A 55-year-old man with COPD and compensated chronic respiratory acidosis has: pH 7.36, PaCO2 68 mmHg, HCO3− 38 mEq/L, Na+ 140, Cl− 90. The expected renal compensation for chronic respiratory acidosis is a rise in HCO3− of approximately:

• A 3.5 mEq/L per 10 mmHg rise in PaCO2 ✓
• B 1 mEq/L per 10 mmHg rise in PaCO2
• C 0.1 mEq/L per 10 mmHg rise in PaCO2
• D 5 mEq/L per 10 mmHg rise in PaCO2

Explanation

For chronic respiratory acidosis, expected renal compensation is an HCO3− increase of approximately 3.5 mEq/L per 10 mmHg rise in PaCO2 (range 3–4). Here, PaCO2 rose from 40 to 68 mmHg (+28 mmHg), so expected HCO3− = 24 + (28/10 × 3.5) = 24 + 9.8 ≈ 34 mEq/L; actual HCO3− of 38 is slightly higher, suggesting a co-existing metabolic alkalosis (e.g., diuretic use). Acute respiratory acidosis compensates only 1 mEq/L per 10 mmHg (immediate bicarbonate buffering).

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

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