Ataxia at 3,600 m: Oxygen, Acetazolamide, or Descent?
A 28-year-old trekker is at 3,600 m on day 3 of an ascent. He has a throbbing headache, nausea, and breathlessness at rest. Respiratory rate is 24/min, heart rate 110/min, blood pressure 118/76 mmHg, temperature 37.0°C, and resting SpO2 is 82% on room air. Arterial blood gas shows pH 7.48, PaCO2 28 mmHg, PaO2 52 mmHg, and HCO3− 20 mmol/L; hematocrit is 52%. He is confused and cannot walk heel-to-toe without staggering. The team has acetazolamide 250 mg tablets, a portable oxygen cylinder delivering 2 L/min for about 4 hours, and a radio. The nearest road is a 6-hour walk; a helicopter is unavailable until morning. The leader proposes giving oxygen and acetazolamide and waiting until morning.
1. Interpret the acid-base and oxygenation data. Why is PaO2 52 mmHg despite an SpO2 of 82%?
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At 3,600 m, barometric pressure is about 480 mmHg and inspired PO2 is roughly 100 mmHg, so alveolar and arterial PO2 fall. Hyperventilation raises alveolar PO2 but blows off CO2: PaCO2 28 mmHg drives pH to 7.48, producing respiratory alkalosis. HCO3− 20 mmol/L is below normal, reflecting renal compensation and a mild metabolic acidosis component. PaO2 52 mmHg with SpO2 82% is consistent with the steep part of the haemoglobin dissociation curve; SpO2 is a poor surrogate for PaO2 because it is affected by temperature, pH, and 2,3-BPG. Hematocrit 52% indicates haemoconcentration and early acclimatisation. The primary problem is hypobaric hypoxia, not a primary acid-base disorder.
2. Explain why ataxia changes management. Prioritise interventions and justify why acetazolamide alone is insufficient.
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Ataxia and confusion at altitude indicate high-altitude cerebral oedema, a medical emergency. Acetazolamide is useful for prevention and mild acute mountain sickness but does not treat cerebral oedema. Oxygen at 2 L/min and immediate descent are the priorities; if available, dexamethasone should be given. Waiting until morning risks coma and death. The team should descend now, even at night, while using oxygen during the descent. The 6-hour walk is long, but descent of even 500–1,000 m can help. Acetazolamide can be started as adjunctive therapy but must not delay descent. Ataxia converts a mountain-sickness problem into a neurological emergency.
3. If descent is delayed, what physiological risks does supplemental oxygen pose, and how should it be used?
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Supplemental oxygen can blunt the hypoxic ventilatory drive, reducing ventilation and raising PaCO2; in a patient with cerebral oedema this may worsen cerebral blood flow and swelling. It should not be used as a substitute for descent. Use the lowest flow that keeps SpO2 above 90% or relieves symptoms, and continue oxygen during descent. Do not stop oxygen abruptly at altitude. If oxygen runs out, descent must continue. Oxygen toxicity is not a practical risk with 4 hours at 2 L/min. The key judgement is that oxygen is a bridge, not a cure; the definitive treatment is descent.