A mountaineer at 4500 m altitude has been acclimatized for 1 week. Which acid-base pattern best describes the steady-state acclimatization?

• A Respiratory alkalosis with incomplete renal compensation (HCO3− partially reduced)
• B Normal pH with elevated HCO3− and high PaCO2
• C Metabolic acidosis with respiratory alkalosis
• D Fully compensated respiratory alkalosis with HCO3− reduced to 18 mEq/L and pH returning to near 7.40 ✓

Explanation

At high altitude, hypoxia stimulates hyperventilation (respiratory alkalosis). Over 3–5 days, the kidneys excrete HCO3− (bicarbonate diuresis via carbonic anhydrase inhibition and reduced HCO3− reabsorption), lowering plasma HCO3−. By 1 week, a fully compensated state is reached with near-normal pH, persistently low PaCO2 (~30 mmHg at 4500 m), and reduced HCO3− (~18 mEq/L). This compensation improves tissue oxygenation by restoring the pH required for optimal hemoglobin function.

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

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