Atrial fibrillation (AF) remains the most common arrhythmia in the world.¹ It is associated with significant morbidity and mortality secondary to its detrimental sequelae: (1) palpitations resulting in patient discomfort and anxiety; (2) loss of atrioventricular (AV) synchrony, which can compromise cardiac hemodynamics, resulting in various degrees of ventricular dysfunction; (3) stasis of blood flow in the left atrium, increasing the risk of thromboembolism and stroke.^2-11

Medical treatment of AF has had many shortcomings including both the inefficacy of many of the antiarrhythmic drugs and their unwanted side effects. Because of this, interest in nonpharmacologic treatment approaches led to the development of catheter-based and surgical techniques beginning in the 1980s. Initial attempts aimed at providing rate control failed to address the detrimental hemodynamic and thromboembolic sequelae of AF. The early attempts at finding a surgical treatment culminated in the introduction of the Maze procedure in 1987, which became the surgical gold standard for decades.

The following sections describe the historical aspects and the current status of surgery for AF, including the introduction of minimally invasive techniques.

The Left Atrial Isolation Procedure

The first surgical procedure designed specifically to eliminate AF, the left atrial isolation, was described in 1980 in the laboratory of Dr. James Cox at Duke University. This approach confined AF to the left atrium, and restored the remainder of the heart to sinus rhythm (Fig. 54-1).¹² This reestablished a regular ventricular rate without requiring a permanent pacemaker. Isolating the left atrium allowed the right atrium and the right ventricle to contract in synchrony, providing a normal right-sided cardiac output. This effectively restored normal hemodynamics.

However, by confining AF to the left atrium, the left atrial isolation procedure only eliminated two of the three detrimental sequelae of AF: an irregular heartbeat and compromised cardiac hemodynamics. It did not eliminate the thromboembolic risk because the left atrium usually remained in fibrillation. This procedure never achieved clinical acceptance, and was only performed in a single patient.

Catheter Ablation of the Atrioventricular Node-His Bundle Complex

In 1982, Scheinman and coworkers introduced catheter fulguration of the His bundle, a procedure that controlled the irregular cardiac rhythm associated with AF and other refractory supraventricular arrhythmias.¹³ This procedure electrically isolated the fibrillation to the atria. Unfortunately, ablating the bundle of His required permanent ventricular pacemaker implantation to restore a normal ventricular rate.

The shortcoming of this intervention was that it only eliminated the irregular heartbeat. Both atria remained in fibrillation, and the risk of thromboembolism persisted. AV contraction remained desynchronized, compromising cardiac hemodynamics. In addition, patients become pacemaker dependant for the remainder of their lives. Nevertheless, AV node ablation has remained a common treatment for medically refractory AF.

The Corridor Procedure

In 1985, Guiraudon and associates developed the corridor procedure for the treatment of AF.¹⁴ This operation isolated a strip of atrial septum harboring both the sinoatrial (SA) node and the AV node, allowing the SA node to drive both the ventricles. This procedure effectively eliminated the irregular heartbeat associated with AF, but both atria either remained in fibrillation or developed their own asynchronous intrinsic rhythm because they were isolated from the septal “corridor.” Furthermore, the atria were isolated from their respective ventricles, thereby preventing AV synchrony. The corridor procedure was abandoned because it had no effect on the hemodynamic compromise or the risk of thromboembolism associated with AF.

The Atrial Transection Procedure

In 1985, Dr. James Cox and associates described the first procedure that attempted to terminate rather than simply isolate or confine AF to the atria.¹⁵ Using a canine model, they found that a single long incision around both atria and down into the septum could terminate AF. This atrial transection procedure effectively prevented AF or atrial flutter in animals.¹⁶ Although this procedure was not effective clinically and was soon abandoned, it laid the foundation for the development of the Cox-Maze procedure.

The Maze procedure was clinically introduced in 1987 by Dr. Cox after extensive animal investigation.^16-18 The Cox-Maze procedure was originally developed to interrupt any and all macro-reentrant circuits that were felt to cause AF, thereby precluding the ability of the atrium to flutter or fibrillate (Fig. 54-2). Unlike previous procedures, the Cox-Maze procedure successfully restored both AV synchrony and sinus rhythm, thus potentially reducing the risk of thromboembolism and stroke.¹⁹ The operation consisted of creating an array of surgical incisions across both the right and left atria. These incisions were placed so that the SA node could still direct the propagation of the sinus impulse throughout both atria. It allowed for most of the atrial myocardium to be activated, resulting in preservation of atrial transport function in animals and in most patients.²⁰

The first iteration of the Maze procedure was modified because of problems with late chronotropic incompetence and a high incidence of pacemaker implantation. The resulting Maze II procedure, however, was technically difficult to perform, and was soon replaced by the Maze III procedure (Fig. 54-3).^21,22

The Cox-Maze III procedure—often referred to as the “cut-and-sew” Maze—became the gold standard for the surgical treatment of AF. In a long-term study of patients who underwent the Cox-Maze III procedure at our institution, 97% of the patients at late follow-up were free of symptomatic AF.²³ These excellent results have been reproduced by other groups.^24-26

Although the Cox-Maze III procedure was effective in eliminating AF, it was technically difficult and required a lengthy period of aortic cross-clamping. This limited adoption and only a handful of cardiac surgeons still perform the cut-and-sew operation.

Over the last 15 years, the field of AF surgery was revolutionized by the introduction of a variety of ablation devices that have been used to replicate the surgical incisions on the atrium. This has made AF ablation much simpler to perform. These ablation-assisted procedures have greatly expanded the field of AF surgery and have resulted in a much wider adoption.²⁷ With present ablation technology, surgery can be performed with low morbidity and mortality and often through less invasive approaches.

The development of surgical ablation technology has transformed a difficult and time-consuming operation that few surgeons were willing to perform into a procedure that is technically easier, shorter, and less invasive. Several ablation technologies exist, each with its relative advantages and disadvantages.

For an ablation technology to successfully replace surgical incisions, it must meet several criteria. First, it must reliably produce bidirectional conduction block across the line of ablation. This is the mechanism by which incisions prevent AF, by either blocking macro-reentrant or micro-reentrant circuits or isolating focal triggers. Our laboratory and others have shown that this requires a transmural lesion, as even small gaps in ablation lines can conduct both sinus and fibrillatory impulses.^28-30 Second, the ablation device must be safe. This requires a precise definition of dose-response curves to limit excessive or inadequate ablation, and potential injury to surrounding vital cardiac structures, such as the coronary sinus, coronary arteries, and valvular structures. Third, the ablation device should make AF surgery simpler and require less time to perform. This requires the device to create lesions rapidly, be simple to use, and have adequate length and flexibility. The following sections will briefly summarize the two current ablation technologies in clinical use: cryoablation and radiofrequency (RF) ablation.

Cryoablation

Cryoablation technology is unique in that it destroys myocardial tissue by freezing rather than heating. It has the benefit of preserving the myocardial fibrous skeleton and collagen structure and is thus one of the safest energy sources available. These devices work by pumping a refrigerant to the electrode tip where it undergoes transformation from a liquid to a gas phase, and in so doing absorbs heat energy from the tissue in contact with the tip. The formation of intracellular and extracellular ice crystals disrupts the cell membrane and causes cell death. There is also evidence that the induction of apoptosis plays a role in late lesion expansion. Lesion size depends on the temperature of the probe and thermal conductivity and temperature of the tissue.³¹

There are currently two commercially available sources of cryothermal energy that are being used in cardiac surgery. The older technology, based on nitrous oxide, is manufactured by AtriCure (Cincinnati, OH). More recently, devices using argon have been developed, and are currently distributed by Medtronic (Minneapolis, MN). At 1 atmosphere of pressure, nitrous oxide is capable of cooling tissue to –89.5°C, whereas argon has a minimum temperature of –185.7°C. The nitrous oxide technology has a well-defined efficacy and safety profile and is generally excellent except around the coronary arteries.^32,33 Experimental and clinical studies have shown intimal hyperplasia and coronary stenosis after cryoablation.^33-35 The potential disadvantage of cryoablation, however, is its relatively long time required to create lesions (1-3 minutes). There is also difficulty in creating lesions on the beating heart because of the “heat sink” of the circulating blood volume.³⁶ Furthermore, if blood is frozen during epicardial ablation on the beating heart, it may coagulate, creating a potential thromboembolic risk.

Radiofrequency Energy

RF energy has been used for cardiac ablation for many years in the electrophysiology laboratory, and was one of the first energy sources to be applied in the operating room.³⁷ Resistive RF energy can be delivered by either unipolar or bipolar electrodes, and the electrodes can be either dry or irrigated. With unipolar RF devices, the energy is dispersed between the electrode tip and an indifferent electrode, usually the grounding pad applied to the patient. In bipolar RF devices, alternating current is passed between two closely approximated electrodes. The lesion size depends on electrode-tissue contact area, the interface temperature, the current and voltage (power), and the duration of delivery. The depth of the lesion can be limited by char formation, epicardial fat, myocardial and endocavity blood flow, and tissue thickness.

There have been numerous unipolar RF devices developed for ablation. These include both dry and irrigated devices and devices that incorporate suction. Although dry unipolar RF has been shown to create transmural lesions on the arrested heart in animals with sufficiently long ablation times, it has not been consistently successful in humans. After 2-minute endocardial ablations during mitral valve surgery, only 20% of the in vivo lesions were transmural.³⁸ Epicardial ablation on the beating heart has been even more problematic. Animal studies have consistently shown that unipolar RF is incapable of creating epicardial transmural lesions on the beating heart.^39,40 Epicardial RF ablation in humans resulted in only 10% of the lesions being transmural.⁴¹ This deficiency of unipolar RF ablation has been felt to be caused by the heat sink of the circulating blood.⁴² This has led industry to examine adding both irrigation and suction to improve lesion formation. Although these additions have improved depth of penetration, there has been no unipolar RF device that has been shown by independent laboratories to be capable of creating reliable transmural lesions on the beating heart.

To overcome this problem, bipolar RF clamps were developed. With bipolar RF, the electrodes are embedded in the jaws of a clamp to focus the delivery of energy. By shielding the electrodes from the circulating blood pool, this improves and shortens lesion formation and limits collateral injury. Bipolar ablation has been shown to be capable of creating transmural lesions on the beating heart both in animals and humans with short ablation times.^43-45 Two companies (AtriCure, West Chester, OH and Medtronic, Minneapolis, MN) currently market bipolar RF devices.

Another advantage of bipolar RF energy over unipolar RF is its safety profile. A number of clinical complications of unipolar RF devices have been reported, including coronary artery injuries, cerebrovascular accidents, and esophageal perforation leading to atrioesophageal fistula.^46-49 Bipolar RF technology has virtually eliminated this collateral damage; there have been no injuries described with these devices despite extensive clinical use. The Cobra Fusion^TM (AtriCure, West Chester, OH) is a new device that combines bipolar and unipolar RF into a single suction-assisted device. Early experimental results have shown consistency in creating transmural epicardial lesions on the beating heart.⁵⁰

Other types of devices utilizing microwave, laser, and ultrasound energy have been used clinically, but limitations of each technology have led to limited use and withdrawal of these devices from the market.^42,51-55

In summary, ablation technology has made significant progress over the past decade and has allowed for a broader application of surgical ablation. As new devices and techniques are developed, continued research investigating their effects on atrial hemodynamics, function, and electrophysiology is imperative.

There are three categories of procedures that presently are performed to surgically treat AF: the Cox-Maze procedure, left atrial lesion sets, and pulmonary vein isolation (PVI). Each of these approaches is described in the following sections.

The Cox-Maze IV Procedure

The original “cut-and-sew” Cox-Maze III procedure is rarely performed today. At most centers, the surgical incisions have been replaced with lines of ablation using a variety of energy sources. At our institution, bipolar RF energy and cryoablation has been used successfully to replace most of the surgical incisions of the Cox-Maze III procedure. Our current RF ablation-assisted procedure, termed the Cox-Maze IV, incorporates the lesions of the Cox-Maze III (Fig. 54-4).⁵⁶ Our clinical results have shown that this modified procedure has significantly shortened operative time while maintaining the high success rate of the original Cox-Maze III procedure.^57,58

The Cox-Maze IV procedure is performed on cardiopulmonary bypass with central or femoral cannulation. The operation can be done either through a median sternotomy or a less invasive (4-5 cm) right minithoracotomy. The right and left pulmonary veins (PVs) are bluntly dissected. If the patient is in AF, amiodarone is administered and the patient is electrically cardioverted. Pacing thresholds are obtained from each PV. Using a bipolar RF ablation device, the PVs are individually isolated by ablating a cuff of atrial tissue surrounding the right and left PVs. Proof of electrical isolation is confirmed after ablation by demonstrating exit block from each PV.

The right atrial lesion set is performed on the beating heart. A bipolar RF clamp is used to create most of the lesions (Fig. 54-5). A unipolar device, either cryoablation or RF energy, is used to complete the ablation lines endocardially down to the tricuspid annulus because of the difficulty of clamping in this area.

The remaining left-sided lesion set (Fig. 54-6) is performed on the arrested heart. First, the left atrial appendage is amputated and an ablation is performed through the amputated left atrial appendage into one of the left PVs. A standard left atriotomy is performed and extended inferiorly around the right inferior PV and superiorly onto the dome of the left atrium. Connecting lesions are made with the bipolar RF device into the left superior and inferior PVs and a final ablation is performed down toward the mitral annulus. Our group has shown that isolating the entire posterior left atrium with ablation lines into both left PVs resulted in a better drug-free freedom from AF at 6 and 12 months than a single connecting lesion.⁵⁹ Unipolar cryoablation is used to connect the lesion to the mitral annulus and complete the left atrial isthmus line.

Left Atrial Procedures

Over the past decade, there have been a number of new surgical procedures introduced in attempt to cure AF. The results have been variable and have had a wide range of success rates.^55,60-67 Various ablation technologies have been used to create a large number of different lesion patterns. All of these procedures have generally involved some subset of the left atrial lesion set of the Cox-Maze procedure. Results have been dependent on the technology used, the lesion set, and the patient population. From a technical standpoint, all of the approaches have attempted to isolate the PVs. The importance of the rest of the left atrial Cox-Maze lesion set has also been generally established. However, Gillinov et al. published a large series demonstrating that the omission of the left atrial isthmus lesion resulted in a significantly higher incidence of recurrent AF in patients with permanent AF.⁶⁸ To complete this lesion, it is mandatory to also ablate the coronary sinus in line with the endocardial lesion. In addition, our clinical results have shown that it is critically important to isolate the entire posterior left atrium and it is not enough just to isolate the PVs.⁵⁹

Pulmonary Vein Isolation

PVI is an attractive therapeutic strategy because the procedure can be done without cardiopulmonary bypass and with minimally invasive techniques, using either a thoracoscopic approach or small incisions. It can also be easily and quickly added to other cardiac surgery procedures (eg, coronary bypass graft or a valve procedure). Based on the original report of Hassaiguerre, it has been well documented that the triggers for paroxysmal AF originate from the PVs in the majority of cases.⁶⁹ However, it is important to remember that up to 30% of triggers may originate outside the PVs.⁷⁰ In an attempt to increase efficacy, some investigators have added ablation of the ganglionic plexi (GP).^71-73

The PVs can be isolated separately or as a box (Fig. 54-7). The most common approach for treatment of lone AF uses an endoscopic, port-based approach to minimize incision size and pain for the patient. At our center, bipolar RF clamps are favored to isolate the PVs, but unipolar RF, cryoablation, and HIFU devices have also been used.^53,74,75 Accumulating data suggest that a box isolation of the entire posterior left atrium is the more effective strategy.^59,76

Patient preparation begins with double-lumen endotracheal intubation. A transesophageal echocardiogram is performed to confirm the absence of thrombus in the left atrial appendage. If a thrombus is found, the procedure is either aborted or converted to an open procedure, in which the risk of systemic thromboembolism from left atrial clot can be minimized. External defibrillator pads are placed on the patient and the patient is positioned with the right side turned upward 45 to 60° and the right arm positioned above the head to expose the right axilla.

An initial port for the thoracoscopic camera is placed at the sixth intercostal space. Under thoracoscopic vision, a small working port can then be placed in either the third or fourth intercostal space at the midaxillary line depending on surgeon preference and patient anatomy. The right phrenic nerve is identified to avoid injury to this structure. An incision is made in the pericardium, anterior and parallel to the phrenic nerve, to expose the heart from the superior vena cava to the diaphragm. Through this pericardiotomy, the space above and below the right PVs is dissected to allow enough room for insertion of a specialized thoracoscopic dissector. This includes opening into the oblique sinus and dissecting the space between the right superior PV and the right pulmonary artery. The dissector and a guiding sheath is introduced through a second port, either lateral or medial to the scope port, and guided into the space between the right superior PV and right pulmonary artery. After the dissector is carefully removed from the chest, the sheath remains in place as a guide for the insertion of the bipolar RF clamp. At this point, the patient is cardioverted into sinus rhythm so that pacing thresholds can be obtained. As with a Cox-Maze IV procedure, it is critical to document pacing thresholds from the PVs before isolation. Some surgeons also use the opportunity provided by surgical exposure to test and ablate ganglionated plexi but there are no data to support this and it is not performed at our institution.