maximal blood pressure as in the case of prior art sphygmomanometers. The above process procedure for measuring blood pressure will be described in greater detail with reference to the graph of Fig. 3 and the flowchart shown in Figs. 4(a) to 4(c). Fig. 3 is a graph which shows variations occurring in the pressure in the cuff 102 during a sequence of blood-pressure measuring steps conducted in accordance with the present embodiment, in which the vertical axis represents cuff pressure and the horizontal axis represents time. The process procedure shown in the flowchart of Figs. 4(a) to 4(c) starts when a switch (not shown) for initiating measurement of blood pressure is actuated. This process procedure (program) is stored in ROM 101a in memory 101. In the following explanation, it is assumed that "150 mmHg" (hereinafter called the "limit value") is preset through the pressure limit value setting switch 117. The period between S and A When the switch (not shown) for initiating measurement of blood pressure is switched on, the process proceeds to Step S1, in which CPU 100 first energizes the valve driver 113 to close the exhaust valve 112 and, at the same time, energizes the valve driver 111 to open the bypass valve 110. Then, in Step S2, CPU 100 causes the pump 107 to run at full power and, in Step S3, waits for the pressure in the cuff 102 to reach 20 mmHg. It is thus possible to speed up the processing between points A and B, as will be described later. The period between A and B When the pressure in the cuff 102 reaches 20 mmHg, the process proceeds to Step S4, in which CPU 100 closes the bypass valve 110, actuates the pulse-wave filter 109, and sets the pump 107 to its normal running state, thereby setting the rate of pressure increase to approximately 50 mmHg/7 seconds. The pressure in the cuff 102 gradually increases at this rate, and during this pressure increase, CPU 100 executes the following processing. First, in Step S5, CPU 100 compares the limit value with the pressure in the cuff 102. If it is determined that the cuff pressure is less than the limit value, the process proceeds to the next step S6, but if not, that is, if the pressure in the cuff 102 exceeds the limit value, the process jumps to Step S21, which will be described later. In Step S6, the amplitude of a pulse-wave signal is detected through the A/D converter 105 and, in Step S7, whether the amplitude of the pulse-wave signal has reached its maximum is determined. If it is determined in Step S7 that the amplitude has not yet reached the maximum, the process returns to Step S5, in which a similar processing is repeated. In practice, variations in the amplitude of the pulse-wave signal are sequentially measured, and the pulse-wave signal measured immediately before the magnitude of the amplitude starts to decrease is utilized as the pulse-wave signal of maximum amplitude. After the pulse-wave signal of maximum amplitude has been measured, the process proceeds to Step S8, where the pressure in the cuff 102 and the amplitude of the pulse-wave signal which are obtained at this time are stored in RAM 101b of the memory 101. In this manner, the pressure in the cuff 102 increases until the amplitude of the pulse-wave signal is detected at its maximum with the cuff pressure being lower than the limit value. The period between B and C If the amplitude of the pulse-wave signal reaches its maximum amplitude while the pressure in the cuff 102 is being increased, it is determined that the cuff pressure has been increased up to the neighborhood of the maximal blood pressure, and measurement of the maximal blood pressure is initiated. In Step S12, the rate of pressure increase is first reduced to 50 mmHg/12 seconds by controlling the pump 107. In the following step S13, a comparison between the cuff pressure and the limit value is made in a manner similar to that executed in Step S5. In Step S14, the measurement of the maximal blood pressure is initiated and, in Step S15, it is determined whether or not the measurement has been completed. If the answer is "NO", the above steps S13 to S15 are repeated until the measurement has been completed. In practice, the maximal blood pressure is the pressure which exists one stroke before the instant that it becomes impossible to detect any K sound through the microphone 106 while the pressure in the cuff 102 is being increased at the aforesaid pressure-increasing rate (50 mmHg/12 seconds). Accordingly, strictly, point C in Fig. 3 corresponds to a pressure whose K sounds exceed the K sounds of the maximal blood pressure by two strokes. Accordingly, since the maximal blood pressure is measured while the pressure in the cuff 102 is being increased, it is not necessary to increase the pressure in the cuff 102 to a value which is greater than the expected maximal blood pressure by approximately 30 mmHg as in the case of prior art apparatus. It is therefore possible to reduce the discomfort inflicted upon the arm of the subject. The measured maximal blood pressure is displayed on a display screen, output in printed form, or stored in a memory device or the like. However it is done, the method of processing measurement results is not to be limited by the present embodiment. In addition, the maximal blood pressure is predicted from the amplitude of the pulse-wave signal. The pressure in the cuff 102 is increased at a relatively high rate immediately before the predicted value and is thereafter increased at a slower rate. It is therefore possible to measure the maximal blood pressure in a short time without causing a reduction in the accuracy of measurement of the maximal blood pressure. The period between C and D When the maximal blood pressure measurement has been completed, the process immediately proceeds to Step S16, in which the pressure in the cuff 102 is rapidly reduced by stopping the pump 107 and opening the exhaust valve 112. In Step S17, it is determined whether or not 10 mmHg plus the value stored in RAM 101b (the value of the pressure in the cuff 102 when the amplitude of the pulse-wave signal is at the maximum) is equal to the value of the pressure in the cuff 102 during the rapid reduction thereof. The process waits for these values to become equal. Incidentally, the value "10 mmHg" is selected by considering the response time of the apparatus, particularly that of a pressure system thereof. The period between D and E If it is determined that the pressure in the cuff 102 has become equal to the aforesaid value, that is, if it is determined that the cuff pressure has reached a level slightly higher than the minimal blood pressure, the exhaust valve 112 is immediately closed in Step S18. Thereafter, the pressure in the cuff 102 is reduced at a rate of approximately 50 mmHg/12 seconds by the action of the constant-rate pressure-reducing valve 103. Then, in steps S18 to S19, the minimal blood pressure is measured. In practice, the minimal blood pressure is obtained at instant that the K sounds from microphone 106 disappear while the pressure in the cuff 102 is being reduced. As described above, in the case of the measurement of the minimal blood pressure as well, the cuff pressure is reduced at a high rate to a pressure level slightly higher than the predicted minimal blood pressure. Thereafter, while the cuff pressure is being decreased at a relatively slow rate, the minimal blood pressure is measured. It is therefore possible to achieve a reduction in time and, hence, minimize the irritation inflicted upon the arm of the subject due to a lengthy measurement period. The period between E and F After the maximal and minimal blood pressures have been measured, the exhaust value 112 and the bypass valve 110 are, in Step S22, fully opened to exhaust the air of the cuff 102 rapidly, thereby completing the measurement. The case where the cuff pressure ≧ the limit value If, in the processing described above, it is determined (Steps S5 and S13) that the pressure in the cuff 102 has been increased to a value higher than the limit value, the operation of a pressure increasing pump is first stopped in Step S21 to rapidly exhaust, in Step S22, the air pumped into the cuff 102, thereby preventing the cuff pressure from increasing accidentally. In accordance with the present invention described above, it is possible to reduce the discomfort to the patient to a minimum since the cuff pressure need not be increased to a pressure which is substantially higher than the maximal blood pressure of the patient during the measurement of the maximal blood pressure. Moreover, it is not necessary to set the value of the pressure increase, and even when the minimal blood pressure is measured, the cuff pressure is rapidly reduced to the neighborhood of the predicted minimal blood pressure. It is therefore possible to achieve a reduction in measuring time, hence reduce the discomfort to the patient to a minimum. In addition, the measurements of the maximal and minimal blood pressures can be combined to minimize the discomfort to the patient due to the application of pressure and a lengthy operation. It is also possible to effect measurement in a short time. Furthermore, even if the pressure in the cuff 102 rises to some excessive level due to an arbitrary cause, it will not exceed the preset limit value. Accordingly, this sphygmomanometer of the invention excels in safety as well.

Claims: An automatic sphygmomanometer comprising: pressure detecting means (104) for detecting a cuff pressure; first pressure-increasing means (107, 108) for increasing said cuff pressure at a high pressure-increasing rate; pulse-wave signal detecting means (109) for detecting the amplitude of the pulse-wave signal of a subject while said first pressure-increasing means (107, 108) is increasing said cuff pressure; maximal blood pressure predicting means (100) for predicting a maximal blood pressure from the peak point of said detected amplitude of said pulse-wave signal; and de-energizing means (112), characterized in that determining means (100, 101) determine whether or not said cuff pressure has been increased to a predetermined level below said maximal blood pressure predicted by said maximal blood pressure predicting means (100); second pressure-increasing means (107, 108) increase said cuff pressure at a rate lower than said pressure-increasing rate of said first pressure-increasing means (107, 108) when said determining means (100, 101) determines that said cuff pressure is a predetermined level below said predicted maximal blood pressure; Korotokoff sound detecting means (106) detect Korotokoff sounds; measuring means (104, 114) measure said maximal blood pressure by using said Korotokoff sound detecting means (106) while said pressure-increasing means (107, 108) is increasing said cuff pressure; and said de-energizing means (112) de-energize said second pressure-increasing means (107, 108) immediately when said measuring means (104, 114) has measured maximal blood pressure, in order to prepare to measure a minimal blood pressure. An automatic sphygmomanometer according to claim 1, wherein said first pressure-increasing means (107, 108) is energized after said cuff pressure has reached approximately 30 mmHg, said cuff pressure being rapidly increased until said cuff pressure reaches approximately 30 mmHg. An automatic sphygmomanometer according to claim 1, wherein said pressure-increasing rate of said first pressure-increasing means (107, 108) is approximately 50 mmHg/7 seconds. An automatic sphygmomanometer according to claim 1, wherein said predicting means (100) predicts said maximal blood pressure by utilizing the proportion of the amplitude of said pulse-wave signal to the maximum amplitude thereof. An automatic sphygmomanometer according to claim 1, wherein said predetermined level below said maximal blood pressure predicted by said maximal blood pressure predicting means (100) is a pressure corresponding to the maximum amplitude of said pulse-wave signal. An automatic sphygmomanometer according to claim 1, wherein the pressure-increasing rate of said second pressure-increasing means is approximately 50 mmHg/12 seconds. An automatic sphygmomanometer according to claim 1, further including a limiter (117) for preventing said cuff pressure from exceeding a predetermined pressure. An automatic sphygmomanometer comprising: pressure detecting means (104) for detecting a cuff pressure; first pressure-increasing means (107, 108) for increasing said cuff pressure at a high pressure-increasing rate; pulse-wave signal detecting means (109) for detecting the amplitude of the pulse-wave signal of a subject while said first pressure-increasing means (107, 108) is increasing said cuff pressure; and predicting means (100) for predicting maximal and minimal blood pressure from the peak point of said detected amplitude of said pulse-wave signal; characterized by first determining means (100, 101) for determining whether or not said cuff pressure has been increased to a predetermined level below said maximal blood pressure predicted by said predicting means (100); second pressure-increasing means (107, 108) for increasing said cuff pressure at a rate lower than said pressure-increasing rate of said first pressure-increasing means (107, 108) when said first determining means (100, 101) determines that said cuff pressure is a predetermined level below said predicted maximal blood pressure; Korotokoff sound detecting means (106) for detecting Korotokoff sounds; maximal blood pressure measuring means (104, 114) for measuring said maximal blood pressure by using said Korotokoff sound detecting means (106), while said second pressure-increasing means is increasing said cuff pressure; first pressure-reducing means (112) for reducing said cuff pressure at a high pressure-reducing rate immediately when said maximal blood pressure has been measured; second determining means (100, 101) for determining whether or not said cuff pressure has been decreased to a predetermined level above said minimal blood pressure predicted by said predicting means (100); second pressure-reducing means for reducing said cuff pressure at a rate lower than said pressure-reducing rate of said first pressure-reducing means (112) when said second determining means (100, 101) determines that said cuff pressure is a predetermined level above said predicted minimal blood pressure; and minimal blood pressure measuring means (104, 114) for measuring said minimal blood pressure by using said Korotokoff sound detecting means (106) while said second pressure-reducing means is reducing said cuff pressure. An automatic sphygmomanometer according to claim 8, wherein said first pressure-increasing means is energized after said cuff pressure has reached approximately 20 mmHg, said cuff pressure being rapidly increased until said cuff pressure reaches approximately 20 mmHg. An automatic sphygmomanometer according to claim 8, wherein said pressure-increasing rate of said first pressure-increasing means is approximately 50 mmHg/7 second. An automatic sphygmomanometer according to claim 8, wherein said predicting means predicts said maximal and minimal blood pressures by utilizing the proportion of the amplitude of said pulse-wave signal to the maximum amplitude thereof. An automatic sphygmomanometer according to claim 8, wherein said predetermined level below said maximal blood pressure predicted by said maximal blood pressure predicting means is a pressure corresponding to the maximum amplitude of said pulse-wave signal. An automatic sphygmomanometer according to claim 8, wherein the pressure-increasing rate of said second pressure-increasing means is approximately 50 mmHg/12 seconds. An automatic sphygmomanometer according to claim 8, wherein said first pressure-reducing means is energized by opening an exhaust valve (112). An automatic sphygmomanometer according to claim 8, wherein said predetermined level above said minimal blood pressure predicted by said predicting means is approximately equal to 10 mmHg plus a pressure corresponding to the maximum amplitude of said pulse-wave signal. An automatic sphygmomanometer according to claim 8, wherein said pressure-reducing rate of said second pressure-reducing means is approximately 50 mmHg/12 seconds. An automatic sphygmomanometer according to claim 8, wherein cuff-charged air is rapidly exhausted after said measuring means has measured said minimal blood pressure. An automatic sphygmomanometer according to claim 8, further including a limiter for preventing said cuff pressure from exceeding a predetermined pressure.

Inventors: SOUMA TAKAHIRO TERUMO KABUSHIK, SOUMA, TAKAHIRO TERUMO KABUSHIKI KAISHA
Applicants: TERUMO CORP, TERUMO KABUSHIKI KAISHA
Application year: 1987
Main classification: A61B5
IPC codes: A61B5