Vascular adjustment and fluid reabsorption in the human forearm during elevation.

Elevation of vascular hydrostatic pressure is known to increase capillary filtration causing, for example orthostatic plasma fluid losses. The present study investigated possible compensatory fluid intravasation in the human forearm during graded elevation, that is during hydrostatic venous collapse. Recordings were made of forearm fluid volume (impedance-plethysmography), forearm blood flow (venous-occlusion-technique), and finger arterial pressure (Finapres). A group of 20 male subjects were seated upright and had their horizontal right forearm passively elevated to 0, 18, 36, and 54 cm above the heart (3rd intercostal space) after equilibration at a reference level 18 cm below the heart. All positions were maintained for 15 min and taken in random order. The vascular volume which drained or refilled within 1.5 min after change of position was found to increase with height. The slow linear volume reduction representing the transcapillary reabsorption rate was found to be almost identical in the three positions above the heart (0.0382, 0.0372, and 0.0398 ml.100 ml-1.min-1). Forearm blood flow reached its highest values at heart level and decreased with height. Calculated total vascular resistance increased with a progressive slope up to about 200% of the value at heart level. As a main finding similar reabsorption rates suggested good maintenance of capillary pressure in positions up to 54 cm above the heart thus contrasting with findings on the calf. The coincidence with increasing total vascular resistance led us to the conclusion that graded venous collapse indicated by grading in venous volume makes for a considerable decrease in pre- to postcapillary resistance ratio with elevation.(ABSTRACT TRUNCATED AT 250 WORDS)