What are the features of life-threatening asthma, and why is a normal CO2 concerning?

Question

Accepted Answer

Features of life-threatening asthma can be grouped into clinical, physiological and gas exchange markers. Clinically: silent chest, cyanosis, poor respiratory effort, inability to speak or exhaustion, and altered conscious level. Physiologically: RR over 25, HR over 110, PEFR under 33% of predicted or best. On ABG: SpO2 under 92%, hypoxia despite oxygen, and importantly a normal or rising PaCO2. In acute asthma patients usually hyperventilate, so a low PaCO2 is expected; a normal or rising CO2 means the patient can no longer maintain adequate ventilation — this represents respiratory muscle fatigue and impending respiratory failure. It is a pre-terminal sign and warrants urgent escalation to ICU.

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