Haemodynamic responses to exercise, ATP infusion and thigh compression in humans:: insight into the role of muscle mechanisms on cardiovascular function

The muscle pump and muscle vasodilatory mechanims are thought to play important roles in increasing and maintaining muscle perfusion and cardiac output (Q) over dot during exercise, but their actual contributions remain uncertain. To evaluate the role of the skeletal muscle pump and vasodilatation on cardiovascular function during exercise, we determined leg and systemic haemodynamic responses in healthy men during (1) incremental one-legged knee-extensor exercise, (2) step-wise femoral artery ATP infusion at rest, (3) passive exercise (n = 10), (4) femoral vein or artery ATP infusion (n = 6), and (5) cyclic thigh compressions at rest and during passive and voluntary exercise (n = 7). Incremental exercise resulted in progressive increases in leg blood flow (Delta LBF 7.4 +/- 0.7 l min(-1)), cardiac output (Delta (Q) over dot 8.7 +/- 0.7 l min(-1)), mean arterial pressure (Delta MAP 51 +/- 5 mmHg), and leg and systemic oxygen delivery and (V) over dot(O2). Arterial ATP infusion resulted in similar increases in (Q) over dot, LBF, and systemic and leg oxygen delivery, but central venouspressureandmuscle metabolism remained unchanged and MAP was reduced. In contrast, femoral vein ATP infusion did not alter LBF, (Q) over dot or MAP. Passive exercise also increased blood flow (Delta LBF 0.7 +/- 0.1 l min(-1)), yet the increase in muscle and systemic perfusion, unrelated to elevations in aerobic metabolism, accounted only for similar to 5% of peak exercise hyperaemia. Likewise, thigh compressions alone or in combination with passive exercise increased blood flow (Delta LBF 0.5 +/- 0.7 l min(-1)) without altering (Q) over dot, MAP or (V) over dot(O2). These findings suggest that the skeletal muscle pump is not obligatory for sustaining venous return, central venous pressure, stroke volume and (Q) over dot or maintaining muscle blood flow during one-legged exercise in humans. Further, its contribution tomuscle and systemic peak exercise hyperaemia appears to be minimal in comparison to the effects of muscle vasodilatation.