In pursuit of the unicorn.

This short review was prompted by The Physiological Society's recent online symposium on variability. It does not deal with a specific methodology, but rather with the myth that certain environmentally-induced clinical conditions can be identified, quantified, simplified and monitored with a single methodology. Although this might be possible with some clinical conditions, others resist the prevailing reductionist approach of minimizing rather than exploring variation in pathogenesis and pathology, and will not be understood fully until the variation in cause and effect are embraced. This is likely to require comprehensive methodologies and collaboration.

Permanence of the habituation of the initial responses to cold-water immersion in humans.

Sudden immersion in cold water initiates an inspiratory gasp response followed by uncontrollable hyperventilation and tachycardia. It is known that this response, termed the "cold shock" response, can be attenuated following repeated immersion. In the present investigation we examined how long this habituation lasts. Twelve healthy male volunteers participated in the experiment, they were divided into a control (C) group (n = 4), and a habituation (H) group (n = 8). In October, each subject undertook two 3-min head-out seated immersions into stirred water at 10 degrees C wearing swimming trunks. These immersions took place at the same time of day, with 4 days separating the two immersions. In the intervening period, the C group were not exposed to cold water, while the H group undertook six, 3-min head-out immersions in water at 15 degrees C. Two months (December), 4 months (February), 7 months (May) and 14 months (January) after their first immersion, all subjects undertook another 3-min head-out immersion in water at 10 degrees C. The H group showed a reduction in respiratory frequency (47 to 24 breaths x min(-1)), inspiratory minute volume (72.2 to 31.3 1 x min(-1)) and heart rate (128 to 109 beats x min(-1)) during the first 30 s of immersion on day 5 compared to day 1. Seven months later these responses were still significantly reduced compared to day 1. After 14 months, heart rate remained attenuated but respiratory frequency and inspiratory minute volume had returned towards pre-habituation levels. The responses of the C group during the first 30 s of immersion were not altered. Both groups showed an attenuation in the responses during the remaining 150 s of immersion following repeated immersions. It is concluded that repeated immersions in cold water result in a longlasting (7-14 months) reduction in the magnitude of the cold shock response. Less frequent immersions produced a decrease in the duration, but not the magnitude of the response.

Habituation of the initial responses to cold water immersion in humans: a central or peripheral mechanism?

1. The initial respiratory and cardiac responses to cold water immersion are thought to be responsible for a significant number of open water deaths each year. Previous research has demonstrated that the magnitude of these responses can be reduced by repeated immersions in cold waterwhether the site of habituation is central or peripheral. 2. Two groups of subjects undertook two 3 min head-out immersions in stirred water at 10 C of the right-hand side of the body (R). Between these two immersions (3 whole days) the control group (n = 7) were not exposed to cold water, but the habituation group (n = 8) undertook a further six 3 min head-out immersions in stirred water at 10 C of the left-hand side of the body (L). 3. Repeated L immersions reduced (P < 0.01) the heart rate, respiratory frequency and volume responses. During the second R immersion a reduction (P < 0.05) in the magnitude of the responses evoked was seen in the habituation group but not in the control group, despite both groups having identical skin temperature profiles. 4. It is concluded that the mechanisms involved in producing habituation of the initial responses are located more centrally than the peripheral receptors.

Temperature dependence of habituation of the initial responses to cold-water immersion.

The initial responses to cold-water immersion, evoked by stimulation of peripheral cold receptors, include tachycardia, a reflex inspiratory gasp and uncontrollable hyperventilation. When immersed naked, the maximum responses are initiated in water at 10 degrees C, with smaller responses being observed following immersion in water at 15 degrees C. Habituation 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 divided into a control (C) group (n = 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 corresponding time of day with 4 days separating the two immersions. In the intervening period the C group were not exposed to cold water, while the H group undertook another six, 3-min, head-out immersions in water at 15 degrees C. Respiratory rate (fR), inspiratory minute volume (VI) and heart rate (fH) were measured continuously throughout each immersion. Following repeated immersions in water at 15 degrees C, the fR, VI and fH responses of the H group over the first 30 s of immersion were reduced (P < 0.01) from 33.3 breaths x min(-1), 50.5 l x min(-1) and 114 beats x min(-1) respectively, to 19.8 breaths x min(-1) 26.41 x min(-1) and 98 beats x min(-1), respectively. In water at 10 degrees C these responses were reduced (P < 0.01) from 47.3 breaths x min(-1), 67.61 x min(-1) and 128 beats x min(-1) to 24.0 breaths x min(-1), 29.5 l x min(-1) and 109 beats x min(-1), respectively over a corresponding period of immersion. Similar reductions were observed during the last 2.5 min of immersions. The initial responses of the C group were unchanged. It is concluded that habituation of the cold shock response can be achieved by immersion in warmer water than that for which protection is required. This suggests that repeated submaximal stimulation of the cutaneous cold receptors is sufficient to attenuate the responses to more maximal stimulation.