A plethysmograph is an instrument for measuring blood flow in the legs by first inflating a cuff on the upper thigh to curtail venous return flow, and then measuring the increase in leg volume via strain guages. Prior to the advent of the above instrument, plethysmography was a time-consuming activity that required recalibrations for each patient; the above unit was totally automatic as a result of a complex inflation subsystem and 4-wire 6-decade digital resistance measurement, both controlled by a 6800-based computer. The plethysmograph involves the middle and lower units in the cabinet.
The upper and lower units in the cabinet combine to form an automated calibration system for Doppler-shifted ultrasound units. A frequency synthesiser, frequency counter, RMS-to-DC converters, phase meter and precision digital voltmeter are used for swept frequency measurements of power and phase across the resonant frequency of the Doppler-shift velocimeter. A block diagram is shown below :
This work was done jointly with Dr.M.G.Taylor in Prof.R.G.Gosling's Blood Flow Group at the Physics Dept., Guy's Hospital Medical School, London, and was funded by the Wellcome Trust.
Photographs courtesy of Tim Eveleigh, Photographic Dept., Guy's Hospital Medical School, London.