A patient on mechanical ventilation has ABG: pH 7.55, PaCO2 28 mmHg, HCO3 24 mEq/L. This is:
- A Acute respiratory alkalosis without metabolic compensation ✓
- B Mixed metabolic alkalosis and respiratory alkalosis
- C Chronic respiratory alkalosis with complete renal compensation
- D Metabolic alkalosis with adequate respiratory compensation
Correct answer: A. Acute respiratory alkalosis without metabolic compensation
Explanation
In acute respiratory alkalosis, HCO3 falls by approximately 2 mEq/L for every 10 mmHg decrease in PaCO2. With PaCO2 12 mmHg below normal (40 − 28 = 12), expected HCO3 drop = 12/10 × 2 = 2.4 mEq/L, predicted HCO3 ≈ 24 − 2.4 ≈ 21.6 mEq/L. The measured HCO3 of 24 is higher than expected, suggesting no or minimal compensation has occurred (consistent with acute onset from mechanical hyperventilation). This is acute respiratory alkalosis; renal compensation takes 2–5 days.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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