A 19 year old male presents to the emergency room at midnight with wheezing and shortness of breath. Although history is difficult to obtain secondary to the patient's tachypnea, he is able to state that his symptoms started one hour ago and have been progressively worsening. Past medical history is significant for seasonal allergies and asthma. Current medications include inhaled albuterol and budesonide. On physical exam, the patient appears anxious and is sitting upright. Vital signs on presentation are pulse 120, respirations 42/minute, BP 118/68, and O2 Sat 93% on room air. Nasal flaring is noted. The skin appears pale. No murmurs, rubs, or gallops are appreciated on cardiac auscultation. Bilateral, symmetrical wheezing is present. ABG obtained on presentation shows pH 7.51, pO2 70 mm Hg, and pCO2 29 mm Hg. Supplemental O2 by nasal cannula is started, and the patient is treated with nebulized albuterol and i.v. methylprednisolone. Thirty minutes after treatment, the patient is reassessed. No wheezing is noted, and breath sounds are equal bilaterally. Vitals are pulse 121, respiratory rate 35/minute, BP is 120/71, which drops to 108/65 on inspiration. Repeat ABG is obtained, showing pH 7.40, pCO2 41 mm Hg, pO2 72 mm Hg. What is the most appropriate next step in management?
A. Obtain chest x-ray
B. Begin i.v. albuterol
C. Begin i.v. theophylline
D. Administer inhaled salmeterol
E. Intubation and mechanical ventilation