Non-isthmus dependent ART (Fig. 12.2) has the same characteristics as isthmus-dependent ART except that the arrhythmia circuit is associated with scar formation and previous atrial incisions.
Cryoablation, radiofrequency, or other energy sources are used to create ablative lesions with the idea of transforming the areas of slow conduction to areas of no conduction. This process eliminates the anatomic and electrophysiological areas that initiate the reentrant circuit. Figure 12.3 demonstrates the ablative therapeutic lesions for normal hearts and has been proposed as a comprehensive therapeutic option for ART to treat virtually all potential areas of slow conduction in the right atrium without disturbing normal conduction from the sinoatrial node to the atrioventricular (AV) node. Some surgeons have noted an increased risk of nodal rhythm when using these lesion sets, prompting some to amend their operative approach with fewer lesions, especially regarding lesions with proximity to the sinoatrial node.
Patients with congenital heartdisease and anatomic variants often present with complex diagnostic and therapeutic dilemmas. The anatomic areas of interest for these patients with ART are the inferior caval vein, the tricuspid valve, the coronary sinus, the atrial appendage, and the fossa ovalis. The intent of these ablative lesion sets is to connect anatomic barriers, resulting in ablating the areas of slow conduction. Unfortunately, anatomic landmarks may be absent or anomalous. These conditions require creative actions based on the ablative techniques. For instance, patients with heterotaxy syndrome may have absence of the coronary sinus (right atrial isomerism), direct atrial entry of the hepatic veins, juxtaposition of the atrial appendages, left superior caval vein to the left atrium, and anomalous pulmonary venous return to the right atrium or superior caval vein. Occasionally, the surgeon is challenged by anomalies that have no antecedent prospective studies to guide the placement of lesion sets to treat the arrhythmia. Figure 12.4 shows most of the aforementioned complex anatomic features in one drawing; any one of these might be encountered in an arrhythmia operation. The ablative lesions shown in the figure are offered as guidelines that, together with the preoperative electrophysiologic study, can help direct the clinician in planning and executing the arrhythmia procedure. The potential solutions that may be required for right-sided and left-sided maze procedures are demonstrated. Although there are many anatomic variants, the principles of ablative therapy are constant and can be used to perform lines of block between two or more anatomic barriers, thereby eliminating the areas of slow conduction.
12.2 Focal or Automatic Atrial Tachycardia
Focal or automatic atrial tachycardia is characterized by a localized area of electrical impulse generation caused by discrete microentry or by automatic focus. Both forms of focal tachycardia inhibit normal sinus function, resulting in atrial tachycardia. The electrical impulse is conducted and propagated throughout the atria, thereby stimulating the AV node and ventricles. Figure 12.5 demonstrates the automatic focus in the right atrium, which can occur at the junction of the superior caval vein and right atrium, the right atrial appendage, or the anterior right atrial wall. Figure 12.6 demonstrates left focal tachycardia with automatic focus in the left atrium, most often associated with the orificesof the pulmonary veins.
Therapeutic management of focal (automatic) atrial tachycardia can usually be accomplished by transcatheter ablative techniques. When this method is not successful, an operative approach with resection or cryoablation of the offending atrial tissue can be performed. Favorable results have been achieved with cryoablation (Fig. 12.7) and excision of automatic foci, which are often heralded by a cobblestone epicardial appearance of the right atrial appendage, as seen in Fig. 12.8. In the case of multiple ectopic arrhythmogenic foci, surgeons have used more extensive and creative ablative techniques such as pulmonary vein isolation, left and right atrial isolation, and (for extreme cases) His bundle cryoablation coupled with pacemaker insertion.
Accessory connection-mediated atrial tachycardia is characterized by an extra electrical pathway (accessory connection) in addition to the normal AV node, which is capable of conducting electrical stimulation from the atrium to the ventricle. This is obviously a dilemma because the extra pathway can direct electrical stimuli in an antegrade or retrograde fashion and cause AV tachycardia. There are two types of accessory connections: the manifest type(with electrocardiogram [ECG] findings) and the concealed type (no ECG findings). The manifest type is known as Wolff-Parkinson-White (WPW) syndrome and is associated with preexcitation as characterized by a delta wave on ECG. The mechanism of action is noted by AV conduction commencing at the AV node and progressing simultaneously to the ventricles through the normal AV node and the accessory connection (bundle of Kent). The accessory connection pathway initiates depolarization of the ventricle first, owing to the intrinsic conduction delay of the AV node. Consequently, the circuit is completed by antegrade conduction through the accessory connection and retrograde conduction through the AV node. This arrhythmia circuit is called antidromic conduction(Fig. 12.9). The orthodromictype is characterized by antegrade conduction through the AV node and retrograde conduction at the accessory connection (Fig. 12.10). Both entities establish an AV circuit that results intachycardia.
Accessory connections are now primarily treated by transcutaneous transcatheter radiofrequency ablation, with excellent results. This strategy is also used before operative therapy in an effort to decrease cross-clamp time, as the ablative procedure is performed before the reparative operation. This staged approach can have important drawbacks, however. Patients with right-to-left shunts, for instance, are at risk for an embolic stroke if the transcatheter approach is used. Once an operative procedure for anatomic and electrophysiologic indications is planned, the operative team must identify all variables that are related to the repair. The considerations are relevant to patients with right-to-left shunts and those with a projected increased cross-clamp time. The experience and facility of the operating team in using multiple ablative procedures for accessory connections and the amount of ventricular dysfunction are all important variables. Collaboration between the electrophysiologist and surgeon determines the best approach for each patient, based on practical wisdom.
12.3.1 Surgical Technique
After a comprehensive preoperative electrophysiologic study, the team may elect to perform intraoperative epicardial or endocardial mapping to confirm the preoperative electrophysiologic findings and to assess the immediate postoperative result. The goal of surgical therapy for WPW syndrome or concealed bypass tract is to divide or ablate accessory connections that are responsible for the reentry phenomenon and clinical tachycardia. Unlike ART, which is treated by ablating the area of slow conduction, accessory connections are specific anatomic entities that are divided and ablated during the operation.
Two surgical approaches can be used: endocardial and epicardial. Cardiopulmonary bypass and aortic cross clamping with cardioplegia is required for the endocardial technique, depending on the anatomic position of the accessory connection. The epicardial technique often also requires cardiopulmonary bypass, depending on the location of the accessory connection. In some cases, however, such as ablation of a right free-wall connection, the dissection can be performed without cardiopulmonary bypass. Whatever the approach, the procedure is performed on the epicardial surface at the AV junction by dividing the atrial end of the connection. Results can be excellent with either approach, depending on the choice and experience of the operating team.
12.3.2 Left Free-Wall Accessory Connections
Ablation of a left free-wall accessory connection is generally performed by the endocardial technique with cardiopulmonary bypass, cardioplegic arrest, and left atrial exposure through the interatrial groove, similar to the approach for mitral valve surgery. Figure 12.11 shows the operative exposure and the dissection location (dotted line) after the mitral valve is identified. A curvilinear incision is made parallel to and 2 mm away from the posterior mitral annulus, extending from the left fibrous trigone to the posterior septum (Fig. 12.12). A dissection plane is then developed between the fat pad of the AV groove and the superior portion of the left ventricle, extending to the epicardial reflection throughout the entire length of the initial incision (Fig. 12.13). The dissection is completed by extending the ends of the incision and the dissection to the mitral annulus to divide any accessory connection that may be located at the juxta-annular area. This dissection exposes the entire left free-wall space to its boundaries, thereby ensuring division of any or all accessory connections. The endocardialincision is then sutured to complete the procedure (Fig. 12.14).
The epicardial approach (Fig. 12.15) is not often used for left free-wall accessory connections. It requires upward and rightward cardiac retraction for proper exposure, which frequently results in severe hemodynamic instability. As a result, most surgeons use cardiopulmonary bypass, although no intracavitary exposure is required for this technique. The epicardial reflection of the atrium is entered, and a plane of dissection is established between the AV groove fat pad and the atrial wall. Coronary sinus tributaries often require ligation and division, and care must be taken to avoid coronary artery injury. The dissection plane is extended to the level of the posterior mitral valve annulus and carried slightly onto the top of the posterior left ventricle. This maneuver divides the atrial end of all accessory connections in this area except those that lie immediately adjacent to the mitral valve annulus. If present, these juxta-annular connections can be ablated by a cryosurgical probe that is placed at the level of the mitral valve annulus. The atrial epicardial reflection is then repaired by suture technique.