A 7-year-old girl admitted with acute severe asthma was initially found to have SpO2 89%, PaO2 60 mmHg, PaCO2 32 mmHg, and pH 7.42. After 4 hours of nebulised salbutamol she becomes drowsy, and repeat blood gas shows PaO2 58 mmHg, PaCO2 52 mmHg, and pH 7.26. What does this change indicate?

• A Improving ventilation with resolving hyperventilation
• B Iatrogenic salbutamol toxicity
• C Compensated metabolic acidosis from dehydration
• D Impending respiratory muscle fatigue requiring ventilatory support ✓

Explanation

In acute asthma, hyperventilation produces a low PaCO2. A rising or normalising PaCO2 in a tiring child means ventilation is failing despite effort, and with drowsiness and respiratory acidosis this signals imminent respiratory arrest, mandating ICU referral and preparation for assisted ventilation. Interpreting the normalising PaCO2 as improvement is the classic fatal error, since true improvement would raise both PaO2 and pH along with clinical recovery.

Reference: Nelson Textbook of Pediatrics, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.