Conventionally, an oscillometric technique is known to measure blood pressure non-invasively. In the oscillometric technique, a cuff is fitted around a limb such as a brachium of a subject to detect pulse waves at the body part by the cuff and a blood pressure is measured based on the fact that the volume of the blood vessel under the cuff changes most at the point when the pressure applied by the cuff to the blood vessel becomes equal to a mean blood pressure thereof.
Specifically, the cuff is inflated to press the body part for avascularization, and as the cuff is being deflated, the cuff pressure is detected until the pressure goes below a diastolic blood pressure thereof to obtain pressure signals to which pulse wave signals are superimposed. The pulse wave signals are separated from the pressure signals by a filter to determine that the cuff pressure value at the point at which the maximum amplitude through the entire period of the separated pulse wave signals (i.e. maximum peak-to-peak value; hereinafter, “an amplitude” is used as a peak-to-peak value) was observed equals to a mean blood pressure thereof. The systolic blood pressure and the diastolic blood pressure are generally determined to be the cuff pressure values at which an amplitude under a predetermined condition is detected respectively based on the maximum amplitude (for example, see Shimazu Hideaki, “Blood Pressure”, Mar. 15, 2001, pp. 112-116).
The automated blood pressure measuring apparatus by the oscillometric technique are widely used since a blood pressure can be measured relatively easily, however, they require a certain time for measurement. This is mainly because a blood pressure value is determined based on the maximum amplitude of detected pulse wave signals in such conventional blood pressure measuring apparatus.
In order to shorten the time for measurement, a cuff deflation rate after avascularization needs to be increased; however, the increased cuff deflation rate makes it difficult to detect the maximum amplitude of pulse waves with high accuracy, which in turn results in increasing errors in blood pressure values.
The pulse wave signals are not detected when the cuff pressure on a body part goes below a diastolic blood pressure thereof. As far as the pulse rate does not change, the higher deflation rate allows only the fewer pulses of pulse wave signals to be detected. Moreover, because the higher deflation rate makes the time around which the cuff pressure equals to the mean blood pressure shorter, the timing of the largest amplitude of the pulse waves may be offset from the timing around which the cuff pressure equals to the mean blood pressure with high possibility.
Meanwhile due to the above factors the increased deflation rate makes it difficult to detect the maximum amplitude with high accuracy, there is a demand from subjects for quick release from the pressure by a cuff after avascularization because the pressure is not comfortable.