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Ann Thorac Surg 2004;78:1056-1059
© 2004 The Society of Thoracic Surgeons
a Department of Cardiovascular Surgery, University of Kyorin, Tokyo, Japan
b Department of Cardiothoracic Surgery, University of Tokyo, Tokyo, Japan
c Department of Cardiac Surgery, Tokyo Metropolitan Fuchu Hospital, Tokyo, Japan
Accepted for publication July 10, 2003.
* Address reprint requests to Dr Kubota, Department of Cardiovascular Surgery, University of Kyorin, 6-20-2, Shinkawa, Mitaka-shi, Tokyo, 181-8611 Japan
kub{at}kyorin-u.ac.jp
Abstract
PURPOSE: After it was shown that a rapidly firing focus in a pulmonary vein (PV) can cause atrial fibrillation, percutaneous endocardial PV isolation using radiofrequency began to be used as a method of treatment. However, this technique is time consuming. It requires fluoroscopy and contrast media to identify the PV, and cardiac tamponade and PV obstruction are major complications. To overcome these drawbacks, we developed a hook-shaped cryoprobe to enable circumferential ablation of PV orifices epicardially. The aim of this experimental study in dogs was to confirm the efficacy of this method electrophysiologically.
DESCRIPTION: Five mongrel dogs (32 PVs) were used. Surrounding tissue was dissected to expose all PVs and their orifices into the left atrium. Each PV was stimulated with an electrode to measure the length of PV which has the same pacing threshold as the left atrium.
EVALUATION: The mean distance from the PV orifice to the pacing boundary line was 8.9 ± 1.3 mm. Encircling cryoablation was performed with a hook-shaped cryoprobe to circumferentially ablate each PV orifice epicardially.
CONCLUSIONS: Cryoablation by this method created a bidirectional conduction block in all PVs. All PVs were electrically isolated, and the PV isolation was achieved epicardially without atriotomy. This method should enable less invasive treatment of AF clinically.
After it was shown that a rapidly firing focus in a pulmonary vein (PV) can cause atrial fibrillation, percutaneous endocardial PV isolation (ablation) using radiofrequency became an established method of treatment. However, the technique is time consuming. It requires fluoroscopy and contrast media to identify the PV, and cardiac tamponade and PV stenosis (obstruction) are major complications. To overcome these drawbacks, we tried using cryoablation. Although the effectiveness of cryoablation has already been demonstrated in the modified maze procedure, its effectiveness by the epicardial approach in the beating heart was unknown. We developed a hook-shaped cryoprobe to circumferentially ablate PV orifices epicardially. The aim of this study was to confirm the efficacy of this method electrophysiologically in an experimental study in dogs.
We used a cryosurgical system PCG12R (CO2, –60 degrees; Spembly Medical Ltd. Hampshire, UK) to create the cryolesions. A novel hook-shaped cryoprobe, 20-mm long and 3-mm wide, was developed to facilitate the PV ablation (Fig 1).
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Five adult mongrel dogs weighing 14.2 ± 2.5 kg were anesthetized with ketamine hydrochloride (20 mg/kg, IM) and sodium pentobarbital (16 mg/kg, IV), and ventilated. All animals received humane care in compliance with the "Guide for the Care and Use of Laboratory Animals" published by the US National Institutes of Health (National Institutes of Health publication 85 to 23, revised 1985). The pericardium was opened through a median sternotomy, and the surrounding tissue was dissected to expose all PVs and their orifices into the left atrium. Since each dog had six or seven PVs, a total of 32 PVs were examined.
Before cryoablation, the pacing threshold of the left atrium was determined by stimulation with a bipolar electrode. Next, each PV was stimulated. The electrode was slid from the ostium to the distal site of the PV little by little, and the area of each PV that had the same pacing threshold as the left atrium was identified (Fig 2). Encircling cryoablation was then performed with the hook-shaped cryoprobe to circumferentially ablate the orifice of each PV epicardially. The circumferential cryolesions were created by passing the probe behind the orifice of the PV. The PV was compressed and flattened by pulling the probe upward when the cryoablation was done. The probe was applied to each orifice for 90 seconds (Fig 3).
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Changes in cardiac rhythm
Before and during ablation, bipolar electrodes were placed on the PV, left atrium, left atrial appendage, and right atrium, and the heart was paced through the electrode on the PV. The electrical potentials at each site were recorded (n = 16), and they were also recorded during PV pacing two hours later.
Results
The mean distance between the PV orifice and the pacing boundary was 8.9 ± 1.3 mm. All PVs had the same pacing threshold as the left atrium at some distance. Twenty-nine of the pacing boundaries were present on the pericardial reflection or in the pleural cavity; the pacing boundaries of only three PVs (left superior) were in the pericardial space. When a pacing site was positioned beyond the boundary, the pacing threshold suddenly became extremely high.
Immediately after commencing the cryoablation, the potential of the PV diminished (Fig 4), and it had not recovered two hours later. Immediately after commencing the cryoablation, the cardiac rhythm converted from the pacing rhythm to the sinus rhythm (Fig 5). The effectiveness of the ablation persisted two hours later.
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Radiofrequency endocardial PV ablation became widely adopted after Haïssaguere and colleagues and Pappone and colleagues [1, 2] reported that rapidly firing foci could be identified in and around the PVs, and that ablation of such foci might lead to resolution of paroxysmal AF. However, the procedure depends on fluoroscopy and the use of contrast medium to identify the PVs. The electrophysiologic studies are time consuming, and cardiac tamponade and PV obstruction are still major problems. Our experiment was designed to overcome these problems.
We first investigated whether the PVs of mongrel dogs have the same electrophysiological characteristics as human PVs. Most dogs have 6 to 7 PVs, and a total of 32 PVs were studied. All PVs were found to have the same electrical characteristics as the left atrium at some distance, and almost all PVs had their electrical boundary on the pericardial reflection on the PVs or after they passed into the thoracic cavity. Although no histopathologic examinations were performed in this study, these findings suggested that the myocardium extends into the PVs of dogs, the same as in humans. Based on these findings, we concluded that mongrel dogs could be used as a model to verify the efficacy of ablation.
Since the PVs were exposed surgically in our experiment, fluoroscopy and contrast medium were not required to identify the PVs, and exposure of the PV orifices was easy. Cryoablation does not injure the endocardium or epicardium, and thus there is no possibility of cardiac tamponade as a result of perforation of the atrial (PV) wall. Moreover, cryolesions do not cause PV obstruction (= myocardial atrophy) even in the chronic phase, because ablating the tissue does not involve any carbonization or vaporization. By contrast, all of the ablation devices that coagulate tissue by means of heat energy, such as radiofrequency, LASER, and infrared coagulators, [3] cause tissue shrinkage, which, in turn, causes PV obstruction.
The potential of the PVs diminished immediately after the start of ablation, and the PV pacing rhythm immediately converted to sinus rhythm. These phenomena demonstrate that the epicardial cryoablation had created a bidirectional conduction block. Although the irreversibility of the effectiveness of ablation was confirmed two hours later, a chronic study is needed to demonstrate the permanence of the effect of the cryolesion.
The effect of epicardial cryoablation is said to not reach the endocardium of the beating heart, because the normothermic blood flow within it weakens the cryoeffect [4]. However, we have shown that the cryoablation in the warm beating heart is effective only at sites that can be compressed by the cryoprobe, thereby eliminating blood flow with it. When the hook probe was pulled upward, the PV was flattened and the blood flow within it was completely eliminated. Flattening a PV can create a "local arrested heart," that may be why cryoablation is able to produce an encircling lesion in a "single" application and create a bidirectional electrical block at PV orifices. Although no histopathologic examinations were performed in our study, the electrophysiologicl findings suggest that the PV wall was ablated transmurally.
These results of our preliminary experiment suggest that cryoablation is capable of overcoming some of the drawbacks of the percutaneous endocardial approach. Modification of the device will allow thoracoscopic epicardial ablation of the PVs.
Conclusions
Using a hook-shaped cryoprobe made it possible to easily create a bidirectional conduction block in PVs. Our method does not require fluoroscopy or contrast medium, and it does not cause cardiac tamponade or PV obstruction. It can be performed epicardially without atriotomy. This minimally invasive method of epicardial cryoablation in the beating heart may provide an effective and secure means of clinically converting isolated atrial fibrillation into sinus rhythm. Experience is needed in a clinical setting.
Disclosures and freedom of investigation
We had full control of the design of the study, methods used, outcome parameters, and production of the written report.
| DisclaimerThe 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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Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, 2000; a grant from the Japan Heart Foundation/Pfizer Grant for Cardiovascular Disease Research, 2000; and a grant from the Fujita Memorial Fund for Medical Research, 2001.
References
This article has been cited by other articles:
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  P. McKeown and A. E. Epstein Future Directions: American College of Chest Physicians Guidelines for the Prevention and Management of Postoperative Atrial Fibrillation After Cardiac Surgery Chest, August 1, 2005; 128(2_suppl): 61S - 64S. [Abstract] [Full Text] [PDF]
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 M. M. Scheinman and E. Keung The year in clinical electrophysiology J. Am. Coll. Cardiol., March 1, 2005; 45(5): 790 - 795. [Full Text] [PDF]
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), which may have arisen from the cryosite, is seen. (LA = left atrium; LAA = left atrial appendage; PV = pulmonary vein; RA = right atrium.)