Insight into the Role of the Potential FCB Region in Other Atrial-related Arrhythmias
In addition to the types of ATs we mentioned, the mechanism of another kind of AT with CL alternans is also related to the FCB, which is a rare phenomenon.13-15 In one type, CL alternans resulted from an intermittent 2:1 conduction block within the channel of the small circuit. The other type, the alternate block and conduction of the 2 channels with different velocities, results in CL alternans.13 The FCB channel of these ATs usually occurs with severe atrial scarring of different causes (prior cardiac surgery, catheter ablation of AF or fibrotic atrial cardiomyopathy). Similar to our study, the FCB region often occurs near the scar area. This may be another expression form of the FCB region.
The DP region means a smaller block area than some of the silent areas because the still potential could be recorded bilaterally. Most DPs or fragmented potentials are more common in AT or AF than in sinus rhythm because the bilateral scar tissue that is recognized in ATs is excited at the same time in sinus rhythm due to the normal conduction pattern, not with a certain direction as atrial flutter (AFL). As we observed in this study, the FCB region participated and maintained the reentrant circuit at different CLs or pathways (Figs. 1 and 2). This phenomenon suggests that some AF patients who still experience recurrence of AF after PVI ablation may also have neglected FCBs, while they have normal conduction substrates after mapping under sinus rhythm. The FCB may maintain the AF under some conditions. More electric anisotropy is related to a more uncertain conduction mode, which may lead to AF. Recently, it has been shown that the ablation of Marshall veins or Bachmann’s bundle may further advance the success rate of AF,12,16 not only for the trigger in these regions but also for reducing the connection with the atrium that decreases the electric anisotropy of conduction. Judging the electric anisotropy region, such as the FCB, may provide new insight into the formulation of the AF ablation strategy.
ATs with over two types of reentrant circuits are often associated with more complex ablation procedures.1 The ablation strategy in our study still depended on the activation mapping results. Despite careful mapping, the reentrant circuit of the second AT could be forecasted before the first ablation in only 1 patient with a double-loop localized reentry who did not show the FCB region (Fig. 4). The instability of the functional zone made the second or third AT unpredictable (Figs. 1 and 2). The occurrence of FCB may depend on the circle length of the ATs, and entrainment mapping may transform tachycardia, confusing the mechanism of ATs. Substrate mapping is also limited by different potentials under different conditions and does not seem to make much sense in optimal ablation strategies. Even though an effective method for identifying a potential FCB region is not yet available, the ablation strategy for addressing this specific substrate is controversial. A future computer simulation may well be used to identify the necessary conditions for FCB and an intervention strategy.