Basic HTML Version
Chapter 21 — 3
CAUSES OF O
2
TOXICITY
For
resuscitation,
100% O
2
should be used for hypoxic diving casualties without any fear of
O
2
toxicity. As mentioned above, the treatment of decompression sickness and air embolism
cases includes 100% O
2
, even before consultation with the diving physician regarding any
potential negative effects.
Oxygen
re-breathing equipment
should be restricted to military, commercial and trained
technical divers use and diving with this should not be attempted by recreational divers. O
2
diving sets have an absolute depth limit of 9 metres for resting dives and 8 metres for
working dives in order to reduce the risk of convulsions. Rebreathing and scuba sets
employing nitrogen/O
2
(
nitrox
) mixtures are limited to depths which produce an inspired O
2
partial pressure of no more than 1.6 atmospheres, and often less.
In
deep diving
operations, gas mixtures of helium, nitrogen and O
2
should have the
composition adjusted so that the inspired partial pressure of O
2
never reaches the toxic range.
Therapeutic recompression
using O
2
tables often involves the compression of the diver to
2.8 atmospheres while breathing 100% O
2
. There is a significant risk of both pulmonary and
cerebral O
2
toxicity and these tables should only be employed on the advice of diving
medical experts.
CLINICAL FEATURES
Cerebral Effects
In this case the earliest symptom may be a convulsion which can develop without any
warning. It may sometimes be preceded by a variety of features such as facial pallor, visual
or auditory disturbances, tunnel vision, faintness, or facial twitching – which are often not
evident underwater. Nausea, retching and even vomiting are common with cerebral O
2
toxicity, as are anxiety and palpitations. There is considerable individual variation in
susceptibility to cerebral O
2
toxicity and an individual may vary in his tolerance from day to
day. It may be increased by anything that increases carbon dioxide levels, such as exercise,
immersion, resistance from breathing equipment and nitrogen narcosis.
During therapeutic recompression using O
2
tables, any convulsion in a diver due to cerebral
O
2
toxicity must be distinguished from a convulsion due to cerebral decompression sickness
or air embolism. Sometimes the convulsions occur soon after the O
2
supply is removed (the
“off effect”).