Catheter Ablation of Ventricular Tachycardia After Repair of Congenital Heart Disease
Electroanatomic Identification of the Critical Right Ventricular Isthmus
Background— Catheter ablation of ventricular tachycardia (VT) after repair of congenital heart disease can be difficult because of nonmappable VTs and complex anatomy. Insights into the relation between anatomic isthmuses identified by delineating unexcitable tissue using substrate mapping techniques and critical reentry circuit isthmuses might facilitate ablation.
Methods and Results— Sinus rhythm voltage mapping of the right ventricle was performed in 11 patients with sustained VT after repair of congenital heart disease. Unexcitable tissue from patch material, valve annulus, or dense fibrosis, identified from bipolar voltage (<0.5 mV) and pacing threshold (>10 mA), was defined as an anatomic isthmus boundary bordering 4 isthmuses between (1) the tricuspid annulus and scar/patch in the anterior right ventricular outflow, (2) the pulmonary annulus and right ventricular free wall scar/patch, (3) the pulmonary annulus and septal scar/patch, and (4) the septal scar/patch and tricuspid annulus. The reentry circuit isthmuses of all induced 15 VTs (mean cycle length, 276±78 ms; 73% poorly tolerated), identified by activation, entrainment, and/or pace mapping, were located in an anatomic isthmus (11 of 15 VTs in anatomic isthmus 1). Transecting the anatomic isthmuses by ablation lesions abolished all VTs. During 30.4±29.3 months of follow-up, 91% of patients remained free of VT.
Conclusions— Reentry circuit isthmuses in VT late after repair of congenital heart disease are located within anatomically defined isthmuses bordered by unexcitable tissue. The boundaries can be identified with 3-dimensional substrate mapping and connected by ablation lines during sinus rhythm. These findings should facilitate catheter and surgical ablation of stable and unstable VTs.
Received June 25, 2007; accepted August 22, 2007.
Surgical treatment has improved the long-term prognosis of patients with congenital heart disease (CHD), but late ventricular arrhythmias remain a risk for patients who have had ventriculotomies.1,2 After repair of tetralogy of Fallot (TOF), the incidence of ventricular tachycardia (VT) is 11.9%, with an 8.3% risk of sudden death by 35 years of follow-up.3 Case reports and small series of intraoperative and catheter mapping have demonstrated reentry as the underlying mechanism of these VTs, although other causes cannot be excluded.4–7 The feasibility of catheter ablation, applying conventional mapping techniques, has been reported.8 However, complex anatomy, hypertrophied myocardium, broad isthmuses, hemodynamic instability, or noninducibility of the VTs may explain ablation failure in 50% of cases.9,10
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Clinical Perspective p 2252
Anatomic regions of conduction block that can potentially define critical isthmuses for reentry include valve annuli, patch material, and surgical incisions. Recently, methods to better characterize the reentry substrate by defining areas of block or unexcitable scar on 3-dimensional (3D) maps during sinus rhythm (SR) have been applied to other types of scar-related reentry.11,12
We hypothesized that substrate mapping techniques provide a 3D reconstruction of anatomic isthmuses after repair of CHD and that pace mapping within these anatomic isthmuses provides a potential indication of the reentry circuit exit location. These methods might then allow ablation strategies designed to transect critical isthmuses during SR.
Review of medical records was approved by the Brigham and Women’s Hospital human subject protection committee. Eleven consecutive patients (8 men; age, 43.7±10.6 years) with recurrent symptomatic VT after repair of CHD who were referred to 2 centers (Brigham and Women’s Hospital and Leiden University Medical Center) for radiofrequency (RF) catheter ablation were investigated between February 1998 and January 2007 (9 of 11 were included after February 2004) (Table 1). Patient 8 has been previously reported.7
Nine patients had undergone surgical repair of TOF at an average age of 10.5±2.6 years; 1 patient had repair of an atrioventricular septal defect by patch closure of an ostium primum atrial septal defect and patch closure of an inlet ventricular septal defect (VSD). The remaining patient had d-transposition of the great arteries associated with VSD without significant pulmonary stenosis that was corrected by an arterial switch operation and patch closure of the VSD.
Three TOF patients had undergone a second operation (pulmonary valve replacement in 2, valvuloplasty in 1). VT occurred at a mean of 34.3±7.6 years after surgical correction. Three patients had recurrent syncope; 1 had been resuscitated after VT degenerated to VF. Three patients had recurrent presyncope, 2 with documented sustained VT and 1 with nonsustained VTs on a Holter recording. The remaining 5 patients had palpitations and documented sustained VT.
Mean QRS duration on the resting ECG of TOF patients was 154±44 ms (range, 100 to 220 ms). Echocardiography was performed in all patients; 5 underwent cardiac magnetic resonance imaging, and 1 had multislice computed tomography imaging.
The right ventricle (RV) was mildly dilated in 7, moderately dilated in 2, and severely dilated (RV inlet >60 mm) in 2 patients, with severely reduced function in 1 patient. Left ventricular function was preserved in all patients (mean ejection fraction, 59±5%). Pulmonary regurgitation was moderate (20% to 40% on magnetic resonance imaging) in 2 patients, and a moderate pressure gradient across the RV outflow tract (RVOT) was evident in 1 patient.13
Electrophysiological Study and Mapping
After informed consent was obtained, conscious sedation was achieved with fentanyl and midazolam. Electrode catheters were positioned in the RV, His-bundle position, and right atrium via femoral veins. Bipolar signals (filtered at 30 to 400 Hz) and 12-lead ECGs were stored on optical disks for offline analysis (Prucka Engineering, GE Healthcare, Piscataway, NJ).
Programmed stimulation was performed from the RV apex and RVOT (3 extrastimuli, 3 basic cycle lengths [CLs]) to induce VT and to obtain the QRS morphology. VTs with a predominantly positive deflection in lead V1 were considered right bundle-branch block morphology tachycardias; those with a predominantly negative deflection in V1 were considered left bundle-branch block morphology tachycardias.
RV mapping during SR was performed with a 3.5-, 4-, or 8-mm-tip catheter with 1-mm (Navi-Star) or 2-mm (Thermocool) interelectrode spacing of the recording electrodes (Thermocool, Navi-Star 4 mm, Navi-Star 8 mm; Biosense Webster, Inc; Diamond Bar, Calif) to construct a 3D voltage map using an electroanatomic mapping system (CARTO; Biosense Webster). Peak-to-peak bipolar amplitudes were displayed color coded, with electrograms <1.5 mV defined as low voltage and electrograms <0.5 mV as very low-voltage electrograms on the basis of mapping data obtained in patients without structural heart disease.11 At low-amplitude sites, unipolar pacing was performed with 10 mA at a 2-ms pulse width between the distal electrode of the ablation catheter (negative pole) and an indifferent electrode (positive pole) located remote from the heart in the inferior vena cava. Sites with pacing threshold >10 mA were tagged as electrically unexcitable scar (EUS) and were shown as gray regions.12 The His-bundle position was marked and the pulmonary valve was localized by advancing the catheter into the pulmonary artery and withdrawing it until an RV electrogram was identified. When the valve was not located in patients with transannular patches and/or adjacent RVOT scar, these regions also were tagged as scar if pacing did not capture. Anatomic boundaries were defined as EUS, pulmonary valve, tricuspid annulus (TA), or very low-voltage electrograms; anatomic isthmuses were defined as areas between these boundaries.
Identification of VT Circuits
After voltage mapping, VT was reinduced. In patients with tolerated VT, activation and entrainment mapping was performed. Reentry circuit isthmus sites were defined by entrainment showing either concealed fusion with a difference between the postpacing interval and the VTCL of ≤30 ms and a stimulus to QRS distance (S-QRS) of ≤70% of the VTCL or sites with diastolic electrical activity during VT and termination and prevention of reinitiation of VT by RF ablation. For VTs that were not mappable because of hemodynamic instability or termination during catheter manipulation or entrainment mapping, reentry circuit isthmuses were defined by pace mapping at sites where the QRS morphology matched that of the VT (≥10 of 12 leads) with an S-QRS delay of >40 ms. If VT could be briefly tolerated, the catheter was moved to the presumed isthmus site during SR, and the VT was reinduced to confirm the position within the circuit either by entrainment mapping or by termination during RF delivery.
RF ablation was performed with an open saline-irrigated catheter (power limit, 50 W) in 8 patients (Thermocool), a closed irrigation catheter in 1 patient (Chilli, Boston Scientific, Natick, Mass), and an 8-mm-tip catheter (power limit, 70 W) in 2 patients (Navi-Star, 8 mm). VT isthmuses were targeted for ablation. Ablation was performed during VT when possible. If VT terminated or slowed during RF application, additional lesions were placed to connect the adjoining anatomic boundaries. In patients with poorly tolerated VT, pace mapping was used to select isthmus sites. RF lesions were placed during SR until unipolar pacing failed to capture. After completion of the lesions, programmed stimulation was repeated, including during isoproterenol infusion (1 to 8 μg/min). If any monomorphic VT was inducible, the mapping and ablation process was repeated. Successful VT ablation was defined as the absence of any inducible sustained (>30 seconds or requiring termination) monomorphic VT.
Morphology of Postmortem Specimens After Repair of TOF
From the Leiden collection of malformed hearts, 8 postmortem specimens with repaired TOF were studied to examine the variation in morphology after corrective surgery and the distribution and number of anatomic isthmuses. The 8 patients (4 male) died at a mean age of 4.1 years (range, 0.4 to 10.4 years). Seven patients died within 11 days after repair; 1 patient survived 5.0 years.
The authors had full access to and take full responsibility for the integrity of the data. All authors have read and agree to the manuscript as written.
SR voltage maps of the RV were constructed from an average of 224±69 sites per patient (range, 162 to 317). RVOT sites showed widespread low-voltage signals indicating abnormal myocardium. Within these low-voltage regions, areas of EUS were identified in the 10 patients tested for EUS; in patient 8 (studied in 1998), isthmus boundaries were defined as areas with very low-voltage electrograms (<0.5 mV).
EUS in a septal location was present in all patients; in 8 patients, the septal EUS was in continuity with the TA. Two additional patterns of EUS could be identified. The first showed EUS of the anterior RVOT in continuity with the pulmonary valve. Patients 3, 4, 5, and 7 who had undergone transannular patch repair, patient 6 with RVOT patch, and patient 1 with resection of a subpulmonary stenosis via RV incision showed this pattern. The second pattern showed EUS of the anterior RVOT or adjacent RV free wall separated from the pulmonary valve by an intervening area where pacing captured. This pattern was found in patients 2, 8, 10, and 11; all had operative correction by an RV incision. The distribution of border-forming areas resulted in 4 distinct anatomic isthmuses located between the RVOT/adjacent RV scar and TA (isthmus 1) found in all patients, between the RV scar and pulmonary valve (isthmus 2) in 4 patients, between the pulmonary valve and septal scar (isthmus 3) in 10 patients, and between the septal scar and TA (isthmus 4) in 3 patients (Figure 1). Figure 2 gives an example of anatomic isthmus 1 and 4 identified by 3D voltage mapping; Figure 3A and 3B shows isthmus 1, 2, and 3 and isthmus 1, 2, and 4, respectively.
Distribution of the Anatomic Isthmuses in Postmortem Specimens
Seven of the 8 specimens had a transannular patch. In 1 specimen, an RVOT patch showed fibrous continuity with the pulmonary valve; thus, no specimen had anatomic isthmus 2 (Figure 4). All specimens had muscular continuity between the transannular/RVOT patch and the TA (isthmus 1), and 7 had muscular continuity between the VSD patch and the pulmonary valve (isthmus 3). In 1 specimen, the VSD patch was bordered by fibrous tissue of the TA and the pulmonary valve excluding anatomic isthmus 3 and 4. Only 1 specimen had closure of a VSD, leaving a small posterior rim of myocardium compatible with anatomic isthmus 4.
ECG Characteristics of the Induced VTs
In 11 patients, 15 different monomorphic VTs (mean CL, 276±78 ms; range, 202 to 504 ms) could be induced; 9 were documented as spontaneous VTs. In patient 5, the spontaneous VT could not be induced. Thirteen VTs had left bundle-branch-block, and 2 had right bundle-branch block morphologies. Frontal plane axis was left inferior in 11, right inferior in 3, and left superior in 1; precordial transition was at V2 in 2, V3 in 6, V4 in 1, V5 in 5, and V6 in 1 (Table 2).
Reentry Circuit Isthmuses Defined by Entrainment, Pace Mapping, and Ablation
Two of the 15 induced VTs were hemodynamically unstable. Nine VTs were briefly tolerated, allowing limited activation and entrainment mapping, with RF application during ongoing VT in only 3 patients. The remaining 4 VTs were well tolerated for mapping and ablation.
Eleven VTs had a reentry circuit isthmus located in anatomic isthmus 1, 1 VT in anatomic isthmus 2, 2 VTs in anatomic isthmus 3, and 1 VT in anatomic isthmus 4. In all but 1 patient, slow conduction within these anatomic isthmuses was evident as a prolonged S-QRS interval exceeding 40 ms during pace mapping (Table 2).14
In patients 3, 9, and 11 (VT3), a reentry isthmus site was identified on the basis of pace mapping during SR and limited VT activation mapping and located within anatomic isthmus 1, 3, and 2, respectively. Activation mapping of VT in patient 3 and VT3 in patient 11 showed a wave front propagating through isthmus 1 in an inferior-to-superior direction. The latter revolved clockwise around the RVOT scar (Figure 3). All 3 VTs were terminated during RF application. Additional RF lesions were applied to connect the boundaries of the anatomic isthmus.
Entrainment mapping demonstrated a central isthmus site in patients 4 and 8 and an exit site in patients 10 and 11 (VT2), all located within isthmus 1. The wave front propagated from superior to inferior through this isthmus in patient 4 and from inferior to superior in patients 8, 10, and 11. Slowing of the VTs occurred during RF delivery at the isthmus site without a change in the QRS morphology. Consecutive RF lesions were applied to connect the initial site of RF delivery to the anatomic boundaries. This resulted in VT termination in the 4 patients. The reentry circuit isthmus of VT1 in patient 11 was localized to isthmus 1 on the basis of pace mapping; this VT was not inducible after these boundaries were connected.
In the remaining 5 patients, RF current was applied only during SR. Localization of reentry circuit isthmuses was based on pace mapping (patients 1 and 2), limited entrainment mapping suggesting an exit site (patients 5 and 6), or both techniques (patient 7) (Figure 5). The site was located within anatomic isthmus 1 in patients 1, 2, 3, and 7; isthmus 3 for VT1, and isthmus 4 for VT2 of patient 6. RF ablation through these isthmuses abolished all 7 VTs. In patient 7, VT1 remained inducible after RF applications within isthmus 1. However, the RVOT/TA line was completed only after a change to an irrigated ablation catheter, which then rendered both VTs not inducible. After completion of these ablation lines with an average of 12.6±7.2 RF lesions (range, 4 to 26), no monomorphic VT was inducible in all patients.
Although reentry circuit isthmuses of 11 of 15 VTs were localized within anatomic isthmus 1, QRS configuration was heterogeneous, and the number of VTs was too small for reliable statistical comparisons. Figure 6 gives 1 ECG example for each axis and transition zone of the VTs sharing isthmus 1 and of the VTs mapped to the anatomic isthmus 2, 3, and 4. The precordial transition tended to be at V3 or later in isthmus 1 VTs but earlier in VTs that used an isthmus bordering on the septum. Furthermore, VTs with an inferior-to-superior wave-front propagation tended to have an earlier precordial transition. The 2 right bundle-branch block VTs propagated from inferior to superior through isthmus 1 and clockwise through isthmus 2. However, the critical isthmus was in isthmus 1 in patient 8 and in isthmus 2 in patient 11.
No complications occurred. Drug therapy was maintained in 5 patients during follow-up (2 amiodarone, 3 sotalol) and discontinued in 2 (side effects in 1, decision of the referring physician in 1); antiarrhythmic drugs were not administered to 4 patients who were not receiving antiarrhythmic drugs at referral. Nine patients received aspirin (100 to 325 mg for 6 weeks); 2 patients continued anticoagulation with warfarin. During long-term follow-up, 5 patients had implantable cardioverter-defibrillators. Three patients underwent implantation of an implantable cardioverter-defibrillator after ablation (patient 2 after being resuscitated, patient 4 after syncope not explained by the induced stable VT, and patient 3 on request of the referring physician); 1 patient (patient 11) had undergone implantable cardioverter-defibrillator implantation before referral after syncope as a result of a documented fast VT (CL, 230 ms). Three patients had a second study. Patients 8 and 9, who were asymptomatic, had no inducible VT 2 days and 6 months after the initial procedure. Patient 7, who had recurrence of palpitations but no documented VT, had inducible VT1 (CL, 243 ms) 10 months after ablation and underwent implantable cardioverter-defibrillator implantation. During an average follow-up of 30.4±29.3 months (range, 4 to 103), no patient had recurrent arrhythmia detected (including patient 7).
We found that reentry circuit isthmuses in VT after repair of CHD are located within distinct anatomic isthmuses bordered by unexcitable tissue. The boundaries can be identified with 3D substrate mapping. Transecting anatomic isthmuses by linear RF lesions during SR abolished all inducible VTs, including poorly tolerated VTs not approachable by conventional activation mapping. Although 4 isthmuses could be identified, isthmus 1, between an area of anterior wall RVOT scar/patch and the TA, was involved in 73% of VTs in this series. The described substrate-based approach achieved long-term success for 91% of the patients.
Limited information is available on mapping and ablation of VTs in patients after repair of CHD (Table 3⇓).4–10,15–24 Small series reported success rates between 50% and 81% for targeting hemodynamically stable VT. More recently, an intention-to-teat analysis revealed procedural success of 50% with a 40% recurrence rate.10
In agreement with our findings, previous reports also have found VT to be due to macroreentry.4,5,7 Misaki et al6 described activation revolving around a myotomy scar with diastolic activity within the lateral RVOT wall consistent with the most common location of reentry circuit isthmuses in the present study.
Although macroreentrant may occur around obstacles in the absence of conduction delay, most studies suggest a slow-conduction area participating in the circuit. Slow conduction within an anatomic isthmus was suggested in 10 of our 11 patients by a prolonged S-QRS during pace mapping or entrainment.14 RV remodeling induced by pressure or volume load might promote hypertrophy and fibrosis with slow conduction, providing the link between impaired hemodynamics and VT.25,26 However, in our series, only 2 patients had undergone a second operation for pulmonary regurgitation and 2 had moderate regurgitation. RV incision and myectomy produce RVOT aneurysmal or akinetic regions in >50% of the patients.27 Misaki et al6 found surviving myocytes embedded in fibrous tissue in resected tissue from an RVOT region with delayed conduction, resembling the histological findings in the border zone of infarcted myocardium.6 Of interest, the postmortem specimen of the patient who survived 5 years after repair showed extreme thinning only of the anterolateral RVOT, consistent with isthmus 1 (Figure 4e).
Identification of scar from voltage mapping and pace mapping is used in unmappable VTs resulting from myocardial infarction and cardiomyopathy.11,12,28 Voltage maps in our patients showed widely distributed low-amplitude electrograms within the RVOT. Sites identified as EUS are likely due to dense fibrosis.12 The size of fibrotic regions identified by this method is not clear, and it may not identify narrow bands of fibrosis that also can serve as conduction block. Despite this limitation, all reentry circuit isthmuses could be assigned to an anatomic isthmus that could be transected during SR. Although we did not routinely test for conduction block across the linear lesion, completion of lesions was confirmed by noncapture with high-output pacing along the line.
The identified anatomic isthmuses were not present in all patients, consistent with the findings in postmortem specimens. Isthmus 2 was not found in the specimens, likely because of the frequent use of transannular patches. The low incidence of isthmus 4 can be explained by predominantly perimembranous VSDs in patients with TOF. The 11 VTs sharing the anatomic isthmus 1 showed significant variation of QRS morphology. Of interest, the 2 VTs with a transition in V2 suggesting a high septal exit were successfully ablated by transecting isthmus 3 or 4. The 2 VTs with right bundle-branch block pattern both used isthmus 2, although 1 also used isthmus 1. Thus, in unmappable VTs with a transition in V2, ablation might target septal isthmuses, and in VTs with a right bundle-branch block pattern, targeting isthmus 2 might be justified.
Although the outcomes for our patients were good, our study cohort is a relatively small, undoubtedly selected population. Postablation management was individualized, with some patients continuing antiarrhythmic medications and some receiving implantable cardioverter-defibrillators. Sudden death can occur in patients with TOF who do not have inducible VT.29 Implantation of a cardioverter-defibrillator is a reasonable consideration for many of the patients that warrants further investigation.
We did not completely define reentry circuits, and >1 anatomic isthmus might be involved in a single tachycardia as demonstrated. Despite this limitation, our results provide an anatomic construct for approaching ablation. It is probable that details of the surgery importantly determine the location of reentry circuit isthmuses and may change in the future. Electrogram characteristics are likely influenced by electrode size, which varied with the mapping catheters used, but our findings of low-amplitude regions were consistent.
Reentry circuit isthmuses in VT after repair of CHD are located within anatomically defined isthmuses bordered by unexcitable tissue. These boundaries can be identified by 3D mapping during SR. RF ablation of the anatomic isthmus during SR is feasible and resulted in noninducibility of all VTs in our population, including those that did not allow mapping during VT. An isthmus between the TA and unexcitable tissue in the RVOT free wall was the most common cause of VT. This finding has potential implications for patients with unstable VTs and those who require repeated surgery for hemodynamic reasons, suggesting that anatomically guided ablation lesions through this isthmus are likely to interrupt VT circuits in a significant number of patients. The results should facilitate catheter and surgical ablation of VT after repair of CHD.
Dr Stevenson has received honoraria from Biosense Webster, Boston Scientific, Medtronic, and St Jude and has served as a consultant for Biosense Webster. The other authors report no conflicts.
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Arenal A, del Castillo S, Gonzalez-Torrecilla E, Atienza F, Ortiz M, Jimenez J, Puchol A, Garcia J, Almendral J. Tachycardia-related channel in the scar tissue in patients with sustained monomorphic ventricular tachycardias: influence of the voltage scar definition. Circulation. 2004; 110: 2568–2574.
Ventricular scars from surgical repair of congenital heart disease are a source of ventricular tachycardia (VT) causing late morbidity and sudden death. The increasing number of patients with corrected congenital heart disease who are entering the adult population is likely to increase the number of patients who require treatment for VT. Implantable cardioverter-defibrillator therapy is effective for terminating VTs, but additional therapy to prevent episodes and to reduce implantable cardioverter-defibrillator shocks is required in some patients. Catheter ablation offers the potential for preventing VT recurrences, but relatively few data exist describing reentry circuit locations and approaches to ablation. In this study, we used 3-dimensional electroanatomic reconstructions to characterize the locations of potential critical isthmuses for reentry circuits in a series of 11 patients after repair of tetralogy of Fallot or ventricular septal defect. The mapping data also were related to morphological analyses of postmortem specimens with repaired tetralogy of Fallot. We describe 4 locations for reentry circuit isthmuses. In most patients, ≥1 VTs involved an anatomic isthmus between the tricuspid valve annulus and an area of scar or patch in the free wall of the right ventricle. Importantly, these isthmuses can be identified during sinus rhythm. Targeting the anatomic isthmuses during sinus rhythm, radiofrequency ablation abolished all inducible VTs and led to long-term success for 91% of our patients. These findings should facilitate catheter and surgical ablation as additional antiarrhythmic therapy in selected patients with recurrent VT late after repair of congenital heart disease.
Guest Editor for this article was Douglas P. Zipes, MD.