Ann Thorac Surg 2008;85:300-303. doi:10.1016/j.athoracsur.2007.05.061
© 2008 The Society of Thoracic Surgeons
New Technology
Evaluation of a Novel Epicardial Atrial Fibrillation Treatment System
Andy C. Kiser, MDa,b,*,
L. Wiley Nifong, MDb,
Jai Raman, MD, PhDc,
Vigneshwar Kasirajan, MDd,
Nigel Campbell, DVM, PhDe,
W. Randolph Chitwood, Jr, MDb
a Division of Cardiothoracic Surgery, Pinehurst Surgical, Pinehurst, North Carolina
b Brody School of Medicine at East Carolina University, Greenville, North Carolina
c Division of Cardiothoracic Surgery, University of Chicago, Chicago, Illinois
d Division of Cardiothoracic Surgery, VCU Medical Center, Richmond, Virginia, North Carolina
e College of Veterinary Medicine, North Carolina State University, Raleigh, North Carolina
Accepted for publication May 22, 2007.
* Address correspondence to Dr Kiser, Pinehurst Surgical, 5 First Village Dr, PO Box 2000, Pinehurst, NC 28374 (Email: akiser{at}pinehurstsurgical.com).
| Drs Kiser, Raman, and Chitwood disclose a financial relationship with nContact Surgical.
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Abstract
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Purpose: Surgical and catheter treatments for atrial fibrillation remain invasive or ineffective for most patients. A novel system developed to create epicardial ablation lesions during beating-heart surgical procedures was evaluated in an in vivo ovine model.
Description: This novel ablation device integrates radiofrequency, suction, and perfusion to create transmural lesions by remaining consistently in contact with the irregular and curved surface of the beating heart.
Evaluation: Two epicardial ablation patterns were generated in five adult sheep: left atrial appendage and left pulmonary vein isolation. The 2-cm and 5-cm coagulation devices generated linear and curved lesions and maintained intimate contact against the epicardium using suction. Significant increases in bipolar pacing thresholds demonstrated trans-lesion conduction block in all animals. Histopathologic examination verified transmurality and showed changes normally observed after coagulation procedures. All lesions demonstrated mural degeneration throughout the lesion. No charring, vaporization, thromboembolic events, nor other complications were observed.
Conclusions: This novel epicardial coagulation system successfully created continuous and transmural atrial lesions in a beating-heart ovine model.
Atrial fibrillation is the most common cardiac arrhythmia. Surgical and catheter ablation procedures have been developed to minimize or eliminate the incisions of the "cut and sew" Cox Maze procedure. The Maze procedure, although highly effective, has significant morbidity and mortality effects [1, 2]. In contrast, less complex ablation procedures have fewer complications, but they are not as successful in most patients with atrial fibrillation [3, 4]. A novel radiofrequency ablation system (nContact Surgical, Morrisville, NC) has been developed integrating vacuum-assisted contact with fluid perfusion. We evaluated the system for ability to safely create transmural lesions on the ovine beating heart.
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Technology
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Coagulation System
The integration of energy, suction, and perfusion in this novel device (Fig 1) assures consistent tissue contact during beating-heart atrial fibrillation treatment (Fig 2). An insulating cover provides directional energy transmission and shields collateral tissue structures. The flexible and low-profile device conforms to the anatomic surface of the heart. Manipulation of the device can be done by manually shaping it or by guiding it with a tether.
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Technique
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Epicardial Coagulation: Treatment Model
Five sheep (40 to 50 kg) were anesthetized and ventilated to maintain a PaCO2 of 35 to 45 mm Hg. Throughout the procedure, analgesia and neuromuscular blockade were maintained and femoral arterial blood pressure was monitored. A left thoracotomy was created along the fourth intercostal space after sterile preparation. Left atrial appendage (LAA) and left pulmonary vein (LPV) lesion sets were generated on the epicardial surface of each beating heart to isolate a region of viable tissue from the remaining atria. The study was done in accordance with the principles stated in the "Guide for the Care and Use of Laboratory Animals" (NIH Publication No. 85-23).
Lesion Generation
Baseline pacing thresholds were established using a bipolar electrode for the LAA and LPV. Lesions were created on the LAA surface and around the LPV with 2-cm and 5-cm silicone-encased unipolar radiofrequency coil electrodes. Both of these maintained continuous myocardial to electrode engagement with the aid of integrated suction. In preliminary canine experiments, initial dose-response relationships (40 to 50 Watts for the 5-cm device and 20 to 30 Watts for the 2-cm device) and ablation durations (>90 seconds) for the creation of transmural myocardial lesions were determined. Energy application was adjusted to maintain <10% to 20% changes in impedance after the initial decrease and plateau seen during ablation procedures. During ablation, saline was aspirated through the coil electrodes and along the epicardium by the suction and thermal energy directed into the myocardium. The perfusion cooled the epicardial surface allowing more uniform energy transmission and deeper thermal penetration without charring the surface. A series of intersecting lesions were defined as: (1) a center of viable tissue within a "diamond" or "triangle" pattern on the LAA (Fig 3) and (2) an electrically isolated LPV. Transmurality and connectivity of the lesions were evaluated through conduction block and histopathology.
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Fig 3. Photograph of a "triangle" lesion pattern on the left atrial appendage with a center of viable myocardium.
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Conduction Evaluation
Using a bipolar electrode, pacing stimuli were transmitted to the center of viable LAA tissue and outside the LPV lesions to verify conduction block. The pacing current was initially set below the baseline-pacing threshold and gradually increased until the capture of atrial pacing, up to a maximum of 20 mA.
Post-Procedural Gross Pathology and Histopathology
After the coagulation and confirmation procedure, the animal was heparinized and euthanized. The lungs and esophagus were macroscopically inspected for immediate damage. The heart was excised en bloc and stained with triphenyltetrazolium chloride (Sigma Product Number T8877; Sigma-Aldrich, St. Louis, MO) to immediately determine the acute lesion extent and allow direct measurements of lesion dimensions. The lesions were bisected along their length and representative LPV specimens were embedded in paraffin, sectioned (5 microns thick), stained with hematoxylin and eosin, and examined microscopically at Charles Rives Laboratories (Frederick, MD) to assess the acute tissue response occurring immediately after tissue coagulation.
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Clinical Experience
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All LAA (n = 5) and LPV orifice (n = 5) lesions were generated successfully on the beating epicardial surface with sequential single applications of the devices. No thrombotic events or damage to the lungs or esophagus were observed. All lesions were visible on the epicardium enabling the creation of continuous lesion patterns. The representative lesions exhibited complete myocardial mural degeneration at histopathologic analysis.
Left Atrial Appendage
The mean ± standard error for final power setting, treatment time, and perfusion rate for each LAA linear lesion were 26.2 ± 3.6 Watts, 106 ± 8 seconds, and 5.5 ± 0.6 mL/min, respectively. The mean baseline pacing threshold for the LAA was 1.0 ± 0.2 mA, whereas the mean post-lesion pacing threshold increased to 18.4 ± 1.6 mA. The average length of the LAA lesion set was 8.4 ± 1.3 cm, and the average lesion width was 5.8 ± 0.3 mm. All LAA lesions were continuous and extended from the epicardium to the endocardium as evidenced by triphenyltetrazolium chloride staining (Sigma Product No. T8877; Sigma-Aldrich) (Fig 4). The mean pacing threshold after isolating a region of tissue was 18 times the baseline values indicating successful conduction block.
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Fig 4. Photograph of the left atrial appendage after triphenyltetrazolium chloride (Sigma Product No. T8877; Sigma-Aldrich, St. Louis, MO) staining of viable tissue. The brown discoloration of the nonviable area extends to the endocardium.
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Left Pulmonary Vein
Two to four intersecting LPV lesions isolated the pulmonary veins from the atria. The mean ± standard error power, treatment time, and perfusion rate were 17.6 ± 2.0 Watts, 120.2 ± 10.1 seconds, and 5.2 ± 0.7 mL/min, respectively. The mean baseline pacing threshold for the LPV was 1.6 ± 0.6 mA, whereas all post-lesion pacing thresholds were >20 mA. The mean lesion set length was 9.7 ± 0.7 cm, and the average lesion width was 5.3 ± 0.2 cm.
Representative histopathologic samples were prepared from the LPV-treated myocardium. Degenerative changes in the muscle (ie, shrunken, hyalinized muscle cells with dense, hyperchromatic nuclei) and denatured collagen were noted, but the mural architecture remained intact (Fig 5). All LPV lesions were continuous and extended from the epicardium to the endocardium as evidenced by the triphenyltetrazolium chloride histopathology and complete electrical isolation of the LPV.
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Fig 5. Representative hematoxylin and eosin stained histopathology of left pulmonary vein treatment regions showing degenerative changes in the muscle and denatured collagen with intact mural architecture post-treatment providing a basis to maintain the physical integrity of the tissue structure (x25).
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Comment
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A device with the ability to create continuous, full-thickness lesions on the epicardium of the beating heart is essential for the surgical treatment of atrial fibrillation. In this study, the efficacy and safety of a novel epicardial coagulation system that incorporates suction, perfusion, and radiofrequency ablation was established in an in vivo ovine model.
The reported means are reflective of the final power values. As tissue is heated, the impedance begins to decrease. If the temperature at the surface increases, tissue impedance will rise. Therefore, we adjusted the power during energy delivery to maintain impedance stabilization. One lesion on the LAA demonstrated atrial capture during conduction evaluation at 12 mA, whereas all the others did not capture below 20 mA. This may have been due to excitation of trabeculae adjacent to one of the lesions at higher voltages. Further, all ablation were performed for greater than 90 seconds with some initial lesions having times greater than 120 seconds. The mean ablation times are reported, but no differences in conduction block or transmurality of the lesions were demonstrated.
Although catheter ablation can damage the esophagus, phrenic nerves, and pulmonary veins [5–7], the targeted epicardial lesions generated on the LAA and along the LPV orifice did not damage adjacent tissue structures, and there was no evidence of thrombus formation on the endocardium. The integrated suction and perfusion features of the device provided constant contact with only the tissue being ablated. The device conformed to the epicardial surface and directed the radiofrequency energy into the myocardium. The ability to generate lesions through the muscle and fat of the epicardium (with the fat’s low electrical and thermal conductivity) can be a significant problem [8]. However, cooling the epicardial surface during energy delivery allows deeper thermal penetration and better creation of continuous and full-thickness lesions [9]. The coagulation device engaged the surface of the fatty tissue around the pulmonary vein without dissection. The ability to perform effective ablation through the fatty tissue demonstrated the effectiveness of the device.
One criticism of current atrial fibrillation treatment devices is the inability to create the continuous and transmural lesion patterns of the complete Cox Maze III procedure on the beating heart. The "clamp" devices are generally limited to pulmonary vein isolation and the "pen" devices are used primarily on the endocardial surface. "Cinches" and similar devices are only able to create "box-type" patterns encircling the pulmonary veins. The device evaluated here creates visible, transmural lesions at any location on the epicardium of a beating heart. This enables the development of a complete and totally epicardial maze pattern that can be confirmed immediately because the heart is beating and remains electrically active.
Many existing technologies lack the flexibility to generate variable lesion patterns and have difficulty producing continuous ablation lines. Another limitation of current devices is the inability to produce transmural lesions consistently. This may be due to inconsistent contact (i.e., inflexible electrodes, electrode instability on the tissue) or to the presence of fatty tissue (i.e., poor energy transmission through the tissue). Unlike these technologies, the novel soft tissue coagulation system described provides the elements that enable beating heart ablation procedures and addresses each of the previously mentioned criticisms. This system is being developed in conjunction with a more minimally invasive surgical approach that allows direct visualization and access, which may provide an effective alternative to catheter-based and open heart surgical treatments.
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Disclosures and Freedom of Investigation
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The authors disclose the financial reimbursement by nContact Surgical for time and expenses incurred during these procedures. Kiser and Chitwood disclose a financial relationship with nContact Surgical as consultants. 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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Acknowledgments
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The authors acknowledge and appreciate the editorial assistance of Cynthia Prichard.
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Footnotes
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Disclaimer The Society of Thoracic Surgeons, the Southern Thoracic Surgical Association, and The Annals of Thoracic Surgery neither endorse nor discourage use of the new technology described in this article.
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References
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- Khargi K, Hutten BA, Lemke B, Deneke T. surgical treatment if atrial fibrillation: a systematic review Eur J Cardiothorac Surg 2005;27:258-265.[Abstract/Free Full Text]
- Gillinov AM, McCarty PM, Marrouche N, Natale A. Contemporary surgical treatment for atrial fibrillation PACE 2003;26:1641-1644.[Medline]
- Tsai C, Tai C, Chen S. Pulmonary vein ablation role in preventing AF Curr Opin Cardiol 2003;18:39-46.[Medline]
- Kanagaratnam L, Tomassoni G, Schweiker R, et al. Empirical pulmonary vein isolation in patients with chronic atrial fibrillation using three-dimensional nonfluoroscopic mapping system: long-term follow-up PACE 2001;24:1774-1779.[Medline]
- Salerno-Uriarte JA, Lanzotti M, De Ponti R, et al. Ablation of paroxysmal and persistent atrial fibrillation Arrhythmias and electrophysiology 2007Available at: http://www.fac.org.ar/tcvc/llave/c014/salerno.pdf. Accessed November 19.
- Chung MK. Current clinical issues in atrial fibrillation. Cleve Clin J Med 70:S6–11.
- Tsao H, Chen S. Evaluation of pulmonary vein stenosis after catheter ablation of atrial fibrillation Card Electrophysiol Rev 2002;6:397-400.[Medline]
- Santiago T, Melo J, Gouveia RH, et al. Epicardial radiofrequency applications: in vitro and in vivo studies on human atrial myocardium Eur J Cardiothorac Surg 2003;24:481-486.[Abstract/Free Full Text]
- Atiga WL, Worley SJ, Hummel J, et al. Prospective randomized comparison of cooled radiofrequency versus standard radiofrequency energy for ablation of typical atrial flutter Pacing Clinical Electrophysiology 2002;25:1172-1178.
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Ann. Thorac. Surg. 2008 85: 303-304.
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303 - 304.
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Abstract
Introduction
