The behaviour of the flow-mediated brachial artery vasodilatation during orthostatic stress in normal man

Flow-mediated brachial artery vasodilatation is an index of endothelial function. Published literature describes only supine data and no study has been performed during vertical displacement. This subject deserves investigation for two main reasons: humans spend the larger part of their life in the upright position; this position has significant effects on neural vascular regulation.

In 21 healthy men (25 +/- 2 years) the flow-dependent brachial artery vasodilating response to distal circulatory arrest was assessed by Doppler ultrasound imaging, while supine and during 20 degrees and 60 degrees head-up tilting (HUT). In 11 of these subjects the vasodilating response to nitroglycerine was also explored.

Absolute and percentage increments in brachial calibre during hyperaemia after deflation of the occluding cuff became increasingly greater at 20 degrees (+0.44 mm) and 60 degrees (+0.92 mm) HUT (P < 0.01), compared with the horizontal position (+0.27 mm), and the arterial dilatation for an increase in flow (0.98 +/- 0.08 and 1.68 +/- 0.06 mm mL(-1) min(-1) x 1000, respectively) was larger (P < 0.01) than occurred while supine (0.41 +/- 0.05 mm mL(-1) min(-1) x 1000). Nitroglycerine-mediated vasodilatation at 60 degrees HUT was similar to that in the supine position.

The orthostatic stimulus is associated with an increase of the flow-mediated brachial artery vasodilatation, which is proportional to the degree of displacement. The mechanism of this effect does not consist of changes in nitric oxide sensitivity.