ECG pre-excited atrial fibrillation

12-lead ECG showing atrial fibrillation in a patient with antegrade conduction through both the AV node and an accessory pathway. The occasional narrow QRS complexes reflect conduction through the AV node, while the wide QRS complexes, with instantaneous rates close to 300 beats per minute, are due to antegrade conduction via an accessory pathway (probably postero-septal based on the QRS morphology).