A woman in her 60s presented to the emergency department with palpitation and dyspnea. She had multiple syncopal episodes months prior. Her blood pressure was 154/78 mm Hg, her pulse was 184 beats per minute, and her arterial oxygen saturation was 98% breathing ambient air. Her electrocardiogram (ECG) is shown (Figure 1A). The arrhythmia did not respond to adenosine. Following amiodarone infusion, a second ECG test was performed (Figure 1B).