A 24-year-old woman with acute asthma has been nebulised twice without relief. She is exhausted and drowsy, her chest is notably quiet on auscultation despite prior loud wheeze, and arterial blood gas shows pH 7.31, PaO2 62 mmHg, PaCO2 44 mmHg on room air. Which single finding best indicates impending ventilatory failure?
- A Persistent loud diffuse wheeze
- B PaCO2 within the normal range during an acute attack ✓
- C Peak expiratory flow 75% of predicted
- D Use of accessory muscles of respiration
Correct answer: B. PaCO2 within the normal range during an acute attack
Explanation
Patients with acute asthma hyperventilate and are normally hypocapnic. A PaCO2 that has risen into the normal range means falling ventilation and fatigue, and it marks transition to life-threatening asthma along with silent chest, cyanosis, bradycardia and exhaustion. Loud wheeze and accessory muscle use indicate moderate severity, not failure, and a PEF of 75% predicted corresponds to a mild to moderate attack.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.