A 55-year-old patient with COPD has ABG: pH 7.34, PaCO2 58 mmHg, HCO3 30 mEq/L, Na 140, Cl 96 mEq/L. What is the CORRECT interpretation?
- A Chronic respiratory acidosis with appropriate metabolic compensation only
- B Mixed respiratory acidosis and metabolic alkalosis of equal magnitude
- C Chronic respiratory acidosis with superimposed metabolic alkalosis ✓
- D Respiratory acidosis with inadequate metabolic compensation
Explanation
For chronic respiratory acidosis, the expected HCO3 compensation is approximately +3.5 mEq/L per 10 mmHg rise in PaCO2. With PaCO2 at 58 (rise of 18 mmHg), expected HCO3 = 24 + (1.8 × 3.5) ≈ 24 + 6.3 = 30.3 mEq/L. Measured HCO3 is exactly 30 — right at the upper limit of expected. However, pH is still 7.34 (acidemic), so compensation is appropriate; but the high serum chloride gap and low Cl (96 vs expected ~102) with elevated HCO3 also suggests a concurrent metabolic alkalosis (urinary chloride loss, often from diuretic use in COPD patients). The anion gap = 140 – (96+30) = 14, normal. The scenario best fits chronic respiratory acidosis with superimposed metabolic alkalosis.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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