Surgery for Atrial Fibrillation and Other Supraventricular Tachycardias

Cryoablation

Cryoablation devices consist of a hollow shaft leading to a conductive metal probe, as well as an integrated thermocouple for temperature recording. Cryogens (nitrous oxide and argon are used clinically) are pumped through a hollow shaft under a high pressure to a distal closed electrode tip. Rapid expansion from the liquid to gas phase absorbs energy, producing cooling in a process known as the Joule-Thompson effect. The resultant gas is aspirated via a vacuum tip and returned through a separate circuit.

Cryoablation is one of the safest energy sources because it preserves fibrous tissue and collagen, maintaining tissue integrity. The technology uses thermal conduction to create direct physical injury, causing sublethal cellular stress response and molecular-based cell death by freezing. Intracellular and extracellular ice crystals form, disrupting the cell membrane. Ice formation occurs along the nonuniform surface of the cells and acts as an insulator to slow heat loss. The ice also causes cryoadhesion between the probe and the tissue, which ensures tissue contact and an area of continuous heat extraction from the tissue. The size and depth of the lesion depend on the probe temperature, probe size, duration and number of freeze cycles, specific heat of the cryogens, and thermal conductivity and temperature of the tissue.

There are currently two commercially available sources of cryothermal energy being used in cardiac surgery: nitrous oxide and argon. At 1 atm pressure, nitrous oxide is capable of achieving a temperature of −89.5°C, whereas argon has a minimum temperature of −185.7°C. The goal of cryoablation is to cause irreversible cell injury while maintaining the extracellular matrix; a temperature of −40°C is required to be certain of intracellular ice formation. ^, Late apoptosis expands the area of cell death. The nitrous oxide technology has been extensively used and is generally safe and efficacious, except around the coronary arteries, where studies have shown late intimal hyperplasia after cryoablation. Disadvantages of cryoablation include the extended duration required for each individual ablation (2–3 minutes) and the difficulty encountered when trying to achieve transmural lesions by applying epicardial cryoablation to the beating heart. The latter shortfall is, in part, a result of dissipation of thermal energy through convective warming from the circulating endocardial blood, which creates a heat-sink effect. Finally, there is a risk for thromboembolism if blood freezes and coagulates during epicardial ablation on the beating heart. This problem can be overcome by placing the probe endocardially and freezing outward; however, this requires cardiopulmonary bypass. Finally, although injury to the coronary arteries and the esophagus are a concern, experience has shown cryothermal energy does not cause permanent damage to valvular tissue or the coronary sinus.

Malleable disposable cryoprobes have been developed that facilitate minimally invasive application and allow for the creation of longer lesions of varied shapes. The ability of one such cryoprobe to reliably produce transmural lesions has been shown to be equivalent to that of a rigid cryoprobe in in vitro and in vivo studies. ^, Most manufacturers recommend a 2-minute ablation time for endocardial ablations.

Radiofrequency Energy

RF technology was one of the first energy sources used in the electrophysiology (EP) laboratory and the operating room for the treatment of AF. RF energy deploys an alternating current with a frequency in the range of 300 kHz to 3 MHz. The high frequency prevents rapid myocardial depolarization and induction of ventricular fibrillation (VF) yet is sufficiently low energy to prevent tissue vaporization and perforation. The lesion size depends on the electrode–tissue contact area, interface temperature, current and voltage (power), topical cooling, and tissue resistance. The tissue immediately adjacent to the probes is directly heated by a process called resistive heating , whereas deeper tissue layers are heated via passive conduction. The heat causes destruction of the myocardial cells through protein denaturation and cellular desiccation. Tissue temperatures of approximately 50°C to 60°C result in coagulation necrosis, followed by destruction of the collagen matrix and replacement with a mature fibrous scar within 1 to 2 months. The depth of the lesion and failure to create a transmural lesion can be limited by char formation, which occurs at temperatures greater than 100ºC, as well as epicardial fat, myocardial and endocardial blood flow, and tissue thickness.

RF energy devices currently used in clinical practice employ either a unipolar or bipolar electrode configuration. Some RF devices use continuous saline irrigation to prevent overheating of the electrodes, to avoid char formation, and to maintain a low-impedance path for energy to penetrate deeper into the tissue. Certain devices intended for epicardial ablation, particularly those intended for use via a minimally invasive approach, employ suction for stabilizing the electrode, in an attempt to promote better tissue contact.

There have been a number of unipolar RF devices developed for ablation. Unipolar RF devices disperse RF energy between an electrode tip on the tissue and a passive electrode, which is typically a grounding pad. Most currently available unipolar devices are irrigated or perfused to facilitate lesion formation. Nonirrigated unipolar RF devices have been shown to create transmural lesions on the arrested heart in animal models after long ablation times of 60 to 120 seconds, but unfortunately this success has not been translated into clinical practice. During mitral valve surgery, only 20% of 2-minute endocardial applications produced transmural lesions. Epicardial ablation on the beating heart has been even more problematic. Animal studies have consistently shown that epicardial application of unipolar RF is incapable of reliably creating transmural lesions on the beating heart, and only 7% of epicardial applications in humans produced transmural lesions. This problem was felt to be caused by the heat-sink effect of circulating blood. In addition, several complications have been described as a result heating adjacent tissue, including coronary artery and esophageal injury. ^, As a consequence, the use of epicardial unipolar RF ablation outside clinical trials is not recommended.

Bipolar RF technology was developed to overcome the shortcomings of unipolar RF ablation. These devices were developed with two different basic designs. The first is a surface bipolar RF device that contains electrodes side-by-side and applies energy to either the endocardial or epicardial surface. The second is the bipolar RF clamp that contains electrodes embedded in its jaws, offering the advantage of applying high-density current to both the endocardial and epicardial surfaces simultaneously. These clamp devices have become the most commonly used surgical ablation devices. An example is shown in Fig. 135.3 .

Unfortunately, there has been concern regarding the transmurality of lesions created with surface bipolar RF devices. Animal studies in our laboratory have shown that despite success in creating acute conduction block, none of the lesions produced with one surface device exhibited conduction block at 4 weeks, even though 76% of histologic sections were transmural. A suction-assisted device was tested by two independent groups in a porcine model; one study showed a 96% rate of transmural lesions, and the other showed a similar rate of transmurality on a per-section basis but only a 68% rate of transmurality for the entire lesion.

In comparison to the surface bipolar devices, the bipolar RF clamp has been much more effective at creating transmural lesions. ^{, ,} Bipolar ablation has been shown to be capable of creating precise transmural lesions on the beating heart in animals, with application times typically between 10 and 20 seconds. ^, Bipolar clamps also have been shown to not damage commonly used polypropylene suture material, although in our clinical experience, suture disruption has been occasionally observed. Presently available devices employ algorithms to predict lesion transmurality from measured tissue impedance and conductance and to optimize safe energy delivery to the tissue. In our clinical experience, it was observed that the devices would occasionally fail to achieve conduction block with a single ablation, which was never observed in animal models. In a recent study, a new model was developed using excised human hearts turned down for transplantation. Although 89% of histologic sections were transmural, only 65% of single ablations resulted in a completely transmural ablation line. This was usually associated with areas of excessive epicardial fat. If two ablation applications are performed in succession without unclamping, however, 100% of lesions were transmural along their entire length. One drawback of bipolar RF clamps is the requirement for the tissue to be clamped. This has limited potential lesion sets, particularly on the beating heart, and requires the use of adjunctive unipolar or cryothermal technology to create a complete Cox-Maze IV lesion set. Careful attention to surgical technique, meticulous testing for entrance and/or exit block when possible, and improvements in device technology are still needed.

Bipolar devices have been shown to be safer than unipolar devices. Although relatively infrequent, reported clinical complications from unipolar RF devices have included coronary artery injuries, cerebrovascular accidents, and esophageal injury leading to atrioesophageal fistula. Bipolar RF technology has virtually eliminated complications from collateral damage by applying the high-density current between the jaws of the clamp. No device-related injuries have been reported in the literature with the use of bipolar RF clamps.

In summary, each ablation technology has its own advantages and disadvantages. Although efforts have been made to indirectly compare the merits of different ablation devices, there are no randomized controlled trial data. Situations exist where the use of each of these devices is appropriate, but it is necessary to understand the advantages and disadvantages of each to ensure that the devices are applied correctly. Failure to do so will result in suboptimal clinical outcomes.

Surgical Technique

The Cox-Maze III procedure is rarely performed at the present time because most centers have replaced the surgical incisions described in the original “cut-and-sew” procedure with lines of ablation created by a variety of different energy sources. At our institution, we have successfully used bipolar RF energy and cryoablation to replace most of the surgical incisions of the Cox-Maze III procedure in an operation termed Cox-Maze IV (see Fig. 135.2 ).

The Cox-Maze IV procedure is performed using cardiopulmonary bypass with either a median sternotomy or a less-invasive right minithoracotomy. The latter is our procedure of choice in patients referred for a stand-alone Cox-Maze procedure or in conjunction with a mitral valve procedure. All patients undergo intraoperative transesophageal echocardiography. Both the right and left pulmonary veins (PVs) are bluntly dissected, as is the intraatrial septum. After PV dissection, if the patient is in AF, attempts should be made to restore sinus rhythm. An IV bolus of amiodarone (150 mg) is given, and cardioversion is performed after excluding the presence of an LAA clot. Pacing thresholds are measured from each PV. The PVs are then isolated using a bipolar RF device, ablating a large cuff of surrounding atrial tissue ( Fig. 135.4 ). RF energy is delivered until the algorithm confirms transmurality. An additional ablation is performed before unclamping the device, and then a second and third set of ablations are performed a few millimeters proximally or distally. The adequacy of electrical isolation is demonstrated by confirming exit block by pacing from each PV in patients who have undergone a full sternotomy and from the right PVs in minithoracotomy patients.

The right atrial lesion set is typically performed on the beating heart during cardiopulmonary bypass through three small purse-string sutures in the minithoracotomy patients and using a small right atriotomy in patients undergoing a median sternotomy ( Fig. 135.5 ). The right atrial free wall and intercaval ablations are performed using a bipolar RF clamp, followed by the two endocardial tricuspid annulus ablations, which are created using a linear cryoprobe. All cryoablations using nitrous oxide devices are performed for 3 minutes at −60°C. With the argon device, a 2-minute ablation application has been sufficient.

The heart is then arrested by cold cardioplegia. The LAA is amputated or oversewn, and the bipolar RF ablation is performed through the appendage site toward either left PV. The remaining ablation lines are then created using the bipolar clamp through a standard left atriotomy (see Fig. 135.4 ). Connecting lesions into the left superior and inferior PVs effectively isolates the entire posterior left atrium, and a linear line of ablation is created toward the mitral annulus. Cryoablation is then used to connect this ablation line to the mitral annulus, completing the left atrial isthmus line. The coronary sinus is ablated using an epicardial cryoprobe. In patients undergoing a right minithoracotomy, cryoablation is more extensively used to complete the posterior left atrial isolation.

Some groups have added ganglionated plexus (GP) ablation to the Cox-Maze IV procedure; however, there has been no clinical evidence to support the use of GP ablation. In fact, a randomized trial of patients undergoing PV isolation (PVI) with or without GP ablation showed no difference in AF recurrence at 1 year but found a higher rate of major complications in patients undergoing GP ablation. Moreover, in animal studies in our laboratory, evidence of early reinnervation at 1 month after GP ablation suggested that permanent denervation of the atrium is not possible with this technology.

Postoperative Care

Postoperatively, the epicardial pacing wires sewn to the right atrium and ventricle before closure of the chest are used for pacing as needed. Junctional rhythms are common after the Cox-Maze IV. In the event that heart block develops, the pacer can be changed to AV sequential pacing to optimize hemodynamics. Patients are started on antiarrhythmic medications once they are in normal sinus rhythm. Patients with persistent bradycardia because of a junctional rhythm are followed for 5 to 7 days to allow for sinus node recovery. If they remain in symptomatic bradycardia, a dual-chamber pacemaker is implanted. In our experience, 5% of patients require permanent pacemaker placement because of persistent heart block or sinus node dysfunction after a lone Cox-Maze IV, which increases in the elderly population and in patients undergoing concomitant valvular surgery or septal myectomy for hypertrophic cardiomyopathy. ^{, ,} Finally, it should be emphasized that the Cox-Maze procedure cannot cause complete heart block (CHB) because there are no ablations in the vicinity of the AV node; thus CHB is the result of the concomitant surgery performed, not the Cox-Maze procedure.

Atrial tachyarrhythmias (ATAs) may occur postoperatively but usually resolve within the first month after the operation. Patients with persistent ATAs receive pharmacologic treatment with class 1 or 3 AADs, as well as cardioversion if necessary. The AADs are usually discontinued at 2 months in patients in sinus rhythm. All patients receiving Cox-Maze IV and without contraindications are started on warfarin or a direct anticoagulant agent postoperatively, and this is continued for 3 to 6 months. Anticoagulation is discontinued when the patient is demonstrated to be free of recurrent ATAs off antiarrhythmic medications, as documented on prolonged monitoring (24-hour or greater Holter recording), who are also shown to have no atrial stasis or thrombus on echocardiography. In patients who have had preoperative embolic neurologic events, our data support lifelong anticoagulation after surgery; this is our current practice.