Discussion
Our study demonstrated the efficacy of new ablation catheters equipped
with MEs compared to conventional ablation catheters in RFA of PSVTs.
The ME catheter was advantageous to the conventional ablation catheter
in terms of the number of RF attempts and RFA time to achieve effective
ablation endpoints. The advantage of the ME catheter was shown for both
SP and AP ablation, although the difference of the primary study
outcomes did not reach statistical significance when analyzed
separately. Notably, the ME catheter was more than twice as effective in
visualization of the pathway electrograms as the conventional ablation
catheter (27/68 vs. 13/68). However, the overall procedure results were
successful in all patients without a significant difference in total
procedure time or acute SVT reinduction rates.
High-density mapping catheters enable the precise identification of
local electrical signals with minimization of far-field signals and
background noise that confers higher resolution mapping in low-voltage
zones and scar areas.6, 7 Currently, high-resolution
electroanatomical mapping is widely used in catheter ablation of
ventricular tachycardias and atrial arrhythmias.8-11The IntellaTip MIFI OI catheter is an ablation catheter that shows only
three additional bipolar ME signals. This catheter may not be suitable
to map the entire arrhythmia circuit in detail, but it can help to
accurately localize catheter tip and to assess ablated and viable
tissue. Previous clinical studies showed that the ablation catheter
equipped with ME was effective in the visualization of local gaps and
avoiding unnecessary ablation during RFA of CTI and
ATs.4, 5, 12 However, Iwasawa et al. reported that the
ME catheter showed worse efficacy compared to the conventional ablation
catheters for CTI ablation in a prospective clinical study. The ME
catheter group had higher RF application numbers and longer ablation
time compared to the 8 mm dumbbell-shaped irrigated-tip catheter or
cryothermal catheter group.13 In that study, there was
a significant difference in the average RF power between the ME group
and conventional ablation catheter group (31.3 ± 9.1W vs. 38.6 ± 7.6W in
the ME and conventional ablation catheter group, respectively). For CTI
ablation, effective transmural lesion formation would be more important
than precise electroanatomical mapping. Although the ME signal showed
viable ablation targets and signal attenuation after ablation, the
temperature-controlled ablation via the small electrode may have
resulted in less RF power and shallow ablated lesions compared to the
irrigated tip ablation catheter. In contrast, RFA of PSVT requires
relatively less power delivery, and detailed mapping with accurate
localization of the ablation target is a more important factor for a
successful procedure.1, 2 Our study first showed that
use of a high-resolution ablation catheter can be helpful in RFA for
PSVTs. The ME catheter showed improved ability to identify pathway
potentials, as well as to localize actual ablation site. SP or AP
potentials are often small and unclear in large electrodes, because it
can be overlapped with far-field atrial and ventricular electrograms. In
our study, ME catheters were highly effective in the recognition of
pathway electrograms (overall 39.7%), compared to the conventional
bipolar ablation catheter (overall 19.1%). The higher effectiveness of
MEs in differentiating pathway potentials is most likely to have driven
the difference in the primary study outcomes.
It is important to point out that the ME groups showed higher average RF
power and lower average temperature in total ablation parameters,
despite identical RF generator setting in the two groups. Because the
IntellaTip MiFi OI catheter was an irrigated-tip catheter, continuous
saline infusion was required to maintain patency. Although the flow rate
was minimalized (2 mL/min) to balance with the Blazer II HTD catheter
which has no irrigation function, it may have affected the temperature
sensed in the ablation tip that would result in altered
temperature-controlled RF power delivery. Irrigated-tip catheter
ablation has been shown to be more effective in AP ablation via
increased power delivery, but previous studies mostly reported that
increased efficacy was observed when more than 17 mL/min of saline
irrigation was applied.14, 15 With the minimal
irrigation in our study, initial RF efficacy does not seem to have been
largely influenced by this difference. Even if the difference in RF
power contributed to the initial ablation time, it would have little
effect on ablation attempt number to achieve ablation endpoint, which
mostly depends on the accuracy of ablation location rather than the
speed of RF energy rising. Traditionally, irrigation mode is not
preferred in AVNRT ablation due to the risk of atrioventricular nodal
injury but irrigated-tip ablation is occasionally used in complex AP
ablation. The use of ME catheters with moderate saline irrigation could
more improve RF efficacy in AVRT/WPW syndrome.
Limitation
The operators could not be blinded to the ablation catheters used, and
awareness of the study group may have biased the study results. Although
the primary outcomes were significantly different between the two groups
in the entire population, the study did not have enough power to
validate statistical significance in the AVNRT population or the
AVRT/WPW syndrome population alone. However, there were similar trends
favoring the ME catheters in both the AVNRT and AVRT/WPW syndrome
groups. As discussed above, although we minimalized the irrigation flow
rate in the ME group, it could have affected the ablation efficacy,
especially for the initial ablation time to achieve the ablation
endpoints.