Causes of loss of consciousness during underwater swimming

TLDR

Underwater swimming can lead to loss of consciousness, as several incidents have been reported. The study aimed to determine the physiological conditions that cause loss of consciousness during underwater swimming. Four breath‑holding protocols were tested—rest, hyperventilation, exercise, and hyperventilation plus exercise—and at the point of loss, maximal expiration was performed and end‑tidal O₂ and CO₂ were measured. Loss of consciousness was associated with hypoxia, as the lowest end‑tidal O₂ (≤34 mm Hg in 4 of 12 subjects) and elevated CO₂ were observed during hyperventilation plus exercise, supporting that hypoxia underlies the phenomenon. Submitted on December 30, 1960.

Abstract

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 O 2 and CO 2 . It was found that when the breaking point was reached, the P co2 was higher and the P o2 lower during exercise than at rest. The lowest P o2 was observed after the subject had exercised following hyperventilation: the P o2 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. Submitted on December 30, 1960