Pathophysiology

Patients with congenital heart disease have a high prevalence of atrial tachycardias (ATs), particularly after they have undergone reparative or palliative surgical procedures. For macroreentrant ATs in adults with repaired congenital heart disease, three right atrial (RA) circuits are generally identified: lateral wall circuits with reentry around or related to the lateral atriotomy scar, septal circuits with reentry around an atrial septal patch, and typical atrial flutter (AFL) circuits using the cavotricuspid isthmus (CTI). Typical clockwise or counterclockwise isthmus-dependent AFL is the most common macroreentrant tachycardia in this patient population. Left atrial (LA) macroreentrant circuits are infrequent in this group of patients. Atrial macroreentry in the right free wall is the most common form of non–isthmus-dependent RA macroreentry (atypical flutter). 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.^1,2

Anatomical factors promoting macroreentry in patients with congenital heart disease include abnormalities of the underlying cardiac anatomy, surgically created anastomoses, and atriotomy scars, resulting in anatomical barriers to impulse propagation. Additionally, extensive cardiac surgery and hemodynamic overload result in myocardial hypertrophy and diffuse areas of atrial scarring with surviving myocardial fibers embedded within scar areas, which provide the substrate for potential reentry circuits.^3,4

The best characterization of macroreentrant AT 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.⁵ Typically, the reentry circuit is located in the lateral RA wall. 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. Participation of the anterior RA wall in the circuit can be confirmed with entrainment mapping. 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). These isthmuses can be areas of slow conduction. Stable pacing of the critical isthmus can be difficult or impossible in RA atriotomy tachycardia because of tachycardia interruption. Isthmus participation in the circuit is often proven by tachycardia interruption with catheter pressure (Fig. 14-3), as well as by tachycardia interruption and noninducibility after radiofrequency (RF) application in the area. A single, wide, fractionated electrogram can be recorded from the lower pivot point of the circuit (Fig. 14-4) in the low lateral RA, close to the IVC, and perhaps also from other isthmuses of the circuit. The line of double potentials or fractionated, low-voltage electrograms can also often be recorded in normal sinus rhythm, to allow tentative localization of the scar and the associated anatomical isthmuses.²

FIGURE 14-1 Three-dimensional electroanatomical (CARTO, Biosense Webster, Inc., Diamond Bar, Calif.) 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.

FIGURE 14-2 Three-dimensional electroanatomical (CARTO, Biosense Webster, Inc., Diamond Bar, Calif.) 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 (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.

FIGURE 14-3 Catheter-induced termination of macroreentrant atrial tachycardia (AT). ECG leads and intracardiac recordings are shown during an episode of non–isthmus-dependent atrial macroreentrant AT in a patient with prior atriotomy. The episode had lasted 3 months at the time of the study, yet catheter pressure at the site from which the distal ablation site (Abl_dist) recording was made terminated AT twice. Note the multicomponent electrogram (blue arrow), the last portion of which is missing when AT terminates (red arrow). Abl_prox = proximal ablation site; CS_dist = distal coronary sinus; CS_prox = proximal coronary sinus; His_dist = distal His bundle; His_prox = proximal His bundle; TA_dist = distal tricuspid annulus; TA_prox = proximal tricuspid annulus.

FIGURE 14-4 Postatriotomy right atrial macroreentry (atypical atrial flutter). ECG leads and intracardiac recordings are shown during non–isthmus-dependent macroreentrant atrial tachycardia (AT) in a patient with a repaired atrial septal defect. A prolonged, fractionated diastolic electrogram is observed at the ablation site. Note the repetitive recording (arrow) that nearly spans the cardiac cycle. The patient had a right atriotomy as the basis of AT and incurred complete heart block during surgery (hence the ventricular pacemaker). Abl_dist = distal ablation site; CS_dist = distal coronary sinus; CS_mid = middle coronary sinus; CS_prox = proximal coronary sinus; His_prox = proximal His bundle; TA_dist = distal tricuspid annulus; TA_prox = proximal tricuspid annulus.

Typical AFL is also often associated with RA atriotomy. In fact, the single most common form of AT among patients with congenital heart disease appears to be isthmus-dependent AFL, particularly in patients with simpler anatomical lesions (e.g., tetralogy of Fallot, atrial and ventricular septal defects) (Fig. 14-5). Moreover, the CTI was found to be part of the reentrant circuit in approximately 70% of patients with postoperative intraatrial reentrant tachycardia. In one report, ablation of this isthmus alone resulted in elimination of the tachycardia in 27% of these patients. Factors such as an atriotomy or atrial fibrosis and hypertrophy serve as promoters for early development of the typical form of AFL.

FIGURE 14-5 Surface ECGs of two types of atrial macroreentrant atrial tachycardia in a patient with previous surgical repair of atrial septal defect. A, A macroreentrant circuit around the atriotomy scar. B, Counterclockwise typical atrial flutter 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; AFL = atrial flutter.

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.⁴

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 reasons behind this laterality are unknown. The mechanism underlying focal AT is unknown. Both triggered and microreentrant mechanisms have been suggested.⁶ 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.³

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. Early postoperative arrhythmias influence the long-term outcome of patients with congenital heart disease and have been found to be a predictor of late complications, such as ventricular dysfunction, late arrhythmias, and late mortality. Whether preventing these arrhythmias will influence the long-term survival of patients with congenital heart disease is unknown.⁷

Clinical Considerations