A patient in their 40s presented to the emergency department with sudden-onset rapid palpitations at rest. The patient recalled similar episodes in the past that had terminated spontaneously. The patient’s heart rate was 210 beats/min at presentation, and blood pressure was 110/70 mm Hg. Carotid sinus massage was attempted to terminate the tachycardia but to no effect. Intravenous adenosine, 12 mg, followed by rapid saline flush was administered while recording the 12-lead electrocardiogram (ECG) in the emergency department (Figure 1).