A 45-year-old woman with a history of asthma presents to the emergency department with increasing wheeze and shortness of breath over 2 hours. She is speaking in short sentences, respiratory rate is 26 breaths per minute, oxygen saturation 93 percent on room air, pulse 110 beats per minute and peak expiratory flow (PEF) is 55 percent of her best. What is the most appropriate initial management?