Effect of oral contraceptives on resting blood flow in the leg.

Resting blood flow in the leg was measured twice per week by venous occlusion plethysmography, for two or more menstrual cycles in 11 normal female volunteers, 5 of whom were taking an oral contraceptive. Both groups showed a wide range of resting flow, with lowest values doubling or occasionally nearly trebling in any one menstrual cycle. In 1 subject a repeatable cyclic variation was found. The mean resting flow of the group taking oral contraceptives was found to be lower (0.1 less than P less than 0.2) and the range of flow values over the menstrual cycle smaller, than those of the subjects not using the pill. The lower flow values found for subjects on the pill suggests a possible mechanism linking use of oral contraceptives with the occurrence of venous thrombosis. However venous distensibilities estimated from plethysmographic observations indicate that the thrombogenic effect of low flow values may be offset by decreased distensibility, and hence it may be possible to maintain normal blood velocities.