A 45-year-old man with known asthma presents with severe breathlessness. He is unable to speak full sentences, RR 34/min, HR 128 bpm, SpO2 90% on room air, and uses accessory muscles. Peak expiratory flow is 30% predicted. He has received multiple salbutamol nebulisers and IV hydrocortisone with little improvement. What is the most appropriate next step in management?