A 35-year-old man with a history of asthma presents with severe breathlessness. He is unable to complete sentences, respiratory rate is 34 breaths per minute, pulse 132 beats per minute, oxygen saturation 90 percent on air and peak expiratory flow is 30 percent of predicted. Arterial blood gases on high-flow oxygen show pH 7.37, PaCO2 5.8 kPa and PaO2 11.0 kPa. Which finding most strongly indicates life-threatening asthma?