Venous mechanics of contracting gastrocnemius muscle and the muscle pump theory.

The time constant of venous drainage (tau v) is an important determinant of the return of blood to the heart. The lower the tau v, the higher the flow for a given volume. To determine whether the tau v of muscle decreases during muscle contraction, we anesthetized dogs with pentobarbital sodium and mechanically ventilated them. We isolated the vasculature of the gastrocnemius muscle and attached the muscle to a force transducer. The muscle was pump perfused, and flows were measured with electromagnetic flow probes on the artery and vein. Pressure in the venous compliant region (Pel) was obtained by simultaneously occluding the artery and vein. Changes in volume (delta V) were produced by changing inflow and obtaining the integral of the difference between inflow and outflow. The tau v was obtained from delta V divided by change in flow. Compliance was calculated from delta V divided by change in Pel from before to after a change in flow. The venous resistance was calculated from tau v divided by venous compliance. The muscle was set at the optimal length, and contractions were produced by stimulating the nerve to the muscle with supramaximal voltage at either 1- or 5-Hz trains with stimulations at 20 Hz, 0.2-ms duration, and duty cycle of 0.25. The tau v at rest was 4.06 +/- 2.16 s and decreased to 2.44 +/- 1.07 s (P < 0.05) at 1 Hz and to 1.81 +/- 0.4 s at 5 Hz. There were no significant changes in venous compliance or venous resistance. In conclusion, muscle contractions can reduce the time constant of venous drainage of muscle and could thereby contribute to the increased venous return and cardiac output during exercise.