Question

An unaccompanied 72-year-old woman is triaged in the acute care unit after experiencing 48 hours of constant chest "fluttering" and progressive exertional breathlessness. She has a known medical history of severe chronic obstructive pulmonary disease (COPD), for which she uses a daily umeclidinium-vilanterol inhaler, and essential hypertension. Upon evaluation, her vital signs are:

- Temperature: $37.0^\circ\text{C}$ ($98.6^\circ\text{F}$)
- Blood pressure: $124/82\text{ mm Hg}$
- Heart rate: $148/\text{min}$ and irregularly irregular
- Respiratory rate: $22/\text{min}$
- Oxygen saturation: $95\%$ on room air

Physical examination reveals a completely irregular cardiac rhythm with absent murmurs, rubs, or gallops. Pulmonary auscultation reveals a prolonged expiratory phase globally, but no focal crackles, rhonchi, or active wheezing are appreciated. There is no jugular venous distension or peripheral edema.

Initial laboratory testing yields the following results:
- Leukocyte count: $7,200/\text{mm}^3$
- Thyroid-stimulating hormone (TSH): $2.8\text{ }\mu\text{U/mL}$ (Normal: $0.4-4.0\text{ }\mu\text{U/mL}$)
- Free T4: $1.1\text{ ng/dL}$ (Normal: $0.9-1.7\text{ ng/dL}$)

Arterial blood gas (room air):
- pH: $7.48$
- PaCO $_2$: $31\text{ mm Hg}$
- PaO $_2$: $76\text{ mm Hg}$

A 12-lead electrocardiogram demonstrates an irregularly irregular rhythm with narrow QRS complexes and an absence of discernible P waves.

Which of the following is the most appropriate initial pharmacologic intervention for this patient's current condition?