Man's responses to breath-hold exercise in air and in water.

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Abstract
The pulse rate responses to exercise with or without breath holding were recorded in 10 subjects in air and in water. The tachycardia of exercise was less in water (30[degree] C) than in air. Although apnea decreased the response to exercise, its effect was different from breath holding without exercise where there is a bradycardia when the subject is in water. Pulse rate responses and pulmonary gas exchanges were also examined in 4 subjects who performed repetitive periods of exercise with apnea in air and water. Thirty-sec, periods of exercise were alternated with 30 sec. of rest. The pulse rate was less during the exercise than during the rest, but there were no significant differences between the experiments in air or in water. Pulmonary gas exchanges were identical in each environment. In contrast to underwater swimming, the PACO2 [arterial carbon dioxide pressure] at the end of the periods of apneic exercise varied with the work load. It is suggested that the low PACO2 seen after underwater swims is the result of the hyperventilation preceding the exercise. In man, as contrasted to other diving vertebrates, the cardiovascular responses to apnea do not result in either the conservation of O2 or retention of CO2.

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