CHAPTER 18 Closed-Circuit Oxygen UBA Diving
C: Convulsions. The first sign of CNS oxygen toxicity may be a convulsion that
occurs with little or no warning.
The most serious symptom of CNS oxygen toxicity is convulsion. Refer to
Chapter 3 for a complete description of a convulsive episode. The following
factors should be noted regarding an oxygen convulsion:
The diver is unable to carry on any effective breathing during the convulsion.
After the diver is brought to the surface, there will be a period of unconscious-
ness or neurologic impairment following the convulsion; these symptoms are
indistinguishable from those of arterial gas embolism.
No attempt should be made to insert any object between the clenched teeth of
a convulsing diver. Although a convulsive diver may suffer a lacerated tongue,
this trauma is preferable to the trauma that may be caused during the insertion
of a foreign object. In addition, the person providing first aid may incur signif-
icant hand injury if bitten by the convulsing diver.
There may be no warning of an impending convulsion to provide the diver the
opportunity to return to the surface. Therefore, buddy lines are essential to safe
closed-circuit oxygen diving.
Causes of CNS Oxygen Toxicity. Factors that increase the likelihood of CNS
oxygen toxicity are:
Increased partial pressure of oxygen. At depths less than 25 fsw, a change in
depth of five fsw increases the risk of oxygen toxicity only slightly, but a sim-
ilar depth increase in the 30-fsw to 50-fsw range may significantly increase the
likelihood of a toxicity episode.
Increased time of exposure
Stress from strenuous physical exercise
Carbon dioxide buildup. The increased tendency toward CNS oxygen toxicity
may occur before the diver is aware of any symptoms of carbon dioxide
Cold stress resulting from shivering or an increased exercise rate as the diver
attempts to keep warm.
Systemic diseases that increase oxygen consumption. Conditions associated
with increased metabolic rates (such as certain thyroid or adrenal disorders)
tend to cause an increase in oxygen sensitivity. Divers with these diseases
should be excluded from oxygen diving.