A patient has an arterial pH 7.28, PaCO₂ 20 mmHg, HCO₃⁻ 9 mEq/L. Expected compensation is PaCO₂ = 1.5 × HCO₃⁻ + 8 (± 2), giving expected PaCO₂ ≈ 21.5 mmHg. What is the acid-base diagnosis?

• A Metabolic acidosis with concomitant respiratory alkalosis (mixed disorder)
• B Metabolic acidosis with inadequate respiratory compensation
• C Simple metabolic acidosis with appropriate respiratory compensation ✓
• D Respiratory alkalosis with metabolic compensation

Explanation

Using Winter's formula: expected PaCO₂ = 1.5 × 9 + 8 = 21.5 ± 2, range 19.5–23.5 mmHg. The measured PaCO₂ of 20 mmHg falls within this predicted range, indicating appropriate hyperventilatory compensation for the metabolic acidosis. There is no superimposed respiratory alkalosis (which would require PaCO₂ below the predicted range). The primary disorder is metabolic acidosis (low pH, low HCO₃⁻, low PaCO₂ as compensation).

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.