Abstract
1. Diving or face immersion bradycardia is a well recognized but incompletely understood reflux which occurs in man and other mammals. 2. In order to investigate the contributions made by voluntary apnoea, face immersion in water and cold exposure, 18 normal subjects were exposed to these challenges separately and in various combination. 3. Tested individually, cold and apnoea caused significant reductions in heart beat (P < 0.01 and 0.002, respectively). Face immersion in thermoneutral water had no effect on heart rate. 4. The bradycardic effect of apnoea at maximal inspiration may be due to stimulation of pulmonary stretch receptors. 5. Cold exposure and voluntary apnoea applied simultaneously caused a summative effect but when tested with face immersion in water there was a synergistic response greater than the sum of individual responses. 6. The results confirm the bradycardic effect of apnoea and cold exposure, whereas immersion in thermoneutral water had little effect, a finding which has been disputed in the literature.
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