A 35-year-old woman with a history of asthma presents to the ED with severe breathlessness, inability to speak full sentences and a peak expiratory flow of 30% of predicted despite multiple doses of inhaled salbutamol and oral prednisolone at home. She has received nebulised salbutamol and ipratropium and IV hydrocortisone in the ED with only partial improvement. What additional treatment is supported by evidence to reduce intubation rates in life-threatening asthma?