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Right arrow Electrophysiology - arrhythmias

Eur J Cardiothorac Surg 2005;28:76-80
© 2005 Elsevier Science NL

Comparison of bipolar and unipolar radiofrequency ablation in an in vivo experimental model

Einar Bugge a , Ian Andrew Nicholson b , Stuart Philip Thomas a , *

Department of Cardiology, Westmead Hospital, Westmead, NSW 2145, Australia
Department of Cardiothoracic Surgery, Westmead Hospital, Westmead, NSW, Australia

Received 29 September 2004; received in revised form 1 February 2005; accepted 2 February 2005.

* Corresponding author. Tel.: +61 2 9845 6795; fax: +61 2 9845 8323. (Email: stuartpr{at}yahoo.com).


   Abstract
Top
 Abstract
1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 References
 
Objective: Linear atrial radiofrequency lesions have been used effectively for the treatment of atrial fibrillation. In most cases an endocardial approach has been suggested. A method for epicardial placement of lesions would reduce the complexity of these procedures. We compared lesions created in ovine hearts in vivo using irrigated bipolar or unipolar handheld radiofrequency ablation devices. Methods: Radiofrequency lesions were produced around a left pulmonary vein, around the left atrial appendage and in the free wall of the right ventricle in ovine hearts. All lesions were created in the beating heart. A bipolar clamping device (n=7) or a handheld unipolar device (n=6) was used. Measurements of local electrograms and pacing thresholds were performed before and after ablation at each site to assess the electrical integrity of lesions. Tetrazolium and digital image analysis were used to assess lesion geometry. Results: In atrial tissue continuous transmural lesions were achieved more often with the bipolar than with the unipolar device (92.3 vs. 33.3%, P<0.02). In atrial tissue the reduction in signal amplitude caused by the lesions was significantly larger with the bipolar than the unipolar device (87.6±9.4% vs. 60.6±23.7% reduction, P<0.01). There was a significant relationship between loss of pacing capture and lesion transmurality (P<0.05). The bipolar device created narrower lesions than the unipolar device (4.1±0.9mm vs. 5.9±2.1mm, P<0.001). Conclusions: The bipolar clamping device produces narrower lesions which are more likely to be transmural and lead to electrical isolation of ablated tissue than those produced by the unipolar device. However, both devices failed to consistently produce transmural lesions using the epicardial beating heart technique studied, particularly in thicker tissues. High output pacing within the ablated tissue partially predicts lesion transmurality and be a guide to the need for further ablation. However, endocardial ablation or transmural bipolar ablation are likely to remain the techniques of choice for linear radiofrequency ablation in the atria until improved techniques are developed.

Key Words: Ablation • Atrium • Arrhythmia • Radiofrequency • Arrhythmia surgery


   1. Introduction
Top
 Abstract
 1. Introduction
2. Materials and methods
 3. Results
 4. Discussion
 References
 
Despite the remarkable results achieved with the incisional maze procedure for treatment of atrial fibrillation [1,2] this method has not gained vast popularity. This could, at least in part, be due to both the complexity of the procedure and the technical skills required and the resulting prolongation in aortic cross clamp and cardiopulmonary bypass times. The success of the Cox Maze procedure and reports demonstrating that paroxysmal atrial fibrillation is usually due to focal electrical activity in the pulmonary veins, [3–5] led to the development of catheter based radiofrequency ablation for treatment of atrial fibrillation. In parallel, modifications of the maze procedure were developed using radiofrequency energy rather then incisions to septate the atria [6–11]. These procedures are technically less demanding and require less cross clamp and cardiopulmonary bypass times.

Electrical isolation of the pulmonary veins is required to prevent electrical activation of the atria from rapid pulmonary vein tachycardias. Continuous transmural lesions are most likely to achieve electrical isolation of the pulmonary veins and adjacent segments of the atria. Furthermore, devices capable of producing these lesions from the epicardial surface would provide a clear clinical advantage over those requiring endocardial ablation to isolate the pulmonary veins. Several handheld unipolar and bipolar devices have been designed for direct myocardial radiofrequency ablation. Choosing the correct device may be important for achieving optimal clinical results. Some reports describe the lesions produced with various probes [12–14], but there are no data comparing lesions produced from the epicardial surface with unipolar and bipolar devices.

The aims of the present study were twofold. Firstly, we compared tissue encircling lesions made from the epicardial surface using two different handheld saline irrigated radiofrequency ablation devices. Secondly, we assessed the usefulness of two simple electrophysiological measurements to predict lesion transmurality.


   2. Materials and methods
Top
 Abstract
 1. Introduction
 2. Materials and methods
3. Results
 4. Discussion
 References
 
Thirteen wether sheep weighing 45.3±5.7kg were used for the study. Anesthesia was induced with intravenous propofol (200mg) after a premedication of intramuscular Xylazine (10mg). Endotracheal intubation was performed, and intermittent positive pressure ventilation maintained with a Harvard Respirator (Harvard Apparatus, MA, USA). Anesthesia was maintained with isoflurane (1–3%). Heart rate, oxygen saturation, expired CO2 levels and cardiac rhythm were continuously monitored throughout the experiments. A thoracotomy was performed at the level of the fourth intercostal space and a pericardial cradle formed to expose the heart. The left pulmonary veins were dissected free to enable placement of the ablation device.

2.1. Ablation procedure and electrophysiological measurements
All ablations were performed on the epicardial surface of the beating heart. Continuous lines of ablation were made in each sheep around the antrum of the left superior pulmonary vein, around the left atrial appendage and, after tilting of the heart, in the free wall of the right ventricle. Before and after creation of each lesion, a decapolar Lasso electrophysiology catheter (Biosense Webster) was held in close contact around the pulmonary vein, around the tip of the left atrial appendage or flat on the free wall of the right ventricle, respectively. Bipolar electrograms were measured using a multichannel recorder (Prucka Engineering) and pacing threshold determined using a Micropace stimulator (Micropace, NSW, Australia). Pacing was performed at the pulmonary vein sheath, at the tip of the atrial appendage and within the ventricular lesion to test the electrical integrity of the lesions. Conduction block was defined as an inability to entrain the atria or ventricles during pacing from within the circular lesion at 25mA.

RF energy was delivered to the hearts using one of two different ablation devices both manufactured by the same company (Cardioblate/Cardioblate BP Surgical Ablation System; Medtronic Inc, MN, USA) one unipolar, one bipolar and both saline irrigated. The unipolar device has been described previously [9]. It consists of a handheld probe with a 4mm tip. Energy is delivered by passing radiofrequency current (30W) between this probe and a large diathermy electrode positioned on the skin. The tip of the device is moved over the epicardial surface of the heart 10 –15mm every second, back and forth at one second intervals over the same tissue for 15s resulting in an average ablation rate of approximately 1mm/s. The bipolar device is also described in detail elsewhere [15]; in summary, it incorporates two electrodes on separate arms of a malleable clamping device. Bipolar RF energy is delivered between the two arms. Power for both probes was delivered from a Cardioblate Generator (Medtronic Inc, MN, USA). The bipolar system monitors tissue impedance, ablating until a plateau in the impedance is reached which indicates the point of lesion transmurality.

2.2. Lesion analysis
After completion of the ablation protocol and electrophysiological measurements the animals were killed and the hearts excised. Sections of the ablated tissue were excised in a plane perpendicular to the long axis of the lesions. The spacing of sections was 5–10mm. Sections of each lesion (a total of 124 sections from 21 lesions, 3.6±1 sections/lesion) were incubated in a phosphate-buffered solution of blue tetrazolium containing succinate at 37°C for 30 to 45min. This technique results in blue staining of viable myocardium, while necrotic tissue remains unstained and appears grey. The sections were then digitally photographed for lesion analysis. Lesion width, depth and tissue thickness were analysed using computerized image analysis software (Scion Image; Scion, MD, USA). A lesion was considered transmural only if all sections from that lesion demonstrated transmurality.

2.3. Statistics
Lesion data and electrophysiological data for pulmonary veins and left atrial appendages are pooled as atrial tissue data for comparisons of the two devices. Electrogram amplitude measurements with the decapolar catheter were averaged for each location before statistical analysis of changes in amplitude. Data are presented as mean±SD. For statistical analysis of the difference in proportion of transmural lesions between devices, differences in the devices’ ability to isolate the tissue and for analysis of electrical isolation vs. lesion transmurality, Fisher's exact t test was used. Other comparisons were performed using Student's t test. Differences were considered statistical significant if the p value was less than 0.05.

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 and the European Convention on Animal Care.


   3. Results
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
4. Discussion
 References
 
3.1. RFA lesions
The duration of RF energy application was significantly shorter with the bipolar vs. the unipolar device (13±4s vs. 103±109s, P<0.0001). The tissue thickness in pulmonary veins (1.9±1.1mm), left atrial appendage (2.2±0.9mm) and right ventricle (4.8±1.9mm) was equal in the two groups. Lesion width and depth are shown in Table 1 . Lesions created with the unipolar device were significantly wider than those created with the bipolar device (4.1±0.9mm vs. 5.9±2.1mm, P<0.0001). As shown in Table 1, there were no differences between the devices concerning depth of the lesions.


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  		Table 1. Geometry of lesions with bipolar clamping device vs. unipolar pen
 
In atrial tissues, lesions created with the bipolar device were more likely to be transmural than those made with the unipolar probe (92.3% vs. 33.3% of lesions transmural, P<0.02). In right ventricular tissue few lesions were transmural with either device (2 of 7 lesions transmural with bipolar vs. 1 of 6 transmural with unipolar device, P=n.s.). Examples of lesions created with the bipolar and the unipolar device are shown in Fig. 1 .



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  		Fig. 1. Examples of lesions created in the right ventricle with the unipolar pen (panel A) and the bipolar clamping device (panel B).

 
3.2. Electrophysiological parameters
Before ablation there were no differences in local electrogram amplitude in any of the ablation sites between the two groups studied. In atrial tissues, the reduction in local electrograms after ablation with bipolar and unipolar devices was 87.6±9.4% and 60.6±23.7%, respectively (P<0.01). Also in atrial tissues, after ablation with the bipolar device 83.3% of lesions resulted in a reduction in signal amplitude of ≥80% while with the unipolar device this was the case in only 16.7% (P<0.01). There were no such differences in electrograms from the right ventricular lesions. Examples of electrograms before and after ablation are shown in Fig. 2 .



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  		Fig. 2. Examples of electrograms recorded from the left pulmonary vein before and after ablation. A and B: Before and after ablation with the bipolar clamping device. C and D: Before and after ablation with the unipolar pen.

 
Pacing thresholds were ≤2mA in all locations prior to ablation. In atrial tissue, 76.9% of lesions created with the bipolar device resulted in an inability to entrain the atria with pacing up to 25mA. This was achieved in only 8.3% of lesions made with the unipolar device (P=0.001). Again, there were no differences in right ventricular lesions. The pacing threshold could not be assessed after ablation in four animals (two in each group) due to ventricular fibrillation.

3.3. Prediction of transmurality
Inability to entrain the LA from the pulmonary vein, the tip of the atrial appendage or within the ventricular lesion with a high stimulator output (≤25mA) significantly predicted transmurality when assessed in all tissues pooled together (P<0.05).The positive and negative predictive values in atrial tissue were 0.92 and 0.58, respectively. In ventricular tissue the test was less reliable with the corresponding values being 0.33 and 0.75. There was no significant relationship between percentage reduction in signal amplitude post ablation and transmurality.


   4. Discussion
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
References
 
4.1. Lesions
In the present study all radiofrequency ablation lesions were performed from the epicardial surface of beating hearts. While the unipolar ablation device was moved around the circumference of the targeted myocardium, the bipolar device was clamped around the pulmonary veins, the left atrial appendage and a section of the right ventricle's free wall, respectively. Thus, while single layers of myocardial tissue were ablated with the unipolar device, a double layer of myocardial tissue was ablated with the bipolar device at each energy delivery. In spite of this important difference we found that the lesions in atrial myocardium resulting from the bipolar device were more likely to be transmural and lead to electrical isolation of the encircled tissue than lesions created with the unipolar device.

Our finding that the bipolar ablation device is able to produce transmural lesions in atrial tissue is in accordance with other studies using similar devices, both saline irrigated [14,15] and non-irrigated [13,16]. Most of these reported 100% continuous and transmural lesions. In these studies ablation was performed only in atrial tissue in experimental animals, and in one of the studies single layers of atrial myocardium were ablated. Transmural lesions were reported in tissue with a thickness up to almost 10mm [15]. One earlier study with ablation in atrial myocardium using a prototype of the bipolar device used in the present study reported lesion transmurality slightly less than 100%, consistent with our results [14].

The finding that lesions in the right ventricular free wall are not likely to be transmural even with the bipolar ablation device is important. Most parts of human atria are less than 5mm thick, but some structures like the crista terminalis, the posterior wall of the left atrium and trabeculated areas in the right atrium can be thicker in hypertrophic hearts [17,18]. The wall thickness of the ablated right ventricle was approximately 5mm in our study, which means that the bipolar device ablated tissue with an average thickness of almost 10mm (two layers of nearly 5mm each) when used on the right ventricle, as well as nearly 4 and 4.5mm thick tissue in the pulmonary veins and left atrial appendages, respectively. This indicates that even the bipolar clamping device cannot be expected to produce consistently transmural lesions in all parts of the human atria if used from the epicardium, clamping two layers of tissue.

The bipolar ablation device utilised in the present study monitors tissue impedance during ablation and uses an algorithm based on changes in impedance to assess lesion transmurality. This is described in detail by others [14,15]. Our findings with regard to lesions in thicker myocardium indicate that this method of assessing lesion transmurality is not sufficiently reliable to be used as the sole parameter for transmurality in thick myocardium if ablated only from the epicardial surface in the beating heart.

The unipolar ablation device created lesions that were not reliably transmural even in the thin atrial myocardium, a finding consistent with several other studies [19–20]. Our group recently reported that radiofrequency ablation lesions created with a handheld, non-irrigated device in an experimental model similar to the one we used are less likely to be transmural if performed from the epicardial rather than the endocardial surface [12]. One explanation for the inferior results for lesion transmurality with the unipolar probe could be the cooling effect from blood circulating in the underlying cavity. This effect is abolished by use of the bipolar clamping device. The absence of a difference in lesion depth between the two probes in the thicker myocardium of the right ventricle in our study is consistent with this hypothesis.

The optimal lesions for linear ablation are deep and narrow, thus providing electrical disruption along the line of ablation without excessive damage to contractile myocardium. It has earlier been shown that linear lesions similar to those produced with the unipolar ablation device in this study reduce atrial contraction in an animal study [21]. This effect is independent of the size of any electrically isolated region. Thus, the significantly less wide lesions produced with the bipolar ablation device may offer an additional advantage compared to the unipolar device. The lesion width with the bipolar device measured in this study is similar to that reported by others using the same device [15].

The radiofrequency ablation maze allows rapid septation of the atria [2,8,9]. The ablation time needed to create each lesion with the bipolar device in our study was very short compared to the unipolar device. However, the bipolar device may be difficult to correctly position in some cases, reducing the importance of this difference. The prolongation of the procedure required for ablation with the bipolar device during concomitant cardiac surgery remains small. Ablation with the unipolar device requires longer ablation time and it may only be effective from the endocardial surface.

Radiofrequency ablation with unipolar probes might lead to adjacent tissue injury, which can have serious clinical implications [22]. However, the bipolar device delivers energy between two closely apposed electrodes which focuses power allowing lesions to be completed within seconds. Also, the electrodes are placed under visual guidance, effectively eliminating the risk of such complications.

4.2. Electrophysiological parameters
The bipolar ablation device caused a greater reduction in local electrogram amplitude and ability to electrically entrain the left atrium than the unipolar device. These changes are likely to be due to the differences in morphology of lesions produced by the two devices. The findings relating to the bipolar device are also consistent with those of others who have studied similar instruments [13,14–16]. Also consistent with earlier studies we found that the ability to entrain the atria from the pulmonary vein by bipolar pacing predicted morphological transmurality of the lesions. However, this measure was not perfect. The reason for this imperfection may be a discrepancy between morphological and electrical integrity of linear lesions. Such a discrepancy has been previously observed experimentally [19]. The effect of lesion oedema on adjacent tissues may be a mechanism for this effect.

Late changes in lesion integrity may not be predicted by acute testing and the presence of electrically intact lesions may not ensure longer term procedural success. It is recognised from catheter-based isolation of pulmonary veins using radiofrequency energy that veins appearing isolated at the time of the initial procedure could demonstrate restored electrical continuity with the left atrium at a later date. However, in the majority of cases the acute result predicts long-term findings. Further it is recognised that the procedures may be successful without complete isolation of all four veins [23]. Nevertheless it is clear that ectopy from the pulmonary veins is usually responsible for initiation of atrial fibrillation and, therefore, electrical isolation of these veins remains a logical procedural endpoint.

4.3. Clinical implications
The morphological analysis performed during this study suggests that epicardial ablation with the devices studied is unlikely to consistently produce transmural lesions. This is particularly the case in diseased human atria which are thicker than those of the sheep used in this study [17,18,24]. Benussi et al. reported excellent surgical results from human procedures using an epicardial approach with several (not all) lesions performed off pump. However, only 50% (54/108) patients were in sinus rhythm and off antiarrhythmic medications after 6 months. The moderate failure rate may be due to a large the high proportion of incomplete lesions one would expect with this technique [25].

In conclusions, we have demonstrated in a sheep model of surgical radiofrequency ablation that a bipolar clamping device is capable of creating lesions significantly less wide and more consistently transmural than a unipolar ablation device. However, both devices were unable to consistently produce transmural lesions. The impedance algorithm used by the bipolar device could not reliably predict transmurality of lesions in thicker myocardium, and further parameters for assessment of transmurality are needed. High output pacing within the ablated tissue predicts lesion transmurality and may be performed as a guide to the need for further ablation. However, endocardial ablation or transmural bipolar ablation are likely to remain the techniques of choice for linear radiofrequency ablation in the atria until improved techniques are developed [12].


   Acknowledgments
 
The skilful technical assistance of Mrs Anita Boyd, Mrs Vickie Eipper and Mr Jim Pouliopoulos was appreciated.


   References
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 References
 

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