A 25-year-old man presents to the Emergency Department with an acute exacerbation of asthma. He is unable to complete sentences, his respiratory rate is 35/min, heart rate is 130 bpm, and his chest is silent on auscultation. His SpO2 is 91% on high-flow oxygen. What is the most appropriate immediate management escalation after initial oxygen, nebulisers, and IV steroids?