VASCULAR ADJUSTMENT AND FLUID REABSORPTION IN THE HUMAN FOREARM DURING ELEVATION

Elevation of vascular hydrostatic pressure is known to increase capill ary filtration causing, for example orthostatic plasma fluid losses. T he present study investigated possible compensatory fluid intravasatio n in the human forearm during graded elevation, that is during hydrost atic venous collapse. Recordings were made of forearm fluid volume (im pedance-plethysmography), forearm blood flow (venous-occlusion-techniq ue), and finger arterial pressure (Finapres(tm)). A group of 20 male s ubjects were seated upright and had their horizontal right forearm pas sively elevated to 0, 18, 36, and 54 cm above the heart (3rd intercost al space) after equilibration at a reference level 18 cm below the hea rt. All positions were maintained for 15 min and taken in random order . The vascular volume which drained or refilled within 1.5 min after c hange of position was found to increase with height. The slow linear v olume 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 reach ed its highest values at heart level and decreased with height. Calcul ated total vascular resistance increased with a progressive slope up t o about 200% of the value at heart level. As a main finding similar re absorption rates suggested good maintenance of capillary pressure in p ositions up to 54 cm above the heart thus contrasting with findings on the calf. The coincidence with increasing total vascular resistance l ed us to the conclusion that graded venous collapse indicated by gradi ng in venous volume makes for a considerable decrease in pre- to postc apillary resistance ratio with elevation. A venous contribution to aut oregulation of capillary pressure may thus limit disadvantageous local fluid losses.