A 28-year-old woman with a known diagnosis of asthma presents with increasing shortness of breath and wheeze over the past 24 hours. Her peak expiratory flow rate (PEFR) is 45% of her best. She is using her salbutamol inhaler every 2 hours with little relief. According to BTS/SIGN guidelines for asthma, what is the most appropriate next step in her management in primary care?