CHAPTER 7

According to the most recent Heart Rhythm Society (HRS)/European Heart Rhythm Association (EHRA)/European Cardiac Arrhythmia Society (ECAS) consensus statement, both endocardial and stand-alone surgical AF ablations can be considered in patients with symptomatic AF refractory or intolerant to at least one Vaughan-Williams Class 1 or 3 AADs, but the current evidence supporting surgical ablation is mainly based on expert consensus and clinical experience. The indication for endocardial ablation is based on results from randomized clinical trials and/or meta-analyses.1 Because both techniques have shortcomings, related to their specific approaches and the available technologies, several groups have tried to combine the endocardial and epicardial procedure to overcome their mutual weaknesses and improve results of AF ablation. In this chapter, we will first briefly review the current status of endocardial and surgical epicardial ablation techniques to better understand the rationale for a hybrid or convergent concept. The second part will focus on the several aspects of the hybrid AF ablation with the emphasis on techniques, follow-up, and outcomes.

ENDOCARDIAL ABLATION

The cornerstone of endocardial catheter procedures for AF is ablation of the ostium or antrum of the pulmonary veins (PVs) with the end point of electrical isolation of these veins from the left atrium (LA). The rationale for this is the seminal observation by Haïssaguerre et al.2 that AF was almost always triggered by ectopic beats arising from the muscle sleeves of PVs. The one-year success rate of this approach is greater than 80% in patients with paroxysmal AF.³ Unipolar radiofrequency (RF) is the most frequently used energy source to perform PV isolation. Cryoablation and laser are two alternatives.^4,5 Whatever energy source is used, one of the most frequent reasons for AF recurrence after PV isolation remains PV reconduction.⁶ Long-term results of catheter ablation for paroxysmal AF are somewhat disappointing: in a recently published prospective study, sinus rhythm was maintained in only 46% of patients after the initial procedure without AAD during a median follow-up period of 5 years.⁷

As AF progresses from paroxysmal to persistent and longstanding persistent AF, the atrial substrate itself plays an increasingly important role in the maintenance of the arrhythmia.⁸ This may explain why PV isolation alone results in success rates of less than 25% in patients with persistent AF.⁹ To improve these results, we can modify the atrial substrate by creating linear lesions and/or ablating complex fractionated atrial electrograms (CFAE). In patients with (longstanding) persistent AF, this approach seems to result in better outcomes than PV isolation alone, but there is an important variation in success rates ranging from 11% to 75%. Furthermore, the incidence of iatrogenic atrial tachycardias (AT) after these procedures is substantial (up to 40%).^10,11 As for paroxysmal AF, there seems to be a slow but steady decline in arrhythmia-free survival after catheter ablation for persistent AF.¹²

The creation of linear lesions was inspired by the surgical Cox-Maze procedure.¹³ These linear lesions are thought to prevent sustained multiple reentry circuits by compartmentalization of the atria. The two most frequently deployed endocardial linear lesions are the roofline connecting both superior PVs and the mitral isthmus line going from the left inferior PV to the mitral annulus. Performing these linear lesions can be very challenging and incomplete lines may act as a substrate for macro-reentrant circuits.¹⁴ Nademanee et al.¹⁵ reported a one-year success rate of 87% in patients with persistent AF (4% with AAD) and 78% in patients with longstanding persistent AF (11% with AAD) after CFAE (defined as fractionated electrograms with 2 or more deflections and a mean cycle length shorter than 120 ms) ablation only. However, those results could not be reproduced by other groups, and the electrophysiological mechanisms underlying CFAEs are still a source of debate.^16–18 Sites demonstrating high-dominant frequency seem to be interesting ablation sites as well. In an animal model, these sites correspond to functional reentry and are called rotors.¹⁹ Recently, localized rotors have been visualized in human AF by computational mapping, and ablation of these rotors results in acute termination of AF or substantial organization of the arrhythmia.^20,21 This discovery could hold the potential for an important step toward a tailored substrate ablation approach, especially in patients with persistent AF.

EPICARDIAL SURGICAL ABLATION

The surgical treatment of AF is based on the Cox-Maze III procedure, also known as the “cut-and-sew” Maze. This operation involves the creation of numerous incisions in both atria to direct the sinus impulse propagation through the atria while interrupting macro-reentrant circuits.22 This procedure results in impressive long-term success rates with 92% of patients free from symptomatic AF and 80% withdrawn from AAD.²³ Because the uses of Holter monitors, pacemaker interrogation, and implantable monitoring devices were not common, the results most likely overestimate the success. An advantage of the surgical approach is the possibility of removing the left atrial appendage (LAA), which may help to decrease the risk for stroke, especially in patients with a contraindication for warfarin.²⁴ According to the HRS/EHRA/ECAS expert consensus statement on catheter and surgical ablation of AF, stand-alone AF surgery should be considered for symptomatic AF patients who are refractory or intolerant to at least one Class 1 or 3 AAD, prefer a surgical approach or have failed one or more attempts at catheter ablation.¹ Nevertheless, the Cox-Maze III procedure did not gain widespread implementation because of its complexity and technical challenge. In an effort to simplify the procedure, several groups replaced these incisions with linear lines of ablation. Shen et al.²⁵ introduced the Cox-Maze IV procedure using bipolar RF devices to isolate the PVs and create linear lesions.²⁵ The results of the Cox-Maze III and IV procedure in terms of rate of freedom from AF at 3, 6, and 12 months are comparable.²⁶ The last decade has been marked by a quest for new surgical ablation devices using unipolar and bipolar RF, microwave, laser, cryoablation, or high-frequency ultrasound that would enable the cardiac surgeon to perform a curative lesion epicardially on the beating heart without the need for cardiopulmonary bypass.²⁷ Unfortunately, none of the currently existing technology is able to guarantee reliable, transmural lesions in all patients. An important shortcoming of the surgical approach is the inability to map and selectively ablate any reentrant or focal tachycardia occurring during surgical AF ablation. Current techniques for the minimally invasive surgical treatment of stand-alone AF result in success rates (defined as freedom from any AF episode longer than 30 seconds off AAD) at 12 months from 65% to 92% in paroxysmal AF and from 67% to 80% in persistent AF.²⁷

HYBRID ABLATION

Given the current knowledge about etiology and pathophysiology of AF, an optimal ablation procedure would (1) isolate the PVs permanently, (2) define the specific properties of the underlying atrial electrical substrate to customize the subsequent ablation strategy, (3) always create completely transmural linear lesions when indicated, and (4) be minimally invasive. To date, neither the endocardial approach nor the epicardial surgical ablation procedure is able to meet all these criteria. However, both techniques seem to be complementary as, performed in combination (hence the nomenclature “hybrid” or “convergent” procedure), they bear the potential to overcome their respective shortcomings (Table 7.1). The epicardial surgical approach seems to result in superior transmurality of the lesions, resulting in long-lasting PV isolation and permanent conduction block, especially when using bipolar RF.^28,29 The endocardial AF ablation procedure on the other hand, using multipolar catheters and three-dimensional electroanatomical mapping systems, is the most efficacious setting to guide substrate modification and ablation of ATs known to occur during the stepwise ablation of persistent AF.³⁰ Another important advantage of the hybrid approach is the possibility of performing endocardial touch-up in case of an epicardial lesion that is not completely transmural.

There appears to be a wide variation in the possible combinations of epicardial surgical and transvenous endocardial techniques, and lesion sets used for hybrid or convergent AF ablation procedures (Table 7.2).^31,32

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