Heart rate responses to apneic underwater diving and to breath holding in man

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Abstract
Bradycardia is a response to apneic diving which man has in common with many other species. Slowing of the heart rate during diving was observed in children as well as adults and was as prominent in poor swimmers as in those subjects who were familiar with the water. The response was independent of depth down to 27 m, but could not be produced by simulated dives in a compression chamber. Diving in water implies several maneuvers, some of which were investigated during breath holding. It was observed that the tachycardia produced by breath holding at different Valsalva pressures was proportional to the increase of intrathoracic pressure. At equal pressures the tachycardia was less when the subject was in water than when in air. Other maneuvers which increased venous return at the beginning of the breath hold produced a bradycardia during the apnea, and conversely when venous return was impaired there was a tachycardia. The hypothesis is presented that diving bradycardia in man might be explainable in terms of already known physiological mechanisms. swimming; submersion Submitted on February 27, 1963

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