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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