Oxygen and Respiration — Page 51, Lesson 76

I want to walk you through the later stages of hypoxic hypoxia — what happens as altitude increases beyond the point where the body can compensate. We've already covered the Indifferent and Compensatory stages; now we enter the Disturbance Stage, which runs from 15 000 ft to 20 000 ft — that's 4572 to 6092 metres. At this stage, the body's physiological compensatory mechanisms — things like increased breathing rate and heart rate — are no longer capable of providing adequate oxygenation to the tissues. The system is overwhelmed. So what symptoms appear? Let me list them as they appear in the syllabus. You get euphoria — a false sense of well-being, which is dangerous because it masks the problem. Dizziness, sleepiness, headache, fatigue. Your intellectual function is impaired — thought processes slow down. Memory impairment sets in. Motor performance — your ability to coordinate movements — is severely impaired. You lose judgement. And you may experience what's called 'grey-out' and tunnelled vision — your visual field narrows and things go grey before you lose sight entirely. Now, above that, from 20 000 ft to 23 000 ft — that's 6092 to 7010 metres — we enter the Critical Stage. Here, mental performance deteriorates rapidly. Within just a few minutes, you get confusion and dizziness. Then — and this is the key point — total incapacitation with loss of consciousness follows with little or no warning. There's no gradual fade; you simply go under. I want to emphasise a note from the text: a healthy person is normally able to compensate for altitudes up to approximately 10 000 to 12 000 ft. That's the ceiling of the Indifferent and Compensatory stages. Above that, the Disturbance and Critical stages take over. Now, what determines how severely an individual is affected by hypoxic hypoxia? The book lists several key factors. First, Altitude — the greater the altitude, the greater the degree of hypoxia, and the more rapid the onset and progression. Simple: higher means worse, faster. Second, Time — the longer you are exposed to high altitude, the greater the effect. Duration matters. Third, Exercise — or workload. Exercise increases the body's demand for oxygen, so it increases the degree of hypoxia. The text says even the smallest physical exertion will significantly reduce the time of useful consciousness — that's the time you have before you become incapacitated. Fourth, Extremes of temperature — both heat and cold place a heavy demand on the circulatory adjustments the body has to make. That lowers your tolerance to hypoxia. Specifically, at low temperatures you shiver to maintain body temperature, and shivering increases oxygen demand, which worsens the hypoxic state. Fifth, Illness and fatigue — both increase the energy demands of the body and lower the threshold at which hypoxia symptoms appear. You become symptomatic at a lower altitude. Sixth, Alcohol and drugs — alcohol affects metabolisation and causes histotoxic hypoxia — that's a different type of hypoxia where the tissues can't use the oxygen even if it's delivered. So alcohol reduces your tolerance to hypoxic hypoxia. Many other drugs have adverse effects on brain function, which may lead to hypoxia as altitude tolerance decreases. So those are the factors: altitude, time, exercise, temperature extremes, illness and fatigue, and alcohol or drugs. Each one either increases oxygen demand or reduces the body's ability to compensate, pushing you into the Disturbance or Critical stage sooner.