Why some people do not drown. Hypothermia versus the diving response.

OBJECTIVE

To examine our present state of knowledge regarding the remarkable survival of some victims from prolonged submersion for an hour or more. Debate continues on the relative importance of the two explanatory theories--diving response and hypothermia.

DATA SOURCES

A wide range of physiological, pathophysiological and clinical papers relating to the diving response, hypothermia and near-drowning, with emphasis on the period 1981-1991.

DATA SYNTHESIS

The normothermic human brain suffers irreversible damage if subjected to acute asphyxia for longer than 10 minutes. Significant resistance of brain tissue to hypoxia occurs only after its temperature has fallen from 37 degrees C to 30 degrees C or less. Body surface cooling depresses core temperature by only one-third of this drop in 10 minutes. Hence an additional factor, other than hypothermia, is required to explain survival from near-drowning. The idea that ingestion and aspiration of large amounts of cold water produce such a temperature drop lacks quantitative evidence. The diving response seen in marine mammals also occurs in humans but to a lesser extent; however, about 15% of volunteers tested exhibit a profound response. This response which starts immediately upon submersion prevents aspiration of water, redistributes oxygen stores to heart and brain, slows cardiac oxygen use and initiates a hypometabolic state. The possible influence of alcohol on these processes is considered.

CONCLUSIONS

Survival from prolonged near-drowning appears to depend upon a specific temporal interplay between the diving response and hypothermia, resulting in a protective state of hypometabolism.