Concomitant Atrial Fibrillation Surgery

Abstract

Atrial fibrillation (AF) is the most common cardiac arrhythmia and is the major cause of stroke and heart failure. The treatment options of AF include medical treatment and catheter-based or surgical ablation. Cox et al. introduced the Cox-Maze procedure (the cut-and-sew Maze) that was first performed clinically in 1987 at Barnes Jewish Hospital, St. Louis, MO. This procedure is characterized by multiple incisions created at both left and right atria to terminate AF while allowing the electrical impulse generated from sinoatrial node to atrioventricular node. The Cox-Maze IV is the latest iteration developed by Damiano Jr. et al., which replaced the previous cut-and-sew Maze with a combination of less invasive linear lesions achieved by new ablation technology, the bipolar radiofrequency (RF), and cryoablation. This chapter describes the operative techniques, preoperative planning, indication for surgery, and future option of surgical treatment.

1. Introduction

Atrial fibrillation (AF) is the most common cardiac arrhythmia and is the major cause of stroke. Over the past several decades, AF surgical Ablation has become a treatment option for refractory and debilitating cardiac arrhythmias. Surgical innovation and ablation technology has rapidly evolved and has revolutionized therapies for AF. Although indications for surgical ablation have narrowed, AF surgery remains an important and effective treatment option.

Medical treatment has had many shortcomings including both the inefficacy and unwanted side effects of many antiarrhythmic drugs [1]. Initial attempts aimed at providing rate control failed the development of catheter and surgical ablation in the 1980s. After a decade of experimental studies, the Maze procedure was introduced in 1987. It has gone through several modifications and has become the gold standard for the treatment of AF [2].

2. Previous surgical ablation

2.1 The left atrial isolation procedure

In 1980, Dr. James Cox developed the left atrial isolation at Duke University. This is the first surgical procedure designed specifically to eliminate AF. This procedure confined AF to the left atrium, allowing the remainder of the heart to be in sinus rhythm [3, 4, 5].

2.2 The Corridor procedure

In 1985, Guiraudon et al. developed the corridor procedure to isolate a strip of atrial septum including both the sinoatrial and atrioventricular nodes to allow the sinoatrial node to pace both right and left ventricles [6]. The shortcomings of the corridor procedure included failure to prevent thromboembolism as well as atrioventricular dyssynchrony. It did have the advantage of preserving atrioventricular conduction [6].

2.3 The atrial transection procedure

Cox et al. first described the atrial transection procedure in 1985. Using a canine model, they found that a single longitudinal incision around both atria and down into the septum would terminate AF [4, 7]. This procedure effectively prevents AF or atrial flutter in animals but not in humans. Although this procedure was abandoned, it represented a transitional step toward the Cox-Maze procedure.

2.4 The Cox-Maze procedure

After extensive animal studies, The Cox-Maze procedure was introduced clinically by James Cox in 1987 [2, 4]. This procedure was designed based on the theory that macro-reentrant circuits cause and propagate AF. The Cox-Maze procedure restored sinus rhythm as well as AV synchrony, reducing the risk of stroke and thromboembolism [8]. The original of Cox-Maze procedure, The Cox-Maze I, consisted of multiple surgical incisions across both atria that allowed for a pathway for the sinus impulse to reach the AV node. It permitted depolarization of most of the atrial myocardium resulting in preservation of most of the atrial contractile function [9].

The Cox-Maze II was formulated because the Cox-Maze I had problems with late chronotropic incompetence with a high incidence of pacemaker implantation [10].

The Cox-Maze III proved to be effective and became the gold standard for surgical treatment of AF [1, 2]. The procedure is often called the cut-and-sew maze, and the surgical results of the Cox-Maze III were excellent, with over 90% of patients free from symptomatic AF at late follow up [11, 12, 13, 14]. Despite its efficacy, the cut-and-sew maze was technically difficult and required a lengthy period of aortic cross clamp time, which limited its widespread adoption. Over the last two decades, modern ablation devices transformed the Cox-Maze III into an easier, shorter, and less invasive procedure, which has been termed the Cox-Maze IV. These have allowed for more widespread adoption [15]. These modern technologies have been used to replace the surgical incisions and have allowed the development of less invasive approaches.

3. Surgical ablation technology

When ablation technology was introduced, the goal was to replace the incision lines of the Cox-Maze III as much as possible. An ablation technology had to have 3 characteristics in order to replicate a surgical incision. First, a device had to create a line of bidirectional conduction block. A pattern of lesions with these properties can prevent AF, either through blocking macro or micro-reentrant circuits or by isolating focal triggers. Only transmural lesions produce reliable conduction block, as even small gaps in ablation lines retain the ability to conduct both sinus and fibrillatory impulses [16, 17]. Second, the ablation device had to be safe. Safety requires that the device delivers a precise dose of energy to minimize both inadequate ablation and potential injury to nearby vital structures by excessive ablation. Lastly, the ablation device had to make AF surgery simpler and require less operative time compared to the original technique.

Several ablation technologies have been developed, and each has its relative advantages and disadvantages. Cryoablation and bipolar radiofrequency (RF) devices have been shown to be the most effective and are the ablation technologies used for the Cox-Maze IV [18].

3.1 Cryoablation

Cryoablation technology creates ablation lines by freezing myocardial issue. Cryoablation preserves the myocardial fibrous skeleton and collagen structure, making it one of the safest energy sources available. These devices work by pumping a liquid refrigerant to the tip of a device where it undergoes evaporation, and in the process absorbing heat from the tissue in contact with the tip. This causes intracellular and extracellular water to freeze. The resulting ice crystals disrupt the plasma membrane and cause early cell death via cell lysis. Lesions also expand due to induced apoptosis. The size of the lesion produced depends on the thermal conductivity and temperature of both the probe and the tissue [19].

Two commercially available sources of cryothermal energy are in clinical use in cardiac surgery. Nitrous oxide-based devices are manufactured by AtriCure (Cincinnati, OH). Devices using Argon have been developed and are currently distributed by Medtronic (Minneapolis, MN). The minimum temperature that can be produced by an ablation device is limited by the thermodynamic properties of the refrigerant used. At 1 atmosphere of pressure, nitrous oxide is capable of cooling tissue to −89.5°C, whereas argon can cool tissue to −185.7°C. Nitrous oxide based cryoablation has a long history of clinical use with a well-defined efficacy and is safe except around the coronary arteries [20, 21]. Experimental and clinical studies have shown intimal hyperplasia and coronary artery stenosis after cryoablation [21, 22, 23]. The disadvantage of cryoablation is the relatively longer time required to create transmural lesion (usually 2–3 minutes). There is also difficulty in creating lesions on beating heart from an epicardial approach due to the circulating blood acts as a heat sink effect [24]. Moreover, freezing of intra-atrial blood poses a potential risk of thromboembolism.

The nitrous oxide technology can be used with both the rigid, reusable, and the flexible, disposable probes. The argon technology is available only as a flexible, disposable ablation device.

3.2 Radiofrequency energy

Radiofrequency (RF) has been used for many years by cardiac electrophysiologists and surgeons to ablate cardiac tissue. RF energy can be delivered using unipolar or bipolar electrodes.

3.3 Unipolar RF

Energy is delivered between the tip of electrode and a grounding pad attached to the patient. Unlike bipolar RF energy, an alternating amount is delivered between 2 jaws of a clamp. Several factors contributed to lesion size such as tissue contact area, interface temperature, the amount of power applied and duration of energy delivery. Several factors can limit the depth of the lesion, potentially preventing successful creation of a transmural lesion with conduction block. These include char formation, epicardial fat, myocardial and endovascular blood flow, and tissue thickness. Several unipolar RF ablation devices have been developed. These include dry, irrigated and suction assisted devices. These devices have had limited applicability in cardiac surgery. Transmural lesions can be created with dry unipolar RF on the arrested heart in animal studies. Unfortunately, this has not been reproducible in clinical practices [25]. There is one study that confirmed that only 20% of transmural lesions achieved after 2 minutes of ablation time during mitral valve surgery. Moreover, the results of epicardial unipolar RF ablation on the beating heart were found to be even less successful. Another animal study has demonstrated the epicardial unipolar RF failed to create transmural lesions on the beating heart [26]. One clinical study has shown only 10% success rate of transmural lesions achieved after epicardial RF ablation [27]. Convection caused by circulating blood that explained the failure of epicardial unipolar RF ablation on the beating heart [28, 29]. No unipolar RF device has been shown by independent laboratories to be capable of reliably creating transmural lesions on the beating heart [30]. The recent expert consensus guidelines mentioned that the use of epicardial unipolar RF ablation outside of clinical trials is not recommended because its efficacy remains questionable [31].

3.4 Bipolar RF

The electrodes are embedded in the jaws of a clamp to focus the delivery of energy. Multiple studies have shown bipolar RF ablation to be able to create transmural lesions on the beating heart in animals and humans with short ablation times [32, 33, 34]. Bipolar RF devices are currently sold by two companies in United States (AtriCure, West Chester, OH and Medtronic, Minneapolis, MN). Both devices have shown similar experimental and clinical efficacy. Bipolar RF energy also has a more favorable safety profile compared to unipolar RF. Some clinical complications of unipolar RF devices have been reported including coronary vessel injuries, stroke and esophageal perforation leading to atrioesophageal fistula [35, 36, 37, 38, 39]. There have been no collateral injuries reported after bipolar RF technology despite its widespread clinical use. Innovation continues within the field of RF ablation technology, including the development of unipolar-bipolar hybrid devices. The Cobra Fusion (AtriCure, West Chester, OH) a suction-assisted device that combines bipolar and unipolar RF, has been shown in early experimental reports to have improved efficacy in creating transmural epicardial lesions on the beating heart [40].

The recent expert consensus guidelines state that the best evidence exists for the use of bipolar radiofrequency (RF) clamps and cryoablation devices, which have become integral parts of many procedures, including pulmonary vein isolation and the Cox-Maze IV procedure.

Bipolar RF clamps or cryoprobes (both reusable and/or disposable) are recommended to be used for PVI in both empty arrested and beating heart with exit block confirmation testing. For beating heart, endocardial cryoablation is recommended for free wall linear ablation instead of epicardial cryoablation due to higher success rate of transmurality. The guidelines also suggest to identify and avoid injury to coronary vessels while doing ablation with any devices [31].

Other energy delivery devices including microwave, laser, and ultrasound have been used clinically but limitations of these technologies have led to limited use and withdrawal of these devices from the market [28, 41, 42, 43, 44, 45].

4. Indication for surgical atrial fibrillation ablation

A recent consensus statement described current indications for surgical ablation of AF include

1. All symptomatic AF patients undergoing concomitant cardiac surgery

2. Selected asymptomatic AF patients undergoing concomitant cardiac surgery which the ablation can be performed by experienced surgeons; and

3. Symptomatic AF patients who have failed medical treatment or have recurrent AF after catheter ablation and prefer stand-alone surgical approach [46].

Strong evidence showing an association between AF and increased mortality and morbidity has led to recent expert consensus guidelines to recommend surgical ablation at the time of concomitant cardiac procedures to restore sinus rhythm with a Class I or IIa indication [31, 47]. Addition of concomitant surgical ablation for AF does improve health-related quality of life (HRQL) and AF-related symptom. It also improves short-term and long-term survival [31].

Some relative indications for surgery that were mentioned in AF surgery expert institutions are persistent AF patients with high risk for stroke and CHADS2 score ≥ 2 and inability to take life-long and anticoagulation and persistent AF patients with proper anticoagulation but still have had a cerebrovascular event [2].

Recent consensus guidelines recommend addition of a concomitant surgical ablation procedure for AF does not change the incidence of perioperative or late stroke/TIA (class IIa) but subgroup analysis of nonrandomized controlled trials found a significant reduction in late stroke/TIA incidence [31].

5. Surgical technique

There are several procedures that are currently performed to treat AF: the Cox-Maze IV procedure, left atrial lesion sets, pulmonary vein isolation (PVI), and hybrid ablations.

5.1 The Cox-Maze IV procedure

The Cox-Maze IV replicates the Cut-and-sew Maze lesion set using bipolar RF energy and cryoablation to replace most of the incisions in the Cox-Maze III [2, 48]. Clinical results have shown that the Cox-Maze IV achieves the high success rate of the Cox-Maze III with significant reduction in operative time and lower complication rates [2, 49]. The Cox-Maze IV procedure requires cardiopulmonary bypass. It can be performed either through a median sternotomy or a minimally invasive right thoracotomy approach. The selection of an approach should be based on the presence of concomitant cardiac pathology, patient-specific anatomic characteristics and the experience of the surgeon [2]. Patients who are in AF at the time of surgery and have no intracardiac thrombosis on intraoperative transesophageal echocardiogram are electrically cardioverted and started on intravenous amiodarone. Pacing thresholds are measured from each pulmonary vein [2, 50].

5.2 Median sternotomy approach

For the standard fashion, the right and left pulmonary veins are bluntly dissected, mobilized and encircled with umbilical tapes. Amiodarone is given and electrical cardioversion is performed if the patient is in AF. Pacing thresholds are performed and pulmonary veins isolation (PVI) is achieved using bipolar clamps. After PVI is completed, exit block is confirmed from each pulmonary vein [2, 50].

To perform the right atrial lesion set, the patient is then cooled to 34°C. Lesions from right atrial lesion set are performed on the beating heart. Two purse-string sutures are created at the base of both right and left atrial appendage which is large enough to place a jaw of bipolar RF clamp. Right atrial free wall ablation is first performed through the previously made purse-string suture down toward the aortic side of the right atrial appendage. Right atriotomy is created vertically toward the atrioventricular (AV) groove. Superior and Inferior vena cava lesions are performed using RF clamp applying from inferior aspect of previous right atriotomy incision. Next, endocardial ablation is created using a linear cryoprobe starting from the right atriotomy down onto the 2 o’clock position of the tricuspid annulus. Then another endocardial ablation lesion is performed using the linear cryoprobe inserted through the previous made purse-string suture down to the 10 o’clock position of tricuspid annulus (this lesion can be omitted in case of small right atrium and no tricuspid regurgitation) [2, 50].

After the aortic clamp is on and the heart is completely arrested, the left atrial lesions set is performed. The left atrial appendage is identified and amputated, and an ablation is performed through the amputated left atrial appendage. The bipolar RF clamp is used to create a connecting lesion into the left inferior or superior pulmonary vein. The left atrial appendage is then oversewn in a double layer. Methylene blue is then used to mark the coronary sinus. Then left atriotomy incision is made, the roof and floor lesions are created with bipolar RF clamp. From the inferior margin of left atriotomy, bipolar RF clamp is applied to create ablation line toward the mitral annulus and across the coronary sinus. A bell-shaped cryoprobe is used to make and endocardial lesion to the mitral annulus at the end of the mitral isthmus lesion. An epicardial cryoablation is performed over the coronary sinus in line with the endocardial lesion to complete the left atrial isthmus lesion [2]. Some institutes use only cryoablation to create the Cox-Maze IV lesion set. The atriotomy is then closed. The patient is weaned from cardiopulmonary bypass and the sternotomy closed in a standard fashion.

5.3 Right minithoracotomy approach

The patient is intubated with a double-lumen endotracheal tube with right lung deflation. Femoral cannulation is obtained for cardiopulmonary bypass. A small minithoracotomy is performed over the fourth intercostal space, midaxillary line. For the right minithoracotomy approach, the ablation lesions set remains the same. The right atrial lesion ablation is performed through 3 purse-string sutures as in a minimally invasive approach. In case of left atrial lesion set, the pattern of ablation also remains the same except for the left pulmonary vein isolation is performed endocardially using cryoablation probe to connect the superior and inferior box lesions. The left atrial appendage exclusion is performed by double layer oversewing endocardially.

5.4 Cox-Maze IV surgical result

The Cox-Maze III procedure had excellent success rates for the treatment of AF. One of the studies at Washington University examined the outcomes of 198 patients who underwent the Cox-Maze III procedure. Their study showed a 97% freedom from symptomatic AF with a mean follow up of 5.4 years and no difference in recurrence when comparing patients who received a stand-alone Cox-Maze III versus patients who received a concomitant procedure [32]. Similar results have been obtained from other studies with the cut-and-sew method [12, 14, 51]. However, very few of these patients had prolonged monitoring or even follow-up electrocardiograms or prolonged monitoring to assess the rhythm.

The modification of the Cox-Maze IV simplified the traditional procedure and made it easier to perform. This allowed for the development of minimally invasive approaches and more widespread adoption, allowing for many more patients to receive surgical ablation at the time of concomitant surgery [52]. In 2018, the number of patients had increased to more than 30,000 by estimates using the Society of Thoracic Surgeons (STS) Adult cardiac surgery database [47, 52]. Badhwar et al reported an overall increase of 50% in performing concomitant surgical ablation from the year of 2011–2014 [47, 52].

The recent study from Damiano group demonstrated an excellent long-term efficacy at maintaining sinus rhythm of the Cox-Maze IV with 77% overall freedom from recurrent ATAs at 10 years follow-up [52]. Moreover, at late follow-up, the results of the Cox-Maze IV remained superior to those reported for catheter ablation and other forms of surgical ablation for AF [52].

The findings from other studies also support the recommendation that the Cox-Maze IV should be considered in all patients undergoing concomitant cardiac surgery if it can be performed without adding morbidity or mortality to the procedure [52, 53, 54, 55, 56].

5.5 Left atrial procedures

Most centers have advocated performing ablation confined to the atrium only to treat AF. Since the majority of paroxysmal AF appears to originate from the pulmonary veins and the posterior of left atrium. Left atrial lesion set typically involves pulmonary vein isolation, with a lesion to the mitral annulus and the left atrial appendage removal/exclusion. The advantage of avoiding right atrial lesions is a potential of lower rate of postoperative pacemaker implantation [57]. However, Gillinov et al. published a large series demonstrating the omission of the left atrial isthmus lesion resulted in a significantly higher incidence of recurrent AF in persistent AF patients [58]. To complete this isthmus lesion, it is important to ablate the coronary sinus in line with the endocardial lesion. Some studies have shown that isolation of the entire posterior left atrium is associated with improved outcomes compared with isolation of the pulmonary veins alone and had significantly higher rate freedom from AF when compared with left atrial set alone [59, 60]. Some studies have shown that AF can originate from the right atrium in up to 30% [61, 62, 63].

5.6 Pulmonary vein isolation (PVI)

Pulmonary vein isolation can be performed without cardiopulmonary bypass with minimally invasive technique via either minithoracotomy or thoracoscopy and can be easily added to other cardiac surgical procedure. Haissaguerre study documented that the triggers for paroxysmal AF originate from pulmonary veins in the majority of cases [64]. However, up to 30% of triggers may originate outside the pulmonary veins [65]. This is further informed by anatomic substrates that could be the generation of AF as extrapulmonary triggers located at the superior vena cava, the ligament of Marshall, and the epicardial ridge between the left pulmonary vein and the left atrial appendage [66]. The pulmonary veins can be isolated separately or as a box lesion. The most common approach for treatment of lone AF uses an endoscopic, port-based approach. Bipolar RF clamps are favored but unipolar RF, cryoablation, and high-intensity focused ultrasound devices have also been used [43, 67, 68]. Although energy sources such as microwave proved not to deliver effective lesions [69]. The application of RF bipolar clamp to create PVI antral pairs via bilateral thoracotomies has been established as a safe procedure with reasonable short-term efficacy [70]. There is a study that demonstrated the late gaps in ablation lines occurred after epicardial PVI ablation regardless of previously exit block confirming intraoperatively [17]. This could be a supporting idea of beneficial combining epicardial PVI with endocardial ablation [31]. The FAST trial, a multicenter randomized trial, compared 63 patients who received linear antral pulmonary vein isolation by catheter ablation and 61 patients who received bipolar RF PVI and ganglion plexus ablation. Most patients had paroxysmal AF. At 1 year, freedom from left atrial arrhythmia without ATA was 66% for surgical ablation versus 36% for catheter ablation [71].

This procedure will be completed until the left atrial appendage has been addressed. In the past, this had been done by stapling across the base of the left atrial appendage. This requires careful surgical technique and attention due to it can result in tears and bleeding [72]. Clip devices have been developed to address this difficulty. They improved efficacy and safety when compared to staplers [73, 74].

5.7 Hybrid ablations

To lessen the invasiveness of surgical ablation, extended epicardial ablation was introduced, which can be placed through thoracoscopic ports. However, these probes have not been able to achieve the same degree of transmurality created by bipolar clamps [30]. This led to the idea of combining endocardial ablation via transcatheter techniques and epicardial ablation via surgical techniques in a hybrid approach. Based on current experience, the hybrid approach with the most effective outcomes and safety profile appears to be bilateral pulmonary vein procedures performed surgically with left atrial appendage management combined with different endocardial ablation protocol [31]. The principles of these approaches are based on the understanding that it is possible to apply mapping techniques from electrophysiologists to surgical epicardial ablation techniques when performed on beating heart [31].

Currently, there are several procedures being performed surgically, combining with endocardial ablation e.g., PVI procedures either bilateral thoracoscopic/minithoracotomy approach or unilateral thoracoscopic PVI posterior encircling box lesion with or without left atrial appendage management. There is also an alternative approach to posterior left atrial wall epicardial ablation lesion (pericardioscopic epicardial debulking ablation procedures, also known as “convergent method”) [31].

The recent expert guideline favors the hybrid approach over percutaneous catheter ablation in terms of results in a subgroup of symptomatic AF patients who have had failed medical and percutaneous catheter ablation treatment [31].

6. Conclusion

The ideal surgical procedure for AF would be a minimally invasive procedure that does not require cardiopulmonary bypass and should preserve normal atrial physiology, have minimal morbidity, and have a high success rate. Achieving this goal will require a better understanding of the mechanism of AF in individual patients and tailoring of treatment approaches. This would have significant advantages and prevent both over and under-ablation. This will require better preoperative diagnostics to identify mechanisms of AF. Non-invasive ECG imaging has a great potential in this era [75, 76] that could be beneficial to tailor lesion sets as well as to decide which specific ablation modalities for individual patients.