A 7-year-old with acute severe asthma was started on continuous nebulized salbutamol, ipratropium, and intravenous hydrocortisone. Initial blood gas showed pH 7.42 and PaCO2 30 mmHg. Two hours later he is drowsier, and repeat blood gas shows pH 7.24 and PaCO2 55 mmHg. What does this change indicate?

• A Improving ventilation, so nebulization can be tapered
• B Impending respiratory failure needing ventilatory support ✓
• C Metabolic acidosis from dehydration requiring fluid bolus
• D Excess beta-agonist causing hypokalemia

Explanation

In acute asthma, a low or normal PaCO2 reflects hyperventilation and is expected. A rising PaCO2 with falling pH signals fatigue and impending respiratory failure, an absolute indication to prepare for intubation and mechanical ventilation. Option A misreads the trend, and hypokalemia from beta-agonists causes weakness and arrhythmia rather than this pattern of hypercapnic acidosis.

Reference: Nelson Textbook of Pediatrics, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.