Blood Gases and a Trekker at 3,800 Metres
A 34-year-old man from Lucknow, at about 200 m, flies to Leh at 3,500 m and treks to 3,800 m over two days. On the second night he has a throbbing headache, nausea, poor sleep and breathlessness at rest. He is fully alert, with no neck stiffness and no ataxia. Respiratory rate is 24/min, pulse 110/min, resting SpO2 82%, and the chest is clear. Arterial blood shows pH 7.48, PaCO2 30 mmHg, PaO2 52 mmHg and bicarbonate 22 mmol/L. He has no fever and no cough. He asks for a sedative to sleep better and insists on continuing the trek in the morning.
1. Explain the blood gas values. Why is PaO2 low with clear lungs, why is PaCO2 low, and what will the kidney do over the next few days?
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Barometric pressure falls with altitude, so the partial pressure of inspired oxygen falls even though the percentage of oxygen in air stays at 21%. At 3,800 m the alveolar oxygen pressure is only about 55–60 mmHg, so the gradient into capillary blood is small and PaO2 settles near 52 mmHg. The lungs are clear because nothing is wrong with the alveoli; the problem is the low driving pressure, not lung disease.
Hypoxaemia stimulates the peripheral chemoreceptors in the carotid and aortic bodies, and ventilation rises from about 12 to 24 breaths per minute. This washes out carbon dioxide faster than it is produced, so PaCO2 falls to 30 mmHg. Since pH is set by the ratio of bicarbonate to dissolved CO2, a fall in CO2 raises pH - respiratory alkalosis.
The bicarbonate of 22 mmol/L indicates that compensation has begun. Over days the kidney excretes bicarbonate and retains hydrogen ions, returning pH towards normal at the cost of a lower plasma bicarbonate. This renal compensation is slow, which is why acclimatisation takes days and why rapid ascent is dangerous.
2. Why must the request for a sedative be refused, why does acetazolamide help, and why is descent the definitive treatment?
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At altitude, ventilation is driven mainly by the hypoxic stimulus. Sedatives depress the respiratory centre and blunt that drive, so ventilation falls, PaCO2 rises, PaO2 falls further and the brain becomes more hypoxic. Sleep at altitude already causes periodic breathing; a sedative can turn this into dangerous hypoventilation and worsen headache and confusion. The request should be refused and the sleep problem treated with acetazolamide and, if needed, oxygen.
Acetazolamide inhibits carbonic anhydrase in the kidney, causing bicarbonate loss and a mild metabolic acidosis. This acidosis stimulates ventilation, raising alveolar and arterial oxygen, and it also reduces cerebrospinal fluid formation and periodic breathing. It is prophylaxis and adjunct therapy, not a rescue.
Descent is definitive because it restores barometric pressure and hence inspired oxygen pressure; no drug can raise the oxygen content of the air. Oxygen and dexamethasone buy time but do not treat the cause. Continuing the trek upward would be the single most dangerous decision in this case.
3. Is this acute mountain sickness or high-altitude pulmonary oedema? Justify with the findings and state what would change the diagnosis.
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The picture fits acute mountain sickness: headache, nausea, poor sleep and breathlessness at rest, occurring after rapid ascent, with a clear chest, no cough, no fever and normal consciousness. Ataxia and altered awareness would suggest high-altitude cerebral oedema; neither is present.
High-altitude pulmonary oedema would be expected to produce breathlessness out of proportion to the oxygen reading, a dry cough progressing to pink frothy sputum, crackles or crepitations in the chest, and often a lower oxygen saturation. The clear chest and absence of cough argue against it.
The SpO2 of 82% is a distractor. At 3,800 m this value can occur in otherwise acclimatising people, and it is not by itself diagnostic of either condition. What matters is the trend and the clinical state: a saturation that falls further despite rest, or new crackles, ataxia or confusion, would change the diagnosis and mandate immediate descent with oxygen.