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Interact CardioVasc Thorac Surg 2007;6:447-450. doi:10.1510/icvts.2006.146688 © 2007 European Association of Cardio-Thoracic Surgery
Microwave surgical ablation for atrial fibrillation during off-pump coronary artery surgery using total arterial-Y-grafts: an early experienceDepartment of Cardiac Surgery, Trent Cardiac Centre, Nottingham University Hospitals, Hucknall Road, Nottingham, NG5 1PB, UK Received 25 October 2006; received in revised form 2 April 2007; accepted 3 April 2007 *Corresponding author. Tel.: +44 115 9691169 extension: 49733; fax: +44 115 8402605
This study demonstrates the efficacy and eligibility of concomitant epicardial microwave AF (MWAF) ablation during off-pump arterial revascularisation using the left internal mammary to radial ‘Y’ graft (OPCABy) in patients with permanent and paroxysmal atrial fibrillation. From June 2004 to December 2005, sixteen consecutive patients were offered MWAF ablation and OPCABy. AF was permanent in 11 cases and paroxysmal in five. The MWAF ablation protocol exploited the use of either the Flex 4 or Flex 10 probe (Afx- Guidant, Santa Clara, CA). Spontaneous cardioversion was used to demonstrate conduction block. Data were collected prospectively. Patients were followed-up in outpatient clinic at 6 weeks, 3 months and 6 months after discharge. Sinus rhythm was seen in 75%, 67% and 71% of patients at conclusion of surgery, and 3 and 6 months postoperatively. Cardioversion to sinus rhythm was seen in 67% of patients with permanent AF and 80% of patients with paroxysmal AF. Spontaneous cardioversion at operation occurred in 12 patients, all of whom were in sinus rhythm at six months. The use of MWAF ablation during concomitant OPCABy surgery is an effective therapy in the short- to medium-term. Spontaneous return to sinus rhythm is a reliable intraoperative indicator of long-term success.
Key Words: Atrial fibrillation; Microwave surgical ablation; Off-pump coronary artery revascularisation
Atrial fibrillation (AF) is one of the commonest pre-existing atrial arrhythmias in patients undergoing coronary artery bypass grafting (CABG), and is recognised as an independent risk factor for early and late mortality after revascularisation [1]. The cut and sew Cox-Maze III operation has been regarded as the gold standard procedure for AF ablation [2]. Limitations of this technique relate to complexity, requirement for permanent pacing and loss of atrial transport in some patients postoperatively. Recent advances in the use of alternative energy sources such as radiofrequency, microwave, ultrasound, laser and modern cryoablation have generated a resurgence in surgical interest in AF ablation because they allow the ability to create lesions similar to those described by Cox, but without the technical difficulties associated with it [3]. Experts have described their experiences with these energy sources, using a variety of different lesion sets, and studies have shown that concomitant surgical AF ablation during on-pump CABG are as effective as in mitral valve disease [4, 5]. Few have described the efficacy of AF ablation during off-pump CABG (OPCABG). Despite our realisation of the limited understanding of the impact, and value of these energy sources, we felt obliged to treat patients undergoing CABG using our preferred technique of off-pump arterial-Y-graft surgery (OPCABy). Available technology existed in the form of the Flex 10 and Flex 4 microwave probes. Ablation rates using microwave (MW) energy sources seem to compare favourably with the Cox-Maze procedure [6]. Furthermore, in its early evolution, the Flex 10/4 systems have been used widely in AF ablation with relative efficacy and safety [7, 8]. This article reports the first experience of the use of epicardial MWAF ablation during OPCABy.
From June 2004 to December 2005, a total of 36 MWAF ablation procedures and 109 OPCABy were performed. Actually, sixteen OPCABy patients underwent MWAF ablation (14.67%). There were no exclusion criteria in this series. Data were collected prospectively. Preoperative and intraoperative details are shown in Table 1. All patients were on multimodal anti-arrhythmic therapy and anticoagulation preoperatively.
Following anaesthesia transoesophageal echocardiography was performed to exclude the presence of atrial thrombus and reassess left atrial (LA) size. The left internal mammary artery (LIMA) and radial artery (RA) was harvested. The RA was anastomosed to an appropriate site on the LIMA to create the arterial-Y-graft. The pericardial reflection between the superior caval vein and the right superior pulmonary vein as well as the inferior caval vein and the right inferior pulmonary vein was dissected. The FLEX 10 catheter was passed around the pulmonary veins and base of the left atrial appendage. The fat in the inter-atrial groove was dissected in order to improve active applicator of the Flex 10/4 devices and the ligament of Marshall was destroyed using unipolar diathermy. In all patients, complete epicardial pulmonary vein encircling box lesion was created (Fig. 1). The FLEX 10 probe was used in all except in two patients. Modulated, unidirectional MW energy at 2.45 GHz was applied at 65 W for 120 s. Using this protocol, off-pump pulmonary vein isolation was used as the default AF ablation protocol, prior to coronary revascularisation. Spontaneous cardioversion was used to signify satisfactory completion of pulmonary vein isolation. If failure to produce this endpoint was seen, further burns were performed in regions near visible epicardial nerve bundles.
All the patients received Amiodarone intravenously as a bolus of 300 mg followed by infusion of 900 mg over 24 h in the first postoperative day. Subsequently the protocol consisted of 200 mg three times daily orally for a week, reduced to 200 mg twice daily for a week followed by 200 mg once daily for four weeks. All the patients were adequately anticoagulated with warfarin (International normalized ratio: 2.0–2.5). If AF recurred, no attempt was made to cardiovert patients to sinus rhythm prior to discharge.
The primary end point of the study was restoration of sinus rhythm. Patients were reviewed in follow-up clinics at 6-weeks, 3-months and 6-months after hospital discharge. All the patients who converted into sinus rhythm had 24-h ECG monitoring on two different occasions during the follow-up. If they remained in sinus rhythm Amiodarone was discontinued. Symptom enquiry was used as a crude indicator for unrecorded episodes. Those who remained in AF were electrically cardioverted whilst remaining on Amiodarone. Cardioversion was repeated if AF recurred at any of the subsequent clinic visits.
Sixteen patients were consecutively offered MWAF ablation during OPCABy. Mean age of patients was 70.6± 8.4 years (range: 57–81 years). Fourteen patients (88%) were males. Ten patients were treated for hypertension (71%). Five patients had left main stem coronary artery disease (31%). Seven had poor left ventricular (LV) function (43%). Eleven patients had permanent AF (69%) while five had paroxysmal AF (31%). All patients suffered from AF for longer than 24 months. Mean LA diameter was 5.1±0.5 cm. Although beating heart MWAF ablation was completed in all patients, one patient required cardiopulmonary bypass to repair LA appendage injury following unsuccessful stapling. LA appendage exclusion was abandoned in all others. There was no evidence of any mediastinal organ damage in these patients. Since the microwave energy release is unidirectional it is very useful for epicardial beating heart ablation, as ablation could be carried out from the dorsal part of the heart without further protective measures. In all patients with permanent AF and in one patient with paroxysmal AF in whom continuous AF was precipitated during pericardiotomy, the spontaneous abolition of AF was used to confirm satisfactory completion of the MWAF ablation. Using this end point, added burns were required in 11 patients. These burns were placed in areas along the left atrial roof and in the inter-atrial groove. AF was eventually abolished spontaneously in 12 patients. In four patients (including the one patient with paroxysmal AF), AF could not be abolished with ablation alone. Unfortunately one patient with EuroSCORE of ten died on the fifth postoperative day from embolic bowel infarction. Preoperative echocardiography excluded intra-cardiac clot and he was anticoagulated from day 1 postoperatively. Another patient died at 15 weeks following surgery from an unrelated cause. At the end of the procedure, 75% (12/16) of patients were in sinus rhythm (Fig. 2). All of these patients were in sinus rhythm at six months. Eighty percent (4/5) of paroxysmal AF patients were converted to sinus rhythm, compared to only 67% (6/9) of those in permanent AF. Anti-arrhythmic medications during follow up are shown in Table 2. All patients who were in sinus rhythm at six months were also in sinus rhythm at six weeks and three months.
Since 2003, we have used OPCABy as our default revascularisation strategy, primarily because of the superior results that we have experienced. This rapid growth in OPCABy surgery in our institution has produced a management dilemma when treating patients with concomitant AF – should we treat AF at the expense of an on-pump revascularisation strategy, or should we consider a beating heart AF strategy? There is a paucity of published data examining this topic. Studies have shown that epicardial microwave pulmonary vein ablation is feasible, and efficacious in the treatment of lone AF [9]. We, therefore, decided to pursue a beating heart strategy for AF ablation during OPCABy. During this study all patients (n=16) with concomitant AF were offered off-pump beating heart epicardial microwave ablation in a non-selective manner. Using this approach, 71% of patients were in sinus rhythm at six months follow-up. Of 9/11 surviving patients in permanent AF, 67% (n=6) were in sinus rhythm at six months. Only one other study, Maessen et al. [10] has described experiences with beating heart microwave AF ablation during OPCABG in seven patients with a success rate of 86%. There are obvious explanations for the superior results seen by Maessen. Firstly, the study described only a very small subset of patients who underwent OPCABG. Secondly, all patients were continued on Sotalol±amiodarone therapy post-ablation; therefore, the relative influences of AF ablation, and anti-arrhythmic therapy is more difficult to separate than in our investigation where anti-arrhythmic therapy was withdrawn very early after surgery. Our anti-arrhythmic protocol involved cessation of amiodarone upon confirmation of sinus rhythm on 24 h Holter and absence of symptoms recorded in patient event diaries; amiodarone was, therefore, stopped at three months. Furthermore, at 6 months, only 6/10 patients in sinus rhythm were on a drug with potential anti-arrhythmic properties, which was a beta-blocker (not Sotalol), continued for the treatment of hypertension rather than rhythm control per second. It is possible that our AF ablation strategy was less efficacious. We did not exploit the use of electrodes to ensure conduction block, for example. We preferred to ablate the pulmonary veins and observe the return to sinus rhythm. Spontaneous cardioversion was observed in 12 of the 16 patients. In those patients where spontaneous cardioversion was not seen, electrical cardioversion produced sinus rhythm that quickly recurred. Failure of therapy was seen in all of these patients. Careful analysis suggests that our ablation protocol was more aggressive than others. Whilst Maessen used a mean of 8.5 applications taking a total mean duration of 13 min, we used 10.8 applications with a mean time in excess of 20 min. We used more prolonged ablation time of 120 s per station. This latter strategy is now recommended for epicardial approach with microwave energy. Finally, different ablation lines were used in the two studies. We used a complete pulmonary vein encircling, as in the treatment of lone AF. Maessen used a ‘bowtie’ ablation strategy isolating left and right pulmonary veins in pairs, and a connecting line across the roof of the left atrium. Further studies will need to be undertaken to investigate any potential differences in these approaches. Results of this cohort are similar to Apkinar et al. study, which used bipolar radiofrequency [11]. At 6 months, 70% of patients were in sinus rhythm with a similar distribution of success in permanent (58%) and paroxysmal (83%) AF patients. This was of great interest to us when considering the importance, or otherwise, of transmural atrial lesions. We now know that epicardial microwave AF ablation does not create reliable transmural lesions in the beating heart, unlike the beliefs surrounding the greater reliability of bipolar radiofrequency in this regard. How do we explain this? Haissaguerre et al. have demonstrated that much of the AF foci arise from the pulmonary veins, and observations during endocardial ablation provide circumstantial evidence that inadvertent injury to the epicardial neural plexuses leads to greater success rates during catheter based AF ablation [12]. Thus, epicardial injury to these neural supplies may be, in part, the reason for the success rates seen with limited epicardial vs. endocardial approaches. It is intriguing that en-bloc isolation of all pulmonary veins leads to induction of sinus rhythm without any atrial tachycardia because a new area of scar where macro re-entry may occur was created. There was no electrophysiological testing performed during the procedure. Moreover, the end point of the procedure was conversion to sinus rhythm, which seems to be a good indicator for mid-term rhythm success. Again it raises the question of ‘how much transmurality is needed to achieve rhythm success?’ Histopathological studies in human atrial tissue by Deneke and colleagues showed low incidence of transmurality in the left atrial isthmus, but high clinical success rate, which suggest that transmural lesions are not always needed to suppress arrhythmias [13] which is in turn again supported by other reports [14]. In this study, patients who remained in sinus rhythm during follow-up had transthoracic echocardiogram performed to assess the atrial transport and only some of them had effective atrial transport. Similarly atrial transport was not detected in some of the patients undergoing pulmonary vein isolation alone in Apkinar's study as well [11]. Failure to regain atrial transport, even with the limited epicardial ablation, needs further evaluation. Furthermore, there is evidence that the presence of atrial transport does not correlate well with efficacy of atrial transport [15]. Thus, the balance between what is perceived to be a successful outcome to sinus rhythm does not correlate with normal atrial contraction, and atrial atony is unlikely in itself to be any better than atrial fibrillation. These are just a few of the many unanswered questions that need careful consideration, in order to improve our understanding of AF control and ablation.
This is a small non-randomised study and should be interpreted as such. Whilst the use of spontaneous cardioversion was our preferred technique for assessing conduction block, it would have been useful to correlate this with demonstrable electrocardiographic evidence. Obliviously the 24-h ECG Holter monitoring has its own limitations to assess paroxysmal atrial fibrillation.
The use of beating heart microwave AF ablation during concomitant OPCABy surgery is an effective therapy in the short- to medium-term. Spontaneous return to sinus rhythm is a reliable intraoperative indicator of long-term success.
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Abstract
1. Introduction
