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The number of patients with severe congenital heart disease (CHD) who are entering the adult population is increasing, likely due to surgical progress resulting in lower morbidity and mortality. The current estimated prevalence of adults with severe congenital heart disease is 0.38 per 1000 adults.1 Patients at high risk for monomorphic VT are those who have undergone a ventriculotomy and patching of a ventricular septal defect (VSD) and VT after repair of tetralogy of Fallot (TOF) can serve as a paradigm for these postoperative arrhythmias. Ventricular arrhythmias can also occur in the setting of a more generalized myopathic process without discrete ventricular scars. This is typical for aortic valve disease or a failing RV after the Mustard or Senning procedure for transposition of the great arteries. These patients are at high risk for sudden cardiac death. Polymorphic VT and VF are the most common documented arrhythmias, whereas monomorphic VT suitable for catheter ablation is less common.

The feasibility of catheter ablation of VT in patients after repair of CHD has been demonstrated, and accordingly, more than 80% of all treated patients were patients after repair of TOF, with an additional 10% having undergone closure of a VSD.

After repair of TOF, the incidence of VT is 11.9% by 35 years of follow-up. The majority of these late VTs are fast and often unstable due to hemodynamic intolerance, therefore requiring a substrate-based ablation approach.²

Case reports and small series of intraoperative and catheter mapping have demonstrated macroreentry as the underlying mechanism of VT after repair of TOF.^3,4 We previously demonstrated that the critical reentry circuit isthmus of a significant number of these macroreentrant VTs is located within anatomically defined isthmuses bordered by unexcitable tissue.⁵ Specific characteristics of the anatomical isthmuses like width and conduction velocity through the isthmus determine its ‘arrhythmogenity’.⁵ Approaches that identify and target these anatomical isthmuses, containing the critical isthmus of the VT without the need for mapping during VT, have been associated with favorable results.^5,6

Preprocedural Planning

Step 1: Knowledge of the Malformation and Review of Operation Records

The potential anatomical isthmuses containing critical parts of the reentry circuit are determined by the malformation and the type of surgical repair. Knowledge of the anatomy and the type of repair is therefore mandatory. We usually make a big effort to obtain all operation records, which are then carefully reviewed. It might be helpful to discuss unclear cases with a surgeon experienced in repair of congenital heart disease or a colleague specialized in grown-up congenital heart disease (GUCH) patients.

The most common underlying disease, TOF, is characterized by a subpulmonary stenosis, a VSD, a dextroposed and overriding aortic orifice, often associated with a clockwise rotation (viewed from the apex) of the aorta resulting in a leftwards displacement of the RCC, and RV hypertrophy. However, it is important to keep in mind that the malformation represents a morphologic spectrum. Reparative surgery involves closing of the VSD, which is subaortic and perimembranous in the majority of TOF patients. The posterior-inferior border of the VSD in these cases is made up of the mitral-tricuspid fibrous continuity, thus unexcitable tissue (Figure 50.1A). In a minority, the defect has a muscular postero-inferior rim (Figure 50.1B). The RVOT obstruction is typically due to an infundibular or subpulmonary stenosis caused by (1) anterior displacement of the outlet septum and (2) hypertrophy of septoparietal trabeculations (anterior) and/or of the trabecula septomarginalis, which requires sometimes extensive resection of the hypertrophied myocardium (Figure 50.1A). A stenosis of the pulmonary orifice, often accompanied by valvular abnormalities, can be relieved by a transannular patch, which disrupts the integrity of the pulmonary valve annulus (Figure 50.2A). Rarely, the stenosis is exclusively infundibular with a pulmonary orifice of normal size. In these cases a RVOT patch might be necessary to augment the restrictive RVOT, sparing the pulmonary valve annulus (Figure 50.2B).

Until 1993, repair was often performed through a right longitudinal (majority) or transverse ventriculotomy. Since then, a combined approach through the RA and the pulmonary artery is used, thereby avoiding a RVOT incision, which can serve as a potential boundary of an anatomical isthmus. A limited RV incision is added, if patch augmentation for the RVOT or pulmonary annulus is needed.

Based on the detailed information on the malformation and the performed reparative surgery, the anatomical boundaries consisting of unexcitable tissue can be predicted; these include the tricuspid annulus, the pulmonary valve, patch material, and surgical incisions. The 4 potential anatomical isthmuses (AI) include the isthmus between (1) tricuspid annulus and scar/patch in the anterior RV outflow; (2) pulmonary annulus and the RV free wall scar/patch; (3) pulmonary annulus and septal scar/patch; and (4) septal scar/patch and tricuspid annulus (Figure 50.3).

Step 2: Evaluation of the Status of the Patient and Assessment of Residual Lesions

As in other patients with structural heart disease who present with VT, a complete workup of the patient is important. This should be performed in specialized GUCH centers. In particular, significant pulmonary regurgitation, which nearly always occurs after transannular patching with subsequent RV enlargement, is a common finding (Figure 50.4; Video 50.1). It is also important to look for residual RVOT obstruction and a residual VSD, which may lead to RV pressure overload and LV volume overload, respectively (Figure 50.5; Video 50.2). These patients may require surgery. Although pulmonary regurgitation is associated with occurrence of VT, simply replacing the pulmonary valve does not appear to eliminate the underlying VT substrate. Therefore, intraoperative ablation should be considered in close cooperation between congenital cardiologist, congenital surgeon, and electrophysiologist.

All patients should undergo cardiac imaging prior to ablation. Although echocardiography is usually helpful, MRI is currently the reference standard to evaluate associated pathology such as branch pulmonary artery stenosis or hypoplasia. Assessment of RV and LV function before ablation helps to identify patients in whom prolonged mapping during VT should be avoided, even if the VT seems to be hemodynamically tolerated. Finally, additional cardiac pathology such as CAD should be considered and excluded.

Step 3: Review of the 12-Lead ECG of the Clinical VT

As in other scar-related VT, the morphology of the 12-lead VT QRS is determined by the VT exit site. However, it is important to keep in mind that each anatomical isthmus can be activated clockwise or counterclockwise, resulting in a different QRS morphology. In some patients, 2 VTs are documented that are morphologically different but share the same anatomical isthmus (Figure 50.6). In these cases, the VT cycle length is often similar or even identical. The precordial transition is usually earlier (e.g., V₂), and a QR in V₁ is typically present in counterclockwise reentry VTs that use isthmus 3 bordering high on the septum. However, VTs with clockwise wavefront propagation through the same isthmus might have a LBBB-like morphology with a late transition (e.g., V₅).

In addition, in the increasing number of patients who have received an ICD for primary or secondary prevention, information on the clinical VT is often restricted to EGM data available from ICD interrogation.

Procedure

Patient Preparation

We usually perform the procedure in conscious sedation achieved with fentanyl and midazolam. The right femoral vein is punctured at least twice and accommodates 1 or 2 sheaths of 6-Fr and an 8-Fr sheath. The 8-Fr sheath that accommodates the mapping and ablation catheter should be placed through its own puncture site to enable easier sheath and catheter manipulation. The 8-Fr sheath can be later replaced with a long sheath. We prefer a long sheath (Swartz™ Braided Transseptal Guiding Introducer [SR-0], St. Jude Medical, St. Paul, MN) for catheter stability in the majority of patients; in patients with severe RV enlargement and/or RVOT aneurysms, we use steerable sheaths (Agilis™ NxT Steerable Introducer, St. Jude Medical) (Figure 50.7).

We use a 5-Fr sheath placed in the right femoral artery for hemodynamic monitoring. This sheath can be easily replaced by an 8-Fr sheath if a retrograde aortic approach becomes necessary. After vascular access a heparin bolus of 100 U/kg is given. An additional bolus of 50 U/kg is given to achieve an ACT of 250 to 300 seconds, which is checked every 30 minutes. Standard 4-polar catheters are positioned in the HRA and in the RV for VT induction. For mapping and ablation, we usually use a 3.5-mm irrigated-tip quadripolar mapping catheter with an F-curve (2-5-2 mm interelectrode spacing, Navistar THERMOCOOL, Biosense Webster, Diamond Bar, CA).

First Step: VT Induction

We always perform electroanatomical bipolar voltage and activation mapping using a 3-dimensional (3D) nonfluoroscopic mapping system (CARTO XP™, Biosense Webster) during sinus rhythm or RV pacing to obtain a 3D reconstruction of all anatomical isthmuses that may contain VT circuit isthmuses by identifying the anatomic boundaries. AIs are present in almost all TOF patients but only those with specific isthmus characteristics constitute the substrate for VT.5 Careful mapping is important to obtain the length of the AI and to determine conduction time through the AI.⁵ During mapping, the RV catheter is withdrawn from the RV, and the maximum positive or negative peak deflection of a surface QRS complex is used as a reference. The onset of the window of interest is usually set before or at the onset of the QRS complex, and its duration should extend beyond the QRS duration to allow for annotation of local EGMs occurring late after the QRS complex.

Peak-to-peak bipolar EGM amplitudes recorded from the distal electrode pair of the mapping catheter (filtered at 30–400 Hz) are displayed color-coded with EGMs greater than 1.5 mV defined as normal voltage and displayed in purple.⁷ We do not define unexcitable tissue by voltage criteria, but use EGMs with amplitudes of less than 0.5 mV, which are displayed in red, as a rough road map for pacing. At these sites we perform bipolar pacing with 10 mA at 2-ms pulse widths. Although bipolar pacing has the potential drawback of anodal capture, the safety settings of the CARTO™ System do not allow easily unipolar pacing from the ablation catheter. If pacing does not capture, the site is tagged as scar or unexcitable tissue likely consistent with an anatomical boundary, and these sites are displayed as gray tags on the map.⁸ If pacing captures, we can at the same time compare the paced QRS morphology with the VT QRS morphology.

We define tricuspid annulus sites by atrial and ventricular EGMs of approximately equal amplitudes (Figure 50.8). The pulmonary valve is identified by advancing the catheter in the pulmonary artery and withdrawing the catheter until we obtain the first RV EGM. It is also helpful to annotate the position of the His bundle to assess the spatial relationship with the anatomical isthmuses, in particular if isthmus 4 bordered by the tricuspid annulus and the VSD patch is targeted.

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