Effects of limb posture on reactive hyperemia

To examine the role of limb posture on vascular conductance during rapid changes in vascular transmural pressure, we determined brachial (n = 10) and femoral (n = 10) artery post-occlusive reactive hyperemic blood Xow (RHBF, ultrasound/Doppler) and vascular conductance in healthy humans with each limb at three diVerent positions-horizontal, up and down. Limb posture was varied by raising or lowering the arm or leg from the horizontal position by 45°. In both limbs, peak RHBF and vascular conductance were highest in the down or horizontal position and lowest in the up position (arm up 338 ± 38, supine 430 ± 52, down 415 ± 52 ml/min, P < 0.05; leg up 1,208 ± 88, supine 1,579 ± 130, down 1,767 ± 149 ml/min, P < 0.05). In contrast, the maximal dynamic fall in blood Xow following peak RHBF (in ml/s/s) in both limbs was highest in the limb-down position and lowest with the limb elevated (P < 0.05). These data suggest that the magnitude and temporal pattern of limb reactive hyperemia is in part related to changes in vascular transmural pressure and independent of systemic blood pressure and sympathetic control.