Elsevier

Heart Rhythm

Volume 5, Issue 9, September 2008, Pages 1296-1301
Heart Rhythm

Original-experimental
Atrial fibrillation propagates through gaps in ablation lines: Implications for ablative treatment of atrial fibrillation

https://doi.org/10.1016/j.hrthm.2008.06.009Get rights and content

Background

It has been hypothesized that atrial lesions must be transmural to successfully cure atrial fibrillation (AF). However, ablation lines often do not extend completely across the atrial wall.

Objective

The purpose of this study was to determine the effect of residual gaps on conduction properties of atrial tissue.

Methods

Canine right atria (n = 13) were isolated, perfused, and mounted on a 250-lead electrode plaque. The atria were divided with a bipolar radiofrequency ablation clamp, leaving a gap that was progressively narrowed. Conduction velocities at varying pacing rates and AF frequencies were measured before and after ablations. AF was induced with an extra stimulus and acetylcholine.

Results

Gap widths from 11.2 to 1.1 mm were examined. Conduction velocities through gaps were dependent cycle length (P = .002) and gap size (P <.001). Overall, 253 (97%) of a total of 260 gaps allowed paced propagation; 51 (91%) of 56 gaps 1–3 mm in width permitted paced propagation, as did 202 (99%) of 204 gaps ≥3.0 mm. Similarly, 253 (97%) of a total of 260 gaps allowed propagation of AF. For AF, 51 (93%) of 55 gaps 1–3 mm allowed AF to pass through, as did 202 (99%) of 205 gaps ≥3.0 mm. Gaps as small as 1.1 mm conducted paced and AF impulses.

Conclusions

Conduction velocities were slowed through residual gaps. However, propagation of wave fronts during pacing and AF occurred through the majority of residual gaps, down to sizes as small as 1.1 mm. Leaving viable tissue in ablation lines for the treatment of AF could account for failures.

Introduction

Atrial fibrillation (AF) is the most common sustained arrhythmia in the world, occurring in 0.4%–2.0% of the general population.1 The Cox-maze procedure (CMP) was developed in the 1980s and has the highest reported long-term success for the treatment of AF.2 Because the CMP is technically challenging and time-consuming to complete, few surgeons choose to incorporate it into their daily practice. Recent advancements in technology have made it possible to replace many of the incisions with linear lines of ablation, making the procedure both shorter and less technically challenging. Various energy sources, including radiofrequency, microwave, laser, cryoablation, and high-frequency ultrasound, have been used.3

There are many contributing factors that can determine success in ablation for either catheter-based or surgical treatment of AF. These include the number and location of the lesions, the mass of atrial tissue ablated, the successful isolation of triggers, and the effectiveness of the particular technology in terms of ablation depth and width. An important question that has arisen is whether having transmural or complete lesions is important or necessary. In a chronic canine model, Ishii et al4 found that conduction block rarely occurred in gaps larger than 5 mm, and decremental slowing was seen in gaps between 5 and 15 mm. In this model, in which gaps were left in surgical incision lines, complete block was seen in gaps smaller than 5 mm. In another canine model in which gaps were left in lines created by unipolar radiofrequency, conduction block was rarely seen with a gap greater than 5 mm and was variable at 2–5 mm of gap.5 Conduction was demonstrated down to a gap of 0.1 mm in an acute canine laser model, substantiating that even very small breaks in ablation lines have the ability to conduct paced impulses.6 In an in vivo study by Inoue and Zipes,7 conduction was found in atrial tissue with a cross-sectional area as little as 1 mm2.

This study was designed to test the following hypotheses: paced conduction would be slowed and eventually blocked with a decrease in gap size and block of conduction of AF through gaps would occur with decreasing gap size.

Section snippets

Methods

All animals received humane care in compliance with the National Academy of Sciences 1996 Guide for the Care and Use of Laboratory Animals. Normal mongrel dogs (n = 13), weighing between 20 and 30 kg, were intravenously anesthetized with 7.5 mg/kg of propofol, intubated, and placed on a positive pressure respirator with 2%–3% isoflurane used throughout the procedure. A median sternotomy was performed, and the heart was cradled in the pericardium. The interatrial groove was dissected, separating

Results

Gap widths were examined from a range of 1.1 to 11.2 mm. Paced conduction velocity slowed as gap size decreased (P <.002; Figure 3). Conduction velocity (Figure 4) was dependent on PCL, as decreased cycle lengths had decreased conduction velocity (P = .001). Pacing direction did not have a significant impact on conduction velocity independent of gap size (P = .192). The mean control ERP was 166 ± 3.9 ms, and the preablation ERP with ACh was 41 ± 3.2 ms. After the fifth ablation, the ERP ACh was

Discussion

This study demonstrated that despite a correlation of decreased conduction velocity with smaller gaps in the ablation line, electrical impulses were still propagated even at very narrow gap widths. The results were similar for paced wave fronts and AF, with consistent conduction persisting in residual gaps greater than the smallest obtainable gap size in this model (∼1 mm). In the vast majority of samples, complete ablation was required to prevent conduction of both paced and AF electrical

Limitations

This model has several limitations. The canine atria were normal, not diseased, and thus results may not be directly comparable to the diseased human atria. However, the canine is an excellent model for testing AF and has been used previously for this purpose.21 This model evaluated residual gaps that included full-thickness intact atrial tissue, which may be different than residual endocardial or epicardial rims of tissue. However, wide superficial ablation lines with residual endocardial or

Conclusion

Propagation of paced signals occurred through narrow gaps left in ablation lines on the atrium, even when conduction velocities were significantly slowed. In addition to propagation of paced signals, the majority of narrow residual gaps conducted AF. A close correlation was seen between the propagation of paced signals and AF, suggesting that pacing can be used as a surrogate to ensure adequacy of a lesion. To guarantee complete conduction block in this model, continuous transmural lines of

References (22)

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      Whatever the underlying heart disease, the described atrial substrate modification may lead to slow-conducting areas that represent the pathophysiological hallmark of AAFL.8,27,28 In an elegant experimental study conducted on canine hearts, Melby and colleagues showed that anatomic gaps left between ablation lines were associated with slow conduction, and the narrower the gap the slower the conduction velocity.27 Similar observations were recorded in human beings undergoing both conventional and three-dimensional electroanatomic mapping for post-CPVA AAFL.8

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    Funded in part by National Institutes of Health grant nos. RO1-HL032257, R01-HL085113, and F32 HL078136-01.

    Dr. Damiano is a consultant for Atricure, Inc., and Medtronic Corporation. Dr. Schuessler is a consultant for Atricure, Inc.

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