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Chapter 4 — 3
Immersion
A neutrally buoyant diver is exempt from the main effects of gravity and this produces
physiological changes in the body. The return of blood flow to the heart and lungs is increased.
The body interprets this as an excess blood volume and compensates by
increasing urine
production (
which may then lead to
dehydration)
.
Cold water exposure produces many reflexes, including a desire to urinate. Temperature
regulation is more difficult. The pressure variations may influence lung function with head out
or vertical positions. Spatial orientation processes are disrupted. Trauma, in the form of
physical injury from water movement, marine infections, dangerous marine animals,
barotraumas, drowning etc. are dealt with in separate chapters.
Dive Reflex
Aquatic mammals display a reflex known as the "
dive reflex
". This is associated with profound
slowing of the heart and redirection of the blood flow away from the muscles and non-essential
organs to give a better blood supply to the heart and brain. It allows for longer submersions.
This reflex is present to a less degree in humans and can be produced by immersing the face or
head in cold water. The heart slowing component of the reflex has been used by physicians to
treat certain cardiac disorders associated with a rapid heart rate. It can also result in heart
arrhythmias.
Other potentially harmful reflexes can be induced by cold, Valsalvas, breath-holding
etc
.
Hypoxic Blackout
This loss of consciousness, due to an inadequate supply of
O
2
to the brain, usually develops
without any warning. Underwater this leads to aspiration of water and drowning. It is a
frequent cause of deaths amongst breath-hold divers. Sometimes the diver arrives on the
surface still alive but in a state almost unconscious, and with some brain damage. This is called
LMC (Loss of Motor Control) and causes unsteadiness and clumsiness. It also may occur after
the diver has been rescued while unconscious or semi-conscious. At other times it may result in
dementia, severe muscle impairment, visual damage or epileptic convulsions. There are two
main types of hypoxic blackout, although they can occur together.
a. Hypoxic Blackout due to Hyperventilation and Breath-holding
There are some people who find the flaunting of safety mechanisms an overwhelming
challenge. They may be trying to swim or stay underwater for as long as possible (such as
swimming the length of a swimming pool, or to impress their peers or girl friend). The break
point can be delayed by
hyperventilating
(taking a succession of rapid deep breaths) before a
dive. This reduces lung and arterial CO
2
so that it takes longer for the blood level to reach the
break point during a dive. During this delay, the blood O
2
level may fall below that necessary
to maintain consciousness and the diver may become unconscious without any warning. This is
one cause of
Hypoxic Blackout
. This can occur at any depth, such as in a one metre deep pool.
Using this method some divers have been able to prolong their breath hold dives for extended
periods — until the body is found!