Peripheral vascular reflexes elicited during lower body negative pressure

To study the interaction between thermal reflexes and baroreflexes on human forearm vasomotor and venomotor control, and to test the hypothesis that peripheral veins are responsive to baroreceptor unloading during gravitational stress, we imposed lower body negative pressure (LBNP) between 10 and 50 mm Hg (Torr) at ambient temperatures (Ta) of 28 and 37 degrees C. We measured arterial and central venous pressures (CVP), heart rate, forearm venous volume, forearm venous pressure, and forearm blood flow in 12 volunteers. Decreases in CVP were relatively large at 10 mm Hg LBNP (p less than 0.01) at both Ta, and less thereafter. Arterial systolic and pulse pressures were not significantly reduced until LBNP exceeded 30 mm Hg (p less than 0.05). With LBNP up to 20 mm Hg, moderate decreases in forearm venous compliance and increases in forearm vascular resistance occurred. Between 30 and 50 mm Hg LBNP, the changes in both compliance and resistance per unit change in CVP were more than tripled. We concluded that unloading of cardiopulmonary mechanoreceptors stimulates increases in both forearm vasomotor and venomotor tone and that addition of arterial baroreceptor unloading adds to these reflex responses.