A 68-year-old COPD patient has: pH 7.32, PaCO2 68 mmHg, HCO3- 34 mEq/L. Using the 'expected compensation' formula, this blood gas most likely represents:

• A Acute respiratory acidosis with no compensation
• B Chronic respiratory acidosis with superimposed metabolic alkalosis
• C Mixed respiratory and metabolic acidosis
• D Chronic respiratory acidosis with appropriate renal compensation ✓

Explanation

For chronic respiratory acidosis, the expected renal compensation is: HCO3- rises by 3.5 mEq/L per 10 mmHg rise in PaCO2. With PaCO2 = 68 mmHg (rise of 28 mmHg from normal 40), expected HCO3- = 24 + (3.5 × 2.8) = 24 + 9.8 ≈ 34 mEq/L. The measured HCO3- of 34 mEq/L matches exactly, confirming chronic respiratory acidosis with appropriate renal compensation (no superimposed metabolic disorder). The pH is low (7.32) because compensation is never complete. If HCO3- were higher than expected, a superimposed metabolic alkalosis would be diagnosed.

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

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