A 32-year-old woman presents to the emergency department with severe dyspnoea and wheeze. She has a history of asthma and has used her salbutamol inhaler multiple times without relief. On examination, she is speaking in single words, respiratory rate 32 breaths/min, pulse 128 beats/min, oxygen saturation 90% on room air and has widespread wheeze with poor air entry. Peak expiratory flow is 25% of predicted. After initial nebulised salbutamol and ipratropium plus systemic corticosteroids, she remains severely breathless. Which additional treatment is most appropriate at this stage?