Atrial fibrillation (AF) affects more than 6% of Americans older than 65. The American Heart Association states that about 2.5 million people in the U.S. have AF and that 300,000 new cases will be discovered each year. The origin of AF has been hotly debated, with some theories involving the tissue of pulmonary vein sleeves. More recently, the role of ganglia located around these veins and the ligament of Marshall (LOM) have been implicated as sites of origination.

       Many approaches to AF ablation have been proposed over the past 5–10 years. Some of these are catheter-based, and some require surgery. Catheter ablation approaches have included pulmonary vein isolation and creation of linear lesions. Most prominent among the surgical approaches has been the open-chest approach of the Cox-Maze procedure.
       Stroke from clots that form predominantly in the left atrial appendage (LAA) during AF has been one of the most feared outcomes of atrial fibrillation. Approaches to management of this problem have included anticoagulation and development of devices to block or amputate the LAA.
       The focus of this article will be a discussion of a new minimally invasive operative procedure for AF that approaches treatment from three different angles. The procedure and equipment used will be discussed in depth, followed by a case report of a patient who underwent the Mini-Maze procedure.

Figure 1.

AtriCure Isolator™ Transpolar™ Clamp. Polished gold electrodes at the tip; comes in standard and long jaw lengths and right- and left -sided curve options for various operative approaches.

Equipment Used in the Mini-Maze
       The Mini-Maze procedure is performed utilizing equipment which has been adapted from open-heart procedures. AtriCure (West Chester, OH) developed a series of bipolar ablation clamps called the Isolator™ and a generator called the AtriCure ASU™, which together deliver radiofrequency current in bipolar fashion. The Isolator (Figure 1) is a unique bipolar ablation system that is responsive to the tissue characteristics. The system couples mechanical and electrical principles to tailor each ablation to the specific targeted tissue. Mechanically, the ablation clamp applies parallel pressure uniformly along the length of the ablation line with pressure related to the volume of tissue within the jaws; the jaw form defines the width of the ablation and includes a thermocouple that measures actual tissue surface temperature. The polished gold electrode allows for a predictable high current density, and allows the system to accurately measure tissue impedance. The energy delivery algorithm measures tissue impedance 50x per second, and adjusts power accordingly to assure the appropriate dose of energy is delivered to the targeted tissue while reducing the risk of lateral thermal spread. Collectively, these mechanical and electrical features allow the system to rapidly recognize impedance rises above a threshold providing confidence that the ablation is transmural throughout the range of 1–10 mm of thickness. The Isolator is an automated system that reliably creates contiguous transmural ablation lines without the risk of thermal spread. Figure 2 is an example of tissue that has undergone ablation with this system. The Isolator™ is available with right and left curved models to accommodate the different operative approaches. Figure 3 shows the AtriCure ASU™.

Figure 2.

Tissue which has undergone ablation with the AtriCure system. Notice the transmurality and the discreet nature of the lesion.

What is the Mini-Maze Procedure?
       Since 1999, the term “Mini-Maze” referred to open-chest “Maze-like” procedures, with fewer lesions than the original Cox-Maze. They still required a median sternotomy, cardiopulmonary bypass, and an endocardial approach to the atria. In 2003, Saltman published the first closed-chest, totally endoscopic, epicardial ablation of AF; he also used microwave energy, so this is sometimes referred to as the “microwave minimaze”. At the same time, another procedure using radiofrequency energy was being developed by Dr. Randall Wolf, Professor of Cardiothoracic Surgery and Director of the Center for Surgical Innovation, University of Cincinnati Surgeons; he published his first report in 2005. The Wolf Mini-Maze is a minimally invasive, off-pump, epicardial bipolar radiofrequency ablation involving isolation of the pulmonary veins, ablation of ganglionated plexuses of the left atrium, removal of the LOM and removal of the LAA (Table 1). Dr. Wolf has a web site devoted to his procedure at www.wolfminimaze.com. A site devoted to all Mini-Maze-like procedures can be found at www.minimaze.org. This site includes a video of the procedure highlighting the partnership between the surgeon and electrophysiologist who work together to map, stimulate and ablate targeted tissues.
       The following describes a typical Mini-Maze procedure. The patient is brought to the operating room, given anesthesia and intubated and turned to a left lateral decubitus position. The right chest is prepped and draped, and intercostal block is performed. Then a trocar is advanced through the sixth or seventh intercostal space. This is followed by a fiberoptic camera, which is advanced to inspect the right hemithorax. A small incision is made through the third intercostal space and used as a working port. Next, the pericardium is opened 3 cm anterior to the phrenic nerve. The area around the right pulmonary veins is dissected using a blunt dissection tool such as the AtriCure Wolf™, Lumitip™ dissector. Then the right pulmonary veins are mapped by the electrophysiologist and the ganglia are stimulated with high frequency. Affirmation of ganglia location occurs when heart block is induced. The Isolator Transpolar Clamp™ is advanced through the port in the seventh intercostal space and placed across the atrial tissue at the atrial cuff or away from the pulmonary vein ostis. A transmural circumferential lesion is created at this point. The ganglionic tissue is clamped and ablated as well (Figure 4). The failure to induce heart block through stimulation of the ganglia post ablation suggests that the ganglia have been successfully ablated. Confirmation of conduction block from the pulmonary veins post ablation is also obtained.

Figure 3.

The AtriCure ASU™ ablation and sensing unit. Tissue conductance is monitored 50 times per second in a continual feedback loop during RF delivery.

Figure 4.

Atricure Isolator™ Transpolar™ Clamp during ablation of a ganglionic plexus.

       The same procedure takes place in the left side of the chest. However, on the left, the LOM is divided and ablated, and the LAA is stapled and excised. The instruments are withdrawn and the operative areas are closed.
       Dr. James McClelland, electrophysiologist with Oregon Cardiology, says that in his experience, there are usually about four to five lesions required on each side to eliminate conduction between the atria and pulmonary veins. He checks conduction by putting a catheter directly on the pulmonary veins, six sites on each side. Most of the ganglionated plexi, perhaps about 85%, are eliminated by these lesions, but stimulation testing is used to determine if any remain; if so, they are eliminated with bipolar RF forceps.
       Post procedure, the pre-procedure antiarrhythmic medications are generally continued. It is expected that some AF will occur for up to three months after the Mini-Maze. Patients are started on blood thinners and a short course of anti-inflammatory drugs on the first post-op day. The antiarrhythmic drugs the patient was on pre-op are weaned after a few weeks. Blood thinners are usually maintained for at least six months.

Theoretical Basis for the Mini-Maze Procedure
Cox-Maze Procedure
       Dr. James Cox pioneered the open-heart “maze” procedure. The Cox-Maze procedure currently included pulmonary vein isolation and linear lesions in the atrium. Amputation of the left atrial appendage is also performed. The procedure has become the “gold standard” for the elimination of AF. The current version, Maze-III, has a success rate greater than 90% and a mortality rate of about 1%. Five percent of patients may require a pacemaker.¹ The development of the Mini-Maze procedure creates opportunities for patients who may not want to undergo an open-chest procedure or who may be considered at higher operative risk from co-morbidities, to undergo a potentially curative procedure. Additionally, the Mini-Maze offers an alternative for catheter ablation.

Ganglia as Targets
       The ganglionated plexuses of the heart have become ablation targets because of the observation that excessive nerve activity can initiate and possibly even sustain AF; therefore, elimination of these nerves may decrease the incidence of AF. The ganglionated plexuses were found to live coincidently in areas where nerves are often cut in the surgical maze procedures or where lesions were created during ablation. The right parasympathetic nerve travels toward the right pulmonary veins and the interatrial groove. The left travels in the direction of the base of the LAA and the left superior pulmonary vein. Atrial regions where ganglionated plexuses are found include the superior surfaces of the right and left atrium, the inferior and lateral aspect of the posterior left atrium, the posterior surface of the right atrium, and the posterior medial surface of the left atrium (Table 2).² Ablation of epicardial fat pad regions in canine work has also led to the creation of denervation. This has occurred in regions between the SVC and aortic root, between the IVC and left atrium, and over the right pulmonary veins.³

Table 1. Components of the Mini-Maze Procedure

- Pulmonary vein isolation Ablation of ganglionated plexuses of left atrium Ablation of ligament of Marshall Stapling of left atrial appendage

Table 2. Atrial Locations of Ganglionated Plexuses

- Superior surface of right atrium Superior surface of left atrium Inferior and lateral aspect of posterior left atrium Posterior surface of right atrium Posterior medial surface of left atrium

The Ligament of Marshall (LOM)
       The LOM is a vestigal remnant of the left common cardinal vein, which becomes a ligament. It extends from the coronary sinus posteriorly and superiorly, and travels along the posterior atrial wall between the LAA and the left pulmonary veins in the epicardial groove. It may serve as a source of adrenergic atrial tachycardia.⁴ In a 1999 study, Doshi et al.⁵ induced an automatic rhythm with isoproterenol in normal canine tissues. The LOM was found to be the origin of the spontaneous activity. Rapid automatic activity transitioned to multiple wavelets of AF. Ablation of the LOM terminated the spontaneous activity and AF was prevented.

The Left Atrial Appendage (LAA)
       The LAA is a major source for development of thromboemboli, which may cause stroke in the AF patient. The open-heart maze procedure includes the amputation of the left atrial appendage, eliminating it as potential clot-developing location. The LAA surface is trabeculated and contains ridges, compared to the smooth surface seen in the rest of the left atrium. Stasis in the LAA is promoted by virtue of this anatomy and the more distanced location of the appendage. Inclusion of the LAA stapling/amputation in the Mini-Maze procedure contributes to favorable post-operative outcomes by eliminating this potential clot source.

Mini-Maze Case Report
       The following is a case report of a patient who underwent the Mini-Maze procedure. Mr. E., a 55-year-old consultant with persistent AF, was referred to Dr. Marc Gerdisch, Director of Cardiac Surgery at Central DuPage Hospital and Clinical Professor of Cardiothoracic Surgery at Loyola University Medical Center in Chicago, for the Mini-Maze procedure. Mr. E. is a golfer with an active lifestyle and a father of two. He experienced breakthrough episodes of AF, which required cardioversion, even though he had been on amiodarone for years. His symptomatology included tightness in his chest and a flutter sensation. He had also been maintained on coumadin. He was seeking a procedure that would eliminate his AF and remove his LAA, thus decreasing his risk of stroke.
       Mr. E. was brought to the operating room, where he was given anesthesia and an endotracheal tube was placed. An intraoperative transesophageal echocardiogram was performed. His LV systolic and diastolic function were found to be preserved and no regional wall abnormalities were seen. He had mild to mild-moderate mitral regurgitation; no clots were visualized in his LAA.
       The Mini-Maze procedure was performed. The post procedural TEE confirmed complete resection of the LAA. He transferred from the OR in stable condition. The procedure was completed in three hours. Mr. E. was discharged home the day after surgery. Three days later he was gardening, and four days later he played a modified game of golf. Mr. E. says he feels more energized and has a decreased level of anxiety. He is doing well three-months post-op, and is being weaned from his antiarrhythmic medication. He will remain on coumadin for at least six months.
       Dr. Gerdisch comments: “This procedure is a substantial advancement over what we have been able to do in the past. The tools used in the Mini-Maze procedure have been shown to consistently create transmural lesions. I have many patients who are undergoing pre-operative testing to determine eligibility for this procedure. Patients have to understand the advantages and the limitations of different treatment approaches. Patients with larger left atrias and therefore greater substrate will usually be better served with the standard maze and open-chest procedure.” Dr. Gerdisch’s first Mini-Maze patient is an active 74-year-old who is now nine-months post procedure and doing well.
       Dr. McClelland states: “I think one has to decide two things when determining the best way to ablate AF. First, what the target is for ablation of AF? If we are doing ‘substrate modification,’ what exactly are we trying to modify? Is it PV-LA conduction? Is it the ganglia? Nademanee's CFAEs (which many believe are markers for the ganglia)? Second, what's the best way to ablate this target? It's clear to me that obtaining conduction block and eliminating the ganglia are far more easily and accurately done epicardially, under direct visualization, than endocardially with a catheter; we’ve done both. We were doing catheter ablation of AF from 1998 to early 2004, but we're doing Wolf's procedure exclusively now. The microwave minimaze doesn't allow proving conduction block, nor that the ganglia were affected, so we chose this. The downside, of course, is that it’s surgery. More discomfort, longer hospital stay (about four days). I suspect that the trade-off is worth it.”

Conclusion
       AF may pose the greatest curative challenge faced by electrophysiologists and surgeons who treat arrhythmias. An entire industry is in search of a magic bullet that will slow the incidence and course of this disease. The Mini-Maze procedure, offering a multifaceted treatment approach packaged into a minimally invasive procedure, may offer hope to some of those seeking relief from a progressive and disabling rhythm.