A patient with chronic COPD and stable hypercapnia has pH 7.38, PaCO2 60 mmHg, HCO3− 35 mEq/L. This is consistent with:

• A Fully compensated chronic respiratory acidosis; expected renal compensation raises HCO3 by 3.5 mEq/L per 10 mmHg rise in PaCO2 ✓
• B Mixed respiratory acidosis and metabolic alkalosis; the HCO3 is higher than expected for compensation alone
• C Fully compensated respiratory acidosis; expected HCO3 = 24 + (1 × 36/10) = 27.6 mEq/L
• D Partially compensated respiratory acidosis; renal compensation is incomplete because PaCO2 is elevated

Explanation

For chronic respiratory acidosis, the renal compensatory rise in HCO3 is 3.5 mEq/L per 10 mmHg increase in PaCO2. The PaCO2 is elevated by 60 − 40 = 20 mmHg. Expected HCO3 = 24 + (3.5 × 20/10) = 24 + 7 = 31, with a range of ±3, giving 28–34 mEq/L. The measured HCO3 of 35 mEq/L is at the upper end/slightly above this range, but pH is 7.38 (near normal), which is consistent with full chronic renal compensation. The diagnosis is fully compensated chronic respiratory acidosis. Option C gives the acute compensation value (1 mEq/L per 10 mmHg), which is incorrect for the chronic scenario.

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

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