Atrial fibrillation (AF) is a relatively common abnormal rhythm of the heart, occurring in 1-2% of the general population (1). The prevalence of AF increases with age and is responsible for 15-20% of ischemic strokes, thus representing a significant health care burden (2). It is present in two forms: paroxysmal or intermittent AF, characterized by episodes with varying frequency and periodicity, and persistent or chronic AF, a sustained rhythm. Consideration of both anti-thrombotic and anti-arrhythmic therapies informs the treatment approach for AF (1).
Pharmacological anti-arrhythmic agents are the first-line therapy for paroxysmal and persistent AF. However, these medications are often poorly tolerated or cause significant morbidity and mortality themselves (1). AF is associated with a decreased quality of life relative to sinus rhythm. Much controversy surrounds the use of rate- or rhythm-controlling agents as optimal therapy. Despite this, rate versus rhythm control trials, utilizing both types of medication, have not been able to conclusively demonstrate an advantage for rhythm control on a quality of life scale (1). As such, clinician preference is often a deciding factor.
Ablation strategies are used with the intent of “curing” AF, removing the requirement for ongoing antiarrhythmic medication. Both catheter and surgical ablation procedures rely on expert staff at specialized institutions for safety. Present guidelines recommend catheter ablation for patients with symptomatic paroxysmal AF resistant to at least one anti-arrhythmic medication (1). Meta-analyses comparing catheter ablation to anti-arrhythmic medication have demonstrated a clear superiority in favor of catheter ablation for rhythm outcomes and freedom from anti-arrhythmic medication (3,4). The reported efficacy of catheter ablation is 61-89% (5) with a complication rate of 6%. Around a third of patients require multiple ablations for successful treatment (5).
Surgical ablation was initially developed with cut and sew lesions and was first described by Cox et al. in 1991 (6). Since its inception, the Cox-Maze procedure has undergone many changes and refinements (7). The cut and sew Maze-III procedure has a reported efficacy of up to 90% when considering the outcome of freedom from symptomatic AF (8). However, the technical difficulty and risks associated with this on-pump, open heart procedure have led to the development of alternative techniques with various energy sources, including cryoablation, radiofrequency ablation and pulmonary vein isolation. Minimally invasive video-assisted thoracoscopic surgery (VATS) off-pump operations utilizing radiofrequency or cryoablation energy delivery devices have been employed to perform epicardial ablation (9). These operations have a reported rate of freedom from AF of 65-82% as reported in small case series (10-13).
While there is clear evidence for the superiority of catheter ablation compared to antiarrhythmic therapy, its relative efficacy compared with surgical ablation is not well established. Thus, this review aims to consolidate a number of smaller studies comparing transcatheter endocardial ablation with epicardial ablation and cut and sew techniques to assess their relative efficacy and safety in clinical practice.
Literature search strategy
A systematic review of studies comparing surgical ablation to catheter ablation for the treatment of AF was performed. Five electronic databases including MEDLINE, PubMed, Embase, Cochrane Central Register of Controlled Trials and the Cochrane Database of Systematic Reviews were searched from January 2000 until August 2013. Appropriate free text and MeSH terms were used to identify all studies: “cardiac catheterisation” OR “endocardial ablation” OR “pulmonary vein isolation” OR “catheter” OR “catheter ablation” and “surgical ablation” OR “epicardial ablation” OR “MAZE” OR “video assisted thoracic surgery” OR “videothoracoscopy” OR “thoracoscopy” and “AF”. Reference lists of all articles found were searched to further identify potentially relevant studies.
The findings from initial scoping searches were used in deciding which outcomes to include in the present review. Freedom from AF was the primary endpoint identified. Secondary outcomes identified include adverse events such as hematoma pacemaker implantation, pneumothorax, myocardial infarction and cerebrovascular events.
Studies eligible for this systematic review directly compared surgical ablation techniques to catheter ablation techniques in patients with AF. Experimental or observational studies were also included. Case reports, series with less than ten patients, abstracts, editorials and expert opinions were excluded. If more than one article had been published from the same center with the same dataset, only the article with the most complete dataset published was used. All studies selected were human trials and in English.
Data extraction and critical appraisal
Three reviewers (WYC, MYH, RS) independently appraised studies from January 2000 to August 2013, using a standard form and extracted data on methodology, quality criteria and outcome measures. All extracted and tabulated data were checked by an additional reviewer (KK). The quality of studies was assessed using assessment criteria recommended by the Centre for Evidence Based Medicine (University of Oxford) (14). Discrepancies between reviewers were resolved by discussion and consensus was reached.
The odds ratio (OR) was used as a summary statistic. In the present study, both fixed- and random-effect models were tested. In the fixed-effects model, it was assumed that treatment effect in each study was the same, whereas in a random-effects model, it was assumed that there were variations between studies. χ2 tests were used to study heterogeneity between trials. I2 statistic was used to estimate the percentage of total variation across studies, owing to heterogeneity rather than chance, with values greater than 50% considered as substantial heterogeneity. I2 can be calculated as: I2 =100% × (Q – df)/Q, with Q defined as Cochrane’s heterogeneity statistics and df defined as degree of freedom (15). If there was substantial heterogeneity, the possible clinical and methodological reasons for this were explored qualitatively. In the present meta-analysis, the results using the random-effects model were presented to take into account the possible clinical diversity and methodological variation between studies. Specific analyses considering confounding factors were not possible because raw data were not available. All P values were 2-sided. All statistical analysis was conducted with Review Manager Version 5.2.1 (Cochrane Collaboration, Software Update, Oxford, United Kingdom).
Quantity of studies
A total of 406 studies were identified from the databases searched. Initial evaluation of the titles and abstracts of the articles found identified seven potentially relevant publications. When the inclusion criteria were applied to these studies, all seven articles remained relevant for assessment (Figure 1).
Quality of evidence
All studies appraised were from specialized tertiary referral centers. Two prospective randomized controlled trials were found (16,17), as well as 5 retrospective analyses (18-22). Six of the 7 were from single institutions (17-22), with one study deriving data from multiple institutions (16). Six of the 7 studies had 99 or more patients (range 99-291, Table 1), with one smaller study (20) presenting data from only 45 patients.
One study, Gu (2013) (17), reported data on a subgroup of patients with rheumatic heart disease undergoing a valvular heart operation concurrently. Similarly, Krakor and colleagues (21) reported on a patient population concurrently undergoing endoscopic mitral valve repair.
Follow-up duration varied between studies from 6 months to a median of 5.6 years for the surgical cases in Stulak and colleagues (18). Three studies reported data for the primary outcome at 12 months, with one at 6 months and the remaining three at 20 months or longer (Table 2).
Differences in surgical intervention methods between studies are described in Table 3. Gu et al. (17) and Stulak et al. (18) both describe an open heart procedure requiring cardiopulmonary bypass and a sternotomy. Stulak is the only study to utilize a cut and sew procedure instead of ablation. The other five studies (16,19-22) use a minimally invasive thoracoscopic procedure not requiring cardiopulmonary bypass. The left atrial appendage was removed or excluded in four of the seven studies (16,17,19,20).
Techniques amongst studies for endocardial ablation also varied (Table 3). Radiofrequency ablation was used in all but one study which favored cryoablation (21). Lesion sets between studies were also variable and are presented in Table 3.
Three of the seven studies included only patients with persistent AF (17,19,20). The prevalence of paroxysmal AF varied from 21% to 85% amongst those who included that patient population (Table 2). Subgroup data analysis for this patient population was only performed for two studies (16,18). This data is included in Table 4.
Duration of AF varied from 2.2 to 7.4 years, with data available for all but one study (18). Looked exclusively at patients suffering from rheumatic heart disease undergoing concomitant valvular surgery.
In five of 7 studies, patients had not previously undergone an endocardial ablation procedure. Stulak (18) and Mahapatra (20) studies both contained patients who had previously had a catheter ablation procedure but did not include data for individual results for these subgroups.
Assessment of efficacy
Seven studies reported the incidence of freedom from AF and demonstrated superior efficacy in the surgical ablation arm compared to catheter ablation at 6 months (73% vs. 61%; OR, 2.19; 95% CI, 1.21-3.96; P=0.01; I2=0%), 12 months (74% vs. 43%; OR, 3.91; 95% CI, 2.38-6.42; P<0.00001; I2=0%), and at the study endpoint (74% vs. 59%; OR, 2.45; 95% CI, 1.74-3.45; P<0.00001; I2=0%). These results are summarized in Figure 2. At study endpoint (1-5.6 years), absolute increase in freedom from AF varied from 8-45% between the studies included (Table 4).
Freedom from AF subgroup data for paroxysmal AF and persistent AF were available for two studies (16,18). In the paroxysmal subgroup, higher freedom from AF was reported in the surgical arm compared to catheter ablation (77% vs. 67%; OR, 2.47; 95% CI, 1.00-6.12; P=0.05; I2=57%). For the persistent AF subgroup, there was a non-significant trend towards higher freedom from AF outcomes in the surgical ablation arm (74% vs. 55%; OR, 2.34; 95% CI, 0.98-5.62; P=0.06; I2=0%). These results are summarized in Figure 3. Similar increases in freedom from AF were also observed for patients with prior failed catheter ablation and those with left atrial dilatation and hypertension, however this was only reported in one study (16).
Assessment of safety
Table 5 depicts all adverse events reported in these seven studies. The most common adverse event was the development of pulmonary vein stenosis, with an incidence of >50% in one study, in 19 of 194 catheter ablation arm patients (18). Pacemaker implantation rates were significantly higher in the surgical ablation arm compared to catheter ablation (5.4% vs. 1.5%; OR, 3.63; 95% CI, 1.30-10.13; P=0.01; I2=0%; Figure 4). No differences between surgery and catheter groups were observed in terms of incidence of stroke/TIA (1.9% vs. 0.7%; OR, 2.34; 95% CI, 0.69-7.91; P=0.17; I2=0%; Figure 5) and cardiac tamponade or pericardial effusion (2.0% vs. 3.0%; OR, 1.16; 95% CI, 0.25-5.41; P=0.85; I2=0%; Figure 6).
Catheter and surgical ablation techniques have been developed over the past 20 years as curative strategies for AF. From initial cut and sew techniques to energy delivery devices applied epicardially and endocardially, the complexity of AF ablation strategies is still evolving. This review covers a spectrum of surgical ablation techniques, from cut and sew techniques as performed by the initial pioneers, to innovative epicardial ablation delivered via a minimally invasive VATS procedure.
According to the European Society of Cardiology guidelines, surgical and catheter ablation procedures are reserved for those failing anti-arrhythmic drug therapy (1). The same guidelines in 2010 further recommended that surgical ablation should be reserved for patients failing catheter ablation. Since that time, six of the seven studies included in this review have been published (16,18-22). Epicardial ablation strategies have traditionally shown better efficacy relative to endocardial ablation (9-13) but have rarely been directly compared.
Freedom from AF is an important clinical outcome from AF treatment, and has been shown to be a predictor of quality of life and survival. The cut and sew procedure demonstrated increased freedom from AF relative to catheter ablation at short-term (6-month), mid-term (12-month) and long-term follow-up periods. Indeed, VATS and epicardial ablation procedures showed an 8-45% absolute increase in freedom from AF (Table 4). These results are consistent with previous studies comparing the efficacy of surgical ablation versus catheter treatment of AF. The lowest increase in efficacy was demonstrated by Krakor (21), however these results may have been influenced by the patient population undergoing concomitant mitral valve repair surgery. In this population, one could hypothesize that there is a different causative mechanism for AF.
Three studies looked exclusively at persistent AF patients: Wang (19), Mahapatra (20) and Gu (17). These studies showed an additional benefit for surgical ablation of 15.7-33.4% (Table 4). They also demonstrated excellent overall freedom from AF, with rates of persistent AF at study endpoint of 74.7-88% (Table 4).
Subgroup analysis demonstrated significantly higher freedom from AF with surgical ablation in paroxysmal AF patients. Boersma (16) had subgroup data for paroxysmal AF showing a 33.8% absolute increase in freedom from AF at 12 months. This is a greater overall benefit than that for persistent AF within this study, which was 22.8%. The surgical and catheter ablation techniques achieved the lowest rates of efficacy in this review (Table 4). We postulate that this may be in part due to the use of a patient population who had previously failed catheter ablation and had been proven to have refractory AF (Table 2). Only Mahapatra (20) also studied this population and while the overall results were better, the in-study procedures showed a greater degree of variation due to its retrospective case-control design. A similar but non-significant trend supporting the superior efficacy of surgical ablation in delivering freedom of AF is also apparent for persistent AF (P=0.06, n=67). Future studies of larger sample sizes with adequate power may potentially prove higher freedom of AF from surgical ablation in persistent AF patients as well.
Previous studies have reported a higher incidence of complications associated with surgical ablation versus catheter ablation. In the current meta-analysis, three studies reported the pacemaker implantation incidence, which was found to be significantly higher in the surgical ablation arm (5.4% vs. 1.5%). Others have suggested that the Cox Maze procedure is linked with sinus atrial node injury and dysfunction, which justifies the higher incidence of pacemaker insertion reported. In a recent meta-analysis by Phan et al. (23), similar pacemaker implantation rates were demonstrated in AF patients with and without surgical ablation. Collectively, these results suggest that catheter ablation may have lower pacemaker implantation rates and thus may be a more suitable treatment modality for patients with contraindications for pacemaker implantation. The incidence of stroke/TIA and of pericardial effusion were reported in 5 studies and were found to be comparable between the surgery and catheter intervention arms (2% vs. 0.7%, P=0.17; 2% vs. 3%, P=0.85, respectively). Previous meta-analyses have suggested that surgical ablation has a protective effect against stroke and thromboembolism, however this trend is not evident in our study.
Other complications, including incidence of respiratory failure, renal failure, haemothorax, rib fracture and wound infections were poorly reported in the current evidence (Table 5), supporting the notion that these adverse effects were generally rare across all studies. Stulak (18) reported 19/124 catheter ablation patients developing >50% pulmonary stenosis, 14 of which required intervention. This finding was unique to this study and may be explained in part by the variety of transcatheter techniques used across the study’s wide timeframe (January 1993—December 2007). While this may also explain the higher (9/124) rate of patients suffering from pericardial tamponade/effusion (18), these elevated morbidity rates were not consistent across the smaller studies. Overall, surgical ablation appears to be a viable treatment method for AF, given the superior efficacy in delivering freedom from AF, as well as the comparable incidence of complications to catheter ablation.
This review was limited by the heterogeneity of the studies included. Only 2 prospective RCTs were found: Boersma and Liu. One of these studies focused exclusively on patients with rheumatic heart disease (17). These 2 RCTs had 124 and 99 patients respectively. The largest study was a retrospective analysis spanning a time period from January 1993 until December 2007, during which time expert techniques developed and procedures were refined.
There was also significant heterogeneity in the study protocols used to compare surgical ablation with catheter ablation (Table 3). Two of seven studies used an on-pump surgical procedure, one of which employed the Cox Maze III procedure (Table 3). The rest of the studies utilized off-pump procedures and may have affected the adverse event profile of the review. Transcatheter ablation procedures also varied in lesions made between studies, which may have affected the efficacy and clinical outcomes of the tested interventions. Furthermore, no definitive conclusion regarding the relative operative risks and clinical outcomes of surgical and catheter ablation could be made due to the poor reporting and small sample sizes of inadequate power. Future RCTs should aim to investigate larger patient populations with a focus on the complication rates, in addition to outcomes of freedom from AF. While all included studies utilised objective measures, no investigator blinding was present. The marked heterogeneity of the included studies’ techniques, patient populations, analysis and designs mean that our results must be interpreted with care.
In conclusion, to best answer the question of surgical ablation versus catheter ablation, a blinded, large, multi-center RCT comparing the efficacy of existing techniques in patients with both paroxysmal and persistent AF is needed. Utilizing the existing evidence presented in this review, surgical ablative techniques appear to demonstrate greater efficacy when compared to catheter-based techniques. Epicardial ablation delivered by VATS showed a higher rate of pacemaker implantation than catheter ablation; stroke and tamponade incidence were comparable between the groups (9-13). This may represent greater technical skill with this new procedure and provides the rationale for further study to clarify these results.
Disclosure: The authors declare no conflict of interest.
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