A 70 kg patient with COPD has ABG: pH 7.35, PaCO2 60 mmHg, HCO3 32 mEq/L. This ABG is consistent with:

• A Acute respiratory acidosis with metabolic compensation
• B Chronic respiratory acidosis with appropriate renal compensation ✓
• C Mixed respiratory acidosis plus metabolic alkalosis
• D Respiratory acidosis with inadequate renal compensation

Explanation

In chronic respiratory acidosis, the kidneys compensate by retaining HCO3: expected increase in HCO3 = 3.5 mEq/L per 10 mmHg rise in PaCO2. With PaCO2 elevated by 20 mmHg (from 40 to 60), expected HCO3 rise = 2 × 3.5 = 7 mEq/L, predicted HCO3 = 24 + 7 = 31 mEq/L. Measured HCO3 of 32 mEq/L matches this prediction. The near-normal pH (7.35) reflects successful chronic compensation. Acute respiratory acidosis compensates only by HCO3 rise of 1 mEq/L per 10 mmHg rise.

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

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