A 28-year-old woman with a history of asthma presents to an urban GP clinic with a 12-hour history of worsening wheeze and shortness of breath after a viral upper respiratory tract infection. She has used her salbutamol inhaler 8 puffs over the last 4 hours. On examination, she is speaking in phrases, respiratory rate is 24 breaths/min, heart rate 108 bpm, oxygen saturation 94% on room air, and there is widespread expiratory wheeze. Peak expiratory flow is 60% of her best. What is the most appropriate next step in management in the clinic?