A 24-year-old woman with known asthma presents to the emergency department with severe breathlessness. She has used her salbutamol inhaler 10 times in the past 2 hours without relief. On examination she is talking in short phrases, respiratory rate is 32 breaths/min, pulse 124 beats/min, oxygen saturation 93% on room air and PEFR is 35% predicted. She has already received nebulised salbutamol from triage. What is the next most appropriate treatment?