Skeletal muscle blood flow responses to hypoperfusion at rest and during rhythmic exercise in humans.

We evaluated the contribution of changes in systemic arterial pressure and local vasodilation to blood flow restoration in contracting human muscles during acute hypoperfusion. Healthy subjects (n=10) performed rhythmic forearm exercise (10% and 20% of maximum) while a balloon in the brachial artery located above the elbow was inflated. Each trial included 3 min of rest, exercise, exercise with balloon inflation, and exercise after balloon deflation. Forearm blood flow (FBF) was measured using Doppler ultrasound. Blood pressure on both sides of the balloon was measured using a brachial artery catheter (distal pressure), and Finometer for proximal (systemic) arterial pressure. Balloon inflation during exercise reduced distal arterial pressure, and FBF fell 37-41%. There was also a surprising acute increase in forearm vascular resistance (distal pressure/FBF). This was followed by recovery of distal arterial pressure and forearm vasodilation that caused a marked (approximately 75%) restoration of flow that was not associated with significant changes in systemic arterial pressure. During validation trials (n=6) at rest and with exercise both balloon and brachial artery diameters were stable when the balloon was inflated. Our findings indicate that at these exercise intensities 1) the restoration of FBF during exercise with hypoperfusion relied primarily on local dilator responses in conjunction with restoration of distal perfusion pressure likely as a result of increased collateral flow around the elbow, and 2) a loss of pulsatile flow and elastic recoil in the forearm may have contributed to the acute increase in vascular resistance seen at the onset of hypoperfusion.