A 36-year-old man with asthma presents with severe breathlessness. He is unable to complete sentences, respiratory rate is 32 breaths/min, heart rate 130 beats/min and oxygen saturation 90% on air. PEF is 30% of his best. He has already had multiple Salbutamol nebulisers and oral Prednisolone from the GP. What is the most appropriate next step in his management?