12-lead electrocardiogram (ECG) showing a left lateral atrioventricular accessory pathway in a patient with Wolff-Parkinson-White (WPW) syndrome

Electrocardiogram during sinus rhythm from a patient with a left lateral accessory pathway.
Panel A: Normal PR interval and a fairly normal QRS complex.
Panel B: With pacing from the coronary sinus at a site close to the accessory pathway, the QRS complexes are maximally preexcited.