Gemma, a healthy fit student, ascends rapidly to Pikes Peak (4300 m) in Colorado. Arterial blood samples are taken on arrival, and after two weeks.
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1. Why is her PaO2 reduced?
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Correct answer:
The higher you go the less the barometric pressure, though the air contains the same fraction of O2. At 4300 m the barometric pressure is about 61 kPa, 60% of that at sea level, so the PO2 of dry inspired air falls from 21 kPa to 13 kPa, leading to the reduced PaO2 and hypoxaemia observed.
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2. What sort of alkalosis is causing the alkalaemia on arrival at altitude?
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At high altitude inspired PCO2 (0 kPa) and resting tissue CO2 production are unchanged; altitude has no direct effect on arterial PCO2 and if alveolar ventilation was unchanged PaCO2 would remain at the sea-level value. However, above about 3000 m (10,000 feet) alveolar ventilation is increased by hypoxic stimulation of the peripheral chemoreceptors of the carotid body (Chapter 12). Initially, this increases the CO2 excretion from the lungs above the rate of tissue CO2 production, leading to a fall in alveolar CO2 fraction, alveolar PCO2 and PaCO2 (hypocapnia). Equilibrium is re-established when alveolar CO2 fraction has fallen sufficiently to compensate for the increased alveolar ventilation so that excretion from the lungs (= alveolar ventilation × FACO2) again equals tissue CO2 production, though of course PaCO2 remains low. The latter causes a respiratory alkalosis and alkalaemia (Chapter 11); Gemma is hyperventilating.
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3. Why does the pH return towards normal after 2 weeks, but the hypercapnia becomes more severe? What is this mechanism called?
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Correct answer:
After 2 weeks the kidneys will have partially compensated for the respiratory alkalosis by excreting HCO3– in the urine. [HCO3–] is therefore reduced, so the [HCO3–]/PaCO2 ratio and thus pH return towards normal. This process represents renal compensation of a respiratory alkalosis. The chemoreceptors also adapt to the chronic alkalaemia (Chapter 12), allowing ventilation to increase further and reducing PaCO2 further.
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4. The PaO2, although increased after 2 weeks, is still much lower than the normal sea level value but O2 content of the blood has returned to normal. How is this possible? What other implication might this have?
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Correct answer:
Hypoxia causes the kidneys to produce erythropoietin, which stimulates production of red cells. Over-time, this leads to polycythaemia, an increased blood haemoglobin content, and so increased O2 carrying capacity. An increase in [haemoglobin] would also increase blood buffering capacity (Chapter 11), thus further limiting any changes in blood pH. Together, all these mechanisms contribute to adaptation to altitude.
