Ablation Of Atrial Flutter:Block (Isthmus
Conduction)
Or Not A Block, That Is The Question?
Ashish Nabar, MD, PhD
Address for correspondence: Ashish
Nabar MD, PhD, Department of Cardiology, Academic Hospital Maastricht,
P. Debyelaan 25, Post box 5800, 6202 AZ, Maastricht, The Netherlands.
E mail: ashish_nabar@hotmail.com
Abstract:
It is important to identify residual slow conduction and minimize the
chance of resumption of conduction after right atrial isthmus ablation
to reduce the chance of recurrence of atrial flutter (AFL). The aim of
this article is to discuss the best possible way of confirming a
bi-directional
isthmus conduction (BIC) block after ablation of an isthmus-dependent
AFL. A combination of activation and double potential mapping seems to
be the most practical way of acutely confirming the BIC block.
Ablation of a macro-reentrant atrial tachycardia
involves: 1) mapping the reentrant circuit to define the conduction
zones 2) creating linear lines within the critical zones of slow
conduction that extend
to anatomical borders and 3) terminating the tachycardia and
demonstrating conduction block. The para-tricuspid course of the
macro-rentrant circuit of isthmus-dependent atrial flutter (AFL) has
been previously well studied.1,2,3,4,5 The low
right atrial isthmus (RAI), lying between the tricuspid annulus (TA),
coronary sinus (CS) os and the Eustachian
ridge, has been for various functional reasons considered the critical
zone of an AFL circuit. These reasons include: 1) induction of AFL
following
an unidirectional block in the RAI 2) repetitive activation of the RAI
by
the AFL macro-reentrant circuit 3) slow RAI conduction during AFL (<
0.5m/s) 4) ability to demonstrate concealed entrainment from this
region
5) termination of the AFL in the RAI during ablation and 6)
non-inducibility of AFL after obtaining a bi-directional isthmus
conduction (BIC) block.
These observations led Poty et al., to recommend that it was not
necessary
to induce AFL and that one could directly proceed to isthmus ablation
if
an AFL with a typical ECG pattern had been clinically documented.6 The aim of this article is to discuss the best
possible way of confirming a BIC block after ablation of an
isthmus-dependent AFL. It is important to identify residual slow
conduction and minimize the chance of resumption of conduction after
successful isthmus ablation. A gap in
the ablation line, as small as 0.8 cm, can lead to conduction slowing
to
0.3 m/s which may be clinically difficult to detect.7
Resumption of RAI conduction occurs in 10%-15% of patients, mostly
within 30 minutes after the last radiofrequency (RF) application.6,8,9,10
Shah et al., suggest that the chance of lesion recovery is far more
common, seen in 50% of the ablated sites, within 1 minute of stopping
the RF application.11 Unfortunately, a
sub-acute resumption of conduction within the first 24 hours has also
been documented.9 In follow-up studies
performed 1-12 months after flutter ablation, recovery of RAI
conduction is seen in 45%-50% of patients, however clinical recurrence
of AFL occurs in only 5%-10% of patients during the first year after
ablation.12,13
Activation mapping - mapping around the ablation line:
During ablation of an AFL we position a duo-decapolar Halo
catheter along the TA, a quadripolar catheter to record His bundle
electrogram
and a decapolar catheter in the coronary sinus. See Figure 1, panels C and D. A unidirectional
right
atrial (RA) activation viz. cranio-caudal, along the lateral RA wall
during
CS pacing and along the septal wall during low lateral RA pacing, after
the last RF application had been proposed by Poty et al., as the
criteria
of BIC block.14 The assessment of
latero-medial
isthmus conduction block is to some extent limited by the less
extensive
mapping of the septum performed using this array of catheters. One has
to rely on the activation sequence, activation of the CS os following
His
bundle activation during low lateral RA pacing. Further, it is often
noted
that the trans-isthmus intervals prolong gradually until the
penultimate
application, and jump to their final prolongation after the last RF
application.7 However, this finding might
not always be reliably observed in patients who continue
anti-arrhythmic drugs at the time of
ablation. Actually, the trans-isthmus intervals before and after
achievement
of BIC block represent activation of different RA structures, and hence
by themselves are not meaningful to identify block versus residual slow
isthmus conduction. Yet Oral et al., have published that 50%
prolongation
of trans-isthmus interval is a sure sign of block.15
An apparent conduction block occurs more often when ablation is
performed
during AFL than during CS pacing.11 We
demonstrated
that the use of isoproterenol infusion, before ending the procedure,
could
rule out an apparent BIC block.8 We further
stated that pacing close to the ablation line may detect persistent
slow
conduction. Other way around, in the presence of a BIC block, an
"apparent
not-block" during CS pacing could be due to conduction behind the
inferior
vena cava (IVC) along the posterior RA wall, which then penetrates the
lower crista terminalis and activates the distal poles of the Halo
catheter.16
Figure 1:
A
(pacing at CS orifice) and B (pacing at low lateral RA) represent
pacing
before ablation to demonstrate BIC. C (pacing at CS orifice) and D
(pacing
at low lateral RA) show BIC block.
Double potentials - mapping
along the ablation line:
That double potentials (DP) separated by an iso-electric interval
indicate a local block under the recording electrode is known from
experimental studies.17 However, Shah et
al., brought this concept to ablation of AFL.18,19 They have recommended creation of a corridor
of DPs, from the TA to the IVC edge of the RAI. This should be
verified,
both during CS and low lateral RA pacing. See Figure
2. Any single or fractionated (un-seperated) potential along the
ablation
line, in an acute setting or when re-ablating for an AFL recurrence,
should
be considered as "gap" and additional RF application delivered. They
found
that, when activation mapping alone would have suggested a BIC block,
in 15% of the patients a persistent conduction was detected by these
criteria.
Further, conduction along the posterior RA wall, behind the IVC, does
not interfere with evaluation of BIC block when using the DP criteria.
Another advantage is, essentially the ablation procedure could be
simplified
to the use of two catheters. Tada et al., attempted to answer, how long
the inter-potential intervals should be to discriminate between
residual
slow conduction and a complete block.20
According to their calculations, a complete block was associated with
an inter-potential interval >110 ms, while an inter-potential
interval <90 ms was always
associated with residual slow isthmus conduction. When the
inter-potential
interval was 90-110 ms, an interposed iso-electric segment and a
negative
polarity of the second component suggested a conduction block. However,
it should not be forgotten that the inter-potential interval is
dependent
on the distance of the recording electrode from the gap, the greater
the
distance the longer the interval.
Figure 2: Demonstration
of DPs along the ablation line. One catheter is used to pace (A: CS
pacing, B: low lateral RA pacing) while the ablation catheter (RF)
records along the ablation line.
Is any one of the two criteria
good enough?
Anselme et al., randomized 76 patients with an isthmus-dependent
AFL to ablation using either activation or DP mapping.13
Activation mapping depends on correct positioning of the Halo
catheter, so that the distal bi-poles lie close and anterior to the
ablation line. In principle, CS pacing after the last RF application
could then rule out any residual slow isthmus conduction. However,
correct positioning of the Halo catheter is difficult in 10% of the
cases, especially so with the re-use of catheters. DP analysis depends
on recording clear-cut widely spaced potentials. Ambiguous potentials
are recorded as often as in 40% of the cases.
See Figure 3, panels A-C. Channel RF represent
sites along the ablation line, recorded in different patients after the
delivery of the last RF application. Panel A: Halo shows reversal of
activation sequence along the lateral RA wall during CS pacing,
suggesting an isthmus conduction block. However, a site on the ablation
line shows unsplit single potential. Decision: ablate at this site
showing a "gap", despite the
fact that the Halo catheter suggests (medio-lateral) isthmus conduction
block. Panel B: Recordings from the Halo catheter suggests residual
slow
isthmus conduction. A site with very small amplitude triple potentials
was present along the ablation line. These small local potentials are
difficult to interpret. What is the significance of a triple potential?
Decision: Helped by the fact that recordings from the Halo catheter
suggest residual slow conduction, one could infer that the local triple
potentials
imply slow conduction and ablate at this particular site. Panel C is
the
most difficult to interpret. The Halo catheter suggests an isthmus
conduction
block. Locally, a triple potential is noted. Does the central potential
suggest slow conduction or bystander activation? After ablation at this
site, if the central potential disappears and the interval between the
pacing spike and the terminal potential remains same as pre-ablation,
it would imply, only retrospectively, that we probably ablated a
bystander
potential and therefore delivered an unnecessary RF application.
However,
if the spike to the terminal potential prolongs further, we could say,
again only retrospectively, that the RF application was justified and
abolished the residual slow conduction.
Figure 3: Is
any one of the 2 criteria good enough to confirm BIC block? See text
for explanations to panel A-C.
Considering these difficulties, Anselme recommended that both methods
should be used concomitantly. We routinely use both the methods, as
shown in Figure 4, to demonstrate BIC block.
CARTO electro-anatomical mapping system, because of its theoretical
ability to map unlimited number of points and a lack of restriction
regarding mappable sites, often experienced with the use of pre-shaped
catheters such as a Halo catheter, could be helpful in difficult cases.
Figure 4:
Concomitant use of activation and local potential mapping to confirm
BIC block. A: CS pacing, B: low lateral RA pacing.
Other methods to confirm BIC block:
Since there is no absolute inter-potential interval that could be
considered as a definitive sign of BIC block, maneuvers involving
pacing close to the ablation line have been developed by Chen et al.,21 and Shah et al.22
Bipolar electrogram polarity reversal to identify a gap in the ablation
line has been reported by Yamabe et al.,23
and Tada et al.24 Lastly, the concept that
unipolar RS morphology is recorded when the wave front propagates under
the recording electrode and a R morphology is recorded at the end of
activation
has been applied to flutter ablation by Villacastin et al.25 They showed in a group of 45 patients that,
after
a successful isthmus ablation, a R or an Rs morphology could be
recorded
in all patients during CS pacing, while a similar morphology can be
documented
in 90% of patients during low lateral RA pacing. The terminal S remains
significant in remaining 10% of the latter patients, probably due to
the
LA activation which continues to move away from the recording electrode
placed posterior to the ablation line.
Conclusion:
We ablate the low RAI during AFL - if this is incessant since the
beginning of the study, or during CS pacing - when the rotation of the
induced AFL has been studied and this can be readily terminated without
the risk of inducing atrial fibrillation. After interruption of the
AFL, we continue to ablate the isthmus during CS pacing along the same
ablation line. On completion of the ablation line, from TA to the IVC
edge, and achievement of a medio-lateral conduction block, as judged by
unidirectional cranio-caudal lateral RA wall activation, we perform
pacing of the low lateral RA to
test for latero-medial conduction block. If this is confirmed, we
record
local potentials along the ablation line, by withdrawing the ablation
catheter in steps, from the TA to the IVC edge while constantly pacing
in the CS. If DPs can be demonstrated all along the ablation line
during both, CS
and low lateral RA pacing, the entire sequence of evaluating BIC block
is repeated under isoproterenol infusion. Additional RF pulses are
delivered
at any sites along the ablation line recording unsplit potentials.
Every
attempt is made to verify non-inducibility of AFL, unless the risk of
inducing
atrial fibrillation is very high. Despite this extensive evaluation and
the known ability of the RF energy to create full-thickness scar in the
isthmus,26 the acute electrophysiological
evidence
of BIC block may reverse with resolution of inflammation and
interstitial edema and may not predict long-term success. Thus the
question is not limited to acute confirmation of BIC block after
isthmus ablation but extends to: how can we be certain that the
conduction block will be maintained over time? Luckily for us, though
isthmus conduction may resume the chance of clinical recurrence of AFL
seems to be very small.
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