Application of a simplified physical model to the plethysmographic study of the leg venous outflow

We describe a simplified physical model of the leg venous outflow. It features an apparent compliance C and outflow resistance R. The venous volume accumulated during a venous occlusion discharges after the release of the compression following an exponential curve (T = time constant = RC). This model fits the curves obtained by strain gauge plethysmography in normal legs and in limbs suffering from chronic venous obstructions. The values for R, C and T are calculated from the plethysmographic measurements. The limbs with chronic obstructions differentiate themselves from the normal mainly by an increase of R but some limbs show values close to normal likely due to a compensation (collaterals or partial recanalization). T (prolonged) and C (decreased) are less affected by chronic venous obstructions. The diagnostic score of this mode of analysis is better than the usual graphic determination of the maximum venous outflow (MVO).