Palpitations and dizzines...

A 23-year-old woman presented with palpitations that began at age 17 years. They occurred intermittently, lasted up to 30 minutes several times throughout the day and night, were worse with activity, and woke her up at night. Her pulse would go up to 150 beats/min on her watch, sometimes with associated dizziness. Echocardiography revealed a structurally normal heart (ejection fraction: 57%). Her resting electrocardiogram (ECG) is shown.