A 19-year-old mountaineer at 4500m altitude has ABG: pH 7.48, PaCO2 28 mmHg, HCO3- 20 mEq/L, PaO2 52 mmHg. Which best characterizes his acid-base state?
- A Acute respiratory alkalosis without compensation
- B Mixed respiratory alkalosis and metabolic acidosis
- C Chronic respiratory alkalosis with appropriate renal compensation; HCO3- has decreased 5 mEq/L per 10 mmHg fall in PaCO2 ✓
- D Metabolic alkalosis compensated by hyperventilation
Explanation
Altitude hypoxia stimulates peripheral chemoreceptors causing hyperventilation (respiratory alkalosis). The expected renal compensation for chronic respiratory alkalosis is a decrease in HCO3- of 5 mEq/L per 10 mmHg fall in PaCO2. ΔPaCO2 = 40-28 = 12 mmHg; expected HCO3- fall = 5 × 1.2 = 6 mEq/L; predicted HCO3- = 24-6 = 18 mEq/L. Observed HCO3- is 20, which is close (renal compensation takes 2–5 days to be complete). The residual pH elevation (7.48) confirms the alkalosis is primary respiratory. Renal compensation involves reduced HCO3- reabsorption, increased HCO3- excretion, and reduced ammonium excretion, and it is this adaptation that largely explains acclimatization.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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