Cardiopulmonary baroreflex control of forearm vascular resistance after acute blood volume expansion

We report the stimulus-response characteristics of cardiopulmonary (CP) baroreflex control of forearm vascular resistance (FVR) in young adult male volunteers before and after: 1) blood volume expansion (8 ml/kg infusion of 5% human serum albumin solution, n = 5) and 2) a redistribution of blood volume toward the heart (6° head-down tilt (HDT), n = 6). We assessed the relationship between reflex stimulus (i.e., changes in central venous pressure (CVP)) and response (i.e., FVR) during unloading of CP mechanoreceptors with lower body negative pressure (0 to -20 mm Hg). Changes in CVP were estimated from changes in venous pressure of a large peripheral vein of the dependent arm with the subject in the right lateral decubitus position. In all conditions, reflex forearm vasoconstriction occurred in response to a reduction in estimated CVP. The absolute change in FVR per unit of CVP was reduced from -4.24 ± 1.68 to -2.15 ± 1.16 units/mm Hg (p < 0.05) following blood volume expansion but was similar before (-3.34 ± 0.89 units/mm Hg) and during 6° HDT (-3.30 ± 0.92 units/mm Hg). The reduced sensitivity of the CP baroreflex following volume expansion was manifested primarily as a smaller FVR response to LBNP (p < 0.05). Blood volume expansion and 6° HDT increased resting estimated CVP by 1.5 and 0.9 mm Hg, respectively (p < 0.05) and resting levels of FVR decreased slightly. The reduction in sensitivity of the CP baroreflex following blood volume expansion could not be attributed to the increase in CVP because similar increases in CVP during 6° HDT did not alter baroreflex control of FVR. These data support the concept that reflexes involved in extracellular volume regulation interact with baroreflex control of vascular tone. In addition, these data support the hypothesis that attenuated baroreflex control of FVR in physically fit individuals is due, in part, to a training-induced blood volume expansion.