Regulation of the Cutaneous Circulation
The cutaneous venoarteriolar response (VAR) decreases blood flow via non-adrenergic mechanisms secondary to local increases in venous pressure. Orthostatic tolerance is reduced in heat stressed individuals. Reduced vasoconstriction during engagement of the VAR in heat stressed individuals may contribute to reduced orthostatic tolerance in this condition.
To identify the effects of whole-body heating on the cutaneous VAR.
12 subjects underwent whole-body heating by perfusing warm water (46°C) through a tube-lined suit. Cutaneous vascular conductance (CVC; dorsal foot and forearm), internal temperature, mean skin temperature, and arterial blood pressure were measured throughout the protocol. During normothermic conditions (mean skin temperature 34°C), and every 10 min during heating until internal temperature increased by 1°C, individuals were exposed to 30° head-up tilt for 2 min. VAR was invoked in the foot with tilting but not in the forearm, as the forearm was maintained at heart level.
Results varied among subjects and among a given increase in internal temperature. Vasoconstriction associated with the VAR during normothermia was present but relatively small. With slight heating (presumably with removal of tonic vasoconstrictor tone) the magnitude of vasoconstriction due to the VAR was augmented. After more pronounced heating, vasoconstriction due to the VAR was either augmented (n = 2), attenuated (n = 2), absent (n = 2), or reversed (i.e., vasodilation in the area of measurement instead of vasoconstriction; n = 6). Forearm CVC showed small decreases during tilt and responses were accentuated as the heat stress progressed.
Pronounced indirect whole-body heating can reduce the magnitude of vasoconstriction of the cutaneous VAR, with the majority of subjects showing either an absence of vasoconstriction or vasodilation during engagement of the VAR in this condition. Funded in part by NIH (HL-61388 & 10488)