Most people can hold their breath approximately 1 minute, but usually not much
longer without training or special preparation. At some time during a breathholding
attempt, the desire to breathe becomes uncontrollable. This demand is
signaled by the respiratory center responding to the increasing levels of carbon
dioxide and acids in the arterial blood and chemoreceptors responding to the
corresponding rise in arterial carbon dioxide.
Breathhold diving shall be confined to tactical
and work situations that cannot be effectively accomplished by the use of underwater
breathing apparatus and applicable diver training situations such as scuba
pool phase and shallow water obstacle/ordnance clearance. Breathhold diving
includes the practice of taking two or three deep breaths prior to the dive. The
diver shall terminate the dive and surface at the first sign of the urge to breath.
Hyperventilation (excessive rate and depth of breathing prior to a dive, as differentiated
from two or three deep breaths prior to a dive) shall not be practiced because
of the high possibility of causing unconsciousness under water.
One of the greatest hazards of breathhold diving is
the possible loss of consciousness during ascent. Air in the lungs during descent is
compressed, raising the oxygen partial pressure. The increased ppO2 readily satisfies
the body’s oxygen demand during descent and while on the bottom, even
though a portion is being consumed by the body. During ascent, the partial pressure
of the remaining oxygen is reduced rapidly as the hydrostatic pressure on the
body lessens. If the ppO2 falls below 11 percent (83.6 mmHg), unconsciousness
may result with its attendant danger. This danger is further heightened when
hyperventilation has eliminated normal body warning signs of carbon dioxide
accumulation.
