1. Field of the Invention
The present invention relates to a pressure-pulse-wave detecting apparatus which presses an artery of a living subject and detects a pressure pulse wave produced from the artery.
2. Related Art Statement
There is known a pressure-pulse-wave detecting apparatus which includes a pressure-pulse-wave sensor adapted to be pressed against an artery via skin and detects, through the sensor, a pressure pulse wave produced from the artery. Various sorts of information can be obtained from a shape or form of the thus detected pressure pulse wave. For example, a pulse-wave propagation velocity, i.e., a velocity at which a pulse wave propagates in a living subject may be determined in such a manner that a pressure-pulse-wave sensor is worn on a first portion of the subject to detect a pressure pulse wave from the first portion; a heartbeat-synchronous signal such as a pulse wave or a heart sound is detected from a second portion of the subject; and the pulse-wave propagation velocity is determined based on a time difference between a time of detection of a characteristic point of the pressure pulse wave, such as a rising point or a peak point, and a time of detection of a characteristic point of the heartbeat-synchronous signal. In addition, an augmentation index (hereinafter, referred to as AI) may be determined based on the form of pressure pulse wave. More specifically described, respective peak points of incident-wave and reflected-wave components of a pressure pulse wave are determined; a pressure difference (xcex94P) obtained by subtracting a magnitude of the pressure pulse wave at the time of occurrence of the peak point of the incident-wave component, from a magnitude of the pressure pulse wave at the time of occurrence of the peak point of the reflected-wave component, is divided by a pulse pressure, PP, of the pressure pulse wave; and an AI value is determined by multiplying the thus obtained value by 100, according to Expression 1:
AI=(xcex94P/PP)xc3x97100(%)xe2x80x83xe2x80x83(Expression 1)
Measuring an accurate pulse-wave propagation velocity needs to determine an accurate characteristic point of a pressure pulse wave; such as a rising point or a peak point. In addition, determining an accurate Al value needs to determine respective accurate magnitudes of a pressure pulse wave at the respective times of occurrence of respective peak points of incident-wave and reflected-wave components of the pressure pulse wave, and an accurate pulse pressure PP of the pressure pulse wave. That is, obtaining accurate pulse-wave propagation velocity and AI value needs to obtain a pressure pulse wave having an accurate form. If the state in which the pressure-pulse-wave sensor is worn is not appropriate, the pressure pulse wave may be attenuated by the arterial wall and/or the skin or subcutaneous tissue. Therefore, obtaining a pressure pulse wave having an accurate form needs to wear the pressure-pulse-wave sensor in an appropriate manner.
However, the conventional pressure-pulse-wave detecting apparatus cannot judge whether a pressure pulse wave detected by itself has an accurate form, or whether a form of a pressure pulse wave accurately represents change of pressure directly measured in an artery. Thus, it has been difficult for a person such as a doctor or a nurse to judge whether physical information obtained based on the form of pressure pulse wave, such as pulse-wave propagation velocity or AI value, is reliable or not.
It is therefore an object of the present invention to provide a pressure-pulse-wave detecting apparatus which can judge whether a form of a pressure pulse wave detected by itself is accurate.
The above object has been achieved by the present invention. According to the present invention, there is provided an apparatus for detecting a pressure pulse wave from a living subject, comprising a pressure-pulse-wave sensor which is adapted to be pressed against a portion of the subject to detect the pressure pulse wave from the subject; a pressing device which presses the pressure-pulse-wave sensor against the portion of the subject; a blood-pressure measuring device which measures at least one blood pressure value of the subject; and a comparison-value determining means for determining a comparison value based on said at least one blood pressure value measured by the blood-pressure measuring device and at least one pressure value corresponding to the pressure pulse wave detected by the pressure-pulse-wave sensor.
The blood-pressure value or values measured by the blood-pressure measuring device can be deemed as accurate intraarterial pressure values. Therefore, if the comparison value determined by the comparison-value determining means indicates that the pressure value corresponding to the pressure pulse wave is near to the blood-pressure value, it can be judged that the pressure value corresponding to the pressure pulse wave accurately indicates an intraarterial pressure value. In this case, it can be judged that there is substantially no attenuation of the pressure pulse wave because of the arterial wall and/or the skin or subcutaneous tissue, and that the form of the pressure pulse wave is accurate.
According to a preferred feature of the present invention, the blood-pressure measuring device measures a systolic blood pressure value and a diastolic blood pressure value of the subject, and the comparison-value determining means determines the comparison value based on a difference between the systolic and diastolic blood pressure values measured by the blood-pressure measuring device and a pulse pressure corresponding to the pressure pulse wave detected by the pressure-pulse-wave sensor.
A pulse pressure is a difference between the highest pressure corresponding to the greatest magnitude of one heartbeat-synchronous pulse of the pressure pulse wave, and the lowest pressure corresponding to the smallest magnitude of the same pulse. The systolic and diastolic blood-pressure values are the highest and lowest intraarterial pressure values, respectively. Thus, the comparison value is determined based on the difference between the highest and lowest pressure values corresponding to the pressure pulse wave, and the difference between the highest and lowest intraarterial pressure values. Therefore, if the comparison value indicates that the pulse pressure is near to the blood-pressure difference, then it can be judged that there is substantially no attenuation of the pressure pulse wave because of the arterial wall and/or the skin or subcutaneous tissue, and that the form of the pressure pulse wave is accurate.
According to another feature of the present invention, the pressure-pulse-wave detecting apparatus further comprises a judging means for judging, based on the comparison value determined by the comparison-value determining means, whether a state in which the pressure-pulse-wave sensor is worn on the portion of the subject is appropriate.
According to this feature, the apparatus can easily judge whether the state in which the pressure-pulse-wave sensor is worn is appropriate.