A 32-year-old man with known asthma presents with severe breathlessness. He is unable to complete sentences, has a respiratory rate of 36/min, pulse 135/min, PEF is 25% of predicted, and oxygen saturation is 88% on air. On examination there is a silent chest and he appears drowsy with a rising PaCO2 on arterial blood gas. What is the most accurate description of his asthma severity?