A 65-year-old man with severe COPD has the following arterial blood gas: pH 7.34, PaCO2 68 mmHg, HCO3 35 mEq/L, Na 140, Cl 98, albumin 4 g/dL. What is the expected compensatory HCO3 for this degree of chronic respiratory acidosis, and is additional metabolic alkalosis present?

• A Expected HCO3 = 29 mEq/L; additional metabolic alkalosis is present
• B Expected HCO3 = 35 mEq/L; compensation is appropriate, no additional disorder ✓
• C Expected HCO3 = 37 mEq/L; compensation is incomplete
• D Expected HCO3 = 33 mEq/L; additional metabolic alkalosis is present

Explanation

For chronic respiratory acidosis the compensation formula is: expected HCO3 = 24 + 3.5 × (PaCO2 − 40)/10. Here: 24 + 3.5 × (68 − 40)/10 = 24 + 3.5 × 2.8 = 24 + 9.8 ≈ 33.8 mEq/L, with an accepted range of ±3 mEq/L giving approximately 31–37 mEq/L. The measured HCO3 of 35 mEq/L falls within this range, indicating pure chronic respiratory acidosis with appropriate renal compensation and no additional metabolic alkalosis.

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

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