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.