A 19-year-old man with asthma presents with severe shortness of breath. He is unable to complete sentences, respiratory rate is 30 breaths/min, pulse 120 beats/min and oxygen saturation 90% on room air. After three back-to-back nebulised salbutamol/ipratropium treatments and 50 mg of oral prednisone, he remains very breathless with peak expiratory flow 35% predicted. What is the most appropriate next step?