12-lead ECG showing the Wolff-Parkinson-White pattern

The two main ECG features of WPW pattern include a short PR interval (<0.12 seconds) and a delta wave (arrows). The QRS complex is wide (>0.12 seconds) and represents a fusion beat; the initial portion (delta wave) results from rapid ventricular activation via the accessory pathway (preexcitation), while the termination of ventricular activation is via the normal conduction system, leading to a fairly normal terminal portion of the QRS.

Normal sinus rhythm at a rate of 71 beats/minute, a P wave axis of 45°, and a PR interval of 0.15 seconds.