A 24-year-old woman with known asthma presents to the emergency department with 2 hours of worsening shortness of breath after exposure to cats. She is speaking in short phrases, respiratory rate is 30 breaths/min, pulse 112 beats/min and oxygen saturation 93% on room air. On examination she has diffuse expiratory wheeze but no accessory muscle fatigue. What is the most appropriate initial management step?