Question

A $64$-year-old man comes to the clinic due to three days of worsening shortness of breath and a productive cough. He has a $40$-pack-year smoking history and was diagnosed with chronic obstructive pulmonary disease (COPD) five years ago. He currently uses a daily tiotropium inhaler and an albuterol inhaler as needed. Over the past three days, he has needed to use his albuterol inhaler every two hours with minimal relief. He also reports that his sputum has increased in volume and changed in color from clear to dark yellow. His temperature is $37.5^\circ\text{C}$ ($99.5^\circ\text{F}$), blood pressure is $130/82\text{ mmHg}$, pulse is $94/\text{min}$, and respiratory rate is $22/\text{min}$. Pulse oximetry is $93\%$ on room air. Physical examination reveals an increased anteroposterior chest diameter. Lung auscultation shows prolonged expiration with diffuse expiratory wheezing bilaterally. Cardiac examination shows a regular rate and rhythm with no murmurs, gallops, or jugular venous distention. There is no lower extremity edema. Arterial blood gas analysis on room air shows:

pH: $7.38$
$\text{pCO}_2$: $48\text{ mmHg}$
$\text{pO}_2$: $68\text{ mmHg}$
$\text{HCO}_3^-$: $27\text{ mEq/L}$

The patient is administered nebulized ipratropium-albuterol and oral prednisone. Which of the following is the most appropriate additional pharmacotherapy for this patient?