A 32-year-old man with asthma presents to the emergency department with severe shortness of breath. He is speaking in single words, respiratory rate is 32 breaths per minute, SpO₂ 89% on room air and peak expiratory flow is 35% of predicted. He has used multiple salbutamol inhalations before arrival. According to Canadian asthma emergency care recommendations, which of the following is the most appropriate immediate management step in addition to controlled oxygen?