DROWNING - Medicine for Mountaineering and Other Wilderness Activities - page 431

holding prevents drowning and provides an oxygen reservoir in the lungs. Selective blood
vessel constriction and progressive hypertension ensure that the heart and brain, which are
the most sensitive to hypoxia, are adequately oxygenated at the expense of less sensitive
organs.
The effectiveness of this reflex in humans, especially the breath holding response, is
doubtful. Immersing adults suddenly in 32°F (0°C) to 95°F (35°C) water disclosed that
colder water greatly decreased breath holding duration and that bradycardia was independ-
ent of water temperature. These studies documented a reduced effectiveness of the dive re-
flex that would actually promote drowning.
In a second study, children four to thirteen years old and adults twenty to sixty-eight
years old were immersed in 84.2°F (29°C) water. Breath holding duration was shorter in
children than adults, decreased progressively with age for children (ten seconds in four-
year-olds),andwouldbeexpectedtobeevenshorterincolderwater.Therelativebradycar-
dia was similar—a 37 percent decrease in heart rate—in both children and adults. This led
the investigators to conclude that their findings did not support the postulate that the dive
response has an important role in the enhanced resuscitability associated with cold water
drowning and shifted emphasis to hypothermia as the mechanism for this phenomenon.
Subsequentstudieshaveconfirmedthedistinctbradycardiainducedbyfacialimmersion
incoldwater.Bradycardiaisinverselyproportionaltowatertemperature,andtheheartrate
reduction reaches about eighteen beats per minute in 50°F (10°C) water.
Interestingly, voluntary breath holding consists of two phases. The first “easy phase”
is not associated with breathing movements. However, increasing arterial carbon dioxide
levelsreachaphysiologicbreakpointandbegintostimulaterespiratorymovements,sothat
duringthesecond“strugglephase”progressiveinvoluntarybreathingmovementsdooccur.
Theprotectiveeffectofcoolingdependsonthecoretemperatureatthecessationofoxy-
gen delivery (asphyxia), and the subsequent rate and extent of the decrease in core temper-
ature. Since the early 1960s surgeons have taken advantage of the first of these principles
by using cardiopulmonary bypass to cool neurosurgical patients to a core temperature of
48°F (9°C), which allows arrest of brain blood flow for at least fifty-five minutes with full
neurologic recovery.
Since core temperature is likely to be near normal at the onset of an accidental submer-
sion, the second principle, rapid cooling after the onset of ischemia, is more significant.
Children have a greater surface-area-to-mass ratio than adults, which is well-known to res-
ult in greater rates of core cooling. However, whether cooling due to conduction alone
could be responsible for a significant core cooling is doubtful.
When cold water (38.2°F or 4°C) drowning was studied in shaved, anesthetized dogs,
coretemperaturesdecreased20°F(11°C)infourminutesinthecompletelysubmerseddogs
compared to only 5.4°F (3°C) in control dogs that were immersed with their heads out of
the water. Body weight and blood measurements indicated that water was actually inhaled