A trauma patient's blood gas shows: pH 7.18, PaCO₂ 32 mmHg, HCO₃⁻ 11 mEq/L, Na⁺ 138, Cl⁻ 100, lactate 8.2 mmol/L. What is the primary acid-base disorder and its expected compensation?

• A Primary metabolic acidosis with appropriate respiratory compensation (Winters' formula: PaCO₂ = 1.5×HCO₃⁻ + 8 ± 2 = 24.5 mmHg expected; current 32 suggests additional respiratory acidosis)
• B Primary respiratory alkalosis with metabolic compensation
• C Primary metabolic acidosis with appropriate respiratory compensation (PaCO₂ 24.5 ± 2 is expected; measured 32 mmHg indicates a superimposed respiratory acidosis) ✓
• D Mixed anion-gap metabolic acidosis and normal anion gap metabolic acidosis

Explanation

Winters' formula for respiratory compensation in metabolic acidosis: expected PaCO₂ = 1.5 × [HCO₃⁻] + 8 ± 2 = 1.5×11 + 8 = 24.5 ± 2 mmHg (range 22.5–26.5). The actual PaCO₂ of 32 mmHg exceeds this expected value, indicating inadequate respiratory compensation — a superimposed respiratory acidosis (e.g., from fatigue, airway compromise). The primary disorder is high-anion-gap metabolic acidosis (AG = 138 − 100 − 11 = 27, elevated, consistent with lactic acidosis from shock). Option A is incorrect in its conclusion about compensation adequacy; C correctly identifies the double disorder.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.