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curity,andgoodswimmersarenotimmunetodrowning.Competitiveswimmersanddivers
can drown as a result of hyperventilation prior to competition. Deliberately “blowing off”
carbon dioxide (hypocapnia) delays onset of the drive to breathe produced by rising blood
concentrations of carbon dioxide, which can allow the development of low oxygen levels
(hypoxia) that result in unconsciousness and underwater attempts to breathe.
MECHANISMS OF DROWNING
A few individuals who drown, such as white-water rafters and kayakers, get pinned or
trapped underwater. Some people dive into shallow water, strike their heads on the bottom
or on submerged objects, and are knocked unconscious or suffer cervical fractures. Indi-
viduals competing to determine who can stay underwater the longest or swim the greatest
distance underwater may lose consciousness, particularly if they have hyperventilated be-
forehand.
The mechanism of death for others is less obvious. Many dive or jump into cold water
and simply do not come up. No struggle of any kind is witnessed. Some individuals
plunged into cold water suffer a cardiac death, usually an abnormal rhythm such as cardiac
arrest or ventricular fibrillation (immersion syndrome). In recent studies of the hearts of
a teenage girl and a nine-year-old boy who died by drowning, Mayo Clinic College of
Medicine investigators found a genetic defect in the electrical conduction system of the
heartsthatapparentlyledtoanabnormalcardiacrhythm,unconsciousness,anddeath.They
warned that such defects could be more widespread.
In many accidents of this type, the sudden contact with cold water apparently prompts a
sudden, uncontrollable gasp that results in aspiration of water. This response (the gasp re-
flex)isessentiallyuniversal,althoughitsometimescanbecontrolled.Manyhaveobserved
that response upon stepping into a cold shower.
Individuals temporarily trapped underwater have experienced an overwhelming com-
pulsiontobreathe.(Thetimeanyonecanholdtheirbreathwhensubmergedincoldwateris
much shorter than when submerged in warm water or on land—usually one-fourth to one-
third as long.) Apparently some individuals give in to this urge, perhaps thinking they can
safely take a single breath to relieve the respiratory drive until they reach the surface.
One inhalation or gasp of water may stop all efforts to reach the surface; complete as-
phyxia may not be necessary. Possibly the water passes without delay through the lungs
into the blood, and the reduced osmotic pressure or some other altered characteristic of the
diluted blood has an immediate effect on the brain. Individuals who have been resuscitated
after drowning have described the sensation as enjoyable, even “orgasmic.” This hypothet-
ical mechanism would explain the drowning of individuals after water washes over their
head in a turbulent stream or ocean surf but does not explain the deaths of individuals who
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