A 67-year-old woman has experienced episodes of paroxysmal palpitation since her early 40s. They are infrequent (one every 3-4 years) and last from a few minutes to a few hours. No precipitating factors have been identified. She denies drinking alcohol. This ECG was recorded in the Emergency Department during one of her episodes of palpitation:

• Atrial tachycardia, 160/min