12-lead electrocardiogram (ECG) showing preexcited atrial fibrillation (AF) due to Mahaim fibers

The transmission of atrial impulses to the ventricle is via an atriofascicular (Mahaim) fiber, resulting in a ventricular rate that averages 350 beats/min. The QRS complexes are wide or aberrant and have a left bundle branch block morphology and a left axis deviation, suggesting an insertion at the right ventricular apex.

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