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Contact Force and Atrial Fibrillation Ablation
Ullah W1, Schilling RJ1, Wong T2
1Cardiology Research Department, Barts Heart Centre, St Bartholomew’s Hospital, Barts Health NHS Trust, London, UK.
2NIHR Cardiovascular Biomedical Research Unit, Institute of Cardiovascular Medicine and Science, The Royal Brompton
and Harefield NHS Foundation Trust & Imperial College London, UK.
Disclosures:
None.
Corresponding Author:
Tom Wong
The Royal Brompton and Harefield NHS Foundation Trust
Sydney Street, London, SW3 6NP. United Kingdom.
Introduction
Traditionally, operators were reliant on indirect measures of the
contact between the tip of the ablation catheter and myocardium,
such as the fluoroscopic appearance of the catheter and tactile
feedback, to guide ablation. With the advent of contact force (CF)sensing catheters, this is no longer the case, as these data are now
directly measured and available in real-time during a procedure. In
this review, we present the preclinical work correlating CF and lesion
sizes, the factors determining catheter contact in the human left
atrium (LA), the impact of CF-sensing on the atrial fibrillation (AF)
ablation procedure, and review work focused on establishing optimal
CF parameters for ablation.
Catheter-Based Contact Force Sensing Technologies
The first of the catheter-based technologies to obtain a CE mark
(2009) was the TactiCath® (St Jude Medical Inc., St Paul, MN,
USA). This catheter uses three optical fibers between the second
and third electrodes of the catheter and an elastic polymer catheter
tip.1 The latter undergoes micro-deformations in response to contact
which changes the wavelength of reflected infrared light transmitted
by the optical fibers. The magnitude and orientation of the contact
force may then be derived from the wavelength of reflected light at
the three fibers.1
The most recently CE marked of the contact force sensing
technologies is the ThermoCool® SmartTouch™ Catheter (Biosense
Webster Inc., Diamond Bar, CA, USA). Here the catheter the
tip electrode is mounted on a precision spring permitting a small
amount of electrode deflection. By measuring this deflection using
location sensor coils at the proximal end of the spring, the system
can calculate the force being exerted (and its orientation) using
the known characteristics of the spring. The SmartTouch catheter
integrates with the Carto3 (Biosense Webster Inc.) electroanatomic
navigation system. During a case, the magnitude of contact force and
its vector are displayed in real-time on the Carto3 display screen as
well as the contact force waveform (Figure 1).
For the SmartTouch catheter, the reported sensitivity reported by
the manufacturer is less than 1g of contact force. Data has been
published for the Tacticath, where a comparison has been made
between the measurements made by the catheter and a calibrated
balance: this demonstrated the measurements by the catheter were
highly sensitive and accurate (mean error ≤1g).1
Contact Force and Ablation
In vitro work with non-irrigated catheters using temperaturewww.jafib.com Feb-Mar 2016| Volume 8| Issue 5
Abstract
Catheters able to measure the force and vector of contact between the catheter tip and myocardium are now available. Pre-clinical work
has established that the degree of contact between the radiofrequency ablation catheter and myocardium correlates with the size of the
delivered lesion. Excess contact is associated with steam pops and perforation. Catheter contact varies within the left atrium secondary
to factors including respiration, location, atrial rhythm and the trans-septal catheter delivery technology used. Compared with procedures
performed without contact force (CF)-sensing, the use of this technology has, in some studies, been found to improve complication rates,
procedure and fluoroscopy times, and success rates. However, for each of these parameters there are also studies suggesting a lack of
difference from the availability of CF data. Nevertheless, CF-sensing technology has been adopted as a standard of care in many institutions.
It is likely that use of CF-sensing technology will allow for the optimization of each individual radiofrequency application to maximize efficacy
and procedural safety. Recent work has attempted to define what these optimal targets should be, and approaches to do this include
assessing for sites of pulmonary vein reconnection after ablation, or comparing the impedance response to ablation. Based on such work,
it is apparent that factors including mean CF, force time integral (the area under the force-time curve) and contact stability are important
determinants of ablation efficacy. Multicenter prospective randomized data are lacking in this field and required to define the CF parameters
required to produce optimal ablation.
Key Words:
Catheter Ablation, Atrial Fibrillation, Contact Force.
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controlled ablation demonstrated a linear relationship between CF
and lesion size, with the largest difference between no contact and
being in contact with tissue.2 Over a wide range of contact forces
(0-400N), there was an increase in lesion size with increasing CF (as
long as the electrode tip temperature was maintained at the pre-set
value and contact was maintained).2 The increase of lesion size with
force is small such that a in a study comparing 10g to 20g of contact,
no difference in lesion size was found.3 Because the power output
of non-irrigated catheters is controlled by the catheter temperature
the effect of changes in contact force is minimized. Poorer contact
entails more power output to reach the same temperatures2 and since
lesion size relates to temperature at the electrode tip,4 similar lesion
sizes are therefore produced.
Conversely, if power controlled ablation is used, there is a significant
relationship between lesion size and tissue contact.5 High contact
and increased power is associated with an increase in steam pops
and impedance rises following rapid, excessive electrode temperature
rises.5 Irrigated ablation is not temperature controlled and thus lesion
size is significantly greater at a force of 30g compared to 10g in vitro.6
Even with temperature-controlled power-limited ablation, in an in
vitro model, where fluid flow external to the catheter is utilized to
cool the catheter tip, lesion size increases with catheter contact before
a plateau is attained.7
These proof of principle studies used catheter rigs where catheter tip
force was extrapolated from the force loaded onto the catheter. Similar
technologies in vivo demonstrate that increased CF is associated with
larger and more transmural lesions8 but also more steam pops.9 These
systems have the limitation that force measurement is not as accurate
because of friction between the catheter and the sheath which varies
with sheath deflection. Irrigated tip force-sensing catheters have
further examined the impact of different levels of CF on lesion
size and also the interplay of this variable with the ablation power.
Increased CF is associated with increased tissue temperature at 3 and
7 mm away from the ETI, and is associated with increased lesion size
at a given level of CF, as is increasing ablation power.1 Increasing CF
is also associated with an increased incidence of thrombus formation
at the electrode edge, especially with increasing power, and a similar
relationship is also apparent for steam pops.1,10
Within the beating heart, even within the fibrillating atrium, it
would be expected that contact will be dynamic rather than static.
Such dynamic contact has been simulated in an in vitro setting
where constant (static) contact has been found to produce the largest
lesions while variable and intermittent contact produce progressively
smaller lesions.11 Therefore, the dynamic quality of contact between
the electrode and tissue is also of great importance in the efficacy of
lesion formation. A measure termed the force time integral (FTI)
has therefore been proposed as a measure of catheter contact during
ablation. This is the area under the force-time curve and incorporates
both the variation in CF during an ablation and also ablation duration.
The FTI has been found to correlate with the lesion volume in in
vitro experiments.11
Contact Force in The Left Atrium
Better electrode-tissue contact results in a greater proportion of
the delivered power contributing to the resistive heating of the tissue,
rather than wasted in the blood stream. Consequently, this can lead
to larger, more likely transmural lesions but also increases the risk
of complications through excessive tissue heating. In an ex vivo
porcine heart study, a force of 417±167g could perforate un-ablated
left atrium, while ablation reduced the force needed to perforate.12
A further study of in vivo porcine hearts found that the lowest CF
recorded to cause perforation was 77g, with a force of 158.4 ± 55.4
needed to perforate unablated left atrium.13 A study in patients
undergoing AF ablation demonstrated that the actual contact forces
exerted on the myocardium vary significantly among operators
during mapping and ablation when they were asked to maintain
what they perceived (without CF-sensing technology) to be ‘good
contact’.14 This included multiple high force events defined as the
contact force exceeded 100g for 200ms, with six of the thirty four
patients having over 40 such instances.14 These episodes occurred
during catheter manipulation as well as ablation. A further study
where operators also blinded to CF measurements mapped around
the pulmonary veins specifically found a significant variation in the
CF by location, with points taken around the left veins having lower
CF than the right side, and the LA/left atrial appendage ridge points
having the lowest CF.15
One would assume that having access to CF data would therefore
reduce the risk of complications such as cardiac perforation, as high
force episodes could be reduced. A retrospective study including
557 patients undergoing AF ablation demonstrated that the use of
CF-sensing catheters was associated with a significant reduction
in the rate of major complications (2.1 vs 7.8%, p=0.01) including
Table 1: Clinical studies assessing ablation efficacy with respect to catheter contact force: methods used to assess efficacy and cut off values for
effective ablation
Author Number of
Patients
Operator
Blinded to CF
AF Subtype Method to judge ablation efficacy Suboptimal Ablation Effective Ablation
Reddy40 32 No PAF 12 months recurrence of symptoms CF <10g; FTI<500g.s CF>20g
FTI>1000g.s
Haldar28 40 In half of
cases
35% PAF Acute PV reconnection in a 7 segment model
per PV pair
CF 14.5g CF 19.6g
Kumar27 12 Yes PAF Acute PV reconnection in a 5 segment model
per PV pair
LPV: CF 9g, FTI 173g.s RPV: CF 11g,
FTI 282g.s
LPV: CF 20g, FTI 436g.s RPV: CF 24g,
FTI 609g.s
Kumar47 20 Yes PAF EGM criteria
for transmurality46
CF>16g, FTI >404g.s
Neuzil39 40 Yes PAF PV reconnection at 3 month protocol-driven
restudy in a 5 segment model per PV pair
CF 15.5g Minimum CF 3.6g Minimum
FTI 118g.s
FTI>400g.s CF 19.5g Minimum CF 8.1g
Minimum FTI 232g.s
Ullah29 60 No Persistent AF Reconnecting segments in a 12 segment model
per PV pair at redo procedure (median 8 months
from index procedure)
CF 11.5g FTI 231g.s CF 12.5g FTI 231g.s
Sohns53 6 No PAF MRI-defined scar in 5mm2 zone >1,200g.s
EGM=Electrogram; PV=Pulmonary Vein; CF=Contact Force; FTI=Force Time Integral; LPV=Left Pulmonary Vein; RPV=Right Pulmonary Vein
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cardiac perforation (0 vs 3.3%, p=0.021).16 However other studies
have shown no reduction in complications with CF-sensing data
available.17–19
There are areas around the pulmonary veins which are more resistant
to electrical isolation20 and have a higher frequency of both acute and
chronic reconnection.21 The intervenous ridges and pulmonary veinleft atrial appendage ridge are important areas in this respect.20,21 The
reason for this locational variation in the efficacy of ablation may
relate variation in wall thickness and texture,22 resulting in differences
in the compliance of different regions,23–26 thus changing the quality
of the contact with the catheter. While availability of CF data could
affect the CF applied by the operator during LA mapping and
ablation, there are other additional factors which also play a role in
determining contact in the LA. Respiratory motion may affect CF,
with lesions delivered during apnea having a higher average contact
force and force time integral than those during ventilation.27 During
wide area circumferential ablation, the CF varies by location, with
lower forces on the left side and anteriorly.26,28 In those ablated with
the operator blinded to CF data, the CF is significantly lower during
ablation around the LA/left atrial appendage ridge compared with
cases where CF data was available.28 The type of trans-septal sheath
used to deliver the ablation catheter during ablation, whether manual
non-steerable or steerable also affected the distribution of contact
forces around the WACA (Figure 2).29
The quality of contact between the catheter and myocardium is not
just reflected by the mean CF but also by the stability of the contact
force waveform, quantified as the CF variability (CFV) (Figure 3).26
The greater the mean CF applied, the greater the CFV. This may be
because at low CF the myocardium has a large capacity for stretching
and can buffer sources of variability such as cardiac motion from
affecting contact with the catheter; at higher CF, CFV increases,
suggesting that less of this variability is buffered by the tissue and
more transmitted to the catheter tip. Other factors have also been
found to increase the CFV: sinus rhythm rather than AF (presumably
because of increased cardiac movement); stiffer robotic sheaths, and
LA location. Apnea has also been found to be associated with a
reduction in the variability of the applied CF.27 This variability of the
CF is of relevance to ablation as there is evidence that for the same
FTI, a higher CFV can result in a lower ablation efficacy (Figure 4).26
Impact of CF-Sensing on Clinical AF Ablation
One single center 38 patient prospective randomized study has
been published assessing the impact of CF-sensing on AF ablation.
A non-randomized prospective single-arm study in which 160
patients were ablated at 21 sites has been also published30 as well
as prospective, non-randomized studies enrolling around 20-30
patients in the CF-sensing arm and comparing with a non-CF
sensing group.28,31–34 Aside from these, multicenter registries,16,17,19
including 200-600 patients have also been published. The results
from these studies are discussed below.
A limitation in all but two18,28 of the two arm studies published
to date is that there has been some variation in the equipment
used in the two study groups, namely the ablation catheter or
mapping systems. This could introduce bias based on the handling
characteristics of the CF-sensing catheter itself compared with
non-CF sensing catheters, rather than its ability to measure CF.
Moreover, the use of CF-sensing generally requires the most up to
date iterations of electroanatomic navigation systems. In non-CF
sensing groups, some of the cases could conceivably be performed
using older systems - previous studies have demonstrated differences
in procedural parameters between different mapping systems35 and
different versions of the same mapping system.36
The use of CF-sensing catheters has been described to be
associated with a reduction in fluoroscopy times,17,19,32,34 though in
two studies fluoroscopy times were found to be longer in the CFsensing group.16,31 Procedure times were also shorter in the CFsensing arms of the above studies,17,32–34 other than in one study
where it was significantly longer where CF data were available.31
In the randomized trial, procedure but not fluoroscopy times were
shorter when CF data were available to the operator.18
While the above procedural parameters are of importance, a key
factor determining the utility of CF-sensing is clinical efficacy.
Success rates have been compared in some of the publications to
date.17–19,31,32,34 In some cases, there was an improvement in success
rates with the use of CF-sensing.17,19,31,32 In one of these studies, a
benefit in terms of success rates was only observed for patients in PAF
rather than persistent AF.19 In another study, exclusively examining
persistent AF patients, an improvement in ablation success rates was
only observed when CF-sensing was used with the remote robotic
navigation system and not when used with manual ablation.17 Other
studies, including the prospective randomized one, have not observed
an improvement in success rates when CF-sensing was used.18,34
CF-sensing catheters have also been found to be associated with a
reduction in procedure times in hybrid epicardial (using a bipolar RF
catheter)/endocardial AF ablation procedures, in a study comparing
the data with a historical cohort where a non CF-sensing catheter was
used for the endocardial ablation.37 On comparison with the second
generation cryoballoon, in a multicenter non-randomized study of
376 patients with PAF, the procedure times for CF-sensing catheters
are significantly longer without a difference in fluoroscopy or overall
complication rates, and with no difference in success rates at 18
Figure 1:
Display of contact force data measured by the SmartTouch Catheter
on the Carto3 screen
Displayed is a left atrial geometry created using the Carto3
electroanatomic navigation system. Highlighted are the contact
force, contact force vector and the contact force waveform for the
contact between the catheter and the myocardium
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integral below 400g.s had a greater chance of reconnection among
PAF patients.39 At 12 months of follow up, the average contact
force, FTI and incidence of low CF during ablation are predictive of
procedural success in PAF patients.40
The aim of a radiofrequency application during AF ablation is the
generation of a transmural lesion. This results in a persistent barrier
to electrical conduction or the elimination of a driver. At a procedural
level, this is best reflected by an improvement in the single procedure
success rate for the ablation. Clearly, this is the most relevant outcome
measure clinically. CF parameters have been compared between
cases with and without a recurrence of symptoms by Reddy et al.,40
(Table 1), with higher CFs during ablation observed in those without
recurrence. An improvement in success rates may not necessarily
mean that the ablation procedure has been more efficient: this would
be reflected by a reduction in the procedure length, for example. In
order to make procedures more efficient, the aim should be for every
radiofrequency application to be contributory to the success of the
procedure. This would lead to shorter procedures and potentially less
risk of complications. Moreover, suboptimal applications may lead
to short term procedural success but long-term failure – by causing
tissue edema and an incomplete transmural lesion. Consequently, it
is useful to be able to assess the efficacy of individual radiofrequency
applications.
In preclinical studies, the efficacy of an individual ablation is
relatively straightforward to judge as histological lesion dimensions
are available.2,41 Lesion histology is not available for clinical studies
though, and therefore alternative measures of the effect of ablation
are used. Classically, the attenuation of the electrogram has been
used to judge the efficacy of an individual ablation. Unipolar atrial
electrogram attenuation has been found to be associated with
transmurality of ablative lesions.42 Significantly more amplitude
reduction in the bipolar signal during sinus rhythm and AF with
transmural lesions is seen in vitro, with a reduction of ≥ 60% having
a high specificity for lesion transmurality.43 In clinical studies, an
≥80% reduction in electrogram amplitude has been targeted.21,44
Electrogram attenuation has been found to correlate poorly with CF
applied during ablation.26,45 Changes in sinus rhythm electrogram months.38
Clearly, further, larger prospective randomized and preferably
multicenter trials are needed to clarify the impact of CF-sensing on
procedure parameters and success rates.
Optimizing CF Parameters During Ablation
An important point with regard to the success rates from AF
ablation procedures is that it is unlikely that simply having CF-data
available will improve outcomes: it more likely that using the data
to maintain optimal ablation CF will make the difference. In this
respect it is interesting that in the SMART-AF trial, cases where
the operator had maintained the CF within their self-determined
optimal range ≥80% of the time were associated with a significant
improvement in success rates, with such procedures over 4 times
as likely to be successful than those where this was not the case.30
This then raises the important question as to what the optimal CF
parameters for ablation should be.
Contact force during ablation predicts acute wide area
circumferential ablation (WACA) reconnection in patients with PAF,
with sites of pulmonary vein reconnection having a lower average
contact force27,28 and FTI during ablation.27 At 3 months’ follow up,
segments within a WACA line ablated with a minimum force time
Figure 2:
Distribution of contact force in the right and left WACA
Upper figure demonstrates a clock face scheme for segmenting each
WACA, left pulmonary veins on the left, right pulmonary veins on
the right. Lower figure demonstrates radar plots of the distribution
of contact forces during initial pulmonary vein encirclement for 60
patients with persistent AF using different trans-septal sheaths.
The outer edge represents the WACA location according to the clock
face segmentation. The further from the center of the plots, the
higher the median contact force in grams
A=Anterior; P=Posterior; Manual-NSS=Manual Non-steerable sheath
group; RRN=Remote Robotic Navigation
*=Significant difference between Manual-NSS and steerable sheath
groups (p<0.05); #=Significant difference between RRN and ManualNSS groups (p=0.009); ~= Significant difference between RRN and
Agilis group (p<0.05); +=Significant difference between indicated
groups (p<0.05)
(Reproduced from Ullah et al,29 Journal of Cardiovascular
Electrophysiology, Wiley, with permission)
Figure 3:
Contact force variability
Contact force (CF) waveform over time at two mean contact forces
– the contact force variability (CFV) is the difference between the
mean trough and peak CF. The mean CF is indicated by the solid
line while the mean peak and trough CFs are indicated by the dotted
lines. On the left of the image is a waveform at low mean CF with
low CFV, while on the right is a waveform at higher CF with higher
CFV
(Reproduced from Ullah et al,26 Journal of Cardiovascular
Electrophysiology, Wiley, with permission)
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ablation efficacy clinically. The drawback here though is that the
efficacy is being assessed at a MRI-zone level (albeit a small zone)
rather than an individual radiofrequency application. This therefore
relies on extremely accurate registration of each radiofrequency
application between the electroanatomic navigation system and the
MRI being used to judge scar. Moreover, this method is unable to
account for any overlap in applications (for example through catheter
drift even during a putative static application). It may be for these
reasons that the threshold for effective ablation is much higher in this
study compared with the other work presented in Table 1.
An alternative approach to clinically assess the efficacy of each
individual radiofrequency application based on the impedance drop/
FTI relationship has also been used.26,45 For persistent AF patients,
the relationship was found to be logarithmic with a plateau at 500g.s.45
This method was also used to investigate the impact of contact force
variability and catheter drift on the efficacy of ablation (Figure 4).26
Based on this work, maximal efficacy is provided by parallel catheter
contact with CFV ≤5g, catheter drift ≤3.5mm and there is no benefit
in terms of biophysical efficacy from ablation beyond 500g.s.
Such detailed optimisation of catheter contact during ablation
is now possible with the introduction of automated lesion marker
placement software such as the Visitag upgrade for Carto.3 In a
study by Anter et al.,54 such an algorithm was used and found to
be associated with lower rates of acute pulmonary vein reconnection
but not improved success rates at 6 months. The limitation of that
study though was that only catheter displacement and impedance
drop were used by the annotation algorithm. The incorporation of
CF parameters could be used to further refine the targets for ablation.
A randomized trial where one group was ablated with optimised CF
morphology have been shown to be predictive of transmurality of
ablation lesions in a porcine model by Otomo et al.,46 In this case,
for unipolar signals a loss of a negative deflection was associated
with transmurality, while in the case of bipolar signals, the changes
associated with the latter were dependent on the orientation of
the catheter to the myocardium. One group has used these criteria
for bipolar signals to judge ablation efficacy and found that CF
parameters were sensitive and specific for identifying transmurality
based on electrogram parameters.47 Another group found no
relationship between CF parameters and transmurality as suggested
by the above electrogram morphology changes.26
The most commonly employed model for assessing ablation
efficacy is reconnection of the WACA lines27,28,39 (Table 1). In
this approach, the ipsilateral WACA is divided into five to twelve
segments and efficacy is based on whether that segment reconnects
or not. The disadvantage here is that target parameters for individual
radiofrequency applications are being assessed based on the response
of a region, quite often with overlapping lesions, to ablation. In
most of these studies, operators were blinded to CF measurements
(Table 1): such blinding serves to exaggerate the differences between
ineffective and effective ablations as a lack of knowledge of CF
allow for a greater range of CF to be applied and therefore makes it
difficult to establish where the actual threshold for effective ablation
lies. Based on these studies, a mean ablation CF of at least 15g and
FTI of >400gs would appear to be associated with a reduced risk of
an ablation being in a reconnecting segment.
While histological lesion parameters are not available for clinical
cases, work has been done using cardiac MRI to attempt to image
ablation lesions. McGann et al., described a methodology for imaging
LA scar using delayed enhancement MRI (DE-MRI) following
pulmonary vein isolation, and the burden of LA scar they observed
correlated with arrhythmia recurrence.48 This group went on further
to demonstrate that areas of DE-MRI enhancement correlate with
areas of electrical scar (R2=0.57) and that DE-MRI imaging could
be used to identify breaks in the pulmonary vein isolation lines.49 In
a blinded analysis using pre- and post-ablation MRI images, another
group found that investigators were able to identify ablated LA
myocardium in only 60% of cases, with a poor ability to distinguish
ostial from circumferential ablation lesions.50 This contrasts with
another report in which ablated myocardium could be identified in
100% of cases on DE-MRI.51 These findings suggest MRI may be
useful in determining the sites of ablation lesions but the difference in
the reported reliabilities may relate to the signal intensity thresholds
being used to assign scar on MRI. To address this, recent work
has correlated macroscopic scar volumes with DE-MRI imaging
scar volumes in the right atria of 8 swine: based on this, DE-MRI
signal intensity thresholds have been proposed which allow the best
approximation of the macroscopic scar volume.52
Contact force parameters have been compared with MRI-imaged
atrial scar by Sohns et al.53 Table 1. In this study of six patients, the
FTI of ablation was correlated with DE-MRI scar. In order for this
comparison to occur though, the FTI was not examined from the
perspective of a single radiofrequency application, but in a subdivision
of 1cm zones. Increasing FTI above 1,200g.s was associated with a
significant increase in the proportion of a 5mm2 region of myocardium
exhibiting DE-MRI scar (below this FTI value, the increase in the
scar burden in that zone with an increase in FTI was small). This
study therefore raises the possibility of using cardiac MRI to assess
Figure 4:
Factors affecting impedance drop during ablation
(A) Contact Force Variability and (B) Catheter drift. Each point is the
mean of at least 10 ablations. *p<0.0005
(Reproduced from Ullah et al,26 Journal of Cardiovascular
Electrophysiology, Wiley, with permission)
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targets and the other without CF sensing could be conducted to
definitively prove the utility of CF-sensing to the ablation procedure.
Conclusions
The availability of real time catheter-based CF-sensing holds great
potential for improving the safety and success rates of AF ablation
procedures by reducing suboptimal and excessive CF during ablation.
Optimal CF parameters for ablation remain to be established, and
one would hope that their adoption would help to optimize each
individual radiofrequency application, improving procedural efficacy.
Multicenter prospective randomized data are lacking in this field and
are required to definitively prove the argument for the adoption of
this technology and the CF thresholds required during ablation.
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