Electrical features of focal activations
Among points with focal activations, approximately two-thirds showed normal bipolar voltage, which is in line with a previous study by Honarbakhsh et al. where almost half of focal activations occurred in the areas of normal bipolar voltage.9 Although the voltage of the DFAs was lower than that of the non-DFAs, the voltage in DFAs was also frequently within the normal voltage range. Focal activation may be less relevant in the LVA. Moreover, the majority of bipolar electrogram morphologies at the points of focal activation were categorized as a discrete type, whereas the fractionated type was rarely found. The majority of discrete types were composed of single potential components. These findings imply that focal activation is located in the normal myocardium with preserved electrical voltage. Furthermore, the nature of focal activation may be different from that of CFAE, which is supported by the absence of a short CL in the focal activation. In this regard, the different locations of the focal activations surrounded by the CFAE area in the CARTOFINDER and ICL maps could additionally support the above-mentioned hypothesis.
Focal activations had relatively short CLSD, and short CLSD was associated with increased focal intensity, suggesting that focal activities may consist of regular activities in temporal characterization. Pappone et al. demonstrated that there were repetitive-regular activities (RRas) during AF mapping, and that ablation targeting RRas in combination with the other indicators resulted in high rates of acute termination and long-term absence of AF.16 Focal activations in our results share some characteristics with the RRas: (1) RRas were defined as bipolar signals with CLSD ≤ 30 ms, and 93.3% of focal activations in our study showed CLSD ≤ 30 ms; and (2) both focal activities and RRas were often located adjacent to the fragmented regions.
Previous works by Hansen et al. described a potential mechanism for focal activation,5 demonstrating that the AF driver was a micro re-entry based on pectinate muscle alignment in optical mapping, and that this regular activity was transformed into fibrillatory conduction. Since the electrical features of the focal activations in our study were consistent with the abovementioned hypothesis, the focal activations may correspond to the exit of the micro re-entry circuit. In contrast, CFAE, which is often surrounded by focal activations, may reveal slow conduction of the circuit. The combination type of bipolar morphology may represent electrograms at the border zone between the focal activation and CFAE areas.