Discussion
This is the first pediatric report of intractable NRFT that was successfully controlled by catheter mapping and ablation. NRFT reportedly occurs only in 2.8% of idiopathic VT cases in adults.3) The mechanism of NRFT was not precisely determined, but was assumed to originate from abnormal automaticity from Purkinje fibers. NRFT is defined in adult patients by the following criteria3,4): (1) Normal ECG and intracardiac conduction interval, (2) QRS morphology with a relatively narrow right bundle branch block waveform, (3) Inducibility by intravenous isoproterenol infusion and burst ventricular stimulation, (4) The absence of criteria of reentrant fascicular VT, such as transient entrainment and verapamil sensitivity, and (5) Successful ablation to the site of the earliest presystolic Purkinje potential. These findings are essential to differentiate NRFT from the reentrant fascicular VT. Because the present case fulfilled all of these criteria, the tachyarrhythmia was finally determined to occur as NRFT.
NRFT is challenging to diagnose as well as to control even after ablation therapy. The long-term success rate of the first ablation therapy was reportedly 74% (11 out of 15) with high recurrence rate3) The reasons for recurrences are thought as follows: First, it is hard to induce NRFT reproducibly and stably and to map the accurate focus of VT. Second, pacemap-guided approach does not always provide the site of the origin of NRFT. Even when identical QRS morphology is obtained by pacemapping at the exit site to myocardium, the VT origin from Purkinje fiber may be escaped from the RF current applied.2) Third, it is not easy to contact the ablation catheter with the site of focal VT origin sufficiently and stably because of the network structure of the Purkinje fibers. In the present case, we performed burst pacing repeatedly from several sites of the ventricles under high doses of isoproterenol to induce NRFT. After the careful diagnostic pacing, we drew VT map using the multi-electrode catheters and identified to record the earliest preceding Purkinje potential. We also performed a transseptal approach before RF application to obtain a better catheter contact with the focal VT origin site and to avoid any injury to the proximal part of the fascicle with the ablation catheter. Thus, we were able to diagnose the VT as NRFT and to provide an effective and safe catheter ablation therapy. Further studies and long-term follow up are needed for the pediatric-onset NRFT.