High-Density Voltage Mapping with the PENTARAY Catheter for Successful Ablation of Tetralogy of Fallot Ventricular Tachycardia

Briefly, TOF is comprised of a ventricular septal defect (VSD), a displaced aorta overriding the septal defect, pulmonary valve stenosis, and right ventricular hypertrophy.² Surgical access to the septal defect is needed, and the right ventricle is incised. As a consequence, monomorphic ventricular tachycardia (VT) can develop many years later in the vicinity of the ventriculotomy. This is attributed to fibrotic and slow conducting tissue, causing reentry around an incisional scar or myocardial patch. Four main types of reentry have been described.^3,4

Radiofrequency ablation is commonly employed to eliminate the reentrant substrate responsible for VT. The success of the procedure hinges on identifying the critical isthmus with careful substrate mapping.⁵ In this particular case, a high-resolution mapping catheter (PENTARAY, Biosense Webster, Inc., a Johnson & Johnson company) was used, allowing for precise electroanatomic characterization detail in the region of the reentrant tract. The affected area was located between the right ventricular outflow tract free wall ventriculotomy and pulmonary valve, which identified the reentrant circuit as a type II isthmus.³

Case Description

A 54-year-old patient with a history of corrective surgery for TOF presented with 7 appropriate shocks for sustained ventricular tachycardia within a 48-hour period. The patient had been previously treated with sotalol for symptomatic atrial tachycardia as well as for VT prevention. The patient had an indwelling implantable cardioverter-defibrillator (ICD) implanted for primary prevention 3 years prior. At that time, the patient had a syncopal spell that prompted electrophysiology study, during which VT was induced when pacing from the right ventricular outflow tract. The patient was symptom free until one year prior to his presentation with electrical storm, after experiencing a remote solitary VT event. Given the sudden intensity and frequency of symptoms, he was referred for catheter ablation.

The procedure was conducted under general anesthesia. VT with a cycle length of 320 ms was easily induced with double extrastimuli at a drive train of 450 milliseconds from beneath the septal aspect of the pulmonic valve (Figure 1). This was the same technique used to provoke VT during his index study prior to ICD implant. The VT was amenable to pace termination, although hemodynamically unstable. The PENTARAY was used to create a three-dimensional shell of the outflow tract and right ventricle. Voltage scaling was using 0.5-1.5 mV to further characterize the region. The VSD patch was identified and noted to have confluent scar superior to it as it merged with the pulmonic valve. Notably, there was dense free wall scarring, which originated from the lateral aspect of the tricuspid valve annulus to the location of the incisional right ventriculotomy (Figure 2). The anatomical circuit was then located and identified as type II isthmus.³ There was no fibrosis between the septal aspect of the tricuspid valve annulus and the VSD patch. Radiofrequency ablation (THERMOCOOL SMARTTOUCH SF, Biosense Webster, Inc., a Johnson & Johnson company) was then used to sever the reentrant circuit using 30 watts for 30-40 seconds while targeting an impedance drop of at least 10 ohms and 10 grams of contact (Figure 3). After a 30-minute period of observation during which programmed ventricular stimulation was repeated, no VT could be induced. At 9-month follow-up, the patient has remained symptom free and without any VT on device interrogation.

Discussion

Late VT after congenital heart disease such as TOF repair is difficult to identify due to the complex anatomy of the right ventricle. The PENTARAY catheter (Figure 4) offers high-resolution mapping with twenty 1-mm electrodes over 5 passive splines covering a surface diameter of 3.5 cm.⁶ Voltage mapping helped to quickly identify the type of reentrant circuit.^3,4 A limitation of substrate mapping is the inability to adequately estimate the depth or thickness of the culprit isthmus. Moore and colleagues demonstrated with post-mortem anatomical samples that tissue girth and length dimensions varied significantly.⁷ In their study, histological examination confirmed the presence of marked fibrosis. This group concluded that a TOF isthmus, such as the one ablated in this case, between the ventriculotomy and tricuspid valve had an average length of 3.9 cm and a mean wall thickness of 1.5 cm.⁷ This has implications on the degree and duration of radiofrequency ablation used to fully and successfully transect the VT circuit.

Conclusion

In this case, the use of the PENTARAY greatly facilitated mapping and identification of the critical isthmus, permitting an effective ablation of a type II TOF circuit.

Disclosures: The authors have no conflicts of interest to report regarding the content herein.  

References

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