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.