Woman Faints During Concert

A19-year-old woman is referred from the emergency department (ED) for follow-up. While attending a concert last weekend, she experienced an episode of syncope, was promptly resuscitated by EMTs on site, and was taken by friends to the ED for evaluation. On arrival, she was alert and oriented, albeit embarrassed that she was responsible for her friends missing the remainder of the concert. She admitted having a couple of beers and thought she may have been dehydrated. Following a history, physical exam, lab work, and an ECG, she was discharged in stable condition and asked to follow up in your clinic. As you ask her to remember the episode, she does not recall becoming light-headed or dizzy, and denies having chest pain or palpitations. She says her medical history is unremarkable; however, upon careful questioning, she admits having had several episodes of near-syncope beginning at age 16, with the most recent occurring six months ago. She is a freshman at a local community college and works as a barista at a local coffeehouse. She has no drug allergies and is currently taking no medications. She drinks socially although she is underage, does not smoke, and denies recreational drug use. Her review of systems is negative, and her physical exam is within normal limits for her gender and age. Her last menstrual period was two weeks ago. Reviewing the medical records from the ED, you discover the blood alcohol content was 0.08 and the toxicology screen was negative. A serum beta human chorionic gonadotropin (β-HCG) for pregnancy was negative as well. The ECG reveals a ventricular rate of 64 beats/min; PR interval, 64 ms; QRS duration, 146 ms; QT/QTc interval, 454/468 ms; P axis, unmeasurable; R axis, 60°; and T axis, 92°. What is your interpretation of this ECG, and what do you suspect is the cause of her syncope?