This 48-year-old man presented to the Emergency Department complaining of palpitation that had started suddenly 1 hour earlier, associated with dyspnea and weakness. He had experienced 3-4 episodes of palpitation in the past 15 years, that lasted 30-60 minutes and subsided spontaneously. He had otherwise enjoyed a good health and exercised frequently without symptoms.

• Atrial fibrillation with rapid ventricular response (182/min)
• Wide QRS (150 ms), except for one narrower complex. The pattern of the wide QRS is compatible with conduction through a left anterolateral accessory pathway (Q waves in I and aVL, broad prominent R wave in V1). The narrower complex has a delta wave in leads II and V5.

Comment: If we were dealing with marked intraventricular conduction abnormality, rather than conduction through a bypass tract, it would be quite unusual for one QRS complex to show such a good normalization of conduction

The patient was treated with IV Amiodarone. Sinus rhythm was restored:

• Sinus rhythm
• Wolff-Parkinson-White
• The presence of q waves (negative delta waves) in I and aVL and positive delta waves with prominent R waves in aVF and V1 are typical of a left anterolateral bypass

EP studies confirmed the presence of a left lateral accessory pathway that was successfully ablated. The following is the post-ablation ECG:

• Sinus rhythm
• Normal ECG