A 28-year-old man presents to the emergency department with acute-onset palpitations and irregular rapid heartbeat. His 12-lead ECG shows an irregularly irregular rhythm with wide QRS complexes (>120 ms) of varying morphology, and a ventricular rate of 280 bpm. The structure marked **A** in the diagram demonstrates the characteristic pre-excitation pattern. Which of the following is the MOST appropriate acute management for this patient?