Leg muscle blood flow during reactive hyperemia. Effects of different body positions, and of subatmospheric pressure.

In normal man at rest transition from the supine to the upright body position is accompanied by autoregualtion of the blood flow to tissues in the dependent extremities. In 11 young healthy males the influence of postural changes and external pressure changes on the blood flow in the anterior tibial muscle during reactive hyperemia was studied. The muscle blood flow was evaluated by means of the Xenon-133 wash-out technique. Transmural pressure changes in the resistance vessels were estimated by measuring the systolic blood pressure at ankle level, using the strain-gauge plethysmograph technique. The mean leg muscle blood flow increased from 48 ml - 100 g-1 - min-1 in a body position with the legs elevated 65 cm above heart level, to 101 ml - 100 g-1 - min-1 in the supine position, and to 151 ml - 100 g-1 - min-1 in a sitting position with dependent legs 70 cm below heart level. The muscle blood flows increased from 92 ml - 100 g-1 - min-1 at ambient pressure to 139 ml - 100 g-1 - min-1 at a subatmospheric pressure of -50 mm Hg. The differences were highly significant (P less than 0.001). Systemic blood pressure measured at heart level did not change during postural changes and external pressure changes. The post-ischemic muscle blood flow was found to increase with the increasing vascular transmural pressure. It is concluded that during reactive hyperemia the normal compensatory vaso-reactions can be inactivated, so that the vessels react passively to changes in the transmural pressure.