A 25-year-old man presents to the ED with acute shortness of breath and wheeze. He has a history of asthma. O/E: RR 28/min, HR 115 bpm, unable to complete sentences, widespread expiratory wheeze. Peak expiratory flow rate (PEFR) is 45% of his best. Oxygen saturation is 93% on air. According to BTS/SIGN guidelines, what is the most appropriate immediate management combination?