A 6-year-old asthmatic child is brought to the emergency room with severe coughing and wheezing during the prior 24 h. The child has been taking theophylline without relief. Physical examination reveals a child who is anxious, has intercostal and suprasternal retractions, expiratory wheezing throughout all lung fields, and RR - 60/min. Initial treatment may include the administration of which of the following?