A 70-year-old with COPD has ABG: pH 7.36, PaCO2 58 mmHg, HCO3- 32 mEq/L. This pattern represents:

• A Combined respiratory acidosis and primary metabolic alkalosis
• B Acute respiratory acidosis with no renal compensation
• C Metabolic alkalosis with respiratory compensation
• D Compensated chronic respiratory acidosis — expected HCO3 rise = 3.5 × (58-40)/10 = 6.3, so expected HCO3 ≈ 30.3, actual 32 mEq/L falls within compensation range ✓

Explanation

In chronic respiratory acidosis, renal compensation is: for every 10 mmHg rise in PaCO2, HCO3− rises by ~3.5 mEq/L. Expected HCO3− = 24 + [3.5 × (58−40)/10] = 24 + 6.3 = 30.3 mEq/L (range 28–32). The measured HCO3− of 32 mEq/L falls within the expected compensatory range and the pH is near-normal (7.36), confirming fully compensated chronic respiratory acidosis — consistent with chronic COPD, not a mixed disorder. In acute respiratory acidosis, HCO3 rises only 1 mEq/L per 10 mmHg.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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