RELATIVE CONTRIBUTIONS OF VOLUNTARY APNEA, EXPOSURE TO COLD AND FACE IMMERSION IN WATER TO DIVING BRADYCARDIA IN HUMANS

1. Diving or face immersion bradycardia is a well recognized but incom pletely understood reflex 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 combinations. 3. Tested individually, cold and apnoea caused significant reductions in heart rate (P < 0.01 and 0.002, respectively). Face immersion in thermoneutr al water had no effect on heart rate. 4. The bradycardic effect of apn oea Pt maximal inspiration may be due to stimulation of pulmonary stre tch receptors. 5. Cold exposure and voluntary apnoea applied simultane ously caused a summative effect but when tested with face immersion in water there was a synergistic response greater than the sum of indivi dual responses. 6. The results confirm the bradycardic effect of apnoe a and cold exposure, whereas immersion in thermoneutral water had litt le effect, a finding which has been disputed in the literature.