Macroreentrant Atrial Tachycardia

For macroreentrant atrial tachycardias (MRATs) in adults with repaired CHD, three RA circuits are generally identified: (1) lateral wall circuits with reentry around or related to the lateral atriotomy scar; (2) septal circuits with reentry around an atrial septal patch; and (3) typical atrial flutter (AFL) circuits using the cavotricuspid isthmus (CTI). Peritricuspid reentry (typical clockwise or counterclockwise CTI-dependent AFL) is the most common single mechanism and usually coexists with other forms. Atrial macroreentry in the RA free wall is the most common form of non-CTI-dependent RA macroreentry. Left atrial (LA) macroreentrant circuits are infrequent in this patient population.

The complexity of the MRAT circuits depends on the underlying congenital anomaly and the complexity of the surgical repair. Very complex or multiple reentry circuits can be seen after placement of an intraatrial baffle (Mustard or Senning correction for transposition of the great vessels) in an extremely dilated RA, after a Fontan procedure, and in patients with a univentricular heart.

Anatomical factors promoting macroreentry in patients with CHD include abnormalities of the underlying cardiac anatomy, surgically created anastomoses, and atriotomy scars, resulting in barriers to impulse propagation and protected isthmuses with adjacent anatomical structures. Conduction abnormalities can be further aggravated by atrial dilatation and scarring secondary to persisting pressure or volume overload after cardiac surgery or because of residual septal defects, valvular abnormalities, or ventricular dysfunction. At greatest risk are patients with single ventricular physiology and Fontan circulation, atrial baffles created by Mustard and Senning procedures for treatment of D-transposition of the great arteries, and repaired tetralogy of Fallot; but patients with simple atrial septal defect repair are also vulnerable years after surgical repair.

The best characterization of MRAT caused by atriotomy is activation around an incision scar in the lateral RA wall, with a main superoinferior axis ( Fig. 14.1 ). This is a common problem in patients who have undergone surgery for congenital or valvular heart disease. The length, location, and orientation of the atriotomy incisions, as well as potential electrical conduction gaps across the atriotomy, are important determinants of arrhythmogenicity. Not only does the central obstacle include the scar, but also functional block can magnify this obstacle to include the superior vena cava (SVC). The anterior RA wall is commonly activated superoinferiorly (descending activation pattern), as in counterclockwise typical AFL. However, the septal wall frequently lacks a clear-cut inferosuperior (ascending) activation pattern. A line of double potentials can be recorded in the lateral RA, extending superoinferiorly. Double potential separation can be more marked and demonstrate a voltage lower than in typical AFL. Narrow passages (isthmuses) in the circuit can be found between the SVC and the superior end of the atriotomy scar, between the inferior vena cava (IVC) and the inferior end of the atriotomy, between the atriotomy scar and the tricuspid annulus, between the atriotomy and the crista terminalis, or even within the scar itself ( Fig. 14.2 ).

Incisional Right Atrial Macroreentry.

Electroanatomic (CARTO) activation map of macroreentrant atrial tachycardia in a patient with previous surgical repair of an atrial septal defect. Gray areas in the posterolateral right atrium represent areas of unexcitable scar related to previous atriotomy, characterized by very low-voltage electrograms. During tachycardia, the activation wavefront travels in a macroreentrant circuit around the atriotomy scar. Ablation lines (red dots) connecting the atriotomy scar to the inferior vena cava (IVC) and superior vena cava (SVC) successfully eliminated the tachycardia.

Right Atrial (RA) Figure-of-8 Macroreentry.

Electroanatomic (CARTO) activation map of macroreentrant atrial tachycardia in a patient with previous surgical repair of an atrial septal defect. Gray areas in the posterolateral RA represent areas of unexcitable scar related to previous atriotomy, characterized by very low-voltage electrograms. During tachycardia, the activation wavefront travels from the midposterior RA superiorly and inferiorly, and both counterclockwise and clockwise wavefronts return to the region proximal to the exit site (purple) to complete the circuit by propagating through a narrow isthmus bounded by two areas of unexcitable scar (figure-of-8 reentry). Radiofrequency ablation targeting the gap in the atriotomy scar successfully eliminated the tachycardia. IVC , Inferior vena cava; SVC , superior vena cava.

Typical AFL is also often associated with RA atriotomy. In fact, the single most common form of AT among patients with CHD appears to be CTI-dependent AFL, accounting for more than 70% of all MRATs, particularly in patients with simpler anatomical lesions (e.g., tetralogy of Fallot, atrial and ventricular septal defects) ( Fig. 14.3 ). The atriotomy scar in the lateral or posterolateral RA forms a fixed posterior barrier to conduction in the superoinferior direction between the vena cavae, a lateral boundary necessary for the development of a peritricuspid reentry (typical AFL) by preventing short-circuiting of the tricuspid annulus via the posterior atrium. Other RA MRATs involving free wall atriotomy become progressively more common with more extensive atrial incisions.

Surface Electrocardiograms of Two Types of Atrial Macroreentrant Atrial Tachycardia in a Patient With Previous Surgical Repair of Atrial Septal Defect.

(A) Macroreentrant circuit around the atriotomy scar. (B) Counterclockwise typical atrial flutter (AFL) that developed following successful ablation of the scar-related macroreentry. Spontaneous premature ventricular complexes allow better visualization of the P wave (flutter wave) morphology. RA , Right atrial.

Reentry circuits can also occur in the sinus node region, possibly as a result of injury related to the superior atrial cannulation site for the bypass pump. These circuits can be quite small, often manifesting as focal tachycardia in the sinus node region, and they frequently can be ablated in a single location without establishing a particular line of block.

More recently, ATs arising from the morphological LA (i.e., the pulmonary venous atrium) have been described after surgical repair of CHD. The incidence of those ATs is higher in patients with univentricular hearts and those with prior ipsilateral atrial surgery. The mechanism of those ATs is more heterogeneous than that arising from the morphologic RA (i.e., the systemic venous atrium). Macroreentry is a less predominant mechanism, accounting for less than 50% of cases.

Focal Atrial Tachycardia

Focal mechanisms underlying postoperative AT have been rarely reported in this patient population. Nonautomatic focal ATs are predominantly found in adults, with most foci in the RA. The mechanism underlying focal AT is unknown. Both triggered and microreentrant mechanisms have been suggested. Viable myocardial fibers embedded within areas of scar tissue, which play a pivotal role in the initiation and perpetuation of macroreentrant tachycardias, can also be the site of origin of a focal AT and thus play an important role in the pathogenesis of these ATs.

Atrial Fibrillation

During long-term follow-up, AF develops in more than one-third of patients with CHD. Compared with MRAT patients, those with AF tend to be older and the arrhythmia develops later after surgery. AF is frequently associated with markers of left-sided heart disease (i.e., left ventricular [LV] systolic dysfunction and LA dilation) and is most commonly seen in patients with congenital aortic stenosis, mitral valve disease, palliated single ventricles, unrepaired heart defects, or end-stage heart disease. Compared to patients without congenital heart defects or with simple congenital heart defects, AF develops at a younger age in patients with complex congenital heart defects. Coexistence of episodes of AF and regular AT has been reported in a considerable number of patients (33%). Regular AT preceded development of AF in approximately two-thirds of patients. Approximately 30% of patients who have previously undergone successful catheter ablation for MRAT develop AF during long-term follow-up.

Early Postoperative Atrial Tachycardia

Arrhythmias are also frequently observed in the early postoperative period after corrective surgery in children, occurring in 14% to 48% in the first few days after surgery. The most common arrhythmia in this period is junctional tachycardia, occurring in 5% to 10% of the operated children and usually self-limiting. Other supraventricular arrhythmias are also seen in 4%. The occurrence of early postoperative arrhythmias seems to be related to procedural factors of cardiac surgery, which are, in turn, related to the complexity of the congenital malformation. Local inflammation, metabolic and hemodynamic stress, as well as inotropic drug therapy can potentially promote automatic and triggered activity focal atrial and junctional tachycardias. Early postoperative arrhythmias influence the long-term outcome of patients with CHD and have been found to be a predictor of late complications, such as ventricular dysfunction, late arrhythmias, and late mortality. However, whether preventing these arrhythmias will influence the long-term survival of patients with CHD is unknown.

Atrial Septal Defect

Atrial septal defects are among the most common congenital heart lesions in adults. In the absence of surgical repair, the prevalence of supraventricular arrhythmias increases with age, with typical AFL being the most common. In the presence of atriotomy incisions, sutures, or patches, non-CTI-dependent MRATs can occur or coexist with typical AFL. Common substrates include macroreentry along the lateral RA wall and double-loop or figure-of-8 circuits. The septal patch itself is rarely a critical conduction obstacle.

The timing of surgical closure of the atrial septal defect appears to affect the incidence of atrial arrhythmias. Approximately 60% of patients who have undergone surgical closure at adult age (older than 40 years) continue to have atrial tachyarrhythmias (AT and AF) during follow-up after surgery. In contrast, surgical closure performed during childhood provides a substantially lower incidence of arrhythmias. The impact of transcatheter atrial septal defect closure on atrial arrhythmias is less clear. In one series, all patients with persistent arrhythmias remained in AF or AFL after closure.

Tetralogy of Fallot

Tetralogy of Fallot is the most common cyanotic congenital heart condition, and it accounts for approximately 10% of the adult CHD patient group. ATs occur commonly (12% to 34%) during extended follow-up after tetralogy of Fallot repair. The observed prevalence of atrial arrhythmias is modestly higher than that of ventricular arrhythmias (15%). The most common atrial circuit is typical AFL. Other circuits often involve the lateral RA wall and may be multiple, often with a double-loop type of reentry. Nonautomatic focal ATs are infrequent and most commonly arise adjacent to suture points, with radial spread of activation. The prevalence of AF increases with advancing age. In the first few decades of life, AF is far less common than MRAT, but it becomes more common (more than 30%) than MRAT after 55 years of age.

Dextro-Transposition of the Great Arteries

Dextro-transposition (D-transposition) of the great arteries accounts for 5% to 7% of congenital heart defects. The Mustard and Senning procedures utilize an intraatrial baffle constructed from prosthetic material or pericardium (Mustard) or from the atrial septum and RA free wall (Senning) to redirect the venous blood from the IVC, and the IVC to the LV via the mitral valve, and the pulmonary venous blood to the RV via the tricuspid valve (“atrial switch procedure”). The “new RA” is called the systemic venous atrium and the “new LA” becomes the pulmonary venous atrium. Both procedures were performed from the early 1960s until approximately 1985 as the major long-term surgical palliation procedures for young children having D-transposition of the great arteries. Hence, there is a population of patients in their early 30s to late 50s who have undergone these operations and who are at an increased risk (15% to 48%) of having supraventricular arrhythmias, with similar rates in patients with Mustard and Senning baffles. Since the mid-1980s, with development of coronary artery reimplantation techniques, arterial switch surgery has supplanted atrial redirection as the procedure of choice for D-transposition of the great arteries, and it has been associated with a significantly lower risk of arrhythmias, with a reported arrhythmia-free survival rate of 97% after 25 years of follow-up.

The most common arrhythmia in patients after Mustard or Senning operations for transposition is atrial macroreentry, occurring in up to 30% of these patients. Typical AFL accounts for up to 75% of all MRATs, but non-CTI-dependent MRATs with critical zones of slow conduction between a suture line and the SVC orifice, mitral valve annulus, and pulmonary vein orifice have all been described. Focal ATs adjacent to suture lines are not uncommon. Importantly, the tricuspid valve is on the pulmonary venous side while the IVC is on the systemic venous side of the circulation; as a consequence, the CTI is necessarily divided between the two sides. Therefore access to the pulmonary venous atrium is almost always necessary for ablation of MRAT and typical AFL, which can be accomplished via a retrograde aortic approach through the tricuspid valve or by transbaffle puncture.

Univentricular Hearts With Fontan Palliation

Among patients with CHD, the incidence of MRAT is highest (up to 50% within a decade of surgery) in older patients who have had older-style palliative surgeries for univentricular hearts, typically varieties of the Fontan procedure. In the older-style Fontan (atriopulmonary [RA appendage-to-pulmonary artery] anastomosis) operations, extensive suture lines, and long-term hemodynamic stress result in marked RA dilation, hypertrophy, and fibrosis. Overall, the most common arrhythmia in these patients is typical AFL, but other MRATs as well as focal ATs are also observed. MRAT circuits in Fontan patients can be complex and multiple, and they represent the most challenging arrhythmias for mapping and ablation. Pericaval circuits have been identified specifically in Fontan patients. In all CTI-dependent circuits, successful ablation requires access to the pulmonary venous atrium (via a fenestration in the intracardiac baffle or a transbaffle puncture) to create the line between the tricuspid annulus and IVC.

The incidence of atrial tachyarrhythmias appears to be reduced by 50% to 70% in patients with total cavopulmonary connections in comparison with classical atriopulmonary connections. Newer Fontan designs (total cavopulmonary connections) bypass the RA to a large extent, using either a lateral tunnel or an extracardiac conduit, thereby avoiding RA dilation and resulting in significant reduction in MRAT incidence to 2% to 7%. In the latter group, however, the macroreentry circuits are typically located on the pulmonary venous side of the tunnel or conduit, requiring complex techniques for accessing the arrhythmia substrate (tunnel/conduit puncture). In patients who have already developed arrhythmias and then undergo lateral caval tunnel conversion to decompress the right atrium, arrhythmias often subside after this procedure (during which various forms of maze surgery can also be performed), but in many cases, atrial arrhythmias remain problematic.

Rate Control

Ventricular rate control during AT is important to prevent hemodynamic instability and improve symptoms in patients with CHD. Oral or IV AVN blockers are utilized for rate control, depending on the severity of symptoms and the degree of hemodynamic compromise caused by the tachycardia. Beta-blockers or nondihydropyridine calcium-channel blockers (verapamil and diltiazem) are the drugs of choice for rate control. Ventricular rate control, however, is often challenging and, for patients with rapid ventricular rates and hemodynamic compromise, hypotension, or acute heart failure, prompt electrical cardioversion is recommended.

Restoration of Sinus Rhythm

Ventricular rate control during AT is usually difficult to achieve, and even with adequate rate control, a subset of patients (e.g., those with a univentricular hearts or systemic right ventricles with decreased contractility) may not tolerate prolonged periods with loss of AV synchrony. Hence, unless contraindicated, restoration and maintenance of NSR is preferred in most patients, particularly in the setting of moderate or complex CHD. Rate control strategy generally is reserved to patients with contraindication to anticoagulation and those with intraatrial thrombi, or those in whom rhythm control strategies have failed.

The timing of attempted cardioversion is influenced by the duration of AT, the severity of the patient’s symptoms, the adequacy of rate control, and the risk of thromboembolism. In stable patients with AT of a duration longer than 48 hours or of unknown duration, any mode of cardioversion (electrical, chemical, pacing, or ablation) should be delayed until the patient has been anticoagulated at appropriate levels for 3 to 4 weeks or transesophageal echocardiography (TEE) has excluded atrial thrombi. Even when the duration of AT is less than 48 hours, TEE also should be considered prior to cardioversion in patients with high thrombotic risk, including those with transposition of the great arteries, tetralogy of Fallot, Eisenmenger syndrome, Ebstein anomaly, intracardiac baffles, Fontan operation, mechanical valve prosthesis, prior thromboembolism, or severe ventricular dysfunction. Patients with Fontan palliation are at a particularly high risk for thromboembolic complications, such that TEE may be considered prior to cardioversion regardless of the anticoagulation status.

If urgency of cardioversion (because of severe symptoms or hemodynamic instability) precludes TEE, therapeutic doses of low-molecular-weight heparin or unfractionated heparin should be administered as soon as possible concurrent with, or preferably prior to, cardioversion.

Several options are available for termination of MRAT, including external direct-current cardioversion, antiarrhythmic drugs, and overdrive atrial pacing. Generally, direct-current cardioversion is preferred to chemical cardioversion given the higher efficacy and the lower low risk of proarrhythmia, especially when the arrhythmia is poorly tolerated and prompt cardioversion is necessary. Intravenous ibutilide is a reasonable option for pharmacological cardioversion. Sotalol is less effective than ibutilide. Data are limited regarding the efficacy of other antiarrhythmic agents. Overdrive pacing is particularly useful in patients with preexisting atrial pacing wires in place (as part of a permanent pacemaker or defibrillator, or temporary epicardial pacing wires following cardiac surgery). For stable patients with adequate heart rate control and minimal symptoms, conversion to sinus rhythm may be deferred until catheter ablation, if this can be performed in a timely manner.

Maintenance of Sinus Rhythm

Prevention of Systemic Embolization

Long-term anticoagulation is recommended in patients with atrial arrhythmias and moderate or complex forms of CHD, regardless of the CHA ₂ DS ₂ -VASc score, since these patients exhibit a particularly high thrombotic risk. On the other hand, for patients with simple nonvalvular forms of CHD, indications for long-term anticoagulation are similar to those in patients with AF ( see Chapter 15 ).

New oral anticoagulants may be considered as an alternative to warfarin in patients with simple forms of CHD and no prosthetic heart valve or hemodynamically significant valve disease. On the other hand, the efficacy and safety of these agents have not been established to allow their utilization in patients with moderate or complex forms of CHD, especially those with Fontan surgery.

Of note, clot formation in the RA is common in this patient population, especially in patients with old-style Fontan procedures, unlike patients without CHD in whom LA thrombus is the major concern.