Cardiac surgeons were the pioneers of curative ablation of AF. Their interest began in the 1980’s when Cox and associates introduced the left atrial isolation procedure, a technique that allowed restoration of regular rhythm and confining AF to the left atrium in dogs.2 Later, in 1985, Guidaron introduced “the corridor procedure”, an open-heart technique that divided the atrium in 3 compartments: right atrium, left atrium and a corridor from the sinus to the atrioventricular node.3 In this procedure sinus rhythm in the corridor was preserved while atrial synchrony was lost. Some of the disadvantages that were pointed to these procedures were the absence of atrioventricular synchronism and the remaining vulnerability to systemic thromboembolism.
The Maze procedure was introduced in humans in 1987 as the first surgical treatment for AF by Cox and colleagues.4 The procedure consisted in interrupting all macro re-entry circuits associated with the development of atrial flutter or AF. The surgical strategy consisted in creating multiple incisions that could block all possible macroreentrant circuits and direct the propagation of the sinus impulse throughout both atria. Lesions were created by a “cut and sew” method (used in the Cox-Maze I to III procedures), performed under direct vision, which had the advantage of increasing the probability of achieving transmurality. Excision of the LAA was also performed alongside. One of the main advantages of this surgery when compared to its previous counterparts was the freedom from stroke.5
Unfortunately, this procedure (the Cox-Maze I) resulted in occasional left atrial dysfunction and the frequent inability to generate adequate sinus tachycardia in response to exercise. In order to overcome these limitations, the Cox-Maze II procedure was developed. It excluded the sinus node incision and relocated the left atrium dome transverse atriotomy to a more posterior location. Later, the necessity of complete transection of the superior vena cava to complete the treatment was confirmed and the initial method was perfected giving birth to the Cox-Maze III. The septal incision posterior to the superior vena cava orifice allowed the long-term preservation of atrial transport and sinus node function, decreasing the need for a pacemaker and the recurrence of arrhythmia, while improving the speed of the procedure.6 In 1999, Cox et al modified the Cox-Maze III to a minimally invasive approach using a 7 cm right submammary incision.7
Despite the proven efficacy of the Cox-Maze III, the procedure was not widely accepted. The reasons for that were its technical complexity and risks for the patient due to the number of atrial incisions. The reported 30-day mortality rates varied from 0-7.2%8 and complications such as iatrogenic injury of the sinus node requiring postoperative atrial pacemaker implantation (6% reported by Cox JL et al.5 and 3.2% in Mayo Clinic experience9), stroke and bleeding were something to be taken into account.
Many attempts were made to improve the simplicity of the treatment. The most obvious change was to replace lines of incision by lines of transmural necrosis using other energy sources. The Maze IV procedure was initially tested in a series of 40 patients from January 2002 to October 2003.10 It preserved the entire lesion set of the Cox-Maze III procedure, but used bipolar radiofrequency instead of the cut-and-sew technique. Unipolar techonology (cryoablation) was used for the valve annuli. There were only two small atriotomies and the LAA could be excised or ligated. It was performed by median sternotomy or a small right thoracotomy and despite the fact that the pulmonary veins could be isolated in the beating heart, it still required a cardiopulmonary bypass for the remaining lesions.
A systematic review from 2005 comprising 3832 patients compared the classical Cox-Maze III with procedures that used alternative sources of energy (cryo and radiofrequency ablation) but found no significant differences as far as postoperative sinus rhythm conversion rates were concerned.11
Simplified (pulmonary vein encircling with connecting lesions) or complete modified Cox-Maze III using a diode-pump laser has also been tried. This was initially described as a single-center experience in a small set of 28 patients with concomitant mitral valve surgery performed in 75%. Results were favorable with > 95% freedom of AF and 76% freedom of all atrial tachycardia after 6 months.12
Minor variations of the Cox-Maze procedure have been proposed over time, namely concerning the extension of the lesion-set. Although full-thickness incisions through the walls of both atria are usually required, Shaff HV et al. reported that incisions could be limited to the right atrium in patients with primary tricuspid valve disease.9 However more recent data suggested that biatrial ablation surgical procedures were more effective in controlling AF than procedures confined to one atrium (mostly the left one).13,14
The presence of left ventricular dysfunction, a feature that initially raised some concerns, is not anymore considered a contraindication for the procedure and restoration of sinus rhythm can improve left ventricular ejection fraction in most patients.9
Pulmonary Vein Isolation, the Minimaze and other Thoracoscopic Procedures
The documentation of spontaneous initiation of AF by ectopic beats originating in the pulmonary veins15 has redirected the focus of interest to the pulmonary veins as the main target. Some years before the development of the Maze IV procedure, Queirós and colleagues developed a strategy aiming to surgically isolate only the pulmonary veins by means of radiofrequency energy. The first procedures were performed through endocardial ablation16,17 and subsequently epicardial ablation was used.18
Some investigators have also proposed a bilateral isolation of the pulmonary veins instead of one box lesion encircling all pulmonary veins.19,20 Nevertheless, a more extensive reduction of substrate mass under the critical level necessary to perpetrate AF may be necessary for AF elimination, especially in dilated atria and, therefore, additional lines between the isolated pulmonary veins and the mitral valve annulus may be considered.19,20 This approach has also been shown to decrease the risk of postoperative atypical atrial flutter.
Ablation of AF through surgical pulmonary vein isolation (PVI) has some advantages compared to the standard Maze procedure.21 Firstly, an atriotomy can be avoided and transmurality may be achieved without significant damage to the endocardium, thus lowering risk of thrombus formation (mainly when linear lesions are made in the left heart, such as during the catheter maze procedure and ventricular tachycardia ablation) and stroke (one of the most feared complications of AF ablation).22
Secondly, ablation performed from the epicardial site limits damage to surrounding tissues (including, but not limited to, the oesophagus), since the energy vector is directed towards the atrial cavum and not away from it. Thirdly, measurements of conduction block are possible during an epicardial beating heart procedure, which may eventually help guide ablation. Additional potential benefits may arise from targeting parasympathetic innervations of the heart (through ablation of epicardial fat pads). Decreasing parasympathetic tone may shorten the atrial effective refractory period, which decreases susceptibility to AF, although the long-term efficacy of this approach is not known, as restoration of autonomic activity may occur early following ablation.
Possible lack of transmurality is one of the main issues in epicardic beating heart ablation. In a histological investigation of microwave epicardial lesions in 3 non-ablation related deaths, only 3 out of 13 samples (23%) showed transmural necrosis.23 This incomplete transmurality of lesions may partly explain the existing difference in success rates between the original Maze operation and other epicardial strategies.24,25
In 2004, Cox defined the minimaze procedure as the minimal set of lesions (“pulmonary vein encircling incision, left atrial isthmus lesion with its attendant coronary sinus lesion, and the right atrial isthmus lesion”) that had to be performed to cure most patients with AF.26 Moreover, he reinforced that in order to achieve this goal, energy had to be applied in the endocardium in order to overcome the presence of the left circumflex artery in the posterior mitral annulus and reach the atrial wall, something that could not be done using neither cryotherapy, unipolar, bipolar or irrigated radiofrequency, microwave or laser energy.
However, in 2002, Saltman had already developed a method for performing epicardial ablation of AF endoscopically in the beating heart, without cardiopulmonary bypass or median sternotomy.27 This method used microwave energy and became known as the microwave minimaze (or micromaze) procedure. Not long after, Wolf and colleages developed a similar procedure using radiofrequency energy instead that became known as the Wolf minimaze procedure.28 In these procedures, the LAA was also frequently removed. The high intensity focused ultrasound minimaze used an ultrasonic device that was positioned epicardically.29 Still, it was performed in conjunction with other cardiac surgical procedures, not being minimally invasive in those cases. These procedures have been almost restricted to paroxysmal AF and long-term results are only based on preliminary reports. Still, short-term and long-term success may range from 67 to 91%.27-29
Thoracoscopic techniques have evolved and became more sophisticated being able to track more severe substrates and severely diseased AF patients. One of the most well-known developments was the “Dallas lesion set”,30 that included a more extensive set of lesions, alongside with partial ganglionated plexi de-enervation. Sirak J and colleagues introduced a true port-access procedure that was able to address both autonomic and anatomic sources of AF, incorporating PVI, mapping of epicardial autonomics, extended linear ablations and ligation of the LAA.31 This technique proved to be a highly effective and safe approach in patients with advanced forms of AF. The same authors also proposed a variant of the thoracoscopic technique, entitled “five-box thoracoscopic maze procedure”, in which a complete dissection of the transverse sinus and exposure of the left atrial floor enabled the creation of contiguous compartments connecting to the anterior mitral trigone and isolating the posterior left atrium, replicating the Cox Maze left atrial pattern.32 This procedure was as effective as the Cox Maze benchmark.
As these are minimally invasive procedures, comparison with other minimally invasive techniques like percutaneous catheter ablation has been performed. The atrial fibrillation catheter ablation versus surgical ablation treatment (FAST) trial was a two-center randomized clinical trial of 124 patients comparing the efficacy and safety of minimally invasive thoracoscopic procedures (Saltman and the Dallas lesion set) with percutaneous catheter ablation in a population comprising either patients with left atrial dilatation and hypertension (33%) or prior failed catheter ablation (67%).33 The surgical treatment group yielded higher efficacy (freedom from left atrial arrhythmia >30 seconds without antiarrhythmic drugs after 12 months 65.6% vs 36.5%; p=0.0022) at expense of a higher adverse event rate (34.4% vs 15.9%; p=0.027).
The Catheter Versus Thoracoscopic Surgical Ablation Strategy in Persistent Atrial Fibrillation (CASA-AF) atrial is currently investigating the safety and efficacy of a thoracoscopically assisted surgical ablation (including PVI, gananglionated plexi ablation and LAA exclusion), while comparing it with catheter ablation. Results of this trial are expected to be available by the end of 2013.34
At present, there is small amount of consistent and reproducible data on the thoracoscopic surgical techniques when compared to the standard Maze ablation, but further research may eventually provide robust data supporting its non-inferiority, allowing these less invasive procedures to become the first line approach in surgical AF ablation.
Hybrid procedures combining minimally invasive epicardial left atrial ablation with the endocardial percutaneous approach have been tried in difficult cases of AF.35 Despite being more time consuming, some theoretical advantages that have been pointed are: maximization of the efficacy of the ablation, avoiding lesion gaps and confirming the conduction block and minimizing potential complications, such as tamponade and thrombus formation
Krul et al. have added the localization and ablation of the ganglionated plexi to this procedure and observed a high success rate of 86%, with no recurrences of AF, atrial flutter or tachycardia, out of antiarrhythmic treatment.36
Pison L et al. have recently demonstrated that in 23% of patients undergoing thoracoscopic procedures the epicardial lesions were not transmural and endocardial percutaneous “touch-up” was necessary.37 The one year success of this hybrid approach was reported to be 90% for persistent and to 93% for paroxysmal AF.