Uncontrolled hyperventilation is caused by overexertion,
panic, and/or fright. A person breathes in and out rapidly but the breaths
are shallow. Little oxygen gets into the lungs. The person feels they are
out of air. The remedy is to relax and "catch your breath." Underwater the
key words are: "Stay calm!" If the diver starts hyperventilating they must
stop what is being done, take deeper breaths, and relax!
Controlled hyperventilation is done to increase the time
one may hold their breath underwater. If it is done to excess it can be very
dangerous. In the case of controlled hyperventilation, every bit of air that
can be exhaled is released from the lungs. This lowers the carbon dioxide
(CO2) in the blood stream. Then a deep breath is taken. This raises the oxygen
in the blood. If this is done enough a person will be able to hold their
breath for a much longer time.
There is a sensor in the carotid artery going into the
brain. This monitors the level of CO2 in the blood. If it rises too high
the sensor sends a signal to a part of the brain that sets in motion all
those things a person goes through when they are not getting fresh air into
the lungs. So, CO2 controls the breathing rate. There is also an oxygen level
sensor in the body, but it is not nearly as effective in stimulating one
to breathe when the oxygen level gets low.
Every time a person consciously hyperventilates they
lower the CO2, but only raise the oxygen a small amount because there is
only 21% in the air. If one hyperventilates 4 or more times there is the
chance the CO2 level gets so low that a person can hold their breath to the
point of blacking out. What happens is, the CO2 level never climbs back to
the point to tell the breath-holder they must breathe before the oxygen level
drops to a point the brain causes a blackout. Swimmers trying to go long
distances underwater while holding their breath after excessive hyperventilation
have blacked out and continued to swim, only to crash into the end of the
pool. The part of the brain causing the blackout is in the cerebrum. The
swimming coordination is controlled by the cerebellum which continues to
function after the blackout.