The Impedance Plethysmographic Sampling Field in the Human Calf

Impedance plethysmography has been widely used clinically because of its convenience and sensitivity, but its use has been questioned by some investigators because of uncertainty concerning the source of the signal. Quantitative definitions of the sensitivity to conductivity changes as a function of position have been lacking. The present study defines the sampling efficiency for impedance measurements made with four electrodes on a uniform cylinder model of the calf. Predictions based on electric field theory compare favorably with results from an in vitro model (average rms error = 8 percent). These studies demonstrate that for a given voltage electrode separation, the proximity of the current electrodes determines the relative sensitivity to conductivity changes within the sampling field. For voltage and current electrode separations of 10 and 18 cm on a 37 cm circumference calf (typical measurements for clinical testing of the lower leg), the sampling field extends about 3 cm beyond the voltage electrodes and the deep vessels are sampled with at least 80 percent of the efficiency of the superficial vessels. These studies also indicate that it is possible to predict the sampling field associated with various electrode configurations, a possibility which should aid the design of electrode configurations for specific applications of impedance plethysmography.