| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Ann Thorac Surg 2004;78:308-311
© 2004 The Society of Thoracic Surgeons
a Department of Cardiovascular Surgery, Saiseikai Utsunomiya Hospital, Tochigi, Japan
Accepted for publication June 3, 2003.
* Address reprint requests to Dr Inoue, Department of Cardiovascular Surgery, Saiseikai Utsunomiya Hospital, 911-1 Takebayashi, Utsunomiya, Tochigi 321-0974, Japan.
e-mail: yosito_inoue{at}saimiya.com
Abstract
PURPOSE: Pulmonary vein isolation is a simple procedure, which has recently been reported as an effective treatment for the termination of atrial fibrillation. Although there are several clinical reports of beating-heart epicardial ablation, the optimal temperature has not been experimentally investigated. We evaluated the effective temperature for the placement of circular lesions around the pulmonary vein–left atrial junction.
DESCRIPTION: Twelve swine underwent epicardial ablation to create linear conduction block lesions around the pulmonary vein–left atrial junction by a seven-electrode ablation catheter. The ablation was performed at 60°C (group I), 70°C (group II), 80°C (group III), and 90°C (group IV) for 120 seconds. The creation of a firm conduction block across the ablated lesion under pacing was compared.
EVALUATION: Complete conduction block was observed in all groups except group I. However, heat injury to adjacent structures in group IV and transient discoloration of the tissue surrounding coronary arteries in groups III and IV were observed.
CONCLUSIONS: The effective temperature for epicardial radiofrequency pulmonary vein isolation was 120 minutes and above 70°C.
Current modification of the treatment of atrial fibrillation (AF), from producing multiple linear conduction block lines inside the atria [1] to minimal ablations for the electrical disconnection of pulmonary veins (PV) [2], is based on the concept of spontaneous initiation of AF by ectopic beats in the pulmonary veins [3]. In the surgical treatment of AF, PV isolation is performed instead of the maze procedure, and 60% to 80% midterm success rate have been reported [4].
Recently, surgical approaches to AF have substituted radiofrequency (RF) energy, in part, for surgical incisions to create conduction block lines. Epicardial application of the RF energy, which is applied for off-pump cases, is performed to treat AF in patients with nonvalvular heart disease, including coronary arterial disease [5].
Although this approach may allow us to widen the indications for the surgical treatment of AF in clinical practice, the epicardial RF ablation is performed without a sufficient fundamental study concerning temperature setting under beating-heart conditions. We evaluated the effective temperature under beating-heart conditions for the placement of circular lesions around the pulmonary vein–left atrial junction in swines.
Material and methods
Twelve domestic swines underwent epicardial RF ablation to create a linear lesion of the conduction block around the pulmonary vein–left atrial junction under beating-heart conditions. The swines (35.7 ± 8.2 kg; range 27 to 58 kg) were premedicated with xylazine (5 mg/kg), ketamine hydrochloride (5 mg/kg), and atropine sulfate (1 mg) administered by intramuscular injection. The animals were intubated and mechanically ventilated. The anesthesia was maintained with 1% halothane plus nitrous oxide gas and oxygen (50%/50%). The level of anesthesia, limb-lead electrocardiogram, femoral atrial pressure, and oxygen saturation were monitored throughout the procedure. All animals received humane care in compliance with the "Guide for the Care and Use of Laboratory Animals" published by the National Institutes of Health.
Via a median sternotomy, the experiment was performed. Target region around the posterior wall of the left atrium (LA) was prepared by the careful dissection of fat and connecting tissue. The pericardial reflections between the superior vena cava and upper right PV, between the oblique and transverse sinus, and between the lower right PV and inferior vena cava, were dissected.
Linear lesions were created with a seven-electrode ablation catheter (Cobra, Boston Scientific-EP Technologies, San Jose, CA) to electrically isolate PV (Fig 1). This circular ablation line was planned for the purpose of securing the area of LA pacing inside the conduction block line. The temperature was set at 60°C (group I, n = 2), 70°C (group II, n = 4), 80°C (group III, n = 3) and 90°C (group IV, n = 3). Radiofrequency energy was delivered for 120 seconds in a temperature-controlled mode by a 500-kHz RF generator (Cobra ESU, Boston Scientific-EP Technologies). Temperature, impedance, and wattage were monitored for each delivery of energy.
|
After completion of the procedure, the animals were sacrificed, and the lesions of LA and mediastinal organs were examined. The LA lesions were bisected longitudinally to assess the continuity and depth of the lesions and to determine whether they were linear and transmural or not.
Results
Epicardial RF ablation was successfully performed under a beating heart in all groups. The ablation procedure was completed within 27.1 ± 8.9 minutes (range, 15 to 47 mintes), and the number of applications of ablation energy was 7.2 ± 2.4. The average number of selected ablation coils at a single energy application was 3.6 ± 0.8 (range, 1 to 7). In group IV, a coagulum formation on the electrode was observed, as a result of overshooting. The RF generator sometimes stopped because of impedance rise, which resulted in the extension of the average procedural time.
Complete conduction block under LA pacing was observed in all groups except group I, by bipolar lead on both atrium and limb lead electrocardiogram.
The postmortem specimen showed linear heat-degenerated tissue around the pulmonary vein–left atrial junction (Fig 2). Pathologic evaluation revealed that ablation above 70°C attained the transmural linear LA lesions. At a higher magnification, myocytes at the ablated lesions were characterized by loss of nuclei, a normal striational pattern, and fiber dissolution (Fig 3). Incomplete transmural segments were observed at the ablated lesions in group I.
|
|
|
Comment
This experiment was performed to determine the optimal temperature for beating-heart epicardial RF ablation. And the results showed above 70°C was the effective temperature in this beating-heart swine model.
The factors affecting transmurality and continuity of the ablated lesion include electrode-tissue contact, duration of energy application, power output, and the tissue temperature. The tissue temperature that causes irreversible heat degeneration is 50°C to 55°C [6], and that required to produce permanent conduction block is 53°C to 58°C. Lesion depth increases in a linear fashion with electrode-tissue interface temperatures going from 50°C to 80°C [6]. The higher the temperature is, the deeper the lesion becomes, but it also requires more energy, and the obtained results indicated that a temperature above 70°C would be necessary to cause a conduction block from the epicardial surface. However, the temperature of the electrode-tissue interface should not exceed 100°C, because it leads to coagulum formation on the ablation electrode, a rapid rise in electrical impedance, and loss of effective tissue heating [6], which lead to interruption of the RF energy delivery, as observed in group IV.
A previous in vitro study indicates at least 120 seconds of energy application are required to maintain a temperature above 55°C with this Cobra RF System in order to create transmural lesion [7]. Although, unlike the interventional approach, there is no circulating blood directly cooling the electrode in this epicardial approach, there is an energy loss because of convective heat loss to the circulating blood pool at the endocardial side under beating-heart conditions. Convective heat loss necessitates increased time of RF application. As deeper tissue heating occurs as a consequence of passive heat conduction from the electrode-tissue interface, the duration of RF energy delivery increases the size of the lesion, as well as tissue temperature.
The results of this experiment, on the other hand, indicated the extension of thermal degeneration to adjacent structures by RF ablation. Among complications observed in this experiment, specific observation was the discoloration of the fat tissue along the coronary arteries during energy application at a temperature above 80°C. This phenomenon was observed during RF energy application at the roof of LA, without any direct contact between any of the ablation electrodes and the coronary arteries. It was different from the injury to adjacent tissue. The development of acute and chronic coronary artery complications after RF current application was reported, and previous experimental data showed chronic coronary artery soft plaque formation by intravascular ultrasonosound scan [8] and the occlusion of left coronary artery by embolic char [9] as a complication of RF energy delivery. The long-term effects of epicardial application of RF energy, which may yield microvascular injury extending beyond the region of acute coagulation necrosis [10], is not known. Although any apparent endothelial damage of the coronary arteries was not observed in the postmortem specimen, the cause and long-term effect of the tissue discoloration observed should be carefully evaluated in future studies.
Our experimental results indicated that epicardial application of RF energy by unipolar catheter potentially damaged adjacent organs above 80°C, especially when it is applied to the roof of LA. Because one of the causes of heat-induced injury is due to RF energy delivered in a unipolar fashion between the tip electrode and an indifferent electrode, the development of the flexible bipolar RF catheter may solve these problems in the future.
The limitations of this experiment include the differences between a normal swine heart and a human heart with chronic AF, which is sometimes thick and contains fibrous scars, especially in patients with valvular heart diseases. The present investigation was conducted in a swine model, which limits the generalization of the conclusions to human AF. And for the purpose of securing the area of atrial pacing inside the conduction bock line, the ablation line is produced circularly around the pulmonary vein–left atrial junction, which is not always the same as the procedure performed in clinical PV isolation. Moreover, the effectiveness of the procedure in the AF animal model was not evaluated. This experiment was performed with a unipolar RF catheter without saline irrigation. And the RF ablation was only performed for a duration of 120 seconds.
In conclusion, at a temperature above 70°C, conduction block was successfully created with the unipolar RF system under the experimental setting, and the histopathological study revealed formation of transmural linear heat degeneration of LA lesions.
Disclosures and freedom of investigation
The authors have performed a free and independent evaluation of this new technology. The authors have no financial relationship with Boston Scientific EP technologies.
Footnotes
Presented at the Poster Session of the Thirty-ninth Annual Meeting of The Society of Thoracic Surgeons, San Diego, CA, Jan 31–Feb 2, 2003.
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.
References
This article has been cited by other articles:
|
|
 |
|
  Y. Inoue Treatment of Atrial Fibrillation With a Right Thoracoscopic Approach Ann. Thorac. Surg., November 1, 2007; 84(5): 1795 - 1795. [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| ANN THORAC SURG | ASIAN CARDIOVASC THORAC ANN | EUR J CARDIOTHORAC SURG |
| J THORAC CARDIOVASC SURG | ICVTS | ALL CTSNet JOURNALS |
