TEMPERATURE-DEPENDENCE OF HABITUATION OF THE INITIAL RESPONSES TO COLD-WATER IMMERSION

The initial responses to cold-water immersion, evoked by stimulation o f peripheral cold receptors, include tachycardia, a reflex inspiratory gasp and uncontrollable hyperventilation. When immersed naked, the ma ximum responses are initiated in water at 10 degrees C with smaller re sponses being observed following immersion in water at 15 degrees C. H abituation of the initial responses can be achieved following repeated immersions, but the specificity of this response with regard to water temperature is not known. Thirteen healthy male volunteers were divid ed into a control (C) group (rt = 5) and a habituation (H) group(n = 8 ). Each subject undertook two 3-min head-out immersions in water at 10 degrees C wearing swimming trunks. These immersions took place at a c orresponding time of day with 4 days separating the two immersions. In the intervening period the C group were not exposed to cold water, wh ile the H group undertook another six, 3-min, head-out immersions in w ater at 15 degrees C. Respiratory rate (f(R)), inspiratory minute volu me (ii) and heart rate (f(H)) were measured continuously throughout ea ch immersion. Following repeated immersions in water at 15 degrees C, the f(R), (V) over dot(1) and f(H) responses of the H group over the f irst 30 s of immersion were reduced (P < 0.01) from 33.3 breaths.min(- 1), 50.5 l.min(-1) and 114 beats.min(-1) respectively, to 19.8 breaths .min(-1), 26.4 l.min(-1) and 98 beats.min(-1), respectively. In water at 10 degrees C these responses were reduced (P < 0.01) from 47.3 brea ths.min(-1), 67.6 l.min(-1) and 128 beats.min(-1) to 24.0 breaths.min( -1), 29.5 l.min(-1) and 109 beats.min(-1), respectively over a corresp onding period of immersion. Similar reductions were observed during th e last 2.5 min of immersions. The initial responses of the C group wer e unchanged. It is concluded that habituation of the cold shock respon se can be achieved by immersion in warmer water than that for which pr otection is required. This suggests that repeated submaximal stimulati on of the cutaneous cold receptors is sufficient to attenuate the resp onses to more maximal stimulation.