4.3 AF rotors and the fibrotic tissue properties
The development and progression of atrial fibrosis are considered as the
substrate for AF perpetuation. LGE-MRI has been developed to visualize
and quantify the extent of atrial fibrosis. The DECAAF multicenter
prospective study demonstrated that atrial fibrosis estimated by LGE-MRI
independently predicted AF recurrence after catheter
ablation.14 Furthermore, computer simulations
demonstrated AF rotors located in boundary zones between fibrotic and
non-fibrotic tissue.4 To quantitatively characterize
the fibrosis spatial patterns in each computer simulation model, the
fibrosis density and fibrosis entropy were calculated. The local
fibrosis density value was calculated as the proportion of fibrotic
elements among all elements within the surrounding sub-volume. The local
fibrosis entropy in each element was calculated as the level of
disorganization within the surrounding sub-volume, quantified based on
the modified Shannon entropy. However, this has not been verified in
clinical use. In this study, the incidence of NPAs was higher in
fibrotic tissue areas with an LGE-entropy of ≧5.7 (G1 and G2) as
compared to that with an LGE-entropy of <5.7 (G3 and G4). No
NPAs could be found in dense fibrotic tissue areas with an LGE-volume
ratio of >50% regardless of the LGE-entropy. This
indicated that the NPAs could be associated with tissue properties with
fibrotic heterogeneity (entropy) rather than the fibrotic volume. The
dense fibrotic tissue areas with an LGE-volume ratio of
>50% appeared to be scar without electrical activity. This
result was consistent with the computer simulation model. We found that
the atrial myocardium consisted of different fibrotic tissue properties.
Although it was not statistically proven, we speculated that the
myocardial fibrosis gradually progressed and the fibrotic heterogeneity
increased, which could have provided heterogenous fibrotic tissue where
the AF rotors were frequently found. Those fibrosis patterns might
correspond to the patchy LGE sites on the LGE-MRI. Further, the fibrosis
progression decreased the fibrotic heterogeneity and increased the
fibrosis volume, which could have provided the homogenous fibrosis
tissue, namely dense scar. This tissue property might play an important
role as an obstacle for macro-reentrant atrial tachycardia but not AF.
Of importance, we might have to pay attention to the early stage
fibrotic tissue properties (G1) that could progress to patchy fibrotic
tissue associated with AF rotors in the future.
AF recurrence was observed in the patients with the slow highest %NP
values where direct RF applications were performed or with the high
highest %NP values where direct RF applications were not performed. The
tissue properties of NPAs with a low highest %NP were characterized as
having a low LGE entropy and LGE-volume ratio, which was categorized as
G3. We speculated that those NPAs were an unmatured AF substrate and the
effect of an NPA-targeted ablation would be less at the NPAs with G3
tissue properties as compared to that with G1 or G2 tissue properties.
As for the catheter intervention for NPAs, direct RF applications were
recommended in the previous study. That was because the real AF drivers
were considered to be contained in the NPAs where rotational activations
were frequently observed.10 When considering the
reconduction of the ablation line, a direct RF application at the NPA
would be necessary even though that area was locked in the PVI or Box
lesion line.