A 24-year-old woman with asthma remains breathless after repeated nebulised salbutamol and ipratropium, intravenous hydrocortisone and oxygen. She can speak only in single words, is becoming drowsy and has very little movement of air on auscultation. Arterial blood gas analysis shows pH 7.22 (7.35–7.45) and PaCO₂ 8.1 kPa (4.6–6.0). Which one of the following is the most appropriate next step?