A 58-year-old with chronic COPD (PaCO2 = 58 mmHg, pH = 7.36, HCO3- = 33 mEq/L) is admitted. His acid-base status and compensation should be characterized as:
- A Acute respiratory acidosis with partial metabolic compensation
- B Chronic respiratory acidosis with appropriate renal compensation; expected HCO3- rise = 3.5 × ΔPaCO2/10 ✓
- C Mixed respiratory acidosis and metabolic alkalosis
- D Chronic respiratory acidosis with inadequate compensation requiring immediate ventilatory support
Explanation
Chronic respiratory acidosis occurs when PaCO2 is elevated for >3–5 days, allowing the kidney to fully compensate by increasing HCO3- reabsorption and net acid excretion. Expected compensation: HCO3- rises 3.5 mEq/L per 10 mmHg rise in PaCO2 (vs. 1 mEq/L per 10 mmHg in acute). ΔPaCO2 = 58-40 = 18 mmHg; expected HCO3- rise ≈ 3.5 × 1.8 = 6.3 mEq/L; predicted HCO3- = 24 + 6.3 = 30.3 mEq/L (observed 33—slightly higher, suggesting possible coexistent mild metabolic alkalosis, but within acceptable range for compensated chronic). The near-normal pH (7.36) indicates effective renal compensation. Ventilatory support would actually risk dangerous alkalosis if CO2 is rapidly corrected.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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