Breathless at 4,300 Metres
A 24-year-old software engineer from Chennai (sea level) flies to Leh (3,500 m) and, against advice, treks to 4,300 m within 36 hours. He develops a throbbing headache, breathlessness at rest and poor sleep. At the clinic: respiratory rate 26/min, heart rate 104/min, SpO2 84% (his sea-level reading was 98%), blood pressure normal. Arterial blood: pH 7.50, PaCO2 28 mmHg, PaO2 48 mmHg, HCO3− 21 mEq/L. Haemoglobin 17.8 g/dL, haematocrit 54%. He has no fever, chest pain or cough, and his chest radiograph is clear. A porter says he has seen this many times and that 'the body simply makes more red cells within a day'. A trainee suggests putting him on 100% oxygen by mask and discharging him, since the mountain air is the only problem.
1. Explain the sequence that produced a pH of 7.50, and predict whether his urine will be acidic or alkaline on days 2–3. Evaluate the porter's claim about red cells.
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Sequence. At 4,300 m the barometric pressure is roughly 60% of sea level, so inspired and alveolar PO2 fall and arterial PO2 drops to 48 mmHg. The carotid and aortic bodies respond to this hypoxic stimulus by driving ventilation up (respiratory rate 26/min). Increased alveolar ventilation washes out CO2, so PaCO2 falls from 40 to 28 mmHg. Since CO2 is the volatile acid, the ratio of bicarbonate to carbonic acid rises, and pH rises to 7.50 - a respiratory alkalosis.
Renal response. Over 1–3 days the kidney compensates by reducing hydrogen ion secretion and reducing bicarbonate reabsorption, excreting bicarbonate. The urine is therefore alkaline, and the plasma bicarbonate falls from about 24 to 21 mEq/L, which partly corrects the pH without eliminating the ventilatory response. This is a compensatory change, not a cure.
Porter's claim. Wrong on timing. A haematocrit of 54% within 36 hours cannot be new red cells: it reflects haemoconcentration as plasma volume falls with altitude-induced diuresis, plus splenic release. Erythropoietin secretion rises within hours, but a measurable increase in red cell mass takes days to weeks. The exam point is that the rapid haemoglobin rise is mostly a change in concentration, not in cell number.
2. His red cells have raised 2,3-BPG. Explain the effect on the oxygen–haemoglobin dissociation curve and decide whether this shift helps or harms oxygen loading in the lungs.
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Effect on the curve. 2,3-BPG binds preferentially to deoxyhaemoglobin, stabilising the T (low-affinity) state. The curve shifts to the right and the P50 rises - at any given PO2 haemoglobin holds less oxygen.
Why it is not harmful at the lungs. At 4,300 m the alveolar and arterial PO2 is still around 50–60 mmHg. That lies on the flat upper plateau of the sigmoid curve, where haemoglobin remains about 85–90% saturated. A rightward shift costs only a few percentage points of saturation at the lung, because the curve is flat there.
Why it helps at the tissues. Tissue PO2 is 25–40 mmHg, on the steep lower part of the curve. Here the same rightward shift releases a much larger quantity of oxygen per unit fall in PO2.
The judgement is therefore that the shift trades a small loss of loading for a large gain in unloading, and it is the sigmoid shape of the curve - flat top, steep bottom - that makes the trade favourable. It also explains why a right shift is dangerous only when alveolar PO2 is already low enough to sit on the steep part, as in severe pneumonia.
3. Critique the plan to give 100% oxygen by mask and discharge the trekker back to his group.
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What oxygen achieves: it raises alveolar and arterial PO2, improves SpO2, reduces hypoxic pulmonary vasoconstriction, and relieves the headache. It is a legitimate first-aid measure.
Why it is not the plan proposed:
- It does not correct the respiratory alkalosis, which continues as long as ventilation stays high.
- It does not accelerate acclimatisation, which depends on ventilatory, renal and erythropoietic changes over days to weeks.
- It can mask deterioration. At altitude a resting SpO2 of 84% with breathlessness should prompt a search for high-altitude pulmonary or cerebral oedema; a clear chest radiograph now does not exclude early pulmonary oedema, and oxygen can make the patient look better than he is.
- During sleep, oxygen may blunt the hypoxic ventilatory drive and worsen periodic breathing, and in a patient with chronic CO2 retention it risks CO2 narcosis.
Safe plan: rest, oxygen, acetazolamide to speed acclimatisation, and above all no further ascent, with immediate descent if symptoms progress. Discharge back to altitude is the unsafe part of the proposal; oxygen is an adjunct, not a discharge criterion.