Effect of positive external pressure on calf volume and local venous haemodynamics.

The pressure external to the calf was increased suddenly in 11 normal subjects and the resulting change in calf volume (considered to be mainly a change of calf vein blood volume) was followed using a mercury-in-rubber strain gauge. For a small increase of pressure the volume decreased exponentially and increased linearly when the pressure was suddenly released. A simple theory was developed which treats the calf veins as a single volume with a single compliance and a single outflow resistance. Using the theory, the following average results were obtained from measured values: effective venous compliance 1-1%kPa (SD 0.3); effective venous outflow resistance 3.8 s kPa/% (SD 1-3); and total flow into the veins from themicrocirculation 0.054%/s (SD 0.020) (% refers to a change in volume expressed as a percentage of the initial volume of the calf). The significance of the findings is considered with special reference to the occurrence of calf vein thrombosis.