A 50-year-old woman with known severe asthma presents to ED with an acute exacerbation. Initial treatment includes nebulised salbutamol and ipratropium, and IV hydrocortisone. Despite this, her PEFR remains <33% predicted, she is cyanosed, and becoming drowsy. Her respiratory rate is 35/min, heart rate 130 bpm, SpO2 88% on high-flow oxygen. What is the single most important next step in management?