FLOW-GENERATING CAPABILITY OF THE ISOLATED SKELETAL-MUSCLE PUMP

We sought to test directly whether the mechanical forces produced duri ng rhythmic muscle contraction and relaxation act on the muscle vascul ature in a manner sufficient to initiate and sustain blood flow To acc omplish this goal, we evaluated the mechanical performance of the isol ated skeletal muscle pump. The hindlimb skeletal muscle pump was isola ted by reversibly connecting the inferior vena cava and terminal aorta with extracorporeal tubing in 15- to 20-kg anesthetized pigs (n = 5). During electrically evoked contractions (1/s), hindlimb muscles were made to perfuse themselves by diverting the venous blood propelled out of the muscles into the shunt tubing, which had been prefilled with f resh arterial blood. This caused arterial blood to be pushed into the distal aorta and then through the muscles (shunt open, proximal aorta and vena cava clamped). In essence, the muscles perfused themselves fo r brief periods by driving blood around a ''short-circuit'' that isola tes muscle from the remainder of the circulation, analogous to isolate d heart-lung preparations. Because the large, short shunt offers a neg ligible resistance to flow, the arterial-venous pressure difference ac ross the limbs was continuously zero, and thus the energy to drive flo w through muscle could come only from the muscle pump. The increase in blood flow during normal heart-perfused contractions (with only the s hunt tubing clamped) was compared with shunt perfused contractions in which the large veins were preloaded with extra blood volume. Muscle b lood flow increased by 87 +/- 11 and 110 +/- 21 (SE) ml/min in the fir st few seconds after the onset of shunt-perfused and heart-perfused co ntractions, respectively (P > 0.4). We conclude that the mechanical fo rces produced by muscle contraction and relaxation act on the muscle v asculature in a manner sufficient to generate a significant flow of bl ood.