Effect of indirect heating on the postural control of skin blood flow in the human foot.

It has been shown in previous studies that skin blood flow in the human foot falls when the extremity is placed below heart level, owing to an increase in precapillary resistance that is probably mediated by a local sympathetic axon reflex or a myogenic response. In order to clarify the influence of the central thermoregulatory mechanisms on this local postural vasoconstrictor response, 12 normal male subjects were studied under standardized conditions, at rest and during heating of the trunk with an electric blanket. Skin blood flow was measured before and during body heating using laser Doppler flowmetry with the foot maintained at heart level and placed passively 50 cm below the heart. Skin blood flow and skin temperature were determined at two sites: the plantar surface of the big toe, an area with a relatively large number of arteriovenous anastomoses, and the dorsum of the same foot, where these anastomoses are few or absent. When the foot was placed in the dependent position, skin blood flow recorded in the dorsum of the foot during indirect heating fell to a level similar to that achieved before heating. In contrast, indirect heating greatly diminished the postural fall in skin blood flow recorded in the plantar surface of the big toe. In conclusion, the partial release of sympathetic vasoconstrictor tone associated with indirect heating appears to over-ride the local postural control of cutaneous vascular tone in areas where arteriovenous anastomoses are relatively numerous.