At high altitude (4500 m), a mountaineer has PaO2 = 48 mmHg and PaCO2 = 28 mmHg after 5 days acclimatisation. Her hemoglobin is now 17 g/dL. Which acclimatisation response most directly increases O2 delivery to tissues in the FIRST 24 hours (before erythropoiesis becomes significant)?

• A Polycythaemia increases O2-carrying capacity
• B Rightward shift of the ODC via increased 2,3-BPG improves O2 unloading at tissues
• C Increased cardiac output from sympathetic activation enhances O2 delivery
• D Hyperventilation-induced hypocapnia raises alveolar PO2 and arterial O2 saturation ✓

Explanation

The earliest (within minutes to hours) response to high-altitude hypoxia is hyperventilation driven by peripheral chemoreceptors sensing low PaO2. Hyperventilation lowers PaCO2 and raises alveolar PO2 (by the alveolar gas equation PAO2 = PiO2 − PaCO2/RQ), increasing arterial O2 saturation and hence O2 content. Polycythaemia takes days to weeks (EPO-driven erythropoiesis). 2,3-BPG rise occurs over 24–48 hours (facilitates O2 unloading but may not increase total delivery). Cardiac output increase is transient. The initial and dominant acclimatisation in the first 24 hours is hyperventilation.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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