Contribution of perfusion pressure to vascular resistance response during head-up tilt

We measured brachial and femoral artery flow velocity in eight subjects and peroneal and median muscle sympathetic nerve activity (MSNA) in five subje cts during tilt testing to 40 degrees. Tilt caused similar increases in MSN A in the peroneal and median nerves. Tilt caused a fall in femoral artery f low velocity, whereas no changes in flow velocity were seen in the brachial artery. Moreover, with tilt, the increase in the vascular resistance emplo yed (blood pressure/flow velocity) was greater and more sustained in the le g than in the arm. The ratio of the percent increase in vascular resistance in leg to arm was 2.5:1. We suggest that the greater vascular resistance e ffects in the leg were due to an interaction between sympathetic nerve acti vity and the myogenic response.