A 12-year-old child is brought to the emergency department with acute severe asthma exacerbation. Physical examination reveals that the child is dyspneic. Breathing is labored; respiratory rate is 50/min. Chest retractions and wheezing are present. Heart rate is 128/min. BP is 130/90 mm Hg. Peak expiratory flow rate is <60% of predicted value. SPO2 is 88%. What is the first drug of choice for treatment of this child