Eur J Cardiothorac Surg 2005;27:456-461
© 2005 Elsevier Science NL
Surgical treatment of atrial fibrillation with diathermy: an in vitro study
B-Khanh Lama,*,
Munir Boodhwania,
John P. Veinotb,
Paul J. Hendrya,
Thierry G. Mesanaa
a Division of Cardiac Surgery, University of Ottawa Heart Institute, Ottawa, ON, Canada
b Department of Pathology, Ottawa Hospital, Ottawa, ON, Canada
Received 11 September 2004;
received in revised form 10 November 2004;
accepted 11 November 2004.
* Corresponding author. Tel.: +1 613 761 5001; fax: +1 613 761 5217. (E-mail: bklam{at}ottawaheart.ca).
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Abstract
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Objective: The utilization of diathermy (electrocautery) as an energy source in the treatment of chronic atrial fibrillation has generated positive early clinical results. Although this technology is available and affordable, it has not been well studied for this indication. The objectives of this study were: (1) to characterize atrial lesions created by diathermy, (2) to determine relationships between power setting, tissue contact time, and lesion depth and (3) to histologically compare diathermy and unipolar radiofrequency lesions. Methods: Fresh bovine atrial tissue samples were used to create endocardial lesions using a unipolar diathermy system with a blade tip. A total of 120 lesions were created at varying power settings and tissue contact times. Subendocardial temperatures were recorded. All lesions were examined grossly, then fixed, sectioned and evaluated histologically by a blinded pathologist. Comparisons were made with saline irrigated unipolar radiofrequency lesions. Results: Gross examination revealed extensive tissue destruction of the endocardial surface at the point of contact. Histological examination showed minimal penetrance of the lesions beyond the destroyed tissue margin of the endocardium. This was corroborated by the finding of minimal thermal penetration beyond the endocardium and superficial myocardium. There was a linear relationship between the power setting (15–55watts), depth of penetrance (2–15mm) at varying contact times (1–5s/cm). Conclusions: In this in vitro model, lesions created by diathermy were not transmural, even with high power settings and prolonged contact times. At these settings, significant tissue destruction was observed that may predispose to atrial perforation without achieving penetration. Diathermy did not constitute an effective energy source in the creation of transmural lesions for atrial fibrillation ablation.
Key Words: Atrial fibrillation • Surgery • Cox-Maze procedure • Diathermy • In vitro study
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1. Introduction
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Atrial fibrillation is the most common cardiac arrhythmia and is associated with increased morbidity and mortality [1–5]. Its medical treatment often yields unsatisfactory results [6,7]. The surgical therapy of atrial fibrillation, as pioneered by Cox and colleagues remains the gold standard, with excellent long-term results [8–11]. However, the Cox Maze III procedure has not gained widespread clinical application due to its perceived complexity. More recently, simpler, less time-consuming ‘partial Maze’ procedures have been developed that have focused on creating surgical lesions in the left atrium predominantly. In this new context, a series of alternative energy sources have been employed including radiofrequency, microwave, ultrasound, laser and cryothermy [12–16]. The utilization of diathermy (electrocautery) as an energy source in the treatment of chronic atrial fibrillation has generated positive early clinical results [17,18]. Although this technology is available and affordable, it has not been well studied for this indication. The objectives of this study were: (1) to characterize atrial lesions created by diathermy, (2) to determine relationships between power setting, tissue contact time, and lesion depth and (3) to histologically compare diathermy and unipolar radiofrequency lesions.
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2. Methods
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2.1. In vitro model
Fresh bovine hearts were obtained and preserved in 4°C saline solution; the left atria were dissected into 7x7cm segments with an average thickness of 5.1±2.1mm. Utilizing different combinations of diathermy delivery mode, power setting and contact time, we created a total of 120 endocardial lesions on 40 atrial samples (Table 1). Each segment of atrial tissue was scored longitudinally three times using a unipolar diathermy system with a blade tip (Valleylab, Tyco Healthcare, Boulder, Co., USA). The length of each lesion was preset at 5cm to facilitate calculation of contact time. Baseline and maximal tissue temperatures were recorded using a Shiley temperature probe (Tyco Healthcare, Boulder, Co., USA) inserted into the atrial tissue, under the path of the diathermy lesions. Radiofrequency lesions were created using the Medtronic Cardioblate Surgical Ablation Pen (Medtronic, Minneapolis, MN, USA) with the generator set on power control. Subsequently, each atrial segment was grossly appraised, photographed and assigned an identification code prior to submission to a blinded pathologist for assessment.
2.2. Histologic evaluation
The atrial specimens were fixed in 10% neutral buffered formalin for several days. Multiple full thickness sections were taken, sampling the lesions that were apparent on the endocardial surface. The tissues were paraffin embedded and 4µ sections prepared and stained with hematoxylin phloxine saffron (HPS), and Masson trichrome stain. The pathologist examining the slides was an experienced Cardiovascular Pathologist (JPV), who was blinded to the treatment.
The slides were examined microscopically and evaluated for the presence and maximum depth of the lesions using a calibrated eye piece micrometer. The depth of the lesions created was measured from the endocardial surface. Tissue damage was described as coagulated tissue (intact but injured adjacent tissue with some coagulative necrosis present) or destroyed tissue (tissue that had vaporized leaving a gap). Total depth of tissue damage (coagulated+destroyed) would represent an estimate of transmurality in this context. All measurements were done with two observers and the pathologist was blinded.
2.3. Data analysis
Simple descriptive statistics were used to summarize data. Continuous variables are presented as mean ± standard deviation. Categorical data were described using frequencies and percentages. When the data had a normal distribution, one-way ANOVA testing was used to assess inter-group variations with the maximum experimentwise error rate (MEER) being controlled by a Bonferroni t-test. When the assumption of normality for an ANOVA was not met, the Kruskal–Wallis nonparametric test was used as an analogue test. Graphic correlation of power setting, contact time and depth of thermal damage were also generated (MS Excel, Microsoft, CA). All analyses were performed using SAS statistical software (SAS v8.2; SAS, Inc., Cary, NC).
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3. Results
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3.1. Gross visualization
Gross visual assessment of the specimens demonstrated the presence of increasing endocardial tissue disruption and penetration as the power setting increased from 15 to 50watts. In contrast, lesions created by radiofrequency energy source did not present any evidence of endocardial disruption (Fig. 1).
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Fig. 1. (A) Diathermy lesions with three power settings: (A) 15, (B) 35, and (C) 55W. (D) Radiofrequency lesion with 25W.
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3.2. Thermal damage
The total tissue damage caused by diathermy was calculated as the sum of coagulated tissue depth and destroyed tissue depth (Table 2). Changes in subendocardial tissue temperatures generally increased proportionally to increases in power setting; great variability was observed, which was likely due to variation in tissue thickness. When using diathermy in spray mode at fixed contact times, the average depth of tissue damage increased significantly as the power setting increased from 15 to 55W (Table 2). Conversely, the average depth of tissue damage increased also when the power setting was kept constant and the contact time increased from 1s/cm to 5s/cm. Under these conditions, extent of tissue damage significantly increased with prolonged contact time for power settings of 15, 25 and 55W (Tables 3 and 4).
3.3. Mode of energy delivery
Atrial lesions were also created using four different diathermy modes: spray, blend, desiccate and fulgurate. To determine the ideal mode, lesions were created using all four modes while controlling for power (40, 50W) and average contact times (2s/cm and 3s/cm). Under these circumstances, the most extensive damage to the endocardium was observed when the mode was either ‘desiccate’ or ‘fulgurate’. We also noted that, irrespective of contact time, there was no significant difference in tissue damage when the power was set at 50W. Our histological analysis also demonstrated that lesions created with ‘spray’ mode had good depth to them with minimal endocardial tissue destruction. Using ‘spray’ mode, relationships between power setting, contact time and extent of tissue damage were established (Fig. 2A); there was a proportional increase in tissue damage observed as power setting and contact time increased. However, we did not detect an increase in the proportion of tissue destroyed for power settings between 25–45W. In addition, proportionally equal amount of coagulation necrosis was observed as power increased from 35 to 55W (Fig. 2B).
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Fig. 2. (A) Relationships between power setting, contact time and depth of tissue damage. (B) Relationships between depth of tissue damage (coagulated and destroyed) and power setting.
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3.4. Comparison with radiofrequency
Radiofrequency lesions were compared to lesions created by all four modes of diathermy. At average power settings (20–35W), significant differences in depth of tissue damage were observed: 3.7±2.4mm (radiofrequency), 8.3±5.2±mm (blend), 9.5±5.8mm (spray), 34.7±17.2mm (desiccate) and 22.6±3.14mm (fulgurate) (P<0.0001). At a higher power setting of 50W, the mean extent of tissue damage was 9.4±2.6mm (radiofrequency), 12.3±1.7mm (blend), 17.2±9.3mm (spray), 20.1±7.2mm (desiccate) and 25.5±7.9mm (fulgurate) respectively (P=0.016). By histology, minimal endocardial disruption was observed on radiofrequency lesions while extensive endocardial necrosis and disruption was seen on diathermic lesions (Fig. 3).
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Fig. 3. (A) Diathermy lesion results in extensive tissue destruction with loss of endocardium and destruction of the underlying myocardium. A small rim of myocardium with coagulative necrosis surrounds the crater (black arrows). (B) In contrast, radiofrequency lesions produced necrotic edematous myocardium (white arrows) with an intact endocardial surface. No significant tissue defect is seen.
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4. Discussion
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Over two million people in North America have atrial fibrillation. With an aging population, there are 160,000 new cases annually [19]. Approximately two thirds of patients have associated cardiovascular disease [20]. In this setting, atrial fibrillation was associated with increased risk of morbidity and mortality [2–4,20,21]. Dissatisfaction with medical therapy has spurred efforts to develop a surgical treatment for AF. The pioneering work of Cox et al. has demonstrated the feasibility of treating AF surgically by interrupting the atrial pathways for multiple reentry circuits, which are necessary for the maintenance of AF [9,10]. Surgery for the treatment of lone atrial fibrillation (without associated coronary or valvular disease) has not been widely adopted, however, because the procedure remains complex, time-consuming, and requires cardiopulmonary bypass. New surgical techniques have focused on the development of potentially less invasive procedures by simplifying the pattern of atrial lesions and evaluating alternative energy sources that can create them quickly, without a cut-and-sew technique [14–16,22–24].
More recently, the reported utilization of diathermy (electrocautery) in the creation of atrial ablation lesions raised the possibility of an affordable and readily available energy source for the surgical treatment of atrial fibrillation. Simha et al. presented their initial experience with 25 patients undergoing the Maze procedure by electrocautery. Lesions similar to those of the Cox-Maze procedure were reproduced using an electrocautery knife tip set on ‘spray’ mode, with a power output of 40W and at a non-specific contact time [17]. There were no complications reported and, sinus rhythm was restored in 96% of patients at a mean follow-up of 3.5 years. Similarly, Desaulniers et al. presented their results using an electrocautery knife tip set on ‘spray’ at a power setting of 26W to reproduce Maze lesions in the left atrium [18]. Using this technique, over 70% of their patients were in sinus rhythm at short-term follow-up.
Despite good clinical outcomes, there is a paucity of data on the morphological effects of diathermy on atrial tissue. Using an in vitro model, we were able to quantify the relationships between diathermy mode, power setting, contact time and tissue damage depth.
4.1. Diathermy relationships
4.1.1. Mode
Through our evaluation of various modes of diathermy, we found that ‘spray’ provided the best results. At a power setting of 30–40W and with 2 and 3s/cm contact times, there was a significant difference in the amount of tissue damage between all four modes evaluated. We noted however, that ‘spray’ mode overall, provided good coagulation necrosis with undue risk of perforation. When the power was increased to 50W, the extent of tissue destruction was so extensive that mode setting was irrelevant.
4.1.2. Power setting and contact time
As anticipated, increased power setting and increased contact time had a direct effect on the depth of tissue damage (Fig. 2A). At power settings below 25W, there was very little destruction of tissue observed, likely precluding this setting in the clinical context of creating atrial lesions by diathermy. Interestingly, by histologic assessment, we did not find that more tissue was destroyed by increasing the power setting from 25 to 45W (Fig. 2B). In addition, higher proportions of coagulated tissue were observed for power settings between 35 and 55W, making these settings more clinically relevant. However, at 45 and 55W, the almost full thickness damage would predispose to undesired atrial wall perforation. If clinically applicable, the best results (high tissue coagulation necrosis, low risk of perforation) would come from power settings between 30 and 35W, along with contact times of 1–3s/cm. Lower settings would not generate sufficient tissue damage in the form of coagulation necrosis while higher settings would likely cause damage to surrounding structures (circumflex artery, coronary sinus, esophagus) and risk atrial perforation.
4.2. Diathermy vs Radiofrequency
Radiofrequency energy uses an alternating current from 350kHz to 1MHz to heat tissue, resulting in thermal injury. The utilization of radiofrequency as an energy source has been well described and the majority of radiofrequency ablation procedures to date have been performed with unipolar systems. In these systems, current flows from the radiofrequency probe to contacted atrial tissue, where thermal energy is released as a result of resistance to conduction. Limitations of the procedure include the unfocused nature of the energy delivered, with elevated local temperatures leading to surface charring, and potential thromboembolic complications. Heat is conducted to surrounding tissues, raising the risk of damage to surrounding structures. Consistent surface application to make uniform lesions is difficult with no indication of transmurality. Recently developed bipolar radiofrequency clamps address these limitations and allow creation of more precise and uniform transmural lesions [25].
Radiofrequency lesions created in this model had no destruction of tissue but increased amounts of coagulated myocyte damage and penetration as the power setting increased from 20 to 50W. In contrast, lesions by diathermy resulted in limited coagulation necrosis, extensive tissue destruction and limited endocardial tissue damage beyond the immediate area of contact. In contrast to the radiofrequency ablation system, diathermy's sinusoidal modulation resulted in a lower repetition frequency which might have predisposed to less subendocardial energy penetration and more endocardial surface damage.
4.3. Conclusions
The utilization of diathermy as an energy source for the creation of atrial ablation lesions in the treatment of atrial fibrillation certainly warrants attention since it is readily available, easy to use and affordable. In this in vitro model, we have established what should be optimal energy delivery mode, power settings and contact times for the application of diathermy in the clinical context of atrial fibrillation treatment. However, there are considerable limitations to this technique: first, we have not found diathermy capable of creating transmural lesions histologically. However, excellent clinical results from reported series certainly call into question the need for transmurality as a prerequisite for electrical isolation [17,18]. Second, the extensive local tissue destruction encountered would certainly predispose to thromboembolic complications, free wall rupture and adjacent tissue injury. We have also not evaluated healed or chronic lesions. Finally, when compared to other energy sources (radiofrequency, microwave, cryothermy), the paucity of in vitro and in vivo data warrants more research on this technique before its standardization and general application.
4.4. Limitations
This in vitro model is limited to a morphological assessment of diathermy as an energy source. Variation in the thickness and histologic content of the various specimens may have caused variant degrees of thermal penetration; however, human atria also have variable thickness, thus making it difficult to predict uniformity of lesion depth in general when using this technique.
This study does not take into consideration the electrophysiological properties of diathermy on atrial tissue. Differences between bovine and human atrial tissues may limit the generalizability of the data; however, these differences may pertain more to electrophysiological properties rather than the morphological aspects of our study. In vivo intact endocardial reaction cannot be assessed as the system is in vitro without blood or coagulation factors. Finally, in some instances, the assumption of normal distribution was made on smaller than usual samples for the ANOVA analysis.
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Appendix A. Conference discussion
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Dr J. Melo (Carnaxide, Portugal): Have you done this with bipolar diathermy? Dr. Pawanavarthan from India has been reporting repeatedly that he is performing his antiarrhythmic procedures with bipolar diathermy, and I've tried that myself in animals and they were not very successful regarding disruption. Because those were anecdotal trials, I suggest that what we need is information if we can do that with bipolar diathermy, which is completely different.
Dr Mesana: We did unipolar.
Dr Eugene M. Baudet (Bordeaux-Pessac, France): When you assert that the endocardial ablation alone is effective enough, on what basis did you assert this opinion?
Dr Mesana: Well, it's just a question. We cannot assert it. If we have this result in clinical settings, which we don't have because we don't do diathermy in our patients, we base it on the idea that transmurality must be achieved, and this study demonstrates that diathermy can fail to achieve transmurality. So the question is, if these guys have good results with diathermy, is the transmurality really needed to achieve a good actual ablation. That's the big question of this study that we ended up with.
Dr Baudet: So what technique did you personally use?
Dr Mesana: We use radiofrequency. We never use electrocautery. We were just intrigued by these papers and said why don't we test this in an in vitro model. But we use radiofrequency. We have probably more than 100 patients already, and we have about the same results as anybody else. But electrocautery just costs nothing to the hospital. That's why we say, well, this is probably interesting to see.
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Footnotes
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☆ Presented at the joint 18th Annual Meeting of the European Association for Cardio-thoracic Surgery and the 12th Annual Meeting of the European Society of Thoracic Surgeons, Leipzig, Germany, September 12–15, 2004.
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Abstract
1. Introduction
