12-lead electrocardiogram showing atrial flutter in preexcitation syndrome

Atrial flutter is generated within the right atrium and the atrial impulses in this case are conducted to the ventricle by an accessory pathway. There is 1:1 conduction with a ventricular rate of 300 beats/min. The QRS complexes are widened or aberrant and have a bundle branch block morphology and a left axis deviation, suggesting the presence of an atriofascicular (Mahaim) accessory pathway.

Normal electrocardiogram showing normal sinus rhythm at a rate of 75 beats/minute, a PR interval of 0.14 seconds, a QRS interval of 0.10 seconds, and a QRS axis of approximately 75°.