A 28-year-old woman with known asthma presents with a severe attack. She cannot complete sentences, has a respiratory rate of 32/min, heart rate 130/min, SpO2 90%, and widespread quiet breath sounds with minimal wheeze. Arterial blood gas shows pH 7.36, PaO2 62 mmHg, PaCO2 42 mmHg. Which feature marks this as life-threatening asthma?
- A Inability to complete sentences
- B A normal or rising PaCO2 with quiet chest ✓
- C Tachycardia above 120/min
- D SpO2 below 92% on room air
Correct answer: B. A normal or rising PaCO2 with quiet chest
Explanation
In acute asthma, hyperventilation normally drives PaCO2 down. A normalising or rising PaCO2 indicates fatigue and failing ventilation, and together with a silent chest defines life-threatening asthma. Inability to complete sentences, tachycardia and hypoxaemia indicate acute severe asthma, but the single most ominous discriminator between severe and life-threatening attacks is the rising CO2 with an exhausted, quiet chest. This patient needs ICU referral, IV magnesium sulphate and possible ventilation.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.