A 45-year-old man presents to the emergency department with wheeze, chest tightness and shortness of breath. He has a history of asthma and has used his salbutamol inhaler 12 puffs in the last hour with minimal relief. On examination, he is speaking only in short phrases, respiratory rate is 30 breaths/min, heart rate 120 beats/min, oxygen saturation 93% on room air, and there is widespread expiratory wheeze. Peak expiratory flow is 45% of his personal best. What is the most appropriate immediate treatment?