The most appropriate next step in treating this patient's respiratory findings is to perform endotracheal intubation. This patient sustained significant thermal injury to his upper airway as evidenced by an edematous oropharynx and monophonic wheeze consistent with inspiratory stridor. Intense heat can cause edema and blistering from the mouth to the larynx, and patients with a visibly damaged airway or stridor are at high risk of complete upper airway obstruction due to swelling. It is difficult to predict which patients with acute inhalational injury will develop complete upper airway obstruction, and even patients with minimally apparent inhalational airway injury may develop worsening obstruction associated with intravenous hydration or analgesia needed for burn treatment. Therefore, ensuring upper airway patency is the first priority in patients with significant smoke exposure, and endotracheal intubation is indicated in this patient at high risk of obstruction.

Because helium is less dense than nitrogen, a mixture of helium and oxygen (heliox) may be used in situations in which there is increased airway resistance. The combination of helium and oxygen improves laminar airflow and may decrease the work of breathing in patients with increased airway resistance. However, it would not treat the underlying inhalation-associated airway obstruction.

Methylprednisolone is useful for COPD exacerbations, but systemic glucocorticoids do not have a role in treating upper airway edema or smoke-induced injury of the lower airways.

Nebulized epinephrine can transiently reduce upper airway edema from allergic or anaphylactic reactions, but it does not have a role in the treatment of inhalational injury.

This patient's expiratory wheezes and acute respiratory acidosis suggest the presence of a COPD exacerbation triggered by smoke inhalation, in which case noninvasive ventilation would be appropriate. However, noninvasive ventilation will not prevent this patient from developing complete airway obstruction, and securing the airway with an endotracheal tube while it is still possible takes precedence.