Mapping Ripples or Waves in Atrial Fibrillation?

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Therapy for persistent atrial fibrillation (AF) is limited by uncertainty in its mechanisms yet, unlike in organized rhythms, mechanisms for AF may vary dramatically with mapping technique. The traditional approach of drawing contour maps of AF by assigning an onset time to each electrogram reveals disordered activity in patients with permanent AF at nonarrhythmia surgery.1 This accounts for AF complexity, but does not explain some clinical observations. Recent studies3 using signal processing to filter far‐field and assist mapping reveal rotational or focal drivers of persistent AF where local ablation can terminate AF, with promising long‐term outcomes in many5 but not all11 studies. It is unclear if divergent results reflect different mapping methods, epicardial versus endocardial1 mapping, patient selection, or intercenter variations in AF ablation success.14 Advances in AF mapping will require comparisons between methods, ideally referenced to clinical endpoints to facilitate interpretation.
In this issue of the Journal, Takahashi et al.15 use “Ripple Mapping,” a novel electrogram analysis previously applied to organized supraventricular16 and ventricular arrhythmias,17 to identify potential focal drivers of persistent AF associated with AF termination by ablation. The authors identified 45 patients in whom persistent AF continued after pulmonary vein isolation (PVI), in whom point‐by‐point left atrial mapping was performed using a high‐density catheter, followed by stepwise ablation. Ripple mapping was applied retrospectively for ≥3 AF cycles at 13 ± 3 sites per patient, compared to manually annotated bipolar electrogram timings in 7 patients. AF propagation maps from ripple‐annotated timings showed organized, mostly focal activation at 14% of sites in persistent AF. During stepwise ablation, lesions determined post‐hoc to pass through these sites were more likely to terminate AF than lesions through nonorganized regions (22% vs. 7%, P = 0.015). The authors conclude that ripple mapping may help future prospective mapping to identify ablation targets in persistent AF.
The authors should be commended for applying ripple analyses in this novel way. Ripple mapping displays activation via “columns” with height proportional to voltage at each location over time, rather than depicting each electrogram by a single onset time and voltage. Thus, a ripple map of macroreentry may show multiple small ripples indicating a site of fractionated signals, or single tall “up and down” columns indicating noncomplex signals.
There are some limitations worthy of discussion. First, the method may be time‐consuming, since it requires moving a multipolar catheter to scores of atrial locations for analysis. Second, since AF sources “wobble” in nearly all descriptions,3 this small sampling approach may lead to underdetection. Third, the accuracy of ripple analysis for AF depends on how bipolar electrograms are handled, which poses some challenges in AF. The amplitude and polarity of bipolar electrograms depends on wavefront direction, which varies rapidly in AF and is unknown a priori. In addition, each pole of a bipolar electrode in AF may detect these different waves of unknown directionality and integrate far‐field activity, as the authors acknowledge.
Comparative mapping studies reveal that conventional marking of bipolar electrograms in AF may not indicate local activation time. Figure 1 shows the juxtaposed catheters (see fluoroscopy) to compare bipolar AF electrograms18 with monophasic action potentials (MAPs) that indicate local activation.19 While some bipolar electrograms (red tracings) clearly reflect local activity (blue arrows on MAP), many do not––indeed, some better match far‐field notches on the MAP. Moreover, many local activations were not reflected in bipolar electrograms––indicated either by very small or no deflections. Traditional (or ripple) analysis of this channel may show disorganized activity, yet MAPs show regularity with far‐field notches.
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