138
Quantitative Magnetic Resonance Imaging Analysis of the
Relationship Between Contact Force and Left Atrial Scar
Formation After Catheter Ablation of Atrial Fibrillation
CHRISTIAN SOHNS, M.D.,∗ ,† RASHED KARIM, Ph.D.,∗ JAMES HARRISON, M.R.C.P.,∗
ARUNA ARUJUNA, M.R.C.P.,∗ NICK LINTON, B.Eng., M.R.C.P.,∗ RICHARD SENNETT, B.Sc.,∗
HENDRIK LAMBERT, Ph.D.,‡ GIOVANNI LEO, M.S.,‡ STEVEN WILLIAMS, M.R.C.P.,∗
REZA RAZAVI, F.R.C.P.,∗ MATT WRIGHT, M.R.C.P., Ph.D.,∗ TOBIAS SCHAEFFTER, Ph.D.,∗
MARK O’NEILL, D.Phil., F.R.C.P., F.H.R.S.,∗ and KAWAL RHODE, Ph.D.∗
From the ∗ Division of Imaging Sciences and Biomedical Engineering, King’s College London, London, UK; †Department of Cardiology
and Pneumology, Heart Center, Georg-August-University of Göttingen, Göttingen, Germany; and ‡Endosense SA, Geneva, Switzerland,
Switzerland
Left Atrial Scar Formation After Contact Force-Guided AF Ablation. Background: Catheter
contact force (CF) is an important determinant of radiofrequency (RF) lesion quality during pulmonary
vein isolation (PVI). Late gadolinium enhancement (LGE) magnetic resonance imaging (MRI) allows good
visualization of ablation lesions.
Objective: This study describes a new technique to examine the relationship between CF during RF
delivery and LGE signal intensity (SI) following PVI.
Methods: Six patients underwent PVI for paroxysmal AF using a CF-sensing catheter and following
preprocedural MRI. During ablation, CF-time integral (FTI) and position was documented for each RF
application. All patients underwent repeat LGE MRI 3 months later. The LGE SIs were projected onto a
MRI-derived 3-dimensional left atrial (LA) shell and a CF map was generated on the same shell. The entire
LA surface was divided into 5 mm2 segments. Force and LGE maps were fused and compared for each
5 mm2 zone. An effective lesion was defined when MRI-defined scar occupied >90% of a 5 mm2 analysis
zone.
Results: Acute PVI was achieved in 100%. Two hundred sixty-eight RF lesions were tagged on the LA
shells and given a lesion-specific FTI. Increasing FTI correlated with increased LGE SI, which was greater
when the FTI was >1,200 gs. Below an FTI of 1,200 gs, an increment in the FTI resulted in only a small
increment in scar, whereas above 1,200 gs an increment in the FTI resulted in a large change of scar.
Conclusion: There is a correlation between FTI and LGE SI in MRI following AF ablation. Real-time
FTI maps are feasible and may prevent inadequate lesion formation. (J Cardiovasc Electrophysiol, Vol. 25,
pp. 138-145, February 2014)
atrial fibrillation, catheter ablation, contact force, left atrium, magnetic resonance imaging, pulmonary vein
isolation
Introduction
Drs. Sohns and Karim contributed equally to this work.
Dr. Sohns is funded by the German Cardiac Society (Deutsche Gesellschaft
für Kardiologie – Herz- und Kreislaufforschung/DGK) through a grant for
clinical research. The authors acknowledge financial support from the Department of Health via the National Institute for Health Research (NIHR)
comprehensive Biomedical Research Centre award to Guy’s & St. Thomas’
NHS Foundation Trust in partnership with King’s College London and
King’s College Hospital NHS Foundation Trust. The views expressed are
those of the author(s) and not necessarily those of the NHS, the NIHR, or
the Department of Health.
C. Sohns reports a small travel grant from Endosense. K.S. Rhode reports research equipment support for this study from Endosense. M. Wright reports
modest honoraria relevant to this study. H. Lampert and L. Giovanni are
employed by Endosense and are owners/inventors listed on the FTI patent.
Other authors: No disclosures.
Some of the data in this manuscript were presented in poster form at the
Heart Rhythm Society sessions in 2013.
Address for correspondence: Christian Sohns, M.D., Division of Imaging Sciences and Biomedical Engineering, 4th Floor, Lambeth Wing, St.
Circumferential pulmonary vein isolation for atrial fibrillation (AF) has evolved over the last decade and is currently
recommended for symptomatic patients refractory to antiarrhythmic drug therapy.1 Long-term freedom from AF can be
achieved in 60 to 80% of patients with additional ablation
procedures often necessary to achieve the best results.1-3 Recurrences of AF are invariably associated with PV reconnection at sites of ineffective lesion formation, a finding that is
corroborated by studies using late gadolinium enhancement
cardiovascular magnetic resonance imaging (LGE-MRI) of
the atrium following catheter ablation for AF.4-8
Lesion formation during RFCA depends on biophysical parameters including the power delivered, duration of
Thomas’ Hospital, London SE1 7EH, UK. Fax: +44-20-7188-5442; E-mail:
[email protected]
Manuscript received 26 July 2013; Revised manuscript received 15 September 2013; Accepted for publication 20 September 2013.
doi: 10.1111/jce.12298
Sohns et al. Left Atrial Scar Formation After Contact Force-Guided AF Ablation
energy application, temperature and ablation catheter tip size
and orientation.9-12 The TOCCATA study described a strong
relationship between contact force (CF) and clinical outcome
after PVI for paroxysmal atrial fibrillation (PAF) with 20 g
described as the optimal mean CF.11
Using quantitative MRI analysis, we sought to evaluate
the hypothesis that catheter CF is a determinant of lesion
formation during catheter ablation for AF.
Methods
Six consecutive patients with drug-refractory, symptomatic PAF were scheduled to undergo PVI and all clinical,
imaging, and procedural data were prospectively recorded.
Written informed consent was obtained from each patient
prior to the procedures. All scans used for the purposes of
data analysis were deemed of adequate quality for analysis
by an experienced cardiovascular MRI operator. Therapeutic
anticoagulation for at least 4 weeks before the procedure was
mandated.
MRI Acquisition
All patients underwent cardiovascular MRI according to
a standard protocol.7 Briefly, MRI was performed in a 1.5Tesla Philips Achieva MR system (Philips Healthcare, Best,
the Netherlands), using either a 32-channel surface coil (Invivo, Orlando, FL, USA) or a large 2-channel flex coil. To
visualize LGE signal intensity (SI), a 3-dimensional ECGtriggered, free-breathing inversion recovery turbo field echo
scan with respiratory navigator motion correction was performed with a pixel resolution of 1.3×1.3×4 mm3 , which
was then reconstructed to 1.3×1.3×2 mm3 . Data were acquired at mid-diastole, with a 150 millisecond acquisition
window and a low-high k-space ordering, as well as spatial
presaturation with inversion recovery fat suppression. The
inversion recovery delay time was determined from a LookLocker sequence and was set at a TI intermediate between
the optimal TIs to null myocardium and blood. Previous
work has validated this method for reproducible visualization of the late enhancement signal from necrotic tissue.13
LGE scans were performed 20 minutes after contrast agent
administration. The number of slices was set for complete
atrial coverage (30–40 slices). To optimize visualization of
the PVs, slice orientation was performed in the 4-chamber
view. Images obtained with this method appear to reflect the
PVs at their maximal size. Identical MRI sequences were
used for images acquired prior to ablation, and 3 months
after PVI.
CF Catheter System
The CF ablation setup (TactiCath Set; Endosense SA,
Geneva, Switzerland) used in this study has been reported
previously.10,11 It consists of the RF ablation catheter TactiCath (CF catheter), a signal processing and display unit (base
station), and a splitter interfacing between CF catheter, base
station, and the RF ablation generator. The CF catheter is
a 7Fr, steerable, RF ablation catheter with open irrigation.
An integrated fibreoptic CF sensor measures the amplitude
and orientation of the force of contact between the catheter
tip electrode and the tissue. The CF sensor has a resolution
and sensitivity of approximately 1 g and can measure both
the lateral force and the axial force independently. The in-
139
dividual forces and resultant force are calculated every 100
milliseconds and are displayed continuously.14 At the end
of each RF application a lesion-specific force–time integral
(FTI) is measured, which represents the total area under the
force–time graph.15
Ablation Procedure
All ablation procedures were performed under sedation
using intravenous midazolam. Blood pressure and oxygen
saturation were continuously monitored. All catheters were
advanced via the right femoral vein. A 6F decapolar reference
catheter was placed in the coronary sinus. A single transseptal
puncture (TSP) was made through which two 8.5F SRO long
sheaths were introduced (St. Jude Medical Inc., St. Paul, MN,
USA). After the TSP, intravenous heparin was immediately
administered to achieve an activated clotting time between
300 and 400 seconds. A 3-dimensional (3D) geometry of the
LA was reconstructed using Velocity (St. Jude Medical Inc.).
A wide-area circumferential ablation was performed of the
right followed by the left PV pair and isolation confirmed
using a circular mapping catheter (Inquiry Optima; St. Jude
Medical Inc.). The RF ablation power settings were as follows: power limited to 30 W on the anterior wall and 25
W on the posterior wall, target temperature 42 ◦ C, constant
flow at 17 mL/min7 . Crucially, all lesions were delivered for
40–60 seconds each at a single position before ablation was
interrupted and the catheter moved to a contiguous ablation
site. During ablation, the CF information was available to the
operator.
For further analysis of the anatomical distribution of CF
and FTI, the ablation line around each PV-pair was divided
into 5 sections (Fig. 1A). Acute PVI after ablation was verified by entrance block only without the use of adenosine
or isoprenaline. In the event of persistent LA-PV conduction after WACA, additional lesions were delivered along
the anatomical ablation line guided by the circular mapping
catheter until entry block in all 4 PVs was confirmed.
Image Processing, Registration, and Analysis
The study protocol is demonstrated in Figure 2. Prior to
PVI, the LA was automatically segmented from a preprocedural whole-heart balanced steady state free precession
(bSSFP) scan using a previously described technique.16 An
MRI surface shell was generated from the segmented LA.
During the procedure, the MRI shell was overlaid on fluoroscopy using the Philips EP Navigator (EPN) (Philips
Healthcare). The position of each RF application was
recorded on the MRI shell using the point tagging feature of
EPN. Alongside, the CF-sensing ablation catheter allowed
the lesion-specific contact FTI to be recorded.
Force Map
The CF and FTI were recorded around the right- and leftsided PVs and the mean CF and FTI per RF application at
each PV site were derived. The mean CF and FTI per patient were also calculated. A force–time integral map was
generated on the preprocedural LA surface shell, which is
composed of vertices (15,141 ± 3,653 vertices per shell), to
each of which can be assigned a force–time value. All ablation lesions were recorded on the MRI surface reconstructed
shell. The FTI associated with each lesion is distributed on
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Journal of Cardiovascular Electrophysiology
Vol. 25, No. 2, February 2014
After fusion of LGE and force maps, both quantitative
and qualitative comparisons were made. For a quantitative
evaluation of force versus LGE, for every recorded ablation
lesion, the proportion of vertices in the 5 mm radius labeled
as LGE were noted. For a qualitative comparison, the force
and LGE maps were visualized using custom made software.
Any similarities of patterns were noted.
Based on experimental data,9,13 the force and LGE-maps
were superimposed and compared for each 5 mm2 zone
(Fig. 2). An effective lesion was defined when scar occupied
>90% of a 5 mm2 analysis zone. All CF and RF ablation
records were reviewed and classified as valid or not valid for
CF analysis according to the suggestions of the TOCCATA
study protocol.10
Follow-Up
Figure 1. A: Pulmonary vein (PV) segments. Definition of 5 sites around
the left (LPV) and right (RPV) pair of pulmonary veins for applying the
contact force (CF) and force–time integral (FTI) during ablation: anterior
superior, anterior inferior, posterior inferior, posterior superior, and carina.
LIPV = left inferior pulmonary vein; LSPV = left superior pulmonary vein;
RIPV = right inferior pulmonary vein; RSPV = right superior pulmonary
vein. Average contact force (CF; [g]) and force–time integral (FTI; [gs])
per ablation site for all 268 ablation lesions. B: Relative distribution of the
average contact force (CF) during ablation.
Acute procedural success was defined as complete
PVI confirmed using a circumferential mapping catheter.
AF/atrial tachycardia (AT) recurrences were defined on the
basis of (1) symptoms with ECG evidence of the presence of
AF/flutter/tachycardia or (2) the presence of symptomatic or
asymptomatic episodes of atrial arrhythmia lasting > 30 seconds on ambulatory cardiac monitoring. Oral anticoagulation
was started the day after PVI with a target INR of 2.0–3.0.
Bridging with unfractionated or low molecular weight heparin was initiated 6 hours after the procedure. After hospital
discharge, all patients were reviewed in the outpatient clinic 3
months after the ablation procedure, or when symptomatic arrhythmic episodes occurred. Upon every follow-up visit, patients were asked for symptoms of AF, documented arrhythmia recurrences, and current medication. Ambulatory 24hour Holter monitoring was performed at 3 months intervals.
Statistical Analysis
a 10 mm diameter region. The FTI map on the MRI surface
shell is shown in Figure 3. The FTI at an ablation lesion
is distributed to its neighboring points, within a 10 mm diameter, on the surface mesh grid (right image), taking into
account catheter, cardiac and respiratory motion as well as
lesion formation. Within this area, the force is distributed
equally to all neighboring vertices such that their total sum
is the FTI (i.e., recorded FTI).
Late Enhancement Map
All patients underwent a repeat cardiac MRI at 3 months
post ablation. As before, the LA was automatically segmented and the LGE-MRI images were manually segmented
using ITK-Snap17 by an experienced observer, blinded to
the clinical course or treatment of the patient. Manually segmented regions were projected on the LA surface using the
Maximum Intensity Projection (MIP) technique.13
Force versus Late-Enhancement Comparison
The force map was available on the preprocedural LA and
the LGE-map on the 3 months LA. These were fused together
using the Iterative Closest Point (ICP) algorithm,18 which
computes an optimal affine-based registration for fusion of
the 2 surfaces. Force could thus be directly analyzed with
LGE data as every vertex on the fused LA shell had an
associated force–time value and LGE label.
Statistical analysis was performed using SPSS for Windows (version 19.0, SPSS Inc., Chicago, IL, USA). Continuous variables are expressed as mean ± standard deviation or
as median with interquartile range if appropriate. Normally
distributed data were compared using the independent Student’s t-test. For categorical variables, the chi-square test or
Fisher’s exact test was performed where appropriate. A P
value < 0.05 was considered as statistically significant. All
tests were 2-tailed.
Results
Patient Characteristics
We studied 6 consecutive patients, aged 58 ± 6 years of
whom 3 were men. Data analysis was performed on a total
of 268 ablation points. The clinical baseline characteristics
of the entire study population are presented in Table 1. A 3month blanking period was observed, during which arrhythmia recurrences were treated with antiarrhythmic drugs or direct current cardioversion. No repeat ablation was performed
within the blanking period. Within the first 3 months after
PVI, clinical AF recurrence was documented in 1 patient.
The median follow-up time per patient was 3 (interquartile
range, 3–4) months.
Sohns et al. Left Atrial Scar Formation After Contact Force-Guided AF Ablation
141
Figure 2. Study protocol: All patients underwent 1.5 T MRI prior to the ablation procedure and 3 months later according to our standard protocol.7 During the pulmonary vein isolation (PVI),
each radiofrequency (RF) ablation point
and the force–time integral (FTI) were
recorded on the preprocedural MRI shell.
After PVI, a scar map was created on the
postprocedural MRI shell using LGE signal intensity. The postablation scar map
was then registered to the preprocedural
FTI map and a scar versus FTI analysis
was performed using custom made software.
Figure 3. A typical example for the force map generation: Ablation lesions are recorded on the MRI surface reconstructed shell. The force–time integral
(FTI) associated with each lesion is distributed on a 10 mm diameter region. The FTI map on the MRI surface shell is shown (left image). The FTI at an
ablation lesion (marked with white circle) is distributed to its neighboring points, within a 10 mm diameter, on the surface mesh grid (right image).
Ablation Procedure
All 4 PVs were successfully isolated in all study patients.
There were 268 ablation records of good diagnostic quality
and therefore subject to further analysis. Average procedure
duration was 214 ± 25 minutes, the mean ablation time was
36 ± 8 minutes, and fluoroscopy time was 43 ± 19 minutes.
A mean of 44 ± 16 RF applications were delivered per patient. There were no major complications. One asymptomatic
pericardial effusion (n = 1) was noted on routine postprocedural echocardiography. Follow-up MRI scans at 3 months
excluded PV stenosis.
cations (>85%) were delivered with a CF of between 5 and
25 g. Overall, the mean FTI per lesion was 1,021 ± 523 gs.
Figure 1A shows the mean CF and FTI applied according to
PV site. The lowest FTI during ablation was applied between
the left-sided PVs (L5 = 768 ± 485 gs) and in the left anterior inferior and superior ridge area (L2 = 813 ± 395 gs,
L3 = 784 ± 400) (Fig. 1A). The highest FTI was applied at
the right anterior superior site (R1 = 1,280 ± 703 gs) and
the left posterior superior site (L1 = 1,429 ± 608 gs), respectively (Fig. 1A). A total number of 84,075 vertices were
analyzed.
Preprocedural CMR
Distribution of CF
CF distribution of the 268 ablation lesions is demonstrated
in Figure 1B. The figure shows that the majority of RF appli-
The median time of preprocedural image acquisition was 3
(interquartile range, 1–10) days. The circumferential burden
of LGE SI detected before any ablation procedure was very
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Vol. 25, No. 2, February 2014
ablation for AF and (2) CF distribution maps can be produced
using CMR/fluoroscopy overlay.
TABLE 1
Baseline Patient Characteristics
Patients (n = 6)
Parameter
Age (years)
Mean ± SD
Range
Sex: Male, n (%)
Paroxysmal AF, n (%)
AF duration (years)
Coronary artery disease, n (%)
Hypertension, n (%)
History of stroke/TIA
Left atrial diameter (mm)
Mean ± SD
Range
Left ventricular ejection fraction
Mean ± SD
CHA2 DS2 -VASc
Mean ± SD
Range
58 ± 6
47–62
3 (50)
6 (100)
6
1 (17)
2 (33)
1 (17)
43 ± 2
42–46
58 ± 2
1±1
1–3
low in comparison to postablation imaging and did not affect
>5% of the PV circumference. In addition, preprocedural
LGE signal localized to the mitral annulus was observed,
which is a common finding due to the fibroelastic nature of
cardiac tissue at this site.19
Association of CF and Left Atrial Scar Formation
All postprocedural imaging were performed 3 months after the ablation procedure. Figure 4 demonstrates an example
for 1 patient of LGE SI appearance after PVI. The left atrial
burden of LGE SI was significantly increased after ablation
in comparison with baseline. In the majority of instances,
there was visually good correlation between FTI and LGE
SI, as can be seen in the anterior view of Figure 4. However,
visual correlation was not always present, as indicated in the
posterior view of the left veins. Semiautomatic analysis of
the circumferential extent revealed that the LGE signal did
not achieve complete encirclement of any vein pair. There
was no significant difference between the amounts of LA
scar burden around the right- and left-sided PVs (P = 0.75).
In this context, increasing FTI correlated with increased LGE
SI, which was higher when the FTI was >1,200 gs. Figure 5A
indicates that only small increments in LGS SI are seen up
to an FTI of 1,200 gs, whereas larger increments are seen at
an FTI > 1,200 gs. This result can also be reproduced when
regarding an effective ablation lesion as an area that is occupied by scar of lower than >90% (range 40–90%) per 5 mm2
analysis zone (Fig. 5A). In addition, Figure 5B shows that
ablation lesions with no scar formation after PVI decrease
when the FTI increases. On postprocedural imaging, gaps in
the ablation lines were predominantly observed in areas of
low CF (e.g., L2, L3, R4), especially at PV sites where the
FTI was <1,200 gs.
Discussion
Main Finding
This study describes a new method to examine the relationship between CF, FTI, and LA scar formation following
PVI. The main findings are as follows: (1) there is a correlation between FTI and CMR LGE SI following catheter
Atrial Scar and PV Reconnection
Durable RF lesion formation is dependent on several
parameters, including catheter tip electrode size, power,
catheter tip temperature, and catheter-to-tissue CF. Previous work evaluating the role of CMR in assessment of the
LA after PVI has used LGE to identify areas of scar before and after ablation.3,5,7,21 During PVI it is necessary to
achieve electrical continuity of circumferential lesions and
in this context recent data demonstrated a strong relationship between CF and lesion formation.14,22,23 Acute PVI in
patients with AF is usually achievable, but recurrence associated with PV reconnection is a common finding.1,20 Recent
studies have evaluated the ability of LGE MRI to evaluate
the integrity of LA scar lesions and this is a potentially useful feedback tool to assess whether a successful lesion was
placed.7,24-27 In this context, Arujuna et al. demonstrated that
acute PVI is achieved by a combination of reversible and irreversible circumferential tissue injury at the junction between
the PVs and the LA with a higher incidence of AF recurrence associated with a greater extent of reversible ablation
injury,7 which in turn is due at least in part to suboptimal
tissue contact. This study shows that gaps in the ablation line
were predominantly observed in areas of low CF (e.g., L2,
L3, R4), especially at PV sites where the FTI was <1,200 gs.
FTI maps (Figs. 2, 3 and 4) are feasible and, by illustrating
areas of low CF to the operator during a procedure, have the
potential to guide adequate lesion formation.
CF Parameters
In the TOCCATA10,11 and EFFICAS I12 studies, an average CF of nearly 20 g was associated with higher long-term
freedom from AF recurrence. Also, ablations with CF < 10
g and FTI < 400 gs were associated with unstable catheter
contact and reduced patient outcome at 12 months. From the
TOCCATA study it is reported that the success rate of PVI
increases significantly with increasing FTI,10,11 which is supported by our findings (Fig. 5). Although we have shown a
strong correlation between FTI and LGE SI when the FTI is
> 1,200 gs, only small increments in LGE SI are observed
up to a FTI of 1,200 gs, suggesting that the relationship is
not as direct as might be expected intuitively. In EFFICAS
I, the authors concluded that to achieve durable successful
PVI, a target CF of 20 g is recommended, with a minimum
CF of 10 g and an absolute minimum FTI of 400 gs per individual ablation lesion.12 Pulmonary vein reconnection is a
common finding at repeat procedures, and may occur because
of atrial injury, which is either nonpermanent but transmural
or nontransmural but permanent, both patterns of which are
inevitably associated with acute atrial edema and cannot possibly be distinguished by CMR at present.28,29 However, the
apparently high FTI (>1,200 gs) at which a firm relationship
exists with LGE SI may be explained by the limited capacity of CMR to distinguish between partial and full thickness
atrial scar: an underestimation of the area of transmural scar
will bias the relationship toward an overestimation of the FTI
at which effective lesions (as defined by CMR SI criteria) are
seen. Although the decline in circumferential extent of LGE
SI between acute and follow-up scans was less in patients
with no AF recurrence than in those with AF recurrence,7 PV
Sohns et al. Left Atrial Scar Formation After Contact Force-Guided AF Ablation
143
Figure 4. Representative example of a patient-specific registration of a postablation left atrial scar map to the force–time integral (FTI) map. For each
patient all RF lesions were tagged on the MR shells and compared to their recorded FTI. A: Individual reconstruction of the preprocedural MRI shell.
B: Axial slice of the left atrium from preprocedural magnetic resonance imaging showing no preablation late gadolinium enhancement. C and D: Typical
example of the FTI, and E and F, left atrial scar maps in the posterior (C and E) and anterior (D and F) view.
Figure 5. Relationship between left atrial lesion formation (%) 3 months after pulmonary vein isolation and contact force (CF) measured as force–time
integral (FTI) (gs). Increasing FTI correlated with increasing LGE signal intensity. For an FTI < 1,200 gs, an increment in the FTI results in a small
increment in scar, whereas for FTI > 1,200 gs an increment in the FTI results in a large change in scar formation. The figure shows different curves depending
on the definition of an effective lesion ranging from 40% to 90% of scar in a 5 mm2 analysis zone. B: Relationship between FTI and percentage of scar
per analysis zone (no amount of scar vs < 70% scar vs > 70% scar) indicating that the amount of scar per zone increases, whereas the amount of no scar
decreases with increasing FTI.
reconnection also occurs in patients without clinical arrhythmia recurrence30 and, therefore, caution must be exercised
in relying on the use of MR-defined scar as a surrogate for
electrophysiological reconnection alone.
Relationship Between LA Anatomy and CF
There is evidence from previous studies that atrial ridges
and muscular folds between the ipsilateral PVs (carina area)
may decrease catheter stability.31 In addition, sites of acute
and chronic PV reconnection are more frequently located
in these areas.32 Our data are consistent with these findings
as lowest average of CF and FTI were observed at the left
anterior inferior ridge (L2, L3; Fig. 1A) and the ipsilateral
left- and right-sided PVs (R5, L5; Fig. 1A), while the posterior wall was represented by higher overall CF and FTI
measurements. However, the applied FTI varied depending
on the perivenous ablation location, with a high degree of
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Vol. 25, No. 2, February 2014
variability in the CF and FTI at the PV sites between the 6
patients as indicated by the relatively large standard deviations (Fig. 1A). In this context, the TOCCATA study and
recent data from Haldar et al.33 have demonstrated that there
is an important clinical impact of the average CF as low
catheter tip-to-tissue CF was associated with a higher rate
of AF recurrence in the follow-up period after PVI.10,11,33
Based on the preclinical data, it is known that lower CFs result in smaller lesions and gaps may occur especially between
these smaller lesions and consequently lead to an absence of
durable PV isolation.11 Focusing on the FTI alone, recent
data indicate that as the FTI increased from 500 to 1,000 gs,
the success rate increased significantly.11 This is absolutely
in line with our data showing that there is a good correlation
between FTI and LGE MRI following AF ablation and the
amount of left atrial scar increases with an FTI > 1,200 gs.
One may speculate that a real-time measurement of FTI
and CF during PVI might facilitate modulation of the ablation lesion size. Subotimal CF could be compensated for
by varying the RF power and/or duration for ablation, although for this a better understanding of lesion formation in
real time is needed, perhaps with the use of tissue ultrasound
or MR thermometry.14,15,22,34 The importance of an effective
first RF application has been shown in the EFFICAS I study,
where the authors demonstrated that the average number of
ablations per segment is inversely correlated to electrical
isolation.12 This supports the idea of edema formation as a
complication for subsequent ablations at the same site and
finally highlights that RF application should be delivered
with adequate CF and FTI levels to ensure a transmural and
permanent ablation lesion.
Study Limitations
This is not an outcome based study. It seeks to demonstrate a new method using cardiovascular MRI to quantify
the relationship between CF, FTI, and LA scar after PVI.
Therefore, the patient numbers are small and follow-up of
the relationship between CF and chronic PV reconnection
was not assessed. However, one very useful outcome of the
findings is that clear correlation between CF, FTI, and LGEMRI demonstrates that LGE-MRI could be used as a surrogate biomarker for clinical trials that seek to investigate the
effects of CF on ablation outcomes.
As mentioned in the results section, the majority of regions
showed good visual correlation between FTI and LGE SI.
However, there were regions where the correlation was less
marked. This can be explained by several factors. First, there
will be inherent errors in the colocalization of the catheterderived FTI data and the imaging-derived LGE data, including the accuracy of the coregistration and compensation of
cardiorespiratory motion. We have tried to minimize these
by recording the FTI information directly on MRI-derived
anatomical models. Second, the formation of lesions is not
purely dependent on FTI but also on other factors such as
the thickness of the substrate, the RF power and application
duration. It is likely that using a parameter that combines
these variables instead of using just FTI alone would generate parameter maps that were more correlated with LGE SI.
Exploration of parameters that are dependent on this multivariable approach will be the focus of future work.
Elimination of the PV potentials recorded from a circular
mapping catheter was the primary endpoint for PVI in this
study, which is in line with the current guidelines.1 In addition, the demonstration of exit block is also considered as an
important end point.1 Furthermore, the administration of isoproterenol or adenosine to identify dormant PV conduction
can be useful.35,36 However, the role of inducibility testing
as an end point in clinical routine is still debated.1
Conclusions
There is a good correlation between FTI and LGE MRI
following catheter ablation of AF. An FTI of 1,200 gs is
associated with atrial scar as defined by CMR. Real-time
FTI maps are feasible and may prevent inadequate lesion
formation.
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