A 26-year-old man with known asthma presents to the emergency department with a 2-day history of worsening wheeze and shortness of breath triggered by an upper respiratory tract infection. He is speaking in phrases, respiratory rate is 26 breaths/min, heart rate 110 beats/min, oxygen saturation 93% on room air and peak expiratory flow is 55% of his best. What is the most appropriate initial management in the emergency department?