Mechanisms of pacing interventions in atrial fibrillation.

Atrial flutter is due to large single reentrant loops, typically oriented around the caval veins and a line of functional conduction block in between. These reentrant circuits can easily be terminated by properly timed extra beats colliding in the antidromic direction with the reentrant orthodromic wavefront and being blocked in the orthodromic direction within the area of slow conduction. Due to the complex situation in atrial fibrillation with multiple, ever-changing wavelets and a marked functional inhomogeneity of the atrial tissue, these simple concepts do not apply. However, regional control of atrial tissue by rapid pacing is feasible during atrial fibrillation, and through a multisite approach, this pacing modality might lead to a situation where the remaining nonentrained atrial tissue is just no longer reaching the critical mass. Single-site, dual-site, and biatrial pacing seems to reduce the incidence of atrial fibrillation recurrences. Specific advantages and disadvantages of each approach still need to be elucidated. Apart from preventing bradycardia, preventive pacing modes reduce intra-atrial conduction times and might reduce intra-atrial dispersion of refractoriness. Still, the specific mechanisms relevant for their effectiveness need further clarification.