A 24-year-old man with known asthma presents with increasing breathlessness. He has used his Salbutamol inhaler hourly at home without relief. On examination, he is speaking in short phrases, respiratory rate 32/min, heart rate 122 bpm, oxygen saturation 91% on air and peak expiratory flow is 35% of predicted. Initial management with high-flow oxygen, nebulised Salbutamol and oral Prednisolone has been started. He remains breathless. What is the most appropriate next step?