Differences in forearm blood flow measured by capacitance and volume plethysmography.

Abstract

A differential fixed-frequency capacitance plethysmograph is described which has a mean spacing from the arm of 1. 25 cm and is capable of determining volume changes to an accuracy of the order of 5%. Comparisons were made with an air plethysmograph and with a conventional water-filled plethysmograph over a wide range of forearm blood flow induced by a number of physiological procedures. Good agreement was obtained when flow was measured by capacitance and air plethysmography but the values obtained by these methods both differed significantly from those obtained with the conventional water-filled method. These discrepancies are ascribed to hydrostatic pressure and local temperature effects imposed by the water-filled plethysmograph. The advantages of free ventilation with the maintenance of normal insensible perspiration and heat exchange, electrical recording, and portability render the capacitance technique an acceptable method of blood flow measurement.