Epicardial microwave ablation for the treatment of permanent atrial fibrillation: 1-year results of a prospective registry study

Background: Atrial fibrillation (AF) which is one of the most common arrhythmias is responsible for a high percentage of strokes. Recently simplified ablation procedures became more relevant in the surgical treatment of AF. One advantage of microwave ablation (MW) is, that it can be used epicardially. We report our first clinical results of the use of epicardial MW as a concomitant procedure during cardiac surgery where opening of the left atrium was not required. Methods: Patients who were scheduled for aortic valve replacement and/or coronary artery bypass grafting suffering from permanent AF, were included into the registry. They received epicardial MW as a concomitant procedure during their operation and were followed over a period of one year. The results were compared to the outcome of a historical patient group with equivalent operations without ablation therapy. Epicardial MW was done on extracorporeal circulation on the beating heart creating box lesions around the pulmonary veins with a connection line on the roof of the left atrium and an additional line to the left atrial appendage. Results: 40 patients were treated with epicardial MW in combination with aortic valve replacement and/or coronary artery bypass grafting. Their preoperative data were as follows: age: 69.6±7.8 years, ejection fraction: 55.7±14.2%, left atrial diameter: 46.2±6.0 mm, duration of AF 6.8±9.8 years. The control group comprised 145 patients (age: 72.6±6.9 years, ejection fraction: 49.7±14.8%, left atrial diameter: 47.4±6.6 mm, AF history: 3.7±5.7 years). One year survival rate was 92.5 % in the ablation group and 86.9 % in the control group. We observed no device-related complications. During the follow-up the sinus rhythm rate ranged between 71.4 and 65.7% in the ablated patients in contrast to 7.4 to 9.6% in the control group (p=0.00001). Conclusion: Our results demonstrate that epicardial ablation, in this study with a microwave ablation device, is an effective treatment option for patients with permanent AF. The procedure is less invasive then the endocardial approach and prolongs concomitant heart surgery only minimally without lengthening ischemic time. Epicardial ablation has become part of the daily routine in our institution for the treatment of AF in patients scheduled for bypass grafting and/or aortic valve replacement. Epicardial microwave ablation for the treatment of permanent atrial fibrillation 285 heart disease is markedly higher. For example, 30% to 60% of the patients who require mitral valve surgery suffer from atrial fibrillation [1]. Especially in the left atrial appendage blood stasis occurs in case of atrial fibrillation when the atrium is no longer capable to actively eject blood into the ventricle by contraction. This results in the formation of thrombi that may embolize. Atrial fibrillation is made responsible for about 15% of all cases of strokes [5, 6]; these patients thus have significantly higher mortality rates than patients without atrial fibrillation [7, 8]. Both in surgical and interventional ablation procedures, the endocardial approach is the method of choice. It can be performed using different energy sources. However, several cardiac surgery procedures do not require opening of the atria (patients with coronary artery bypass grafting and/or aortic valve surgery). An additional opening of the left atrium to perform the endocardial ablation would necessitate additional bicaval cannulation and prolong the entire operation. In order to avoid this and still be able to treat atrial fibrillation, ablation strategies have been developed for epicardially ablation in recent years. Not all endocardially utilized energy sources are suitable for the epicardial approach, the microwave technique, however, has proved to be usable in the endocardial and epicardial approach. In this study we investigated the efficacy of epicardial microwave ablation during one year of follow-up in patients with permanent atrial fibrillation who were scheduled for coronary artery bypass grafting and/or aortic valve surgery. 2. Materials and methods This registry study was conducted following the rules of the Declaration of Helsinki, Somerset West 1996 and the guidelines of the European Forum for Good Clinical Practice [9]. The Ethics Committee of the Dresden University gave approval for the study. The purpose of this pilot study was to investigate the feasibility and efficacy of epicardial microwave ablation. The obtained data were compared to the outcomes of patients with permanent atrial fibrillation who had undergone comparable operations without an ablation procedure (Dresden registry). Primary endpoints of the study were restoration of sinus rhythm and death. The general inclusion criteria to the study for both groups were: permanent atrial fibrillation with duration of at least one month, patients older than 18 years, and indication for coronary bypass graft and/or aortic valve surgery. Patients with paroxysmal or persistent atrial fibrillation were excluded. The preoperative medication regimen was continued until the day of surgery (including aspirin and clopidogrel). Only patients on oral anticoagulation therapy were switched to low molecular weight heparin at the appropriate time. 2.1 Peri-operative protocol Left atrial dimensions were measured by transthoracic or transoesophageal echocardiography prior to the operation. Furthermore, thrombus formation in the left atrial appendage was excluded by an intraoperative transesophageal echocardiogram. 2.2 Ablation procedure All patients were operated using extracorporeal circulation. The ablation treatment was performed on-pump on the beating heart before aortic cross-clamping. The pericardial reflections at the pulmonary veins, superior and inferior vena cavae and the fibro-fatty tissue of the interatrial groove were bluntly dissected. Then the transverse pericardial sinus was exposed. Epicardial microwave ablation was performed using a FLEX 4® probe. A continuous box lesion was created around each pulmonary vein pair. These two box lesions were connected by an ablation line along the left atrial roof through the transverse pericardial sinus. The ablation procedure was completed by a lesion line from the left-sided box lesion to the apex of the left atrial appendage (see Figure 1). 2.3 Microwave ablation system The FLEX 4TM microwave surgical ablation device (Guidant Corp.; Santa Clara, CA, USA) was used to produce linear lesions on the epicardial surface. Ablations were performed for 90 s each with an output of 65 W. Details of the endocardial method have been described previously [10]. 2.4 Postoperative protocol Starting from the first postoperative day, patients received metoprolol to stabilize rhythm and, after re286 M. Knaut, S. Brose, R. Forker, F. Jung, K. Matschke moval of the chest tubes, anticoagulation therapy was switched from heparin to phenprocoumon aiming at a target INR of 2.0 to 2.5 for at least 6 months. Heart rhythm was monitored daily up to postoperative day 10. Follow-up examinations were carried out at day 30, 90, 180 and 360 after surgery. Cardiac rhythm was evaluated with a 12-channel ECG and a continuous 24-hour Holter ECG at day 180 and 360. Early recurrences of atrial fibrillation up to postoperative day 90 were treated with up to two cardioversions. 2.5 Statistical analysis Mean values and standard deviations were given for all continuous data, and classified variables are presented in percentage frequency. Probability of survival over time and overall survival rates were calculated using Kaplan-Meier analysis and compared with the (Mantel-Cox) log-rank test. Chi-square and two-tailed, unpaired Student’s t-test (exact Fisher’s test) were used for two sample analysis. Probability values less than 0.05 indicate a statistically significant difference (at the 5% level) between the ablation and the control group outcome.