Irregular Rhythm in a Middle-Aged Man Presenting With New-Onset Heart Failure

A man in his 50s with recurrent palpitations, fatigue, and progressive exertion dyspnea had irregular narrow-QRS tachycardia in an incessant, repetitive fashion with heart rates up to 180 beats per minute and occasional short runs of wide-QRS tachycardia on Holter monitoring and 12-lead electrocardiogram. What would you do next?