FLUID BALANCE VERSUS BLOOD-FLOW AUTOREGULATION IN THE ELEVATED HUMAN LIMB - THE ROLE OF VENOUS COLLAPSE

This study evaluated the postural vascular adjustment in the human for earm which may be responsible for the recent observation that transcap illary fluid balance is maintained above the level of the heart while blood flow decreases in a linear fashion. In this study further eviden ce was provided that a posturally graded profile of collapsed veins ho lds for both an overall increase of resistance with height and compens ation for hydrostatic effects on capillary pressure. This was achieved by manipulating peripheral venous profile/volume: a proximal outlet r esistance (upper arm cuff) was used for re-opening of collapsed distal veins. In test (a), 12 healthy subjects underwent recordings of fluid reabsorption rate and blood flow in a 20-cm segment of their forearm horizontally placed at 36 cm above heart level (third intercostal spac e). Applying upper arm cuff pressures randomly between 0 and 25 mmHg ( 0-3.33 kPa) for 15 min led to maxima of blood flow and reabsorption ra tes at inflations of 5 or 10 mmHg (0.67 or 1.33 kPa). This was attribu ted to minima in postcapillary resistance facilitating flow and reduci ng capillary pressure. In test (b) the flow-maximizing outlet resistan ce found was studied for its effect in different forearm positions (-1 8, 0, 18, 36, 54 cm relative to heart level). Blood flow then showed a shift of its maximum from heart level to 36 cm above heart level, whi le the reabsorption rate increased above 18-cm height - in contrast to previous findings with a free circulation. It was therefore concluded that the venous profile in the forearm adjusts postcapillary resistan ce in such a way that local dehydration is confined at the cost of blo od supply. Thicker and less collapsable veins may ensure better flow a utoregulation during impaired fluid balance - as seen in the legs.