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Ann Thorac Surg 2006;82:2260-2264
© 2006 The Society of Thoracic Surgeons
Division of Cardiothoracic Surgery, Department of Surgery, College of Physicians and Surgeons of Columbia University, New York, New York
Accepted for publication April 7, 2006.
* Address correspondence to Dr Williams, Surgical Arrhythmia Program, 177 Ft Washington Ave, MHB 7-435, New York, NY 10032. (Email: mw365{at}columbia.edu).
| Drs Williams and Argenziano disclose that they have a financial relationship with Edwards Lifesciences.
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Abstract |
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DESCRIPTION: The surgical atrial fibrillation ablation procedure was performed through a left atriotomy; the pulmonary veins were encircled in 16 dogs using a diode laser (980 nm) with a diffusing tip that permits linear ablation perpendicular to the fiber direction. Lesion durations were 45 seconds with a power density of 3.8 or 4.5 W/cm. Six animals were allowed to survive 4 hours after the procedure, with the remainder sacrificed at 1 week (n = 1), 4 weeks (n = 3), and 6 weeks (n = 6). Electrophysiologic effectiveness was judged using unipolar or bipolar pacing from the pulmonary veins after attempting isolation. Hearts were harvested for histologic examination using standard trichrome staining.
EVALUATION: All animals tolerated the procedure. The animals required an average of 5.6 ± 0.82 lesions to complete the procedure. All animals had confirmed isolation of the pulmonary veins as judged by unipolar or bipolar pacing, and this isolation persisted in those animals that were allowed to survive. Pathology revealed all lesions to be transmural with an average tissue thickness of 3.62 ± 1.50 mm (range, 0.95 mm to 7.06 mm).
CONCLUSIONS: Diffusing tip laser technology reproducibly makes rapid, transmural, and electrophysiologically effective atrial lesions.
During recent years, the use of alternative energy sources, including cryoablation, radiofrequency, and microwave have been adopted as a means of performing Cox’s maze faster and less invasively. By decreasing operative times, technical difficulty, and the risks to the patient, the use of energy as an alternative to the scalpel has resulted in the resurgence of surgical treatment of arrhythmias, especially atrial fibrillation [1, 2].
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Technology |
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The purpose of this study is to evaluate the diffusing tip laser technology in surgical atrial fibrillation ablation using a canine model. This is the first study to describe the pathologic and histologic findings using a laser energy source.
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Technique |
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Healthy adult canines weighing between 25 and 35 kg were used. All animals were anesthetized using 17 mg/kg of intravenous Pentothal (Abbott Laboratories, Abbott Park, IL) and maintained with 1.5% to 2.0% of inhaled isofluorane. Using standard surgical procedures, an inferior sternotomy was performed. A baseline electrophysiology study was done. The animals were then placed on cardiopulmonary bypass through an ascending aortic and bi-caval cannulation. The left atrium was entered through a standard left atriotomy anterior to the right pulmonary veins. The heart was then arrested using 4 to 1 blood cardioplegia given in the aortic root. Next, endocardial lesions were created using preformed handheld delivery devices varying in length from 1 to 3.5 cm. The lesion set included circumferential isolation of the posterior left atrium including all the pulmonary veins with the left atriotomy serving as the right portion of the isolation. An additional lesion was made from the circumferential lesion to the mitral valve annulus. Retrograde cardioplegia was given during the creation of this lesion to prevent potential damage to the circumflex coronary artery. The left atrium was then closed in a standard fashion. After closure, warm blood was infused through the retrograde coronary sinus catheter and the aortic cross clamp was removed. After achieving a stable rhythm the animals were weaned from cardiopulmonary bypass and allowed to recover in normal sinus rhythm, without the aid of any inotropic agents. After this, a repeat electrophysiology study was performed. If the animals were intended to survive longer than 4 hours, they were closed in a standard fashion.
Sixteen canines were survived for periods of 2 to 4 hours (n = 6), 1 week (n = 1), 4 weeks (n = 3), and 6 weeks (n = 6). There was 1 canine mortality in the 4-week survival group. Differing survival times were established to assess electrophysiology at various points in tissue remodeling. At the time of sacrifice, a follow-up electrophysiology study was performed.
Laser and Diffusing Tip
All lesions were created using a diode laser with a wavelength of 980 nm (Optiwave 980 Laser Console, Edwards LifeSciences, Irvine, CA). The laser is coupled fiber optically to preformed diffusing probes (Optiwave 980 Endocardial Device [Edwards LifeSciences]). The diffusing tip on the probe contains scattering particles in a silicone matrix that directs the energy radially and perpendicular to the fiber direction. Continuous saline irrigation (ie, approximately 80 cc/hr) was delivered through the device to cool the gold foil that provides directional ablation. The energy was delivered for 45 seconds at an average power density of 3.8 W/cm for the first 12 animals and 4.5 W/cm for the last 4 animals.
Electrophysiology Protocol
Two methods were used to determine electrophysiologic effectiveness of the lesions. The first method used unipolar pacing from the isolated segment and was performed in the first 12 animals. It was observed that field conduction was likely with unipolar pacing. Therefore, to eliminate this effect and simplify data interpretation, bipolar pacing was used for the final 4 animals.
Unipolar protocol
Prior to placing the animal on bypass, unipolar pacing was performed on 4 extra-pericardial pulmonary veins using a pulse generator (Medtronic 3475 [Medtronic, Inc, Minneapolis, MN]). Baseline measurements were recorded prior to ablation and defined as the threshold required to capture the entire atrium from the pulmonary veins. Only sites capturing at a threshold less than 4 mA were used for subsequent pacing at the time points: 1 was immediately post-procedure and 2 was at time of sacrifice. To ensure that changes in subsequent threshold measurements were not induced by cardiopulmonary bypass and inflammatory processes, control threshold measurements were also recorded from the tip of the right atrial appendage. Because monopolar pacing was used, the possibility of field conduction existed. Thus, post-procedure capture greater than 7 mA was believed to be supraphysiologic and indicative of a successful isolation.
Bipolar protocol
Prior to initiating cardiopulmonary bypass, bipolar electrodes were secured to the right atrial appendage and to the left atrium near the right superior pulmonary vein. Bipolar pacing was then undertaken from additional pulmonary veins and the right atrial appendage. Confirmation of capture and suitability of the recording sites was then verified. After the procedure, but pre-sacrifice of the animal, pacing was again undertaken from the same sites. Isolation was considered successful if the recording electrodes on the pulmonary veins demonstrated only far field signals during a sinus or right atrially paced beat or a local cardiogram during pulmonary vein pacing without propagation to the remainder of the left atrium.
Lesion Analysis
After sacrifice of the animals, all hearts were harvested and characterized. Gross examination included inspection for signs of epicardial or endocardial wall disruption and charring. In addition, the surrounding tissue was examined for evidence of collateral thermal injury; if suspicious areas were found, they were also sectioned for histologic analysis. After completion of gross examination, the heart was sectioned for histologic analysis using trichrome staining.
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Pacing Measurements
Unipolar pacing
Of the sites with sufficient capture at baseline, pacing block was present after the isolation procedure in all sites; this block persisted in all animals that were allowed to survive. By our definition, pacing block never occurred in the right atrium, thus demonstrating that threshold increases were not solely due to post-procedure changes. By paired analysis, the absolute thresholds increased significantly from the pulmonary veins but not the right atrial appendage. This same difference persisted in the animals allowed to survive (Tables 1,
2).
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One animal received a burn at 4.5 W/cm on the pericardium with superficial damage to the overlying pulmonary pleura, but without evidence of parenchymal lung involvement. Despite the thin left atrial tissue (1.02 mm), there was no perforation at this site.
Microscopic evaluation revealed all lesions to be transmural. Acute lesions demonstrated evidence of early cell necrosis with some intramural hemorrhage. Chronic lesions had undergone fibrotic replacement of the necrotic myocardium with occasional associated inflammatory changes at the border zone (Fig 3). The transition between lesion and atrium was well defined with minimal collateral atrial injury. Some epicardial inflammation consistent with expected postoperative processes was present particularly in the 4-week specimens. The average lesion depth was 3.62 ± 1.50 mm (range, 0.95 mm to 7.06 mm).
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Comment |
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Laser energy represents another modality that can be used to effectively ablate cardiac tissue. In this canine atrial ablation model, laser energy coupled with diffusing tip technology consistently created electrophysiologically effective lesions based on unipolar as well as bipolar pacing studies. In addition, histologic analysis revealed 100% of the canine lesions to be transmural, validating this promising electrophysiologic data. These lesions were shown to be thin and well demarcated, with no peripheral spread. This indicates that the focused nature of the energy below the tissue device interface minimizes the collateral atrial tissue damage that can compromise contractility and induce pro-arrhythmia. This thermal mechanism does not create an obvious visual lesion, and as a result, greater care needs to be taken to ensure that the lesions are created continuously.
The future of this technology will be determined by its clinical efficacy. Among the first 8 patients at our institution to undergo surgical atrial fibrillation ablation using the laser energy source, 6 patients (75%) are in sinus rhythm at 1 year follow-up. Although small in number, these numbers suggest that laser energy may afford important advantages to previously described modalities. With more follow-up, the clinical safety and efficacy of this energy source can be determined.
Limitations
Several limitations of this canine model must be addressed. First, this study used two methods to determine electrophysiologic effectiveness. Although both methods provided satisfactory data on lesions, the bipolar protocol provided data that was easier to interpret. With the defined unipolar pacing measurements, block was demonstrated in all animals; however there was capture at higher thresholds. This was due to the ability of unipolar pacing to induce field conduction across lesions. To simplify the definition of electrophysiologic effectiveness, the protocol was changed to bipolar pacing, where the demonstration of a local pacing capture in the isolated segment without propagation to the remainder atrium is indisputable evidence of electrical isolation.
Two different laser measurements were also used: 3.8 W/cm and 4.5 W/cm. In the early studies we noted that the lesions were difficult to see after creation, which can compromise the completion and effectiveness of the intended lesion set. We sought to determine if a higher power would permit easier visualization of the lesions. The 4.5 W/cm setting resulted in more distinct lesions, but induced superficial damage on the lung of 1 canine in our study. This occurred in an area where the atrium was particularly thin near the inferior pulmonary veins. Although this is of concern to us, it is a potential problem with any energy source (including standard electrocautery), and has been reported in the clinical setting [3–5]. Human atrial tissue is thicker and requires a higher power to ensure transmurality of the lesions; the decided optimal setting is 4.3 W/cm for the Food and Drug Administration-approved laser probe clinically used. To date we have not experienced any adverse events in our clinical practice from any energy source. With all of the energy sources, care must be taken when creating lesions to avoid damaging structures directly behind the left atrium, particularly the esophagus. To prevent collateral injury, an insulating material, such as a sponge should be placed in the oblique sinus. A further safety precaution is to ensure that transatrial lesions do not directly overlie the esophagus by making them closer to the dome when superior and closer to the mitral annulus when inferior.
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Disclosures and Freedom of Investigation |
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The authors had full control of the design of the study, methods used, outcome measurements, analysis of data, and production of the written report.
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