This 30-year-old factory worker is referred because of numerous nonspecific symptoms (dyspnea on exertion, rapid palpitation of gradual onset and offset, lightheadedness, sharp localized precordial pain). The ECG is abnormal, and there are no evidence of structural heart disease by echocardiography:

• Sinus Rhythm
• Wolff Parkinson White (WPW)
• Left-sided accessory pathway

Comments: The presence of an isoelectric delta wave in lead I (and a negative delta wave in aVL) identifies a left free wall accessory pathway. As the delta wave is positive in aVF, the pathway is left lateral or left anterolateral. A loop recorder showed only sinus tachycardia during symptoms. The patient underwent EP investigation that proved the presence of a left lateral accessory pathway that was successfully ablated.