A novel ablation catheter with an insulated tip
In this issue of the Journal of Cardiovascular Electrophysiology, Aryana
et al. (6) evaluated the outcome of RF ablation using a novel
insulated-tip catheter (SMT, Sirona Medical Technologies, Windsor, CT)
versus a conventional RF ablation (RFC) in silico, ex vivo (n=267) and
in vivo (n=81) using porcine model. Briefly, the SMT ablation catheter
is an open-irrigated ablation catheter with a novel design that consists
of a central electrode, surrounded by 4 peripheral electrodes for both
RF and pulse field ablation. The design is intended to insulate the
central electrode which is surrounded by an insulating dielectric as
well as a flexible metallic braid. The key concept of this catheter is
to minimize the passage of RF energy into the bloodstream and the
surrounding tissue and to focus a greater magnitude of the delivered
energy into the targeted tissue directly. It does this by creating a
Faraday cage by virtue of insulating the central electrode which is
surrounded by an insulating dielectric as well as a flexible metallic
braid. Further, the ablation catheter can be deployed and used in
‘vector’ or ‘linear’ configurations. The vector configuration intended
to provide greater catheter stability and tissue contact during ablation
procedure by using the four flexible wings as a “landing gear”.
Although the catheter is capable of both RF or pulse field ablation,
only RF was evaluated in this study.
In this study, RF ablation using SMT proved capable of directing 66.8%
of the RF energy into the targeted tissue, while RFC only 13.7% by
analysis of computational model (in silico). This was further validated
by the evidence from the ex vivo and in vivo studies. Low power-low
irrigation (8-12W, 2ml/min) RF ablation using SMT is capable of creating
lesions comparable in size to those with RFC at 30W, but with
significantly greater impedance drops. On the other hand, high
power-high irrigation (15W, 20ml/min) RF ablation using SMT can yield
lesions that are significantly larger and deeper than with RFC at 30W.
RF ablation using SMT was consistently associated with lower incidence
of steam pop both ex vivo and in vivo study. Of note, lesions created
with SMT were more homogenous and sharper transition zones than those
created with RFC.
The results of the study by Aryana et al. provide novel evidence
supporting the effectiveness of RF ablation using SMT catheter. Clinical
importance of RF application is how efficiently can RF energy be
transferred directly to the target tissue without causing unfavorable
complications. In that meaning, SMT may be superior to RFC based on the
result of this study that approximately 67% of the RF energy directly
into the target tissue, requiring lower power to create comparable
lesions provided by RFC. Considering the structure and mechanism of SMT,
RF ablation using SMT reduces the opposing force (convective cooling)
and improves the catheter stability when used in ‘vector’ configuration.
As a result, power and duration of energy delivery can be reduced, which
leads to a lower likelihood of complications such as steam pop and
coagulum formation. The uniform and homogeneous lesion formation of SMT
is also of critical importance during ablation procedures since
non-uniform and heterogenous ablation lesions may lead to arrhythmia
recurrence or in the worst case, iatrogenic arrhythmias. The findings of
this study provide hope that the insulation technology may refine RF
ablation in the future to minimize the risk of iatrogenic complications.
The main question is whether these promising results support the idea
that SMT should be used in clinical practice. The significant limitation
of this study is that it is only confirmed by animal experiments under
the fixed setting. All RF ablation lesions were created with
perpendicular orientation to the tissue with preset power and irrigation
flow, which are high power-high irrigation (15W, 20ml/min) and low
power-low irrigation (8-12W, 2ml/min). The effect of varying
orientations in a beating heart, and of varying tissue architecture
remain unknown.
During RF energy application, a myriad of factors should be considered
in addition to traditional ablation indices of power and RF duration.
For example, catheter orientation, catheter stability, and tissue
thickness (tissue architecture) also play an important role in
determining RF ablation lesion formation. Recently, modification of
irrigation flow rate has been proposed to assist in altering geometry.
Open irrigation can change lesion geometry such that the maximal lesion
width, corresponding to the depth of hottest tissue temperatures. This
is particularly relevant for thin-walled structures such as posterior
left atrium that lie in close proximity to structures vulnerable to
collateral injury such as the esophagus (7).
Another important concern is the contact force. In this study, contact
force was not described since control experiments were only conducted
using the ThermoCool RFA catheter. It is well accepted that contact
force is a key determinant of lesion size, volume, and depth (8). The
optimal contact force required to make transmural lesions has been
examined in animal models. Transmural lesions at cavotricuspid isthmus
(CTI) of swine were seen with average contact force >20g
but never with contact force <10g (mean depth 0.75mm) (9).
Further, a number of studies demonstrated that contact force sensing
reduced procedural and fluoroscopy time, improved impedance falls,
reduced RF ablation time when compared with non-CF sensing catheters
(10). The results of these studies potentially suggest the importance of
contact quality. Shah et al. demonstrated that constant compared with
variable and intermittent contact resulted in greater lesion size (11).
A lower average contact force made it more likely that contact was
intermittent, with 51% lesions exhibiting diastolic loss of contact
force (0g) if the average contact force was <4g compared with
3% if average contact force was >20g (12).
For a comprehensive interpretation of ablation lesion formation by using
SMT, the effects of these factors need to be verified in addition to
power and duration of energy delivery. Although it is still unclear
whether the results of the SMT catheter can be translated to a human
beating heart, the data for SMT catheter of this study are very
promising. We eagerly await further studies to validate with
comprehensive factors and show if this technology can be clinically
applicable. For the moment, however, the authors ought to be
congratulated for an important advance in ablation technologies using
elegant computational, ex-vivo and in-vivo experimental work.