A 35-year-old man with known asthma presents with severe dyspnoea, diffuse wheeze, inability to speak full sentences and a respiratory rate of 32 breaths per minute. His oxygen saturation is 88 percent on room air. Initial treatment with high-dose inhaled salbutamol and ipratropium via nebuliser plus systemic corticosteroids is commenced. After 30 minutes, there is minimal improvement and he is tiring. What is the best next step in management?