Correspondence
Name: Professor Dhiraj Gupta, MD
https://orcid.org/0000-0002-3490-090X
Address: Department of Cardiology, Liverpool Heart and Chest Hospital,
UK L14 3PE
Phone: +44 151 600 1793; Fax: +44 151 600 1696
Email:
Dhiraj.gupta@lhch.nhs.uk
Despite being first described over 30 years ago, focal radiofrequency
(RF) continues to be the most widely used energy modality for catheter
ablation. The fact that it has managed to hold its own against stiff
competition from alternative energy sources used for pulmonary vein
isolation (PVI) is down to continuous evolution based on enhancements in
our understanding of its biophysical principles. In particular, the
advent of contact-force (CF) based integrated indices such as Ablation
Index has improved both efficacy and safety1. However,
a significant limitation of this approach is the absence of tissue
feedback during lesion creation, which results in a blunt
‘one-size-fits-all’ approach. This limitation has been further brought
into focus by the recent appreciation of the much greater importance of
circuit impedance rather than delivered power as a fundamental
determinant of RF lesion size2.
Historically, electrophysiologists have monitored the change in
generator impedance (GI) in real time as a surrogate of RF lesion
quality. However, GI is confounded by individual patient factors within
the ablation circuit including subcutaneous tissue and hydration
status2,3. The large contribution of these variables
dwarfs the relatively low magnitude of GI change from ablation itself,
thereby limiting its clinical usefulness. Catheter local impedance (LI)
is a novel tool that attempts to minimize the influence of these
confounding variables in the impedance circuit, potentially offering a
more accurate representation of changes occurring close to the
catheter-myocardial interface. A local potential field is generated by
driving a non-stimulatory current between a distal and a more proximal
catheter electrode; proximity to physical structures such as myocardium
causes distortions in this local field which can be measured with one or
more intermediate electrodes, allowing derivation of a LI
value4. Bench work has shown that during RF
application, LI drop dynamically follows the rate of intramural
temperature rise (Figure 1)5. The resulting tissue
feedback can potentially allow operators to individualize each RF
application, and several groups have already reported on their clinical
experience (Table 1) with a LI-sensing catheter.
In this issue of the Journal, Francesco and colleagues present the
findings from the multicentre Italian CHARISMA registry, a study of 153
consecutive patients undergoing RF catheter ablation for AF using the
MiFiTM OI catheter (Boston
Scientific)6. Ablation lesions were delivered at 30-35
W, targeting a LI drop of at least 10 Ω over 30 seconds, up to a maximum
of 40 Ω, and an inter-lesion distance (ILD) of 6 mm or less. After
first-pass pulmonary vein encirclement, lesion success was defined by a
failure of pacing capture at the lesion site. Offline assessment of LI
and GI in 3556 first-pass ablation lesions showed that successful
lesions were associated with higher LI and GI drops than unsuccessful
ones, and ROC curve analysis found LI to be a far better discriminator
than GI. Every 5-point drop in LI had an OR of 3.1 (CI 2.7-3.6) for
lesion success, plateauing at 15 Ω. No steam pops or ablation-related
adverse outcomes were observed. In their conclusions, the authors
suggest a 15 Ω drop as an indicator of probable lesion effectiveness,
with an upper safety limit of 30 Ω in order to avoid steam pops.
The authors should be congratulated for their study, which is the
largest reported to date using LI, and the first one to use a
LI-targeted approach for AF ablation. Although the study population was
heterogeneous, combining both first time and redo cases as well as both
paroxysmal and persistent AF, the low rate of arrhythmia recurrence on
follow up is encouraging, and suggests potential for real world use for
this novel technology.
It is notable that the results of the present study are consistent with
suggested LI targets from the recently published multicentre LOCALIZE
study, where the same catheter was used for first-time PVI in paroxysmal
AF patients whilst blinded to LI7. Gaps in the ablated
segments were assessed after a protocol-mandated 20-minute wait
following first pass PV encirclement; a LI drop of 16.1 Ω in the
anterior/roof segments had a positive predictive value (PPV) of 96.3%
for absence of gap, while for posterior/inferior segments a LI drop of
12.3 Ω had a PPV of 98.1%. The largest LI drop associated with a
conduction gap when targeting an ILD of 6 mm or less was 20.1Ω. The
threshold value of approximately 15-20 Ω as being predictive of lack of
gap was also observed by other groups, and so it appears that the
technology is reproducible and predictive across a range of power and CF
values.
In addition to being a marker of lesion quality, LI may also serve as an
indicator of underlying substrate. LI is known to be independent of
catheter orientation4 and activation wavefront
direction8, and yet has been shown in the present
study6 and others8,9 to be
positively correlated with electrogram amplitude. Lower baseline LI
values, as well as significantly lower response to RF ablation are seen
in scarred as opposed to healthy tissue10,11. It is
notable that Solimene and colleagues also observed a lower baseline mean
LI in patients with persistent AF as compared to paroxysmal AF, although
there was no difference in mean LI values between de novo and redo
cases.
Successful evolution necessitates improvement beyond that which is
currently available. Particularly with a very well-established procedure
such as PVI, and with competition from single-shot and established RF
energy delivery techniques, procedural efficiencies such as first pass
PVI are very important. In this context, it would have been useful for
the current study to include such data, so that the effectiveness of
this technology could be put into context with that of other
contemporary techniques. For instance, a recent large multicentre study
systematically employing the CLOSE protocol reported a first-pass PVI
rate of 82.4%1. Whether a LI- based approach can
match such results, and preferably do so with more tailored but less
overall RF energy delivery remains to be determined.
In conclusion, Solimene and colleagues provide real world clinical
evidence in a sizeable multicentre real-life cohort of AF patients that
supports the use of local impedance in guiding RF ablation for PVI.
However as is often the case for such a rapidly evolving field,
obsolescence for the OI MiFi catheter may already be on the horizon in
the form of the recently released StablepointTM(Boston Scientific) catheter that has the ability to simultaneously
measure both CF and LI12. It remains to be seen how
our recently acquired knowledge of LI will translate to this new
platform, and whether LI data provide incremental benefit beyond what
can be achieved already with standard composite metrics. Either way, it
appears that humble focal RF energy may still have a lot to offer
electrophysiologists, and its obituaries may be premature.