A patient with chronic renal failure has the following ABG: pH 7.30, PaCO2 28 mmHg, HCO3- 13 mEq/L, Na 138, Cl 105. Calculated anion gap = 20 mEq/L (high). Expected respiratory compensation for metabolic acidosis using Winters formula is PaCO2 = 1.5 × HCO3- + 8 ± 2. Expected PaCO2 = 27.5 ± 2. The actual PaCO2 is 28 mmHg. This means:

• A There is an additional primary respiratory alkalosis superimposed
• B Respiratory compensation is appropriate; this is a simple high-anion gap metabolic acidosis ✓
• C There is an additional primary respiratory acidosis superimposed on metabolic acidosis
• D Delta-delta ratio indicates an additional normal anion gap metabolic acidosis

Explanation

Using Winter's formula: expected PaCO2 = 1.5 × 13 + 8 ± 2 = 19.5 + 8 ± 2 = 27.5 ± 2 = 25.5–29.5 mmHg. The measured PaCO2 of 28 mmHg falls exactly within the expected range, confirming appropriate respiratory compensation—the patient is a pure high-anion gap metabolic acidosis (likely uraemic in the context of chronic renal failure). If PaCO2 were lower than expected, respiratory alkalosis would be superimposed; if higher, respiratory acidosis would be co-existing. The delta-delta ratio (ΔAG/ΔHCO3-) would help detect mixed non-gap acidosis, but the question asks specifically about respiratory compensation adequacy.

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

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