Acid-Base Status and Ascent Decisions at 4,200 Metres
A 29-year-old trekker from Kathmandu flies to Lukla and walks to 4,200 m over three days. On the third morning she has a mild headache but no breathlessness at rest. Measurements: SpO2 82% (sea-level baseline 98%), respiratory rate 24/min, arterial pH 7.49, PaCO2 29 mmHg, HCO3- 21 mmol/L, hematocrit 51%, temperature 36.8 degrees C, chest clear. She wants to continue to 5,000 m the next day and asks for acetazolamide 'to be safe'. The lodge owner says a previous guest with similar oxygen saturation was fine. The trekking doctor must decide whether to continue, descend, or stay and acclimatize; whether to start acetazolamide; and what would change the decision. A portable ultrasound is not available; a pulse oximeter and acetazolamide are.
1. Interpret the acid-base and oxygenation data. Explain the physiological chain from altitude to pH 7.49 and PaCO2 29 mmHg, and the trade-off for tissue oxygen unloading.
Read the worked answer
At 4,200 m, inspired PO2 is about 60% of sea-level, so alveolar and arterial PO2 fall. Peripheral chemoreceptors detect hypoxia and increase ventilation (rate 24/min). More ventilation washes out CO2, so PaCO2 falls from about 40 to 29 mmHg. This is respiratory alkalosis: pH rises to 7.49 because HCO3- (21 mmol/L) has not yet fallen much. The kidney compensates over days by excreting HCO3-, which lowers pH toward normal; the value of 21 mmol/L suggests early renal compensation. Hyperventilation raises alveolar PO2, improving saturation, but the alkalosis shifts the oxyhaemoglobin dissociation curve to the left, increasing affinity and impairing O2 unloading at tissues. Later, increased 2,3-BPG and acclimatization shift the curve back. Hematocrit 51% reflects haemoconcentration plus erythropoietin-driven red cell production. The headache is consistent with mild acute mountain sickness.
2. Decide whether she should continue, stay, or descend, and whether acetazolamide should be started. Justify with the evidence.
Read the worked answer
She has mild headache but no ataxia, no breathlessness at rest, and a clear chest. This is mild acute mountain sickness, not HAPE. Continuing to 5,000 m the next day is unsafe: ascent with symptoms risks progression. Best: stay at 4,200 m and acclimatize for 24-48 hours, with rest, fluids, and no alcohol; descend if headache worsens, vomiting, ataxia, or breathlessness at rest develops. Acetazolamide is reasonable as an adjunct: it inhibits carbonic anhydrase, causes bicarbonate diuresis and metabolic acidosis, and stimulates ventilation, accelerating acclimatization; it also reduces periodic breathing. It does not replace descent and is not a licence to ascend rapidly. Reassess after 24 hours: if symptoms resolve and SpO2 is stable, a slow ascent with a rest day is acceptable; if not, descend. The previous guest's experience is irrelevant because individual susceptibility varies.
3. Identify two findings that would indicate early HAPE and explain why SpO2 alone is unreliable.
Read the worked answer
Early HAPE findings: (1) breathlessness at rest or on minimal exertion, with a rising respiratory rate; (2) fine crackles in one lung, cough, or frothy pink sputum. Other signs include ataxia, cyanosis, and disproportionate desaturation. SpO2 alone is unreliable because normal values vary between individuals; pulse oximeters are affected by cold fingers, poor perfusion, nail polish, and altitude; some acclimatized people have low SpO2 without HAPE, and some with HAPE may have a value that looks normal for altitude. A falling trend with symptoms is more useful than a single reading. Clinical status is the deciding factor.