Hypoxia, Hyperventilation and Cabin Atmosphere
Hypoxia
The four types are hypoxic (reduced oxygen partial pressure, the aviation type), hypaemic (reduced oxygen-carrying capacity, e.g. carbon monoxide or anaemia), stagnant (poor circulation) and histotoxic (cells cannot use oxygen, e.g. alcohol or cyanide). Hypoxic hypoxia produces euphoria, impaired judgement, tunnel vision, cyanosis and eventually unconsciousness. Time of useful consciousness falls rapidly with altitude: minutes at 25,000 ft, seconds at 40,000 ft. The insidious feature is that the pilot often feels fine while judgement is already degraded.
Hyperventilation
Rapid breathing lowers carbon dioxide, causing dizziness, tingling, cramps and a feeling of breathlessness - symptoms that overlap with hypoxia. The treatment is to slow the breathing rate, not to breathe faster. If hypoxia is suspected at altitude, use oxygen first, then control breathing.
Cabin atmosphere
Cabin altitude is normally kept below about 8,000 ft by pressurisation. A loss of pressurisation requires oxygen masks and an emergency descent. Traps include assuming hypoxia always causes obvious symptoms (it does not) and treating hyperventilation with more air. Remember that alcohol, fatigue and smoking increase susceptibility.
Worked example
A pilot at 14,000 ft feels euphoric and makes arithmetic errors. This is hypoxic hypoxia; the correct action is oxygen and descent. Rule of thumb: if in doubt at altitude, put on oxygen before debating the cause.