A 35-year-old man with known asthma presents to the Emergency Department with acute shortness of breath and wheeze. His peak expiratory flow rate (PEFR) is 45% of his best. He is speaking in short sentences, respiratory rate is 28/min, and heart rate is 115 bpm. Oxygen saturation is 93% on air. According to BTS/SIGN guidelines, what is the most appropriate immediate management?