A 26-year-old woman presents to the emergency department with severe breathlessness, unable to speak full sentences. She has a history of asthma but has been using only a salbutamol inhaler. On arrival: respiratory rate 32 breaths/min, pulse 128 beats/min, oxygen saturation 91% on room air, widespread wheeze, and she is agitated. Initial management with high-dose inhaled salbutamol via spacer, ipratropium bromide and systemic corticosteroids is commenced. Despite this, she remains very breathless with saturations 92% on 8 L/min oxygen. According to Australian asthma emergency guidance, what is the next most appropriate pharmacological step?