Forearm vascular responses to lower body negative pressure and orthostasis.

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Abstract
Forearm blood flow, forearm venous tone, and forearm vascular resistance were measured in normal subjects both during 60 mm Hg supine lower body negative pressure (LBNP) and during upright tilting. Tilting measurements were done while standing passively erect and while hanging with feet and legs dependent. Forearm blood flow was measured with mercury-in-rubber strain gauges and forearm vascular resistance was calculated as mean arterial blood pressure/forearm blood flow. Venous tone was evaluated by: forearm pressure-volume curves, venous pressure changes in isolated arms where the circulation was arrested, changes in pressure in isolated vein segments, and volume of blood contained in the forearm at a constant collecting cuff pressure. Forearm blood flow fell and forearm vascular resistance increased during both LBNP and orthostasis. All 4 measures of venous tone indicated a significant rise during LBNP and upright tilting. No differences were seen in the forearm vascular responses due to passive standing as compared to upright tilting with the feet and legs dependent.

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