Acute Effects of Radiofrequency Ablation upon Atrial Conduction in Proximity to the Lesion Site

The electrophysiological effects of RF ablation upon the areas in proximity to the lesioned zones have not yet been well characterized. An experimental model is used to investigate atrial conduction in the boundaries of RF damaged zones. In 11 isolated and perfused rabbit hearts, endocardial atrial electrograms were recorded using an 80-Iead multiple electrode positioned in the left atrium. Both before and after the RF application (5 W, 8 s, 1-mm diameter unipolar epicardial electrode) in the mid-portion of the free left atrial wall, measurements were made of conduction time from the pacing zone (posterior wall of the left atrium) to three points between 7.5 and 7.9 mm distal to the damaged zone. Conduction velocity and the direction of the activation propagation vector were determined in ten groups of four electrodes positioned around the damaged zone, and at the left atrial appendage. The mean diameter (± SEM) of the transmural lesions produced by RF ablation and defined by macroscopic examination was 4.2 ± 0.2 mm. The conduction times to the three points distal to the lesion site were significantly prolonged as a result of RF ablation: 7.6 ± 0.4, 7.4 ± 0.5, and 6.9 ± 1.0 ms (control); and 11.3 ± 1.0 (P ≤ 0.01), 11.1 ± 1.3 (P < 0.01), 10.6 ± 1.4 ms (P < 0.05) (post-RF). The differences between the conduction velocities determined in the areas surrounding the lesion, before and after RF application, failed to reach statistical significance: 86.2 ± 6.5 cm/s (control) versus 75.5 ± 5.7 cm/s (post-RF) (NS). After RF, significant variations were only observed in the direction of impulse propagation in the proximal-inferior quadrant adjacent to the lesion site, the difference being -61°± 18° (P < 0.02). In 2 of 4 experiments in which the lesion size was increased by a second RF application (5 W, 16 s), tachycardias with activation sequence around the lesion could be induced, with cycle lengths of 56 and 50 ms, respectively. In the atrial wall, the conduction times to the regions distal to the RF lesion are significantly prolonged. No significant changes are observed in conduction velocity in the areas in proximity to the lesion. Prolonged conduction to the areas distal to the ablation site is due to the lengthened pathway traveled by the impulses in reaching these areas. Tachycardias with activation patterns that suggest reentry around the RF damaged zone may be induced.