Cited Passage
3-18 U.S. Navy Diving Manual — Volume 1
Because the first sign of hypercapnia may be unconsciousness and it may not be
readily apparent whether the cause is hypoxia or hypercapnia. It is important to
rule out hypoxia first because of the significant potential for brain damage in hy
poxia. Hypercapnia may cause unconsciousness, but by itself will not injure the
brain permanently.
3‑5.2.4 Prevention of Hypercapnia. In surface supplied diving, hypercapnia is prevented
by ensuring that gas supplies do not contain excess carbon dioxide, by maintaining
proper manifold pressure during the dive and by ventilating the helmet frequently
with fresh gas. For dives deeper than 150 fsw, helium oxygen mixtures should be
used to reduce breathing resistance. In closed or semiclosed circuit UBAs, hyper
capnia is prevented by carefully filling the CO
2
absorbent canister and limiting
dive duration to estab lished canister duration limits. For dives deeper than 150 fsw,
helium oxygen mixtures should be used to reduce breathing resistance.
3-5.3 Asphyxia. Asphyxia is a condition where breathing stops and both hypoxia and
hypercapnia occur simultaneously. Asphyxia will occur when there is no gas to
breathe, when the airway is completely obstructed, when the respiratory muscles
become para lyzed, or when the respiratory center fails to send out impulses to
breathe. Running out of air is a common cause of asphyxia in SCUBA diving.
Loss of the gas supply may also be due to equipment failure, for example regulator
freeze up. Divers who become unconscious as a result of hypoxia, hypercapnia,
or oxygen toxicity may lose the mouthpiece and suffer asphyxia. Obstruction of
the airway can be caused by injury to the windpipe, the tongue falling back in the
throat during unconsciousness, or the inhalation of water, saliva, vomitus or a for
eign body. Paralysis of the respiratory muscles may occur with high cervical spinal
cord injury due to trauma or decompression sickness. The respiratory center in the
brain stem may become non functional during a prolonged episode of hypoxia.
3-5.4 Drowning/Near Drowning . Drowning is fluid induced asphyxia. Near drowning is
the term used when a victim is successfully resuscitated following a drowning epi
sode.
3 - 5.4.1 Causes of Drowning. A swimmer or diver can fall victim to drowning because of
overexertion, panic, inability to cope with rough water, exhaustion, or the effects
of cold water or heat loss. Drowning in a hard hat diving rig is rare. It can happen
if the helmet is not properly secured and comes off, or if the diver is trapped in a
head down position with a water leak in the helmet. Normally, as long as the diver
is in an upright position and has a supply of air, water can be kept out of the helmet
regardless of the condition of the suit. Divers wearing lightweight or SCUBA gear
can drown if they lose or ditch their mask or mouthpiece, run out of air, or inhale
even small quantities of water. This could be the direct result of failure of the air
supply, or panic in a hazardous situation. The SCUBA diver, because of direct
exposure to the environment, can be affected by the same conditions that may
cause a swimmer to drown.
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