Responses evoked in the forearm vasculature on normal human subjects on repetition of mild, indirect cooling

We have studied responses evoked in the forearm vasculature of normal subjects by mild cooling of the contralateral hand; it was transferred from thermoneutral water at ~33°C to water at 16°C for 2 min. Total forearm blood flow was measured by venous occlusion plethysmography, cutaneous red cell flux was monitored with a laser Doppler flowmeter and arterial pressure was recorded by semi-automatic sphygmomanometry. In each of two groups (I and II) of twelve and 15 subjects, mild cooling evoked a rise in mean arterial pressure, but in each group, approximately half (six and eight respectively) showed a decrease in total forearm vascular resistance (FVR) in response to the first cool immersion (dilator group), while the remainder showed an increase in FVR (constrictor group), apart from one of group II who showed no change in FVR. Cutaneous vascular resistance tended to increase in both the dilator and constrictor groups, but this did not reach statistical significance. When cooling was repeated six times in group II, the decrease in FVR (-18% from control) in the dilator group reversed to an increase in FVR (+25%) by the sixth immersion, while the increase in FVR (+55%) in the constrictor group persisted through to the sixth immersion (+23%). We propose that mild cooling evokes a primary reflex vasoconstrictor response in forearm muscles, but that this can be overcome in some subjects by the characteristic muscle vasodilatation of the alerting response which is evoked by novel, or noxious stimuli; the latter is known to habituate on repetition of the stimulus.