Voltage clamp method for the use of electrical admittance plethysmography in human body segments.

Abstract

ELECTRICAL IMPEDANCE (NYBOER, 1970; KUBICEK et aL, 1970) or admittance plethysmography (ITO et al., 1976; YAMAKOSHI et aL, 1978) may still be one of the simplest and most convenient methods for the non-invasive measurement of blood volume change and/or blood flow, requiring only the attachment of electrodes on the body segment concerned. There have therefore been many studies on the non-invasive measurement of cardiac output (CO) (or stroke volume, SV) and regional blood flow (LBF), in conjunction with the venous occlusion technique, in human limbs particularly using the impedance method (GEDOES and BAKER, 1989). In contrast, we have previously used the adamittance plethysmography techifique, which bears an inverse relation to that of impedance. This method is more improved with respect to simplicity and practicability than the impedance method, as the variable to be measured is only the first derivative of admittance pulsations (dY/dt) for SV determination (ITo et aL, 1976; BUKHA~ et al., 1981) and only the admittauce change (6Y) following the venous occlusion for LBF measurement (YAMAKOSm et al., 1978; 1979; t980; SHIMAZU et al., 1982). However, the highly accurate measurement of the bio-admittanee signal has been much more difficult to obtain with electrical circuitry than the measurement of the bio-'mlpedance signal obtained by the relatively easy technique of the eomtant current ~areuit, because an analogue IC divider was conventionally used to perform the reciprocal conversion of the bio-impedanee signal CtTO et aL, 1976; YAMAKOSI-a et aL, 1979; BUKHAm et al., 1981). In this work, we therefore propose a voltage clamp method to obtain the bio-admittance signal with fairly high accuracy, but with very simple circuitry without such an analogue divider. The outline, main electric circuits and measurement accuracy of a device designed.by this method are presented, together with an application to CO and LBF measurements.