A patient with chronic obstructive pulmonary disease on home oxygen presents with ABG: pH 7.38, PaCO2 58 mmHg, HCO3− 33 mEq/L. How should this be interpreted?
- A Mixed respiratory acidosis with metabolic alkalosis — pH within normal range suggests over-correction
- B Fully compensated respiratory acidosis — the elevated HCO3− is an appropriate renal compensation ✓
- C Primary metabolic alkalosis with respiratory compensation
- D Normal ABG for age with mild hypercapnia
Correct answer: B. Fully compensated respiratory acidosis — the elevated HCO3− is an appropriate renal compensation
Explanation
In chronic respiratory acidosis, renal compensation generates HCO3−: for every 10 mmHg rise in PaCO2 above 40, HCO3− rises by ~3.5 mEq/L. Here PaCO2 is 58 (18 mmHg above normal), so expected HCO3− = 24 + (18/10 × 3.5) = 24 + 6.3 ≈ 30.3 mEq/L; the observed 33 is close (within normal range of chronic compensation). pH returning to 7.38 confirms full compensation. This is a fully compensated chronic respiratory acidosis — a common pattern in stable severe COPD.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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