Abstract
Monomorphic ventricular tachycardia in postoperative congenital heart disease patients is based upon morphologic/anatomic variants of the heart defect itself or caused by ventricular incisions and patches that allow initiation and perpetuation of a stable ventricular macroreentrant circuit allowing endocardial mapping and catheter ablation. Most experience has been gained in patients with postoperative tetralogy of Fallot. With the use of the 3-dimensional mapping systems, endocardial mapping is performed by detailed right ventricular substrate mapping with the focus on identification of anatomic markers as the ventricular septal defect patch and the right ventricular outflow tract patch. Radiofrequency catheter ablation using cooled-tip or irrigated-tip catheters is the preferred energy mode for ablation in this setting. Four main discrete anatomic isthmuses that support initiation and perpetuation of ventricular tachycardia in patients with postoperative congenital heart disease have been identified mainly involving the right ventricular outflow tract. Aim of radiofrequency catheter ablation is to interrupt electrical conduction through these anatomic critical isthmuses by transsection the isthmus. Procedural success is proven by noninducibility of the tachycardia and demonstration of complete conduction block along the ablation lesion line. Several case series reported very satisfactory results on mapping and catheter ablation of postoperative ventricular tachycardia, but recurrences were not insignificant. Implantable cardioverter-defibrillator implantation has been advised also in patients with a successful ablation procedure accordingly. With increasing experience and expertise, however, catheter ablation of ventricular tachycardia isthmus ablation can be curative in patients with repaired congenital heart disease with preserved ventricular function and isthmus-dependent reentrant circuit.
Keywords
anatomic isthmus, catheter ablation, congenital heart disease, endocardial mapping, ventricular tachycardia
Key Points
Tachycardia Mechanism
- •
Ventricular tachycardia based upon morphologic/anatomic variants of the congenital heart defect or ventricular incisions and patches/scar tissue
- •
Best example tetralogy of Fallot: right ventricular macroreentrant tachycardia through anatomically defined isthmuses, often involving the right ventricular outflow tract
Mapping
- •
Substrate mapping combined with activation mapping and pace mapping to localize right ventricular anatomic isthmuses
- •
Proof of electrophysiologic findings by concealed entrainment and stimulus-to-QRS-delay
Ablation Targets
- •
Transection of anatomic isthmuses (often multiple)
- •
Ablation during tachycardia (if tolerated) or during sinus rhythm
- •
Proof of completeness of radiofrequency current lesion line
- •
Noninducibility of ventricular tachycardia
Special Equipment
- •
Three-dimensional mapping system
- •
Cooled tip or irrigated tip radiofrequency current ablation catheters
- •
Steerable or long preshaped sheaths
- •
Cryoenergy catheter
Sources of Difficulty
- •
Complex anatomy
- •
Limited vascular access to target
- •
Unstable hemodynamics during ventricular tachycardia
- •
Insufficient lesion formation in thickened and fibrosed right ventricular myocardium
- •
Often multiple anatomic isthmuses
Introduction
Ventricular tachycardia (VT) is a well-known late consequence after surgical repair of a variety of congenital heart defects. For the purpose of this chapter it seems appropriate to make a distinction between two different forms of VT in this population. One type of VT is based upon morphologic/anatomic variants of the heart defect itself or to ventricular incisions and patches that allow initiation and perpetuation of a stable ventricular macroreentrant circuit resulting in a stable monomorphic VT. Because of the underlying electrophysiologic mechanism, these tachycardias are amenable to endocardial mapping and catheter ablation, which will be discussed in detail within this chapter. The best example of this clinical entity is unoperated/native and postoperative tetralogy of Fallot and its variants. The second type of VT mainly occurs in severely diseased ventricular myocardium with significant fibrosis and myocardial disarray resulting in less organized, rapid polymorphic VT and ventricular fibrillation with the risk of sudden cardiac death. Because of the underlying electrophysiologic mechanism, these tachycardias cannot be treated sufficiently by catheter ablation. Accordingly, implantable cardioverter-defibrillator (ICD) implantation is the recommended therapy in these patients. Common varieties of congenital heart defects associated with these types of ventricular tachyarrhythmias are obstructive left ventricular outflow tract lesions, d-transposition of the great arteries after atrial switch procedure with failing systemic right ventricle, tetralogy of Fallot with significantly impaired right ventricular function, and univentricular hearts with a Fontan circulation. In selected patients, however, both types of VT may be present. Accordingly, patients with significantly impaired hemodynamics after ICD implantation for fast VT may benefit from endocardial mapping and catheter ablation with the aim to decrease number of ICD shocks as monomorphic VT may trigger polymorphic rapid VT and ventricular fibrillation. In selected patients, ablation of premature ventricular beats that trigger ventricular fibrillation may result in a significant reduction of ICD shocks.
Ventricular tachyarrhythmias are a clinical problem that is encountered in daily routine practice of all physicians involved in the care of adult patients with congenital heart defects. In a multicenter trial involving 793 patients with repaired tetralogy of Fallot, sustained monomorphic VT occurred in 4.5% of the patients during a mean postoperative follow-up of 21 years. Sudden cardiac death was noted in another 2% of the patients. In a second multicenter trial covering 556 adult patients with tetralogy of Fallot ventricular arrhythmias were prevalent in 14.6%.
Because nowadays almost 90% of all newborns born with congenital heart defects survive with adequate quality of life into adulthood, the number of patients presenting with congenital heart defects and VT will increase over time.
Anatomy
Most experience on pathophysiology and management of patients with VT and congenital heart disease has been gathered on tetralogy of Fallot. Even in patients with a favorable result after surgical repair, VT is often associated with significant symptoms like syncope and sudden cardiac death. In general, annual incidence of sudden cardiac death after repair of tetralogy of Fallot has been estimated to be 0.15% with increasing risk in adulthood. Several risk factors including anatomic, surgical, hemodynamic, and electrophysiologic parameters have been identified, but the positive predictive value is quite low. For all congenital heart defects, the combination of anatomic abnormalities, surgical scars, and patches/conduits as well as chronic pressure/volume overload leading to myocardial fibrosis and scarring may finally result in formation of a substrate for the development of VT.
Tetralogy of Fallot includes a spectrum of delicate morphologic features that may serve as a substrate for a macroreentrant tachycardia even in the unoperated/native state. The muscular outlet/conal septum is deviated anteriorly and superiorly relative to the remaining interventricular septum giving rise to a large ventricular septal defect with overriding of the aorta, which is partly committed to the hypertrophied right ventricle. Hypertrophied septoparietal trabeculations result in subpulmonary obstruction. The conal septum may extend toward the ventricular–infundibular fold, a thin sheet of muscle interposed between the inlet and outlet portions of the right ventricle. Accordingly, a substrate for a macroreentrant circuit incorporating the conal septum is present even in the unoperated state ( Fig. 35.1 ).
Surgical repair of tetralogy of Fallot should result in complete closure of the ventricular septal defect and preservation of right ventricular form and function with an unobstructed right ventricular outflow tract incorporating a competent pulmonary valve. Surgical repair has made consistent progress over the last 50 years. In the beginning, surgical techniques were restricted to palliative procedures by augmenting pulmonary blood flow through creation of systemic-to-pulmonary artery shunts. Significant for the development of ventricular arrhythmia substrates, early repair techniques included closure of the ventricular septal defect and relief of right ventricular outflow tract obstruction by extensive resection of right ventricular outflow tract muscle via a large right ventriculotomy. In addition, repair after shunting was performed in childhood after long-persisting cyanosis and systemic right ventricular systolic pressure.
Today, corrective surgery is performed early in infancy (within the first 6 months of life) via a transatrial/transpulmonary approach thereby avoiding right ventriculotomy and its associated scarring and risk for the development of VTs. Closure of the ventricular defect with an artificial patch while paying special attention to the specialized conduction system and resection of hypertrophied right ventricular muscle is typically performed via the tricuspid valve. Pulmonary valvotomy and resection of subpulmonary muscle bundles is accomplished through an incision in the main pulmonary artery. Nowadays, when relieving right ventricular outflow tract obstruction, attention is focused on preservation of the pulmonary valve even at the expense of some residual stenosis to reduce the risk of late pulmonary insufficiency and right ventricular outflow tract aneurysm formation. Depending on the individual anatomy, surgeons try to avoid right ventricular outflow tract patch enlargement as well as transannular patch augmentation because of the association with late VTs ( Fig. 35.2 ).
Pathophysiology
Electrophysiologic studies in patients with monomorphic VT after surgery for tetralogy of Fallot have proven that the main tachycardia mechanism is a macroreentrant circuit involving the right ventricular outflow tract (see Fig. 35.2 ). Major sites involved within the circuits are the incision in the right ventricular outflow tract, the ventricular septal defect patch, and the tricuspid valve annulus. Transient entrainment can often be achieved during pacing, with constant fusion at the paced cycle length and progressive fusion at decreasing cycle lengths. Postpacing intervals equaling spontaneous tachycardia cycle length suggests that the pacing site in the right ventricular outflow tract is part of the reentrant circuit ( Fig. 35.3 ).
Diagnosis (and Differential Diagnosis)
The first step during electrophysiologic study is induction of the tachycardia by programmed ventricular stimulation to confirm the diagnosis and to rule out supra-VT with aberrancy ( Box 35.1 ). Monomorphic VT involving the right ventricular outflow tract in postoperative tetralogy of Fallot exhibits an inferior QRS complex frontal plane axis and left bundle branch block morphology in the majority of the patients, but QRS complex morphology may vary. It is of note that QRS morphology of macroreentrant VT involving the right ventricular outflow tract is influenced by the direction of the rotation (clockwise vs. counterclockwise) around the anatomic obstacle. In case of VT involving the right ventricular free wall QRS morphology may exhibit a right bundle branch block pattern.
Pathophysiology —in tetralogy of Fallot: macroreentrant circuit often involving the right ventricular outflow tract, major sites involved: incisions in the right ventricular outflow tract, ventricular septal defect patch, and tricuspid valve annulus.
Arrhythmia Diagnosis and Differential Diagnosis —induction of ventricular tachycardia by programmed ventricular stimulation, rule out supraventricular tachycardia with aberrancy, typical QRS complex pattern of ventricular tachycardia: inferior axis and left bundle branch block morphology.
Mapping
It should be emphasized that the prerequisite for successful and safe endocardial mapping and catheter ablation is the understanding and knowledge of the anatomic abnormalities, the surgical details, and the frequently encountered arrhythmogenic substrates and targets in the individual patient studied. In this context, it is important to gather complete information on history, anatomy, and hemodynamics as well as previous surgical and interventional procedures and the ventricular tachyarrhythmia encountered before starting the procedure.
Early reports used conventional contact mapping combined with pace mapping and entrainment mapping (see Fig. 35.3 ) achieving significant success rates. Entrainment depends on the presence of an excitable gap, which may not be reproduced in tachycardias that are very rapid and not hemodynamically tolerated. In addition, entrainment mapping may not be precise as pacing close to the exit site from a zone of slow conduction does not allow to satisfy any of the criteria for entrainment.
The use of the modern 3-dimensional mapping systems has proven to be extremely useful to overcome those limitations of conventional contact mapping. As the first step, endocardial mapping is performed during sinus or baseline rhythm by detailed right ventricular substrate mapping with the focus on identification of anatomic markers as the ventricular septal defect patch and the right ventricular outflow tract patch. If monomorphic VT is hemodynamically tolerated, right ventricular endocardial mapping is performed subsequently during ongoing tachycardia. If the tachycardia is not tolerated, voltage mapping alone during sinus rhythm may be sufficient to gain important information on the arrhythmia reentrant circuit. In the past, the noncontact mapping system was very useful in this entity, as it particularly allows for complete activation and substrate mapping of fast and hemodynamically unstable VT as well of nonsustained VT.
Zeppenfeld and coworkers used with outstanding expertise the electroanatomic mapping system (CARTO) for identification of the critical right ventricular isthmuses. Right ventricular sinus rhythm voltage mapping was performed and unexcitable tissue from patch material, valve annulus, or dense fibrosis was defined as areas with voltage lower than 0.5 mV. If pacing at 10 mA with pulse width of 2 ms failed to capture, sites were tagged as unexcitable scar. Natural anatomic regions as His bundle, pulmonary valve, and tricuspid valve were annotated. Anatomic isthmuses were defined as viable ventricular myocardium between areas of unexcitable scar (for example ventricular septal defect patch, ventriculotomy incision or right ventricular outflow tract patch), pulmonary valve, tricuspid valve annulus, and areas of very low voltage (<0.5 mV, Fig. 35.4 ). In patients with hemodynamically stable VT, activation and entrainment mapping was performed subsequently (see Fig. 35.4 and Fig. 34.5 ). Reentry circuit isthmus sites were defined by concealed entrainment and a difference of the postpacing interval and VT cycle length less than 30 ms. Additional criteria included a stimulus to QRS interval during pacing less than 70% of tachycardia cycle length or sites with diastolic electrical activity during tachycardia and termination and prevention of reinduction of tachycardia after radiofrequency current application. If VT was not amenable to mapping, reentry circuit isthmuses were defined by pace mapping at sites where the QRS morphology matched that of the VT (>10/12 leads match) with a stimulus-to-QRS delay of more than 40 ms.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
