The most appropriate next step in treatment of this patient is to initiate β-blocker therapy. He has paroxysmal atrial fibrillation with aberrant conduction, resulting in a wide-complex tachycardia. The electrocardiogram demonstrates a normal sinus beat followed by a run of atrial fibrillation with right bundle branch block. Note the irregularly irregular nature of the tachycardia and the QRS morphology consistent with typical right bundle branch block. Given his rapid ventricular response and his symptoms of palpitations and dyspnea, the atrial fibrillation requires treatment. β-Blocker therapy is the preferred atrioventricular nodal blocking agent given the patient's history of coronary artery disease.

Assessment of the need for anticoagulation therapy is also indicated in this patient with atrial fibrillation. Current guidelines recommend the use of the CHA₂DS₂-VASc score for this purpose, replacing the CHADS₂ score because of its ability to more clearly discriminate stroke risk. This patient has a CHA₂DS₂-VASc score of 4 (1 point for hypertension, 2 points for age, and 1 point for coronary artery disease), placing him at moderate risk for stroke. Therefore, initiation of oral anticoagulation also is appropriate.

Emergent cardioversion is not necessary because the patient is hemodynamically stable and appears to be having self-terminating paroxysms of tachycardia. If the patient had a sustained arrhythmia accompanied by hemodynamic instability, emergent cardioversion would be indicated regardless of the specific etiology of the arrhythmia (that is, supraventricular versus ventricular).

Intravenous amiodarone would be an appropriate treatment for recurrent ventricular tachycardia. The electrocardiogram appearance is consistent with right bundle branch block. There is an rSR pattern in lead V₁ and a terminal S wave in leads I and V₆. Right axis deviation is present (QRS axis 123 degrees); however, there is also evidence of left posterior fascicular block (small r waves and deep S waves in leads I and aVL; qR complexes in leads II, III, and aVF). Thus, these features are most consistent with aberrant conduction in the setting of atrial fibrillation rather than ventricular tachycardia.

Intravenous procainamide would be the agent of choice if this tachycardia were preexcited (Wolff-Parkinson-White syndrome). Preexcitation is evidenced by the presence of a delta wave. This patient's electrocardiogram does not demonstrate preexcitation in either the sinus beat or the tachycardia.