A woman in her 60s was evaluated for palpitations. She had a history of supraventricular tachycardia (SVT) for 20 years. The episodes were previously controlled with verapamil, but sustained symptoms associated with shortness of breath recurred 6 months before presentation. Her symptoms did not respond to amiodarone therapy. On presentation, she had a regular heart rate of 122 beats per minute and blood pressure of 135/85 mm Hg. Cardiovascular examination demonstrated regular rhythm with cannon A waves in the jugular venous waveform. The 12-lead electrocardiogram (ECG) is shown in Figure 1A. A 24-hour Holter monitor showed sustained tachycardia throughout the monitoring period. Her echocardiogram results were within normal limits.