A 30-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 using his salbutamol inhaler every 30 minutes with little relief. His respiratory rate is 28/min, heart rate 115/min, and SpO2 93% on air. He is able to speak in short sentences. According to BTS/SIGN guidelines, what is the most appropriate initial management in the ED?