Fehmi Keçe

Chapter 1 14 1.2 Pathophysiology and ablation Techniques The pathophysiology of AF is difficult and still not completely elucidated. The first theory is that of multiple wavelets, suggesting that AF results frommultiple unstable re-entry circuits in the atria. The second theory in contrast, is of focal sources in the atria triggering and sustaining a chaotic rhythm. This theory gained support based on the role of the pulmonary veins showing short refractory periods and changes in myocyte fibre orientation as an ideal substrate for AF. Pulmonary vein isolation became therefore the cornerstone of AF ablation (8). However, in progressed AF also other sources outside the pulmonary veins sustaining AF have been identified. To define or improve a treatment strategy for AF, better understanding of the potential mechanisms of AF is of importance. 1.2.1 Pathophysiology Mechanisms underlying AF can be divided in mechanisms responsible for its initiation (triggers) and in mechanisms responsible for its perpetuation. This is important to define ablation targets. 1.2.2 The role of the pulmonary vein electrophysiology The role of the pulmonary veins with focal discharges initiating AF, firstly described by Haissaguerre in 1998 (9), is now the base of the ablation procedure. In histological studies muscle extensions, so called sleeves extending from the left atrium into the pulmonary veins with complex muscular architecture and fibre orientation producing great non- uniform anisotropic properties, act as an anatomical substrate for local re-entry (10). Also the presence of ectopic pacemaker tissue in the pulmonary vein myocardium, harbouring cells with pacemaker function (Cajal cells) may play a role in the arrhythmogenicity (11, 12). Pulmonary veinmediated arrhythmogenesis (PV-triggers) is based on re-entry, automaticity and triggered activity. 1.2.3 Functional re-entry The functional re-entry as the ‘leading circle model’ was firstly described by Allessie et al . in 1976 (13). A circus movement of the impulse through a small area of atrial muscle results in a constant activation, making it continuously refractory. This area acts like a functional barrier, like a scar, which can sustain re-entry. A circus movement in one direction is initiated by an unidirectional block. The bordering myocardium and the center of the circuit become activated by the simultaneously spreading impulse. The number of reentrant circuits that can be sustained is dependent on wavelength and atrial size.