Automatic quantitation of venous hemodynamics with an electrically calibrated strain gauge plethysmograph

Strain gauge plethysmography has permitted noninvasive quantitation of altered venous hemodynamics in the legs of patients with venous disease. A new plethysmograph has been developed which avoids previous problems of mechanical calibration of gauges and the necessity for a recorder for calculating blood flows. This instrument permits accurate electrical calibration of mercury-in-Silastic gauges which are effectively at the corners of the Wheatstone bridge and are supplied by constant current for linear output. A timer and follow-and-hold circuit allow automatic calculation of rates of change in limb circumference and, thus, blood flow, from a panel meter. The accuracy of the automatic versus the graphic methods of quantitating venous outflow and reflux flow was determined in 20 normal subjects. Venous outflows were 15.1 +/- 0.5 (mean +/- 1 S.E.M.) and 15.9 +/- 0.4 cc/min/100 cc by automatic and graphic methods, respectively (r = 0.96, p less than 0.01). Reflux flows were 2.1 +/- 0.2 and 2.3 +/- 0.2 cc/min/100 cc by the automatic and graphic methods, respectively (r = 0.93, p less than 0.01). This plethysmograph may prove to be a simple, rapid, and accurate device for evaluating altered hemodynamics in venous disease.