3-44 U.S. Navy Diving Manual — Volume 1 limits. Allowable oxygen exposure limits for each system are discussed in later chapters. 3‑9.2.2.2 Symptoms of CNS Oxygen Toxicity. The most serious direct consequence of oxygen toxicity is convulsions. Some times recognition of early symptoms may provide sufficient warning to permit reduction in oxygen partial pressure and prevent the onset of more serious symp toms. The warning symptoms most often encountered also may be remembered by the mnemonic VENTIDC: V: Visual symptoms. Tunnel vision, a decrease in diver’s peripheral vision, and other symptoms, such as blurred vision, may occur. E: Ear symptoms. Tinnitus, any sound perceived by the ears but not resulting from an external stimulus, may resemble bells ringing, roaring, or a machinery ­ like pulsing sound. N: Nausea or spasmodic vomiting. These symptoms may be intermittent. T: Twitching and tingling symptoms. Any of the small facial muscles, lips, or muscles of the extremities may be affected. These are the most frequent and clearest symptoms. I: Irritability. Any change in the diver ’s mental status including confusion, agitation, and anxiety. D: Dizziness. Symptoms include clumsiness, incoordination, and unusual fatigue. C: Convulsions. The first sign of CNS oxygen toxicity may be convulsions that occur with little or no warning. Warning symptoms may not always appear and most are not exclusively symp toms of oxygen toxicity. Muscle twitching is perhaps the clearest warning, but it may occur late, if at all. If any of these warning symptoms occur, the diver should take immediate action to lower the oxygen partial pressure. A convulsion, the most serious direct consequence of CNS oxygen toxicity, may occur suddenly without being preceded by any other symptom. During a convul­ sion, the individual loses consciousness and his brain sends out uncontrolled nerve impulses to his muscles. At the height of the seizure, all of the muscles are stimu­ lated at once and lock the body into a state of rigidity. This is referred to as the tonic phase of the convulsion. The brain soon fatigues and the number of impulses slows. This is the clonic phase and the random impulses to various muscles may cause violent thrashing and jerking for a minute or so. After the convulsive phase, brain activity is depressed and a postconvulsive (postictal) depression follows. During this phase, the patient is usually uncon­ scious and quiet for a while, then semiconscious and very restless. He will then usually sleep on and off, waking up occasionally though still not fully rational. The

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