A 62-year-old man with severe COPD has a PaCO2 of 60 mmHg on room air. Arterial pH is 7.36 and serum HCO3 is 31 mEq/L. Based on the expected renal compensatory response, what does this picture indicate?

• A Acute respiratory acidosis with appropriate compensation
• B Acute respiratory acidosis with excessive compensation suggesting a metabolic process
• C Chronic respiratory acidosis with appropriate renal compensation ✓
• D Mixed metabolic and respiratory acidosis

Explanation

In chronic respiratory acidosis, the kidneys generate new bicarbonate over several days, raising serum HCO3 by roughly 4 mEq/L (range 3.5 to 5) for each sustained 10 mmHg rise in PaCO2. For a 20 mmHg rise here, the expected HCO3 is about 30 to 34, matching the measured 31. In acute respiratory acidosis, only chemical buffering operates and HCO3 rises by about 1 mEq/L per 10 mmHg, which would predict around 26.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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