The Coolrail was compared with an existing technology that has been widely used in the clinical treatment of atrial fibrillation. The Guidant Flex4 microwave device failed to reliably create transmural lesions a majority of time, even at the manufacturer’s recommended ablation time of 90 seconds (Fig. 6). Only 38% of lesions were transmural. Fifty-eight percent of lesions in tissue less than 4-mm thick were transmural for the microwave device and none of the lesions in tissue thicker than 4-mm were transmural.
Comparing the lesions at the maximum time tested for each device in a side by side manner, the Coolrail performed more reliably than the Flex4 in creating transmural lesions (Fig. 7). Similarly, the Coolrail created deeper lesions than the Flex 4, averaging 2.8 ± 1.1 mm in depth versus 2.0 ± 1.1 mm for the Flex 4 device (P = 0.005).
The traditional incisions of the Cox-Maze III have been replaced in clinical practice by linear lines of ablation using various energy sources. Many of these devices were used clinically before dose-response data were published.17,22 Subsequent studies have shown that some did not consistently create transmural lesions.21,23–27 As the demand for minimally invasive techniques has led to the development of new technologies, establishing dose-response before widespread use is paramount. Knowledge of device performance characteristics is vital to effectively treat patients with atrial fibrillation.
This study demonstrated that the Coolrail device was capable of creating transmural lesions 91% of the time in atrial tissue up to 4-mm thick; however, it did not perform well in thicker atrial tissue. Above 4 mm, only 21% and 48% of lesions were transmural at 40 and 50 seconds, respectively. At shorter ablation times, even thin tissue sections presented a challenge for the Coolrail. Twenty percent of lesions between 2.0 and 2.5 mm were not transmural at ablation times of 20 and 30 seconds.
This laboratory has previously published our animal work with microwave devices, including the one tested in this study.21 The results of the this study were similar. The microwave device had a difficult time creating a transmural lesion in the beating heart with less than 40% of lesions transmural at any time tested. A follow-up study in this laboratory demonstrated the dependence of lesion thickness on intracavitary blood flow and cardiac output, which may serve as a heat sink in the beating heart, and thus shielding the endocardium from thermal damage and preventing transmural ablation.28
This study demonstrated that the internally cooled, bipolar radiofrequency-based Coolrail can overcome some of the limitations of earlier generation devices like the Flex 4, penetrating deeper into beating atrial tissue and achieving more consistent transmurality, especially in thin atrial tissue. Forty-two percent of all lesions less than 4-mm thick were not transmural with the microwave device as compared with only 9% for the Coolrail device at the maximum time tested.
With the introduction of technology capable of creating transmural lesions epicardially on the beating heart, the goal of achieving a minimally invasive procedure for all patients with atrial fibrillation becomes more achievable. To date, most published reports on minimally invasive atrial fibrillation surgery have limited their procedures to pulmonary vein isolation with or without ablation of the ganglionated plexuses.29–33 Although early results have been encouraging in patients with paroxysmal atrial fibrillation, success rates have been poor for patients with long-standing or persistent atrial fibrillation. In these subsets, the most effective option remains a full Cox-Maze lesion set.1,34
This study demonstrated that the Coolrail can create linear, transmural lesions on the beating heart, up to 4 mm. However, in thicker tissue, it did not have sufficient efficacy. Because it is usually not practical to determine tissue thickness at every proposed ablation site intraoperatively, this is a significant shortcoming of this technology. Pathologic atria can achieve thicknesses greater than 4 mm,35 and even normal atria get very thick in areas such as the crista terminalis and Bachman’s bundle.36,37 This would suggest that it will be necessary to test lesion integrity in the operating room at the time of surgical ablation when this device is used.
Electrophysiologic testing of ablations represents a difficult challenge. Initial success with late recurrences has been observed consistently in follow up after ablation procedures. Electrophysiologic study at the time of ablation procedure consistently documents isolation of the pulmonary veins in catheter-based treatments, but repeat studies of these patients after treatment failure demonstrate recovery from isolation.38–40 A reliable, expedient method for documenting permanent conduction block remains to be elucidated, although several groups are working on this challenge.41–43 A number of centers are exploring a hybrid approach in which surgeons and electrophysiologists collaborate closely in the performance and testing of surgical ablation procedures. Until reliable methods for documenting conduction block are available, application of any energy source, including the Coolrail, on the epicardial surface of the beating heart must be done with caution.
The main limitation of this study is that ablations were examined in the acute setting. Chronic results with the Coolrail may be different.26 However, TTC staining has been shown to reliably delineate the extent of necrosis, and experience in our laboratory has demonstrated similar results in the acute and chronic setting using the techniques of sampling and analysis used in this study.13,44,45
A second limitation of the study is that the healthy, normal porcine atria do not completely mimic the clinical situation. Average tissue thickness was less than 4 mm whereas pathologic atria can be greater than 10-mm thick in humans and may contain significant tissue fibrosis, both of which can limit the extent of lesion formation. In addition, epicardial fat has been shown to limit the penetration of radiofrequency energy.46 The placement of lesions in this study specifically avoided the epicardial fat pads present in normal porcine anatomy. Furthermore, these lesions were planned so that the maximum number of lesions could be created in any one animal to reduce the number of animals needed to acquire sufficient dosimetry data and not to simulate clinical lesion sets designed to treat atrial fibrillation. For example, no lesions were made to encircle the pulmonary veins in this study as the anatomy of the pig made this area difficult to ablate. This lesion has been critical in both the surgical and catheter-based treatment of atrial fibrillation. This has electrophysiologic implications, but no attempt was made to document the electrophysiological consequences of the lesions in this study. Finally, as lesions were sectioned every 5 mm, the contiguity of the lesions were not measured along the entire length of the ablation.
The authors acknowledge the technical assistance of Diane Toeniskoetter and Naomi Still.
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Surgeons performing surgical procedures for atrial fibrillation should be familiar with both the lesion pattern required to achieve the highest success rate and with the ablation device in use. Each of the ablation devices being used by surgeons has some limitation that seems to be ignored by many of us. This is mainly related to our inability to apply an epicardial transmural lesion on a full beating heart using a nonclamp radiofrequency or cryogenic technology. This article from Washington University nicely tests the performance of a new surgical ablation device that uses a cooled nonclamped bipolar radiofrequency. The authors found that the new device has a linear dose response relationship to the extent that a longer application resulted in a deeper and a wider lesion; however, the performance was found to be poor on tissue thicker than 4 mm; when only 91% of the applications were transmural and applied on a 4-mm tissue for 50 seconds. The take home message from this study is that although it seems that this device may perform a little better than other devices it is still not reliable in creating reproducible transmural lesions when applied on a full heart epicardially. As surgeons, we should be aware and careful when using such a device under the same clinical settings because we can not guarantee transmurality and therefore should expect much higher failure rates.
Niv Ad, MD, is the guest editor.