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and absorbed by the submersed dogs and that rapid cooling was produced by convective
heat exchange in the lungs. Dogs immersed with their heads out were cooled only by sur-
face conduction.
Although these studies have clearly demonstrated continued ventilation of cold water
after submersion in dogs, this response in humans has not been documented. However,
reaching the struggle phase of breath holding during which involuntary breathing move-
ments predominate, coupled with unconsciousness, would be expected to result in inhala-
tion of water in at least some circumstances. Experimental and anecdotal evidence in hu-
mans is rare. However, one helicopter crash survivor reported that, after being trapped un-
derwater for some time, he recalled feeling he was about to die and that he was breathing
water in and out just prior to escaping the cockpit.
Protection from Anoxia by Cooling
Hypothermia provides an advantage during anoxic periods such as cold water submer-
sion because it produces a situation called a “metabolic ice-box.” Whole body or focal hy-
pothermia has been used to extend the survival time during surgery under ischemic con-
ditions for years. For heart surgery to repair congenital defects, circulatory arrest has been
induced for sixty minutes at core temperatures of 59° to 64°F (15° to 18°C) without any
subsequent cerebral dysfunction.
This cerebral protection has commonly been attributed to the decreased cerebral meta-
bolic requirements for oxygen that follow the Q
10
principle, the change of physical and
chemical rates of reaction with an 18°F (10°C) change of temperature. The Q
10
of the
whole body is about 2, although higher values are attributed to the brain. One investigator
hasshownthattheQ
10
ofthebrainincreasesfrom~3between98°and80°F(37°and27°C)
to 4.8 between 80° and 64°F (27° and 18°C). Based on these values, if the brain could sur-
vive an ischemic insult for five minutes at 97.6°F (37°C), cooling to 81°F (27°C) and 63°F
(17°C) would provide fifteen and seventy-two minutes of protection, based solely on the
decreased cerebral metabolic requirements for oxygen.
Althoughlongsurvivaltimesatbraintemperaturesbelow68°F(20°C)maybepredicted
onthebasisofincreasingQ
10
anddiminishingcerebralmetabolicrequirementsforoxygen,
additional mechanisms may be required to explain intact survival after prolonged submer-
sion in which core temperatures are above 86°F (30°C). Over the past decade several stud-
ies directed mainly toward protection of the brain during or following cerebral ischemic
events such as strokes have demonstrated that moderate brain cooling provides substan-
tial cerebral protection from ischemic insult. Various groups of investigators have demon-
strated that moderate cooling of the rat brain to 95°F (35°C) and 91°F (33°C) greatly re-
ducesoreveneliminatesneuronaldamagecausedbytentotwentyminutesoftotalcerebral
ischemia. Functional outcome is improved by cooling; the response to electrical stimula-
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