The physiological consequences of hypoxia in general medicine are well known and are not discussed here.
The symptoms and signs of hypoxia become obvious when the PaO2 drops below about 50 mm Hg. This corresponds to an inspired concentration at sea level of 8 to 10 per cent. If the fall in PO2 is rapid, then loss of consciousness may be unheralded. With slower falls, an observer may note lack of coordination or poor job performance. Euphoria, overconfidence and apathy are also been reported. Memory is defective and judgement impaired, leading to inappropriate or dangerous reactions to the emergency that may also endanger others. The diver may complain of fatigue, headache or blurred vision.
There are rarely any symptoms to warn the diver of impending unconsciousness from hypoxia.
Hyperventilation may develop in some cases, but it is usually minimal if the arterial CO2 tension (PaCO2) is normal or low.
There are marked individual differences in susceptibility to hypoxia. When combined with hypocapnia or hypercapnia, hypoxia will impair mental performance earlier than if the diver is normocapnic; mental performance may not be severely impaired until the alveolar-arterial PO2 falls below 40 mm Hg. Hypoxia may precipitate or exacerbate other pathological conditions, such as coronary or cerebral ischaemia.
Cyanosis of the lips and nail beds may be difficult to determine in the peripherally vasoconstricted ‘cold and blue’ diver. Generalized convulsions or other neurological manifestations may be the first signs. Masseter spasm is common and may interfere with resuscitation. Eventually, respiratory failure, cardiac arrest and death supervene.
Diagnostic errors may arise because some of the foregoing manifestations are common to nitrogen narcosis, O2 toxicity and CO2 retention. The attending physician should also consider cerebral arterial gas embolism and decompression sickness (DCS), should the previously described features develop during or after ascent by a diver breathing compressed gases.