Causes of loss of consciousness during underwater swimming

¹ Department of Physiology, University of Rochester, School of Medicine and Dentistry, Rochester, New York

Several incidents of losing consciousness while swimming underwater are known to the author. Experiments were designed to find out how this could happen. Four types of breath holding were executed: a) at rest, b) after hyperventilation, c) during mild exercise, and d) after hyperventilation and during exercise. At the breaking point the subject made a maximal expiration, and the end tidal air was analyzed for O₂ and CO₂. It was found that when the breaking point was reached, the Pco₂ was higher and the Po₂ lower during exercise than at rest. The lowest Po₂ was observed after the subject had exercised following hyperventilation: the Po₂ was 34 mm or below in 4 of the 12 subjects, a degree of hypoxia often associated with unconsciousness. Other experiments including underwater swimming support the conclusion that loss of consciousness after hyperventilation and during exercise is possible and is probably due to hypoxia.