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Ann Thorac Surg 2005;80:2263-2270
© 2005 The Society of Thoracic Surgeons
a Division of Cardiovascular Surgery, Mayo Clinic, Rochester, Minnesota
b Medtronic, Inc, Minneapolis, Minnesota
Accepted for publication June 3, 2005.
* Address correspondence to Dr Schaff, Division of Cardiovascular Surgery, Mayo Clinic, 200 First St SW, Rochester, MN 55905 (Email: schaff{at}mayo.edu).
| Ms Lutterman and Mr Francischelli disclose that they have a financial relationship with Medtronic, Inc.
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Abstract |
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 Top Abstract IntroductionMaterial and MethodsResultsCommentThe Society of Thoracic...AcknowledgmentsReferences |
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METHODS: A modified Cox maze procedure was performed in 8 adult sheep during cardiopulmonary bypass using irrigated bipolar and unipolar radiofrequency ablation. For bipolar radiofrequency ablation, atrial tissues were clamped between opposing electrodes; ablation was terminated at the plateau in impedance decline. Unipolar radiofrequency ablation lesions were painted on the endocardium, and transmurality was assessed visually. Animals survived 30 days.
RESULTS: Bipolar lesions (n = 48) were thinner (7.4 ± 2.4 mm versus 12.7 ± 3.2 mm) and required less time (14.1 ± 3.4 seconds versus 41.4 ± 21.8 seconds) and energy (377.5 ± 99.2 W · s versus 995.1 ± 547.1 W · s) to create despite being longer (31.7 ± 8.6 mm versus 19.2 ± 5.6 mm) than unipolar lesions (n = 26). The left atrial pacing threshold across selected bipolar lesions increased at least fivefold above baseline (1.6 ± 0.2 mA) at 1 hour (18.4 ± 4.6 mA; n = 8; p < 0.001) and 30 days (17.2 ± 5.2 mA; n = 6; p < 0.001), indicating functional conduction block. Bipolar lesions had no adherent thrombus or endocardial defects. Cross-section examination confirmed transmurality in 100% of bipolar lesions and 98.7% of unipolar lesions.
CONCLUSIONS: Irrigated bipolar radiofrequency ablation with impedance monitoring safely and reliably produces electrically isolating, transmural Cox maze lesions in vivo.
The Cox maze procedure is a surgical technique developed to eliminate atrial fibrillation and reduce the risk of associated morbidity, especially stroke. A successful Cox maze procedure partitions the atria with multiple electrically isolating scars that create a preferential path for electrical signals to travel directly to the atrioventricular node while maintaining synchronized activation of the atrial myocardium. Favorable results have been reported in patients undergoing this procedure [1–3]. However, the risks associated with open heart surgery and cardiopulmonary bypass (CPB), as well as the technical complexity of the procedure, have prohibited its widespread application.
Recent interest has focused on simplifying the Cox maze procedure by replacing the multiple surgical incisions with linear lesions created by alternative energy sources. Handheld instruments have been developed that deliver focused bipolar radiofrequency ablation (RFA) energy between two opposing electrodes embedded in the jaws of a clamp [4, 5]. Focused energy delivery shortens ablation times, minimizes lesion width, and reduces the potential for adjacent tissue injury. In addition, these devices can be coupled with computer software platforms that monitor tissue impedance at the site of ablation, providing an immediate assessment of lesion transmurality.
Although bipolar RFA devices have shown promising results in large animal models [6, 7] and preliminary clinical trials [8], the utility of nonirrigated systems may be complicated by "coagulum" and "microbubble" formation at the tissue–electrode interface; both phenomena have been observed in previous studies of nonirrigated unipolar RFA [9, 10]. Coagulum may interfere with energy transfer to ablated tissue, resulting in partial-thickness or discontinuous lesions, whereas microbubbles cleave tissue planes and may disrupt tissue surfaces. Therefore, our laboratory has focused on irrigated RFA techniques to create transmural lesions [4, 11], and initial ex vivo results with a bipolar RFA device indicate transmural lesions can be created reliably with the assistance of continuous impedance monitoring to predict lesion transmurality [4]. The present study aimed to evaluate the feasibility and reliability of irrigated RFA and impedance monitoring to produce electrically isolating Cox maze lesions in a large animal model of open heart surgery.
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Material and Methods |
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Radiofrequency Ablation Equipment
All lesions were created with either a US Food and Drug Administration–approved irrigated bipolar or unipolar RFA instrument (Cardioblate Surgical Ablation System; Medtronic, Inc, Minneapolis, Minnesota). The bipolar device consisted of a handheld surgical clamp mounted on an articulated, rotating platform. The articulated platform provides approximately 90 degrees of flexion away from the long-axis orientation and 300 degrees of axial rotation. The clamp jaws were made with two opposing malleable electrodes that allow the surgeon to conform their shape to a desired orientation. Preliminary ex vivo studies have shown that slight jaw misalignment can be tolerated, up to 4 mm, without jeopardizing transmural ablation (unpublished data). Electrodes were fabricated from a stainless-steel hypotube and were embedded in a nonconductive porous polymer such that metal of the electrode would not directly contact tissue surfaces. During ablation, each electrode was infused continuously with 0.9% normal saline solution (3 to 4 mL/min) flowing outward from the electrode through the polymer from an external pressurized source. Radiofrequency ablation energy, therefore, was transferred from the entire length of each electrode to tissue surfaces through the irrigation fluid. To create each bipolar lesion, the area of atrial myocardium to be ablated was manually positioned between the clamp jaws to avoid tissue bunching or gapping. Firm clamp pressure was applied while irrigated RFA energy was delivered between the electrodes.
The specifications of the irrigated unipolar device have been reported previously [11]. A 10- x 10-cm ground plate was placed in contact with the left hemithorax of animals during the procedure to complete the ablation circuit. Irrigated RFA energy was delivered through steady oscillating movements of the electrode tip over a 1-cm area of tissue. The electrode tip was applied to the endocardial surface with firm pressure to ensure a continuous electrode–tissue interface.
A single RFA generator supplied energy to the devices during ablation. The surgeon controlled energy flow by operating a foot pedal connected to the generator. During irrigated bipolar ablation, a proprietary transmurality feedback program within the RFA generator monitored tissue impedance between electrodes and varied the power delivery during the time course of ablation according to a preset algorithm. Ablation was terminated when the transmurality feedback program detected a steady-state plateau in tissue impedance, indicating full-thickness ablation (Fig 1). During unipolar ablation, the generator delivered 25 W of power to the electrode continuously until ablation was terminated. Unipolar RFA lesion transmurality was determined by visual inspection of the atrial tissue.
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The heart was then paced from the right atrial (RA) body and PVs, and the baseline pacing threshold was recorded. The animal was then anticoagulated with intravenous heparin (300 U/kg), and an activated clotting time longer than 300 seconds was maintained throughout the procedure. The inferior vena cava, SVC, and left femoral artery were cannulated for CPB. A cardioplegia needle was secured to the aortic root, and mild hypothermic (32°C) CPB was initiated at 50 to 70 mL · kg–1 · min–1, titrated to maintain a mean arterial pressure greater than 50 mm Hg. The aortic root was cross-clamped, and the heart was arrested with a cold crystalloid cardioplegia bolus (10 mL/kg); a second dose (5 mL/kg) was given 20 minutes after the first to maintain arrest. Because the animals consistently had a persistent left-sided SVC producing torrential blood flow through the coronary sinus, the coronary sinus ostium was temporarily occluded once the RA was opened to provide a dry operative field.
Cox Maze Procedure
Exposure of the atrial endocardial surface was achieved through a 4-cm right atriotomy through the RA body and appendage and a left atriotomy in the interatrial groove. The right atriotomy stopped approximately 1 cm from the anterior tricuspid valve leaflet. The left atriotomy extended from the inferior border of the left PV confluence, around both right PVs, to the superior border of the left PV confluence (Fig 2). A 1-cm counterincision was also created at the confluence of the inferior vena cava and SVC to facilitate creation of bipolar lesions on the RA.
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Unipolar lesions were created next on the RA and LA. Two lesions were created on the RA: posterior tricuspid valve lesion, which originated from the distal extent of the bipolar RA body lesion, and anterior tricuspid valve lesion, which originated from the anterior corner of the right atriotomy. These lesions ran down to the tricuspid annulus. A single LA lesion was fashioned across the mitral isthmus, running from the inferior corner of the left atriotomy to the posterior mitral valve annulus. An additional small LA lesion was added to connect the distal extent of both PV lesions as needed to completely isolate the PVs.
Surgical Procedure Conclusion
After completion of the Cox maze lesions, air was removed from the left side of the heart, and the left atriotomy was closed. The aortic cross-clamp and coronary sinus tourniquet were released. Next, air was removed from the right side of the heart, and the right atriotomy and counterincision were closed. Ventricular fibrillation ensued in all cases and was converted to normal sinus rhythm with direct-current cardioversion and an intravenous lidocaine bolus (1 to 2 mg/kg), repeated as necessary. The animal was weaned from CPB, and the CPB cannulas were removed. Anticoagulation was reversed with intravenous protamine sulfate administration (1 mg/100 U heparin). Animals were allowed 1 hour to stabilize hemodynamically before further investigations were conducted.
Functional assessment of acute conduction block produced by the PV lesions was performed by measuring the pacing threshold from the right PVs. The pacing generator rate was set at least 20 beats per minute higher than the intrinsic heart rate, and pacing amplitude was gradually increased until either maximum amplitude (20 mA) was reached or atrial capture outside the lesion occurred. The thoracotomy was then closed in layers, and animals were allowed to recover from anesthesia.
Terminal Procedure
Animals were allowed to survive for 30 days. Studies of RFA lesions have indicated that in this time span myocardial changes mature to a point resembling their final appearance [12]. Animals were again anesthetized, and the heart and PVs were exposed through a left fourth intercostal space anterolateral thoracotomy. Long-term conduction block produced by the PV lesions was assessed by repeating pacing threshold measurements from the right PVs. A 14-gauge angiocatheter connected to a pressure transducer was inserted into the LA to obtain an amplified tracing of the central pressure waveform during 20 electrocardiographic cycles. This tracing was assessed qualitatively for the presence of atrial 
waves as an indicator of preserved LA contractile function.
Animals were euthanized, and the heart and great vessels were excised en bloc.
Lesion Evaluation
Hearts and venae cavae were examined grossly for the presence of endocardial or intimal disruption, respectively, or thrombus formation associated with any of the lesions. The coronary arteries, coronary sinus, and persistent SVC were examined for evidence of ablation and stricture. Hearts were then stained in 1% triphenyltetrazolium chloride [13–15] for at least 45 minutes. The atria and venae cavae were sectioned, taking care not to disrupt lesions. Each lesion and a small cuff of nonablated tissue was dissected from the surrounding atrial and venous tissue and sectioned into 5-mm cross sections along the entire length of the lesion. Standard histologic slides were prepared from each lesion cross section stained with Masson trichrome stain.
Each lesion cross section was assessed grossly and microscopically for transmurality. Digital static photographs of the endocardial or intimal surface of all lesions in situ as well as lesion cross sections were prepared using a ruler as reference for scale. Lesion width and length were measured from the digital images using a computerized software platform (SPOT RT Advanced Mode; Meyer Instruments, Houston, TX).
Data Analysis
The duration and total energy (power x time) for ablation as well as dimensions for each lesion were summarized as mean ± standard deviation (range). Lesion transmurality is reported as the percentage (95% confidence interval) of lesion cross sections that were completely transmural. For assessment of functional conduction block across the PV lesions, acute and long-term pacing thresholds (milliamperes) were compared with baseline values using Student's t test for paired results. Values of p less than 0.05 were considered statistically significant.
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Results |
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TopAbstractIntroductionMaterial and MethodsResultsCommentThe Society of Thoracic...AcknowledgmentsReferences |
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Devices were able to access all areas of the atria needed to successfully create each lesion in a single application. However, large LA size in 2 animals necessitated the addition of a small unipolar lesion connecting the distal extents of the PV lesions to completely isolate the PVs from the LA. The RFA system functioned reliably in each trial with no equipment failures occurring throughout the study.
Efficacy
Total irrigated RFA energy delivered to tissues was 5,561.8 ± 1,407.9 W · s (3,340.0 to 7,068.0 W · s). Less total time and energy were required during complete ablation for bipolar lesions than for unipolar lesions (Table 1); however, a direct statistical comparison between bipolar and unipolar lesions is invalid because different lesions were created using the two devices.
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waves on central venous pressure tracings.
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show representative gross photographs of intact lesions and lesion cross sections and micrographs of lesion cross sections.
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No endocardial thrombus or full-thickness atrial disruption was associated with any of the lesions. Both the left circumflex and right coronary arteries were patent and uninvolved by RFA in all animals. The back wall of the coronary sinus was involved by the mitral isthmus unipolar lesion, and the persistent SVC was circumferentially involved by the inferior PV bipolar lesion in all animals. Neither vessel had any thrombus or stricture produced from this injury.
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Comment |
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TopAbstractIntroductionMaterial and MethodsResultsCommentThe Society of Thoracic...AcknowledgmentsReferences |
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The Cox maze procedure has been performed for more than 25 years to treat atrial fibrillation, with multiple institutions (including our own) reporting excellent long-term results [1–3]. Surgeons have not widely used this highly successful procedure, however, because of its invasiveness, its technical complexity, and the potential risk of morbidity associated with CPB and open heart surgery. To overcome these limitations, simpler alternative means have been investigated for creating full-thickness lesions to replace the Cox maze incisions while maintaining the efficacy of the technique. Previous studies have demonstrated the effectiveness of both irrigated and dry unipolar RFA energy to create transmural, linear ablation lines in the preclinical [11] and clinical [16, 17] settings. But uncertainty regarding the depth of ablation and recent reports of serious adjacent organ injury using dry RFA, including esophageal perforation, have called into question its clinical utility [18, 19]. As a result, our laboratory, as well as those of others, has begun examining bipolar RFA energy sources for application in the Cox maze procedure.
Advantages of Bipolar Radiofrequency Ablation
Several advantages of bipolar RFA technology make it an appealing alternative to unipolar systems. Bipolar devices focus ablation energy between two closely opposed electrodes. As the present study demonstrates, focusing the energy in this way reduces the overall energy needed to create bipolar lesions and minimizes the lesion width. Focused ablation energy causes tissue to quickly reach therapeutic temperatures (50° to 60°C) capable of producing irreversible tissue damage while limiting the thermal spread beyond the region of the electrodes. Indeed, previous ex vivo studies with the bipolar device used in this investigation demonstrate temperatures in adjacent atrial myocardium remain less than 50°C during energy transfer [20]. Limited thermal spread helps minimize the possibility of adjacent organ or vessel injury seen with unfocused unipolar sources [7, 21]. The clamp design of bipolar instruments ensures consistent tissue contact over the entire length of the electrodes, resulting in more-uniform transmural ablations performed in seconds, not minutes [5, 7, 22].
Advantages of Irrigated Radiofrequency Ablation
For this study we chose an irrigated bipolar RFA energy source on the basis of previous experiments with unipolar instruments demonstrating superiority of irrigated systems. When electrode–tissue interface temperature exceeds a critical value during nonirrigated ablation, tissue desiccation and protein denaturation form an insulating layer referred to as coagulum on the dry electrode surface. Coagulum interferes with energy transfer, resulting in an impedance rise and interruption of further tissue ablation. Furthermore, coagulum is potentially thrombogenic, forming a nidus for future emboli [23]. Microbubble formation can also accompany increasing tissue–electrode temperatures from evaporating interstitial fluid. Release of accumulated microbubbles cleaves tissue planes and can disrupt tissue surfaces. Continuous irrigation during RFA can prevent or prolong time to coagulum formation, impedance rise, and microbubble formation [24], likely because of dissipation of heat within the irrigation fluid.
Disadvantages of Irrigated Radiofrequency Ablation
Compared with a similar bipolar RFA system, total time for ablation and overall width were greater for lesions in our model [5]. This discrepancy may be related to differences in design and size of the electrode filament as well as to the study model. However, dissipation of heat with irrigated RFA energy transfer is associated with slower rise in tissue temperature and higher maximum applied power. This is known to result in longer ablation times and more voluminous lesions than nonirrigated energy sources [9]. Therefore, we would expect longer ablation times and wider lesions than reported for a nonirrigated bipolar RFA system. Because the discrepancies in time and width are quite small, we would not expect these experimental differences to have a profound impact in the clinical setting.
Efficacy
In the present study, both irrigated RFA systems functioned reliably without mechanical failure, and all desired lesions were easily created in the open heart. Functional conduction block was achieved in all animals as demonstrated by the significant rise in pacing threshold across the PV-isolating lesions. This is consistent with findings from similar studies of nonirrigated bipolar or unipolar RFA [6, 8, 11]. A single unipolar lesion applied in the region of the anterior tricuspid valve leaflet was found not to be fully transmural. Unfortunately, the only indicator of transmurality for unipolar lesions at the time of operation is visual inspection of ablated tissue. This finding highlights the imperfection of unipolar RFA systems and underscores the advantage of bipolar systems that provide reliable indicators of transmurality. As in our previous studies, the plateau in impedance decline consistently predicted lesion transmurality for irrigated bipolar lesions. Histopathologic examination verified the completeness and transmurality of these bipolar lesions.
In no instance was atrial tissue dehiscence or thrombus formation associated with any of the lesions. Also, adjacent structures, including the left circumflex and right coronary artery, were not involved by ablation. Interestingly, all animals had a persistent SVC that coursed adjacent to the left inferior pulmonary vein, and in every case, this vessel was circumferentially ablated by the bipolar instrument during creation of the inferior pulmonary vein lesion. At study termination, the vessel remained patent without evidence of stricture, thrombus, or aneurysm. This provides preliminary indication that irrigated bipolar RFA may be applied across adjacent venous structures such as the coronary sinus. However, our study design does not allow us to draw the same conclusion in regard to the coronary arteries or valve annuli because bipolar RF energy was not applied directly across these structures. Future studies addressing this limitation are being planned because the ability to safely apply bipolar RFA across coronary vessels and valves would simplify the technique even further by negating the need for additional unipolar lesions and would improve the feasibility of irrigated bipolar RFA for off-pump, beating-heart procedures.
Study Limitations
This in vivo study has several limitations requiring cautious application of results to the clinical setting. Human and sheep anatomy differ considerably, including orientation of mediastinal structures and relative distances to adjacent tissues. Also, normal atrial tissues were studied, which vary in thickness from diseased human atrial tissue. Although we did qualitatively assess chronic atrial contractility in all animals, this model did not permit a thorough quantitative assessment of LA transport function. This function is an important indicator of the degree of collateral atrial damage produced by RFA energy sources during creation of Cox maze lesions. Finally, functional conduction block was produced by irrigated RFA lesions in this model. Although we did not attempt to induce atrial fibrillation in these animals to assess whether these lesions could prevent this arrhythmia because we re-created all lesions of the Cox-maze III, we anticipate lesions created by irrigated RFA will be efficacious.
Conclusion
In summary, this study demonstrated an efficacious method for creating electrically isolating atrial lesions during CPB using a combination of irrigated unipolar and bipolar RFA. Real-time impedance monitoring during ablation reliably predicted transmurality of irrigated bipolar RFA lesions. Preliminary evidence suggests that irrigated bipolar RFA may be applied effectively across coronary venous structures without deleterious effect. Further studies are needed to determine the safety and efficacy of bipolar RFA applied across coronary arteries and valve annuli, as this technique would negate the need for additional unipolar lesions and improve the feasibility of irrigated bipolar RFA for off-pump, beating-heart procedures.
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