A 24-year-old woman with known asthma presents with an acute attack. She is exhausted, cannot complete a sentence, and has a silent chest with minimal air entry. Pulse is 128/min. Arterial blood gas shows pH 7.31, PaO2 58 mm Hg, PaCO2 54 mm Hg, and PEF is 25% of predicted. Which finding in this patient specifically marks LIFE-THREATENING asthma?
- A PaCO2 of 54 mm Hg ✓
- B Tachycardia above 120/min
- C Marked accessory muscle use earlier in the attack
- D Loud diffuse wheeze
Correct answer: A. PaCO2 of 54 mm Hg
Explanation
In acute asthma, falling PaCO2 from hyperventilation is expected. B normalising then rising PaCO2 signals respiratory muscle fatigue and impending ventilatory failure, defining life-threatening asthma that needs ICU referral and possible ventilation. Tachycardia, accessory muscle use, and audible wheeze indicate acute severe asthma but not the fatigue stage; indeed a silent chest means airflow is too poor to generate wheeze. The single decisive fact is that any PaCO2 above normal in acute asthma is an ominous sign.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.