A 24-year-old woman with known asthma presents to an urban emergency department with severe breathlessness after a viral upper respiratory infection. She is speaking in single words, respiratory rate is 32/min, heart rate 122/min, oxygen saturation 89% on room air and there is widespread expiratory wheeze with use of accessory muscles. Her peak expiratory flow is 35% of predicted. According to Australian guidance for acute asthma in adults, which immediate management step is most appropriate?