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©2013 UpToDate ® Print Email Atrial fibrillation in preexcitation The transmission of atrial impulses to the ventricle is via a atriofascicular (Mahaim) fiber, resulting in a venticular 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. Courtesy of Morton Arnsdorf, MD. Normal ECG Normal sinus rhythm at a rate of 71 beats/min, a P wave axis of 45°, and a PR interval of 0.15 sec. Courtesy of Morton Arnsdorf, MD.