**2. Electrophysiologic basis of atrial fibrillation**

It was documented that all AF is characterized by the presence of two or more large macroreentrant circuits in the atria simultaneously [19]. Haïssaguerre et al. first detected the focal triggers of atrial ectopic beats [20]. They noted that the ectopic foci are mainly (>90%) located in and around the orifices of the pulmonary veins and the remaining are located in other sites such as right atrium, left atrium, crista terminalis, and left atrial appendage. The findings of their paper, which reported that the triggers were successfully treated with radiofrequency ablation, led to an explosion of efforts by a number of cardiologists and cardiac surgeons to treat AF with catheter ablation and surgical techniques, respectively. The concept of treating AF was originally focused on isolating the pulmonary veins, either by catheters or surgical devices.

However, AF produces unfavourable changes in atrial function and structure, which is called remodeling [21]. After many years of paroxysmal AF, the macroreentrant circuits of AF can become self-perpetuating. At this point, paroxysmal AF can become long-standing or persistent AF, and the underlying electrophysiologic culprit is no longer the focal triggers, but rather the macroreentrant circuits themselves. Therefore, for long-standing or persistent AF, simple isolation of pulmonary veins is not an effective treatment because the focal triggers do not account for onset of AF for this type of AF. In these patients, it is necessary to interrupt the macroreentrant circuits by placing additional linear lesions in the atria. This concept has led to a surgical ablation technique called Cox-Maze procedure.
