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.