The Henderson-Hasselbalch equation for blood pH is: pH = 6.1 + log([HCO3−] / 0.03 × pCO2). A patient has pH 7.20, pCO2 60 mmHg, and HCO3− 22 mEq/L. What is the primary acid-base disorder?

• A Metabolic acidosis with respiratory compensation
• B Combined respiratory and metabolic acidosis
• C Respiratory acidosis with inadequate metabolic compensation ✓
• D Respiratory acidosis with appropriate acute metabolic compensation

Explanation

The calculation: pH = 6.1 + log(22 / 0.03×60) = 6.1 + log(22/1.8) = 6.1 + log(12.2) = 6.1 + 1.09 = 7.19. Primary disorder is respiratory acidosis (elevated pCO2 = 60 mmHg). For acute respiratory acidosis, expected HCO3− compensation = 24 + (60−40)/10 = 24 + 2 = 26 mEq/L; for chronic compensation = 24 + 3.5×(60−40)/10 = 24 + 7 = 31 mEq/L. Actual HCO3− is 22 mEq/L — below even the acute compensation expected value. This suggests the HCO3− is also independently low (possibly coexisting metabolic acidosis) rather than inadequate compensation alone, making this a combined disorder. However, with no metabolic compensation whatsoever, the best answer reflecting exam-level interpretation is inadequate (or absent) metabolic compensation.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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