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Ann Thorac Surg 2003;75:543-548
© 2003 The Society of Thoracic Surgeons
a Department of Cardiology, Westmead Hospital, Westmead, New South Wales, Australia
b Cardiothoracic Surgery, Westmead Hospital, Westmead, New South Wales, Australia
Accepted for publication August 19, 2002.
* Address reprint requests to Dr Thomas, Department of Cardiology, Westmead Hospital, Westmead, New South Wales 2145, Australia.
e-mail: stuartpt{at}yahoo.com
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Abstract |
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METHODS: Radiofrequency lesions were produced using a multielectrode malleable handheld probe in ovine myocardium with three wall thicknesses. Detailed analysis of lesion dimensions was used to assess the effects of site of ablation, muscle thickness, and duration of ablation.
RESULTS: After epicardial atrial ablation, myocardial lesions were detected in all sections without macroscopically visible epicardial fat (n = 10), but only 43% (6/14) of sections with epicardial fat. Three of 24 atrial epicardial sections (13%) and 92% (23/25) of endocardial atrial lesion sections were clearly transmural. In thicker tissues lesion depth was independent of endocardial (right ventricle: 3.9 ± 1.1 mm, left ventricle: 3.8 ± 0.7 mm) or epicardial (right ventricle: 3.4 ± 0.6 mm, left ventricle: 4.3 ± 0.9 mm) ablation site. Epicardial lesions are less deep in thinner areas of myocardium (p = 0.003). Lesions were all wider than they were deep. There was no significant increase in lesion depth with the increase in ablation duration from 1 to 2 minutes.
CONCLUSIONS: Lesions were unlikely to be transmural with either technique when the wall thickness was greater than about 4 mm. Epicardial fat has an important negative effect on epicardial lesion formation. Where epicardial fat is absent epicardially produced lesions penetrate less deeply when the wall thickness is small, possibly due to endocardial cooling by circulating blood. Prolongation of the duration of ablation from 1 to 2 minutes does not significantly increase lesion depth.
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Introduction |
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An endocardial approach was used in early studies of linear radiofrequency ablation for atrial fibrillation. A single or multielectrode handheld probe was inserted through an atriotomy to produce lesions by passing radiofrequency current between the electrodes and a large indifferent electrode placed on the skin or between the endocardial and an epicardial electrode [6]. Recently some investigators have produced linear lesions with the ablation probe positioned on the epicardial surfaces of the atria [1, 5]. This technique may simplify the procedure by allowing the lesions to be formed on a beating heart. The effects of this alternative approach on the effectiveness of lesion formation are unknown. Severalfactors including the structure of the epicardium, the presence of blood perfusing the myocardium, and the presence of blood cooling the endocardial surface during ablation may alter lesion formation.
The purpose of this study was to compare the site of ablation, endocardial versus epicardial, on radiofrequency energy induced lesion formation in myocardium. We also examined the effect of myocardial wall thickness and duration of ablation on lesion formation.
A sheep model was used to compare lesions formed either epicardially on the beating heart or on the endocardial surface after cardioplegia. The atria of sheep are very thin and structurally different from those in humans. The wall thickness of the sheep right ventricle is closer to that of diseased human atria. The prominent trabeculation of the ovine right ventricle also mimics that of the human trabeculated atrium. Therefore, atrial and right ventricular lesions were produced. Lesions were also produced in left ventricular tissue. The thick left ventricular tissue allows comparison of lesion size unobstructed by substrate boundaries and independent of the endocardial blood cooling effect.
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Material and methods |
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In all animals a thoracotomy was performed at the fourth left intercostal space to expose the heart. In 5 sheep, ablation was performed directly on the epicardial surface of the beating heart. Two long linear lesions were possible on the epicardial surface of the atria (right and left), two lesions were produced in the right ventricle, and a further two lesions were produced in the left ventricle of each animal. In the remaining 5 sheep the aorta was cross-clamped and cardioplegia solution (Solution A, Baxter, Deerfield, IL) administered antegradely at a temperature of less than 5°C through an aortic cannula until electromechanical arrest was achieved. The cardiac chambers were opened and vented where appropriate to allow access to the endocardial surface. Two long linear lesions were possible in the atria (left and right), two lesions were produced in the right ventricle and a further two lesions were produced in the left ventricle of each animal. Care was taken in all cases to ensure a large area of viable tissue separated the linear lesions. The positioning of lesions was consistent throughout but the ablation durations were alternated at each site to prevent systematic bias. Lesions were approximately 50 mm long.
Ablation procedure
A multielectrode handheld ablation device was used to produce all lesions (Cobra, Boston Scientific, Boston Scientific-EPT, San Jose, CA). This probe had seven electrodes including six 12.5 mm coil electrodes wound around a 7F circular shaft with an interelectrode distance of 2 mm. The distal electrode was 8 mm long. Simultaneous, in-phase, unipolar ablation was performed between selected electrodes and a large surface electrode. Radiofrequency current was provided by a 150-W generator (Boston Scientific). The electrode temperatures during ablation were controlled using two thermocouples attached to the margins of each electrode. A target temperature of 85°C was used for each energy delivery. The duration of radiofrequency energy application was either 1 or 2 minutes.
Lesion analysis
After completion of the ablation protocol the animals were killed and the hearts excised. Lesions were examined macroscopically and microscopically. Sections were excised in a plane perpendicular to the long axis of the lesions. The spacing of sections was 5 to 10 mm. Sections of each lesion (2.8 ± 0.5 sections/lesion) were incubated in a phosphate-buffered solution of blue tetrazolium containing succinate at 37°C for 30 to 45 minutes. This technique results in blue staining of viable myocardium. The sections were digitally photographed. Analysis of lesion width and depth were performed with computerized image analysis software (Scion Image; Scion, Frederick, MD) by two blinded observers. Where there was doubt about the extent of the lesion the sections were placed in formalin and sectioned further for microscopic analysis. Eight lesions were also examined microscopically to characterize the histopathological features of the lesions. Four sections were made from each lesion. Gomori’s Trichrome stain was used to identify the lesion margins. The width and depth of lesions were measured.
The study was approved by the Western Sydney Area Health Service Animal Ethics Committee and conducted in a manner conforming to the ethical and scientific principles set out by the National Health and Medical Research Council of Australia. All animals received humane care in accordance with the "Guide for the Care and Use of Laboratory Animals" (National Institutes of Health publication 85-23, revised 1985).
Statistics
Sections were averaged for each lesion before statistical analysis of lesion sizes. Expressions of transmurality describe individual sections. Analysis of variance was used to assess the effects of lesions site and duration of ablation on lesion volume. Differences were considered statistically significant if the p value was less than 0.05. Tissue thickness comparisons were performed using Student’s t test. Continuous variables are expressed as mean ± standard deviation.
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Results |
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The presence of epicardial fat modified lesion formation during epicardial application of radiofrequency energy. Fourteen of the 24 (58%) sections analyzed after application of epicardial radiofrequency current contained macroscopically visible epicardial fat. The thickness of the fat layer in these sections was 3.4 ± 2.0 mm. Myocardial lesions were detected in only six of the sections containing fat (43%). Lesions were detected in all sections in which no epicardial fat was present. Only three of 24 atrial epicardial lesions were transmural (13%). Where lesions were present the maximum lesion depth and width were 2.1 ± 1.1 and 6.0 ± 2.1 mm, respectively.
In contrast to the findings after epicardial ablation 92% (23/25) of endocardial atrial lesion sections were clearly transmural (Fig 2B). The two nontransmural sections were in unusually thick left atrial myocardium (5.7 ± 0.7 mm). The lesion depth in those cases was 4.4 ± 0.3 mm. The width of the endocardial lesions was 8.6 ± 2.0 mm.
Lesion formation in myocardium of moderate wall thickness: ovine right ventricle
No fat was noted on the epicardial surface at the ventricular lesion sites. The wall thickness of the right ventricles was 6.6 ± 1.5 mm. The epicardial lesion depth in right ventricular tissue (3.4 ± 0.6 mm) was smaller than the endocardial lesion depth in the right ventricle (3.9 ± 1.1 mm, n = 20) but this difference was not statistically significant. The epicardial lesion depth in the right ventricle was significantly smaller than the epicardial lesion depth in the left ventricle (4.3 ± 0.9 mm, p = 0.02, n = 20). There was a significant proportional relationship between right and left ventricular wall thickness and lesion depth during epicardial ablation (p = 0.003). This relationship was not observed in endocardial lesions. This finding suggests the cooling effect of endocardial blood flow may reduce the depth of lesions in thinner tissues. No significant difference was noted in the width of lesions produced by the epicardial (8.9 ± 1.7 mm) or endocardial (9.6 ± 3.7 mm) approach.
Lesion formation in very thick tissue: ovine left ventricle
The thickness of the left ventricular myocardium was 13.9 ± 2.6 mm. In left ventricular tissue, the depth of lesions produced by endocardial ablation was not significantly different to those produced by epicardial ablation. The endocardial and epicardial lesion depths were 3.8 ± 0.7 and 4.3 ± 0.9 mm, respectively. A trend was noted toward a reduction in lesion depth for endocardial lesions, but this difference was not statistically significant. This trend may be due to the lower baseline tissue temperature during endocardial ablation. The lower temperature during ablation is due to the prior administration of cold cardioplegia and the lack of blood perfusion. The widths of endocardial and epicardial lesions in the left ventricle were 8.1 ± 1.6 and 9.1 ± 2.7 mm, respectively (p = NS).
Effect of duration of ablation on lesion size
The small wall thickness of myocardium and the confounding influence of epicardial fat prevented the analysis of duration of ablation in the ovine atrial tissue. Therefore, analysis of the effects of wall thickness and duration of ablation on lesion development was performed using measurements of lesions from the right and left ventricles.
The target electrode temperatures were achieved in all radiofrequency energy applications. In all cases there was a small overshoot in the temperature. No significant increase was noted in lesion depth or width with the increase in ablation duration from 1 to 2 minutes. These results are illustrated in Figure 3. This finding was statistically independent of the endocardial or epicardial placement of the lesions or the position of the lesion in the right or left ventricular myocardium.
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Comment |
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Even when no epicardial fat was present, the epicardial lesions in atrial tissue were still smaller than those produced by application of radiofrequency energy on the endocardial surface and frequently not transmural. Epicardial radiofrequency application also produced smaller lesions in the ovine right ventricles, but the difference in lesion depth was less marked and did not achieve statistical significance. Endocardial and epicardial lesions were similar in size in the thicker left ventricular tissue, but epicardial lesions were smaller in the right ventricle than similar lesions in the left ventricle. These findings suggest the relative reduction in lesion size with epicardial ablation is inversely related to the myocardial thickness. This relationship may be due to endocardial cooling in the thinner myocardium. The cooling effect of circulating blood in the underlying cavity is likely to reduce lesion size during epicardial ablation. In the human atrial myocardium this effect may prevent lesions from penetrating the full thickness of myocardial tissue and create discontinuities in lines of ablation.
Duration of ablation
Increasing the duration of ablation from 60 to 120 seconds did not significantly increase lesion size. This finding is consistent with previous studies of lesion formation using radiofrequency energy in the myocardium [9–11]. These studies showed that most of the lesion depth is achieved in the first 15 to 30 seconds and that increases in duration of ablation beyond 60 seconds produce only a small increment in lesion size.
Lesion width
The optimal lesions for linear ablation are deep and narrow. Such lesions would provide electrical disruption along the line of ablation without excessive damage to contractile myocardium. We have previously shown that linear lesions similar to those produced in this study reduce atrial contraction [12]. All lesions in the present study were wider than they were deep. Width of lesions was independent of the thickness of tissue and the duration of ablation.
Optimal lesion depth
The Cox maze procedures septate the atria causing electrical isolation of adjacent areas of myocardium. When radiofrequency ablation was initially used to replace the incisions of the Cox operations, it was assumed that continuous transmural lesions would be required. The present study demonstrates that radiofrequency ablation applied through the handheld probe electrodes to the endocardial surface during cardioplegia produces lesions 3.9 ± 1.1 mm in depth in the right ventricle and 3.8 ± 0.7 mm in the left ventricle. Where lesions were not transmural in atrial tissue because the atrial wall was thick, lesion depth was identical to lesion depths in ventricular tissue. These findings indicate when radiofrequency energy is applied to the endocardial surface the probe is capable of consistently producing lesions that would be transmural in thin parts of the atria. Most of the human atrium is less than 5 mm thick, but structures such as the crista terminalis, the posterior left atrial free wall, and the trabeculations of the right atrial free wall in hypertrophic hearts are up to 6 mm thick [13, 14]. The results of the present study indicate that lesions in the thicker parts of the human atria are unlikely to be transmural even when produced from the endocardial surface. Patterns of linear radiofrequency lesions should be designed with the regional differences in atrial wall thickness in mind to reduce the risk of discontinuities in lines of ablation.
Even where there was no epicardial fat, lesions produced by epicardial ablation were often not transmural in the atria. Previous studies have demonstrated that conduction can proceed over a small surviving isthmus of tissue [15, 16]. Most linear ablation procedures for atrial fibrillation incorporate lesions designed to isolate the pulmonary veins electrically. Early experience with transvenous catheter techniques for pulmonary vein isolation has demonstrated that failure to isolate the veins adequately often leads to arrhythmia recurrence. This principle is also likely to apply to the surgical isolation of the pulmonary veins. Furthermore, gaps in linear lesions may also lead to the development of fixed reentrant circuits that manifest clinically as atrial flutter [2].
Implications for linear ablation in humans
The lesions created in this study were in normal ovine myocardium. Long-standing atrial fibrillation is characterized histopathologically by diffuse interstitial fibrosis. Scarring in the myocardium may alter the biophysics of radiofrequency ablation. However, a study of radiofrequency ablation in normal and scarred myocardium performed using the percutaneous catheter technique suggested this effect is small and unlikely to be clinically significant [17]. Another important difference between ovine and human atrial myocardium is the prominence of endocardial fibrosis in diseased human atria. The fibrosis may reduce the penetration of lesions formed from the endocardial surface.
The limited depth of lesions produced by radiofrequency energy restricts the patterns of lesions that may be used and may limit the use of this device for epicardial ablation. Probes capable of producing deeper lesions are necessary to provide greater versatility with lesion placement. Devices incorporating irrigated electrodes produce deeper lesions in the setting of percutaneous transvenous catheter ablation [18]. Further studies are required to confirm that this finding also applies to intraoperative ablation and determine whether the increased lesion depth results in improved outcomes.
Study limitations
The myocardial tissue was not reperfused before sacrifice of the animals in this study. The major determinant of lesion formation is direct thermal injury but damage to vessels coursing through the lesion and supplying adjacent areas of nonablated myocardium may produce ischemia and extension of the lesion on reperfusion. Nath and colleagues [19] demonstrated evidence of relative ischemia in the region around the limits of the pathologic scar produced by radiofrequency ablation. However, the effect of this relative ischemia on lesion size is unclear. Another study by the same group comparing lesion size in perfused and nonperfused myocardium showed lesion formation was independent of flow [11]. Large areas of infarction outside the expected limits of ablation have not been observed in chronic animal studies of linear ablation. Therefore any effect is likely to be small. Accurate determination of the influence of this mechanism on lesion size would require further study.
Conclusions
Lesions produced in this study were limited in depth and unlikely to be transmural in the thickest parts of the atria. Epicardial fat has an important negative effect on lesion formation. Where fat is absent, epicardially delivered radiofrequency energy still results in smaller lesions, possibly due to endocardial cooling by circulating blood. Radiofrequency lesions were wider than they were deep, so deep penetration of lesions into the myocardium can only be achieved by producing broad lesions with large volumes. Prolongation of the duration of ablation from 1 to 2 minutes does not significantly increase lesion depth. Patterns of lesions need to be designed that avoid areas of the atrial myocardium with a wall thickness of more than 3.5 mm and regions where epicardial fat is present.
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