A 68-year-old woman presents to the emergency department with sudden-onset dyspnoea and wheeze. She has a history of asthma. Vital signs: temperature 37.0°C, respiratory rate 26/min, oxygen saturation 92% on room air, heart rate 110 beats/min, blood pressure 135/80 mm Hg. She is speaking in full sentences but has widespread wheezes. According to Canadian Thoracic Society guidance, what is the most appropriate initial pharmacologic treatment?