Question

A 34-year-old man is evaluated in the emergency department four hours after experiencing a witnessed generalized tonic-clonic seizure at a local restaurant. Bystanders report that he vomited during the postictal period before paramedics arrived. The patient is now fully awake and oriented but complains of worsening shortness of breath and a dry cough. His medical history is notable for epilepsy, though he has not taken his levetiracetam for the past week.

His temperature is $37.6^\circ\text{C}$ ($99.7^\circ\text{F}$), blood pressure is $118/76\text{ mm Hg}$, pulse is $110/\text{min}$, and respirations are $26/\text{min}$. Pulse oximetry is $89\%$ on room air. Physical examination shows no jugular venous distention. Lung auscultation reveals focal crackles over the right mid-to-lower posterior lung fields. Cardiac examination is normal with no murmurs or gallops.

Laboratory results:
Leukocyte count: $11,500/\text{mm}^3$
Serum lactate: $4.2\text{ mEq/L}$

Arterial blood gas (room air):
pH: $7.28$
PaCO2: $30\text{ mm Hg}$
PaO2: $58\text{ mm Hg}$
HCO3-: $14\text{ mEq/L}$

A chest radiograph shows a new patchy alveolar infiltrate in the superior segment of the right lower lobe.

Which of the following is the most appropriate next step in the management of this patient's respiratory symptoms?