DISCUSSION
We have tested an ablation strategy for unstable reentrant ATs based on
focal rotor ablation. To our knowledge, this is the first study that
uses driver ablation in patients with reentrant ATs in order to
stabilize unstable circuits. Most patients showed rotors or sites with
STD plus non-continuous fractionation that could be ablated to achieve
AT stabilization or termination with high success.
Ablation of reentrant ATs, excluding CTI-dependent atrial flutter, has
shown limited one-year freedom from atrial arrhythmias in recent reports
(51%-77%).1,5-7 Moreover, acute procedural success,
defined as AT termination into sinus rhythm or conversion to another AT,
is not achieved in approximately 10-15% of
cases.1,5-7 We have obtained, in a cohort of patients
classically excluded from other series and considered as especially
complex, similar rates of procedural success (88.9%) and one-year
freedom from atrial arrhythmias (66.7%). In patients with mappable
reentrant ATs ablated in the same period of time in our Centre, results
were also comparable. Interestingly, included patients had less dilated
atria than these patients with mappable reentrant ATs; maybe, in more
enlarged atrium, with more dense-scarred tissue and less conduction
speed, reentrant circuits are more prone to stabilize.
Rotor identification has to date been based mostly on the use of
dedicated catheters and/or software that imply additional costs and time
consumption during ablation procedures.14-17 On the
contrary, STD analysis as a tool to find rotational activity can be
performed with conventional high-density mapping catheters and is
subjectively performed by operators according to visually detectable
electrical patterns which show very high (94.3%) interobserver
concordance.11 Of these patterns, the most specific is
the finding of a fractionated continuous signal in a single bipole of
the mapping catheter, present in almost all regions with dispersion and
in almost no regions without dispersion.11 We
hypothesize that fractionated continuous (or quasi-continuous) activity
should be visualized within the majority of rotor cores on bipolar
signals, as true functional microreentries (Figure 1, panel A); in fact,
fragmentation to consider ‘conventional’ anatomical microreentry has
been defined as that comprising >75% of the tachycardia
cycle length.9,18 In peripheral regions, however, we
believe that spatiotemporal distribution of signals (i.e. all the AF
cycle length comprised within the different bipoles of the mapping
catheter), which may exhibit some fractionation but not on a continuous
manner, could be the most frequent pattern. Besides, finding the rotor
pivoting point (i.e. core) would allow focal ablation, which should be
sufficient to stop rotational activity without the need of performing
more extensive ablation of all the rotor ‘region’. For these reasons, we
decided to use the finding of fractionated quasi-continuous signals on a
limited number of adjacent bipoles of the mapping catheter as our
definition of rotor. This definition, whether representing or not the
true core of rotors, allowed focal ablation with high efficacy in our
series.
Another pattern, electrical STD plus non-continuous fragmentation on
single bipoles, was used as the marker of rotational activity for
patients in which the former pattern could not be found. We hypothesize
that, if the rotor core slightly travels with a rotational pattern,
fractionated but not continuous activity could be visualized with single
bipoles of the mapping catheter located within the travelling trajectory
of the rotor core (Figure 1, panel B).
In patients with AF, conversion rates with driver ablation ranges
between one third19 and two thirds of cases in most
reports17, with a pooled success rate of 39.6% in a
recent meta-analysis,10 although higher efficacies up
to 95% have been reported.11 The high conversion rate
observed in our series could be related to the nature of the arrhythmia;
in the continuum between stable reentrant AT and AF, unstable reentrant
ATs might exhibit atrial conduction properties less prone to chaotic
conduction than AF, and, hence, easier to stabilize through ablation.
Also, the mean number of rotors we found in our cohort (2.0 ± 1.23
rotors per patient) is in the low range of reported data on AF driver
ablation (which ranges from 1.8 ± 1.23 to 5 ± 1.5 drivers per
patient)11,20, although no clear relationship between
the number of drivers and ablation success to convert AF seems to
exist.10 The main location of rotors was the pulmonary
vein antra (42%), which is a common place for AF drivers. In a study
that specifically addressed locations where AF termination was achieved
in a cohort of AF patients16, 27.5% of terminations
occurred ablating within the pulmonary vein antra, and in 91% of these
locations a rotor was detected.
Another reason that may justify our rotor ablation success rate is a
higher short-term spatiotemporal stability of rotors in the context of
unstable AT than in AF. Although this affirmation is hypothetical, all
rotors that were found through initial mapping in our cohort persisted
in the same location when mapping was complete several minutes
afterwards. The 10-seconds criterion to determine temporal stability of
rotors in our series was arbitrarily set between reported time criteria,
which ranges between 2.5 s and 30 s in most
series.11,21 However, spatiotemporal stability of
drivers in the medium or long term in AF patients has not been
demonstrated,22 which might account at least partially
(along with atrial disease progression) for arrhythmia recurrences after
driver ablation both in AF and unstable AT patients. Indeed, 37.5% of
recurrences in our cohort were AF instead of AT.