At 3,500 m: acclimatisation or emergency?
A 24-year-old sea-level athlete travels to 3,500 m. At rest on day 1: breathing rate 20 min⁻¹ (sea level 12), tidal volume 0.6 L (0.5 L), heart rate 95 (58), arterial oxygen saturation 88% (98%), arterial pH 7.47 (7.40), haematocrit 45% (42%). After 10 days: breathing rate 17, saturation 91%, haemoglobin 17.8 g/dL (15.2), pH 7.44. On day 3 he has a headache, nausea and cannot answer simple questions. A companion wants to give him oxygen and stay one more night; another wants to descend immediately. The nearest clinic is 6 hours' walk down. He is otherwise healthy, takes no medication, and has no history of lung disease. The barometric pressure at 3,500 m is about 66 kPa, so the partial pressure of oxygen in inspired air is about 14 kPa, compared with 21 kPa at sea level.
1. Explain why the climber's breathing rate and arterial pH change on day 1.
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At 3,500 m the barometric pressure is about 66 kPa, so the partial pressure of oxygen in inspired air is about 14 kPa instead of 21 kPa. The lower PO2 is detected by peripheral chemoreceptors (carotid and aortic bodies), which increase ventilation. Breathing rate rises from 12 to 20 per minute and tidal volume from 0.5 to 0.6 L, so alveolar ventilation increases. More carbon dioxide is exhaled, so arterial PCO2 falls. CO2 dissolves to form carbonic acid, so less CO2 means less H⁺ and a higher pH: 7.47 instead of 7.40. This is respiratory alkalosis. The increased pH is a side effect of the response, not the trigger; the trigger is low oxygen. The kidneys later compensate by excreting bicarbonate.
2. Explain the changes after 10 days and why they improve oxygen transport.
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Sustained low oxygen stimulates the kidneys to release erythropoietin, which increases red blood cell production in bone marrow. Haemoglobin rises from 15.2 to 17.8 g/dL and haematocrit from 42% to 45%, so the blood carries more oxygen per litre. Arterial saturation improves slightly from 88% to 91%, partly because of improved matching of ventilation to perfusion and the higher haemoglobin. The kidneys excrete bicarbonate to compensate for the respiratory alkalosis, so pH falls from 7.47 to 7.44. Breathing rate falls from 20 to 17 because the bicarbonate compensation reduces the pH stimulus to the central chemoreceptors; the peripheral oxygen stimulus remains. This is acclimatisation, not a cure: the climber is still hypoxic compared with sea level.
3. Evaluate the two proposals for the climber with headache, nausea and confusion.
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Headache, nausea and confusion at altitude suggest high-altitude cerebral oedema (HACE), a medical emergency. The first proposal (oxygen and stay another night) is unsafe: oxygen may relieve symptoms temporarily but does not treat the underlying problem, and staying at altitude allows the swelling to worsen. The second (descend immediately) is the correct priority: descent is the definitive treatment because it increases the partial pressure of oxygen. The 6-hour walk is a constraint, but descent should start now, with oxygen and, if available, acetazolamide or dexamethasone as adjuncts, not substitutes. The climber should not walk alone; a companion should monitor confusion and breathing. The judgement is that symptoms of brain dysfunction outweigh the inconvenience of a night-time descent.