Epidemiology and Natural History

IART is common in patients with CHD, with a spectrum of arrhythmia presentations ranging from occult, asymptomatic arrhythmia to sudden death. Incessant or recurrent arrhythmia may cause gradual hemodynamic deterioration, and vice versa, often resulting in a vicious cycle of clinical decompensation. Thrombosis and thromboembolic events and likely increased risk of mortality are also associated with IART ( Fig. 13.1 ). Symptoms, frequent need for hospitalization, and the management of cardiac devices and antiarrhythmic drugs constitute a significant burden on quality of life.

Electrophysiologic and clinical correlates of intraatrial reentrant tachycardia in patients with congenital heart disease. CHF, Congestive heart failure.

An early, retrospective multicenter study of young patients with atrial flutter showed that over 80% had associated repaired or unrepaired CHD, and also reported a mortality rate caused by sudden death of 10% over 6.5 years. ATs in an adult CHD population are associated with a twofold increased risk of mortality ; independent predictors for mortality are poor functional class, single ventricle physiology, pulmonary hypertension, and valvular heart disease. Other studies have come to differing conclusions regarding the direct association of IART with mortality, but have noted that it co-occurs with congestive heart failure and thrombosis. Regardless, it seems clear that IART is a fellow traveler with a variety of adverse late sequelae of CHD.

The natural history of IART is characterized by gradual loss of normal sinus node function over years to a decade or more, followed by increasingly frequent recurrences of tachycardia. Risk factors for IART are older age at operation, preoperative and perioperative occurrence of arrhythmia, and longer follow-up. A study of nearly 500 early survivors of the Mustard procedure identified an IART prevalence of 27% at 20-years follow-up, and 60% had sinus node dysfunction, frequently in association with IART. Sudden death occurred in 6.5% of patients over a mean follow-up of 11.6 years. A retrospective study on the occurrence of atrial flutter in 53 patients with tetralogy of Fallot was performed by Roos-Hesselink and coworkers. They found that sinus node dysfunction and IART each were present in about a third of the population after mean follow-up of 18 years, more prevalent than ventricular tachycardia (VT), and more likely to be associated with symptoms. Among patients who have undergone the Fontan procedure for single ventricle physiology, the natural history of atrial arrhythmia is described by retrospective studies from several large cardiac surgical centers totaling around 1400 patients. These indicate that between 25% and 50% of patients who have undergone the Fontan procedure will have clinical documentation of IART by 10 years of follow-up.

Mechanisms of Intraatrial Reentrant Tachycardia

In addition to the anatomic differences observed in CHD patients, the atrial myocardium itself is often markedly thickened and fibrotic arising from lifelong exposure to abnormal hemodynamic stresses, cyanosis in many cases, and intermittent inflammatory effects of surgical intervention ( Fig. 13.2 ). Electrophysiologic testing in patients after Fontan repair of single ventricles and Mustard repair for transposition of the great arteries demonstrates prolongation of atrial refractoriness and areas of intraatrial conduction delay. Progression of the aforementioned arrhythmia findings may be associated with gradual prolongation of the P wave, and measurements of the atrial histology of patients with a variety of complex CHD lesions suggest that increasing hypertrophy and fibrosis are also present. Finally, all patients with CHD are also predictably vulnerable to the automatic and triggered atrial arrhythmias in the perioperative setting, in the context of local inflammation, metabolic stress, inotropic support, and alterations in hemodynamic loading. Each of these factors may complicate diagnosis and management of atrial arrhythmias.

Varying degrees and patterns of atrial myocardial fibrosis observed in patients with congenital heart disease. Masson trichrome stain highlights fibrous tissue in blue green.

The most common mechanism underlying IART in CHD is macroreentry within the atrial musculature. Anatomic structures, areas of scar tissue, long suture lines, cannulation sites, or surgically inserted prosthetic materials are often the boundaries of these reentrant circuits. Separation of atrial muscle bundles by fibrous tissue enhances the complexity of reentrant circuits as they form multiple corridors within areas of scar tissue. Ectopic ATs are less frequently observed. Their underlying mechanism is unclear, although focal activity originates from low-voltage areas and mapping studies are suggestive of microreentry.

Animal Models of Intraatrial Reentrant Tachycardia

Animal models have been used to mimic the atrial anatomic changes associated with the surgical palliation of CHD. These models are highly arrhythmogenic and appear relevant to the understanding of postoperative atrial reentrant tachycardias. Acute and chronic models of the classic and lateral tunnel varieties of the Fontan procedure have shown that extended atrial suture lines and surgical incisions serve as common pathway for many observed tachycardias. In carefully mapped preparations, the suture line associated with the baffle appeared to serve as the primary determinant of the arrhythmia circuit. Inclusion of the crista terminalis in the suture line further increased vulnerability to arrhythmia. These arrhythmogenic substrates could be rendered noninducible by surgically anchoring the suture line to the nonconductive boundary of the tricuspid annulus.

Electrocardiographic Manifestations of Intraatrial Reentrant Tachycardia

IART typically has a stable cycle length and P wave morphology, suggesting that it is organized by a fixed myocardial substrate. Diagnostic criteria for IART in patients with CHD are listed in Box 13.1 . A typical, ambulatory electrocardiographic example is shown in Fig. 13.3 , demonstrating the frequent association of sinus node dysfunction and variable atrioventricular (AV) conduction. Although IART may sometimes share the electrocardiographic characteristics of common atrial flutter, atypical electrocardiographic morphologies are common. P waves are frequently discernible, separated by relatively long segments of isoelectric electrocardiographic baseline. Cycle length is typically significantly longer than that seen in atrial flutter, especially in Fontan patients, and may permit 1:1 AV conduction.

Continuous Holter strip from adult patient with congenital heart disease status post-Mustard procedure, demonstrating concomitant sinus node dysfunction and atrial tachycardia with variable atrioventricular nodal conduction.

Although any specific occurrence of IART is generally stable, multiple, different IART morphologies may be recorded from a single patient over time. This may represent reversal of activation of a given IART circuit, use of an alternate circuit, or changes in passive activation of the atrium outside the arrhythmia circuit itself. Schoels and coworkers performed high-density epicardial mapping on animal preparations of a variety of atypical atrial flutters induced using a sterile pericarditis model. After classifying these as either flutter or P wave tachycardias, they demonstrated that periods of isoelectric atrial diastole were correlated with activation of narrow corridors of slowly conducting atrial tissue, whereas maps of flutter wave tachycardias were less likely to display such features. This electrocardiographic discrimination between flutter wave and P wave morphologies may be practically useful with respect to designing ablation strategies in these patients ( Fig. 13.4 ).

Top trace demonstrates a sawtooth pattern commonly encountered in isthmus-based atrial reentry. Bottom trace demonstrates a P wave tachycardia commonly associated with atypical atrial reentry dependent on scarred myocardium.

Anatomic Complexity of Arrhythmia Phenotypes in Congenital Heart Disease

The presence of CHD complicates understanding of the mechanisms of atrial macroreentrant circuits. Unfamiliar anatomic relations, arising from both the congenital malformations themselves and the variety of palliative surgical procedures used to redirect blood flow and septate the heart, may have significant impact on management and ablative strategies. In addition to analysis of the targeted arrhythmia, the physician must have complete and specific knowledge of the patient’s cardiovascular anatomy and the consequences of that anatomy and subsequent surgical modifications on cardiovascular function.

There are also specific subtypes of CHD that are associated with other forms of supraventricular tachycardias (SVTs) that may complicate diagnosis and management or even co-occur with atrial macroreentrant tachycardias. Ebstein’s anomaly has a high prevalence of accessory pathway–mediated tachycardias with and without associated preexcitation. Patients with anatomically complex forms of heterotaxy syndrome may sometimes have AV reciprocating tachycardias based on twin AV nodes. Finally, a variety of complex lesions may rarely demonstrate atypical AV nodal reentrant tachycardias, in a setting that often makes it impossible to ascertain the position or anatomic relationships of the AV node and its inputs.

Surgical Anatomy of Repairs

Certain combinations of CHD diagnoses and surgeries are associated with increased prevalence of IART and/or special considerations with respect to ablative treatment. From the point of view of arrhythmia diagnosis and management, one can classify most lesions into one of the four broad groups ( Table 13.2 ).

The first group consists of those patients whose surgical repair left them with a normally septated heart and normal venoarterial connections (e.g., atrial septal defect [ASD] or ventricular septal defect, tetralogy of Fallot, many endocardial cushion defects).

The second group includes patients who have undergone an atrial switch procedure, such as the Mustard or Senning procedure and patients who have undergone many of the baffle variants of the Fontan procedure for palliation of single ventricle physiology. In these procedures, the surgeon creates a tissue or prosthetic baffle to redirect venous blood into the pulmonary circulation and arterial blood into the systemic circulation. This results in extended atrial suture lines and typically leaves a significant portion of the right atrium (often including the cavotricuspid isthmus) located on the pulmonary venous side of the baffle.

The third group consists of a dwindling but still an important group of patients who underwent one of the older variants of the Fontan procedure, most commonly the atriopulmonary anastomosis. These patients have very enlarged, fibrotic, and surgically scarred right atria that are highly arrhythmogenic and which present unusual and specific challenges to ablation.

The final group consists of those patients who are complicated by virtue of their anatomy but are unpalliated, including many patients with Ebstein’s anomaly or patients with unrepaired single ventricle. In this heterogeneous group, the anatomy that determines the arrhythmia is by definition native. Because these patients may also be cyanotic, their atrial myocardium is exposed to long-term stress, which may enhance its arrhythmogenic potential.

Studies of Clinical Mechanism

Because IART is usually well tolerated hemodynamically, it has been possible to study clinical tachycardia mechanisms in detail. This has allowed evaluation of hypotheses derived from animal models and electrocardiographic observations and made it possible to correlate anatomic and electrophysiologic mechanisms. In turn, empirical anatomic approaches to the treatment and prophylaxis of IART based on these observations are being developed and tested. These techniques may use either linear ablative techniques in the catheterization laboratory or application of linear cryoablative lesions in the operating room, with varying degrees of direct electrophysiologic guidance.

Interruption of a tachycardia during ablation provides evidence that the ablation site is within the tachycardia circuit, and initial evaluation of successful IART ablation sites in patients with CHD demonstrated that critical sites for circuit interruption are dependent to some degree on the primary congenital heart lesion. In patients with CHD, the great majority of IART circuits are located in the right atrium, and most of those are based on a limited set of common arrhythmia substrates. An important rule is that, among the great majority of patients who have an isthmus defined by the inferior vena cava and a right-sided AV valve (e.g., biventricular repairs, Mustard and Senning patients), the most common IART is AV valve–caval isthmus dependent, similar to patients with the common form of atrial flutter. Although the AV valve may be either mitral or tricuspid and located in either the systemic or pulmonary venous atria, successful ablation for these IART circuits typically targets the area anterior to the Eustachian ridge ( Fig. 13.5 ). Reentrant circuits using the isthmus but anchored to the ostium of the inferior vena cava (pericaval reentry) have also been described.

Schematic illustration of the different anatomic expressions of the cavotricuspid isthmus, which in the case of ventricular inversion is actually the cavomitral isthmus, and in patients who have undergone surgical intraatrial baffle procedures is located on the pulmonary venous side of the baffle. SLL, Situs solitus, L-looped ventricles and levo position of great arteries.

Analysis of successful ablation sites along the lateral right atrial wall has also highlighted the importance of conduction block caused by right atriotomy scars in the genesis of incisional ATs. Such scars are virtually ubiquitous among patients with CHD and may result in tachycardias arising from the free wall or as dual-loop tachycardias in conjunction with an isthmus-dependent circuit. The exact location of atrial scar may be difficult to determine, and a useful approach to defining their location is to identify lines of double or split potentials. Clusters of double potentials represent continuous atrial scar acting as a central obstacle that defines the IART circuit, and which can be functionally eliminated using ablation to extend the scar to a nonconductive boundary. Love and coworkers characterized a variety of anatomic and electrophysiologic central obstacles, which served to anchor IART circuits, including the right AV valve (when present), ASDs, and surgical scars located on the free wall of the right atrium, a finding which has been further defined recently in patients with tetralogy of Fallot. Examples of such IART circuits have been characterized in their entirety in a number of cases by anatomic mapping of the response to entrainment pacing.

Certain patients, notably those with the older variants of the Fontan procedure, have patchy areas of atrial scar in the atrial free wall separated by channels of viable myocardium, which appear to be the critical substrate for the associated IARTs ( Fig. 13.6 ). The overall electrical amplitude of endocardial signals measured from the right atrium is markedly decreased in patients with CHD, particularly in the area surrounding the crista and at nearby sites of surgical intervention. Critical channels of active myocardium, coursing amidst complex islands of atrial scar defined by arbitrary voltage criteria, have also been successfully targeted for ablation.

Scar-based lateral wall channel (yellow arrow) defined by electroanatomic mapping in an adult patient with a Fontan created by atriopulmonary anastomosis.

Focal ATs have also been observed to occur with some frequency among these patients. These tachycardias most commonly arise from the right atrium and are triggered by pacing. Careful mapping studies by De Groot and colleagues suggest that these IARTs represent areas of atrial microreentry occurring in circumscribed areas of diseased tissue. Clinical demonstration of microreentrant circuits consistent with these findings, delineated by high-density catheter-based mapping arrays and associated with long, fractionated electrograms and termination by nonpropagated extrastimulus, has recently been demonstrated in case report.

Prevalence/Significance of Atrial Fibrillation

Atrial fibrillation (AF) is less prevalent in CHD than IART, but may occur in as many as 25% to 30% of patients with AT, more commonly among those with residual left-sided obstructive lesions and incompletely palliated complex heart disease, and to a less degree in those patients who have undergone the Fontan procedure. A study of cardioversions performed on patients with CHD over a 10-year period identified 31% of patients as having AF, 20% as their sole presentation (i.e., without other presentations of AT). AF is a well-recognized sequel of large, unrepaired ASDs in adults, and closure of the ASD with or without adjunctive surgical maze procedure reduces its prevalence postoperatively.

It is unknown whether the mechanisms of AF in adult patients with congenital heart defects are similar to those seen in patients with normal cardiac anatomy. It is reasonable to assume that principles of cellular activation, wave front propagation, and effects of myocellular hypertrophy and interstitial fibrosis are the same in both groups, but the effects of surgical intervention, aberrant anatomy, and chronic cyanosis on AF are largely unknown. It also appears that limited atrial maze procedures may have proarrhythmic effects, particularly with respect to atypical atrial reentry circuits, whereas more extensive maze procedures are antiarrhythmic. The apparent predilection for AF in CHD patients with left-sided lesions suggests that dilation of the left ventricle and left atrium promotes AF in patients with acquired ventricular dysfunction. By contrast, the chronic hemodynamic loads imposed on the right atrium by CHD, in combination with surgical scarring, more commonly results in right atrial arrhythmias in this patient group.

General Considerations

The presence of CHD in an arrhythmia patient significantly alters the potential severity of the arrhythmia complaint as well as the safety and feasibility of various treatments. In particular, the physician must be able to tolerate the hemodynamic stresses of atrial arrhythmia, both acutely and chronically, and the potential for associated thrombotic and thromboembolic complications.

A variety of clinical factors will affect the clinical decision as to whether to intervene acutely to treat an episode of IART in a CHD patient or to provide chronic prophylaxis, or both. Tachycardias may be transient, symptoms may vary widely from one episode to another, and specific decisions regarding how to intervene are often a matter of clinical judgment. It is difficult to measure and compare the outcomes of various treatment options, because much of the available outcome data have been collected in small, retrospective series of patients with varying degrees of illness. Prospective studies will need to involve more structured monitoring for transient atrial events, such as those applied to the study of interventions for AF. Scales incorporating arrhythmia frequency, severity of associated symptoms, and the risk of associated morbidity are in development, but must still be validated.

Cardioversion

The mainstay of acute therapy of IART is cardioversion. This is typically performed by synchronous direct current shock. Gandhi and coworkers showed that minimum successful biphasic shock energies for these atrial reentrant arrhythmias ranged from 0.25 to 0.5 J per kg. Alternative modes of cardioversion are available, including pacing by a pacemaker or transesophageal route, and the use of ibutilide. In the case of the latter, similar efficacy rates and risk profile for ventricular proarrhythmia as with adult patients are described. Because echocardiographic observation of atrial thrombi is not infrequent, echocardiographic screening is generally performed for patients who have been in tachycardia without anticoagulation for more than 48 to 72 hours. In contrast to patients without CHD in whom left atrial thrombus is the major concern, right atrial thrombus is common especially in patients with early version Fontan repairs.

Importance of Anticoagulation

Reports of stroke after cardioversion of IART in CHD patients are rare. However, intravascular and intracardiac thromboses are often reported in patients with CHD ( Fig. 13.7 ). Recent studies have estimated the rate of stroke in patients who have undergone the Fontan procedure to be 2 to 3:1000 patient-years, and the rate of all thromboses to be approximately 4 times that. Co-occurrence of IART may further promote thrombosis, and a prevalence of intracardiac thrombi in 42% of patients undergoing echocardiography before cardioversion has been reported.

Mural thrombus removed from the right atrium of an adult patient status post-Fontan with chronic intraatrial reentrant tachycardia refractory to medical management.

It is not clear whether ATs promote such events or are simply a common comorbidity in these sick patients, as well as whether the use of anticoagulant regimens alters this risk. Nonetheless, the frequent occurrence of thrombosis in adult patients with CHD and AT suggests that warfarin or other potent anticoagulant therapy is indicated in most of these patients.

Use of Antiarrhythmic Drugs in Intraatrial Reentrant Tachycardia

Antiarrhythmic drugs of all classes have been used as the first line of prophylaxis for IART. Symptomatic arrhythmias can be suppressed in some individual IART and/or AF patients with CHD using Class Ic or Class III antiarrhythmic agents. Small case series have indicated that certain agents such as sotalol and amiodarone may decrease the frequency of tachycardia recurrence. However, there are no studies that convincingly demonstrate their efficacy or safety, and antiarrhythmic drug therapy is generally considered to be unlikely to prevent IART recurrences during long-term treatment in most patients. Proarrhythmia and adverse effects on ventricular and nodal function may further limit the value of these agents in CHD patients. AV nodal blocking drugs may also be used but are often difficult to titrate because of the relatively slow cycle length and fixed conduction ratios often seen in IART.

Pacing Therapy for Intraatrial Reentrant Tachycardia

Early perioperative sinus node dysfunction is common in patients undergoing congenital heart surgeries, as is gradual, progressive loss of sinus rhythm on late clinical follow-up, especially those undergoing extensive dissection and modification of the right atrium. Patients with sinus venosus defects or heterotaxy syndromes, particularly left atrial isomerism, may also have congenital abnormalities of the sinus node. Although chronic bradycardia is often tolerated, pacing may sometimes alleviate signs of congestive heart failure and symptoms such as fatigue, exercise intolerance, dizziness, or syncope in patients with junctional escape rhythms, severe resting bradycardia, chronotropic incompetence, and/or prolonged pauses. Pacing may also be necessary to permit therapy with antiarrhythmic medications.

Conversion of IART can often be achieved with rapid atrial pacing leading to temporary block of the tachycardia circuit, and devices capable of performing this type of automatic tachycardia recognition and burst pacing have been developed and shown to be efficacious in some patients. However, although pacing may sometimes result in symptomatic improvement and decreased tachycardia frequency, neither antibradycardia nor antitachycardia pacing is reliably useful for prophylaxis of IART. Practical limitations often require that the choice of system be adapted to patient-specific problems faced with lead placement and maintenance, and many of the benefits that might be derived from atrial pacing may be offset by the difficulties encountered in positioning an effective pacing lead. Accessible endocardial or epicardial sites that are suitable for such an approach (i.e., capable of generating sensed electrograms of sufficient magnitude and quality to ensure reliable atrial sensing) may be few in number, and endovascular placement of atrial leads may increase risk of thrombosis and paradoxical embolism, as well as exacerbate inadequate baffle orifices (baffle stenosis) in Mustard/Senning patients. In addition, limited choices of atrial sites for pacing may require placement of leads close enough to ventricular muscle that far-field R wave sensing becomes problematic.

General Considerations for Catheter Ablation in Congenital Heart Disease Patients

Thorough preprocedural evaluation should be performed. The operator must be familiar with the clinical documentation of IARTs with respect to P wave morphology and cycle length, other arrhythmia diagnoses, the underlying cardiovascular anatomy and its implications for arrhythmia substrate, any vascular or intracardiac anomalies that may affect catheter access, and the general hemodynamic status of the patient. All arrhythmias documented by surface electrocardiogram, Holter recordings, and/or implantable loop device should be reviewed, as well as reports of previous surgical procedures or catheter ablation procedures, and 3-dimensional cardiac anatomy obtained by echocardiography, magnetic resonance imaging (MRI), and/or computed tomographic (CT) scan. If there are significant noncardiac comorbidities or ventricular dysfunction that predicts possible postprocedure cardiorespiratory instability, with need for invasive monitoring or advanced nursing care, arrangement should be made in advance for an intensive care unit bed. Members of the multidisciplinary team involved with complex ablations should be familiar with the special issues around performing ablation in patients with complex CHD, and any special technologies or expertise likely to be necessary to accomplish the planned procedure should be available. These may include use of advance mapping systems with the ability to incorporate multimodal imaging, application of irrigated ablation, and the capability to perform advanced vascular access and transseptal techniques.

Mapping and Ablation of Intraatrial Reentrant Tachycardia in Congenital Heart Disease

Mapping of IART has been enhanced by the advent of technologies that allow precise correlations to be drawn between the arrhythmia circuit and underlying anatomy. These have shown that IARTs are often either isthmus dependent or are related to known anatomic malformations and/or surgical interventions. Knowledge of these prevalent circuits makes the task of identifying the mechanism in a given patient much simpler, opens the door to the possibility of standardized, empirical, and effective ablation interventions, and makes it easier to recognize atypical situations when they do arise. For example, because the cavotricuspid or cavomitral isthmus is so likely to be a substrate for IART, barring specific contraindications, ablation at this site may be considered a routine component of IART ablation, regardless of where other targeted clinical IART circuits are identified.

Although it is possible to ablate atrial flutter with fluoroscopic guidance alone, use of 3-dimensional electroanatomic mapping systems for guiding ablative therapy is generally recognized to be necessary to optimize the outcomes of complex procedures. In fact, similar to interest expressed for mapping and ablation of arrhythmias in normal cardiac anatomy, the feasibility of less fluoroscopic ablation of IART has recently been demonstrated. Atrial anatomy is typically severely distorted by underlying disease and prior cardiac surgery, and mapping allows for visualization of anatomic structures and relationships, areas of scar tissue, and prosthetic material. Target sites areas for ablation are selected by combining mapping of electrical activation and endocardial voltage with entrainment pacing maneuvers, and this is discussed in the following sections.

Activation Mapping

Activation mapping is the process of iteratively measuring the timing of endocardial electrograms measured at the catheter-tip electrodes, in relation to a fixed reference time in the tachycardia cycle. Having a stable and easily identified atrial signal for timing reference is very important for effective mapping, as it allows mapped points to be classified as earlier or later in the tachycardia cycle. For IART mapping, this is usually defined by an intracardiac electrogram recorded from a stable location by a second catheter, as it is often impossible to ascertain features of the surface P wave, which are of adequate quality to make such measurements. The QRS complex may sometimes be used in the presence of a stable AV relationship.

Measuring the electrogram timing and catheter location point-by-point across the endocardial surface allows the gradual creation of a comprehensive map of the anatomy of the arrhythmia. It also permits the operator to classify the mechanism as focal (activation originates from a single point and spreads centrifugally; Fig. 13.8 ) or reentrant (activation appears to create a spatial and temporal loop with no single earliest site; Fig. 13.9 ). Although the IART mechanism sometimes becomes clear quite quickly in the mapping process, deceptive patterns sometimes emerge. To avoid missing large portions of the IART circuit in an electroanatomic map, it is good practice before coming to a conclusion about the substrate of a given IART to make sure that all of the accessible endocardial anatomy has been sampled, and that the electrogram timings of the points that have been measured span most (i.e., ≥ 90%) of the tachycardia cycle length. This approach has recently been advanced to allow for more rapid and high-density mapping using a variety of multielectrode arrays and algorithmic approaches to automatic annotation of activation events.

Focal pattern of activation demonstrated using electroanatomic mapping in a patient who has undergone a Mustard procedure and has a focal form of atrial tachycardia located at the anteroseptal aspect of the mitral valve, which in this patient is a systemic venous structure. Red color indicates earliest activation, and purple latest activation in relation to a fixed temporal reference time. Note that activation radiates centrifugally from the red area; deep red markers indicate site of successful ablation. IVC, Inferior vena cava; SVC, superior vena cava.

Macroreentrant pattern of activation demonstrated using electroanatomic mapping in the same patient. Note that both left (pulmonary venous) and right (systemic venous) atria are shown. Here, deep red markers again indicated the site of a cavotricuspid isthmus ablation, which in this patient is one of the pulmonary venous side of the atrial baffle. The blue line indicates the margin of the systemic tricuspid annulus. Again, red color indicates earliest activation, and purple latest activation in relation to a fixed temporal reference time; in this tachycardia, activation propagates counterclockwise about the tricuspid annulus in the manner of common atrial flutter. IVC, Inferior vena cava; SVC, superior vena cava.

An alternative method of mapping, demonstrated previously but currently less frequently used in IART, involves the use of electrical catheter navigation to create a virtual model of the endocardial surface of the chamber of interest ( Fig. 13.10 ). Once this model is completed, a balloon-mounted endocavitary electrode array of known geometry is used to record far-field electrical potentials and to compute instantaneous isopotential maps, which can then be studied in sequence to determine activation patterns. This approach has the advantage of being able to collect useful data in a single beat, but is somewhat difficult to use in CHD patients with IART caused by geometric constraints and the low amplitude of endocardial potentials that are frequently encountered.

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