Reflex control of the cutaneous circulation during passive body core heating in humans

The impact of body core heating on the interaction between the cutaneous and central circulation during blood pressure challenges was examined in eight adults. Subjects were exposed to -10 to -90 mmHg lower body negative pressure (LBNP) in thermoneutral conditions and -10 to -60 mmHg LBNP during heat stress. We measured forearm vascular conductance (FVC; ml · min^-1 · 100 ml^-1 · mmHg^-1) by plethysmography; cutaneous vascular conductance (CVC) by laser-Doppler techniques; and central venous pressure, arterial blood pressure, and cardiac output by impedance cardiography. Heat stress increased FVC from 5.7 ± 0.9 to 18.8 ± 1.3 conductance units (CU) and CVC from 0.21 ± 0.07 to 1.02 ± 0.20 CU. The FVC-CVP relationship was linear over the entire range of LBNP and was shifted upward during heat stress with a slope increase from 0.46 ± 0.10 to 1.57 ± 0.3 CU/mmHg CVP (P < 0.05). Resting CVP was lower during heat stress (6.3 ± 0.6 vs. 7.7 ± 0.6 mmHg; P < 0.05) but fell to similar levels during LBNP as in normothermic conditions. Data analysis indicates an increased capacity, but not sensitivity, of peripheral baroreflex responses during heat stress. Laser-Doppler techniques detected thermoregulatory responses in the skin, but no significant change in CVC occurred during mild-to-moderate LBNP. Interestingly, very high levels of LBNP produced cutaneous vasodilation in some subjects.