A 23-year-old woman with known asthma presents to the emergency department with worsening wheeze and shortness of breath over 2 hours. She has used her salbutamol metered dose inhaler 10 puffs via spacer at home without relief. On arrival she is speaking in short phrases, respiratory rate is 30 breaths/min, heart rate 120 beats/min and oxygen saturation 93% on room air. Auscultation reveals widespread wheeze. What is the most appropriate next step in management?