You are a PA working in a GP surgery and you are seeing a child who has a past medical history of asthma. This child is having an acute exacerbation which is life-threatening. An ambulance has been called and you are awaiting its arrival. You have given the child nebulised salbutamol but they have not responded well and are still struggling to breathe. What is the next most appropriate step in management while you wait for the ambulance to arrive?