Acute changes produced by the different ablation energies
In our study, the LA wall thickness was found to increase similarly at the level the PV ostia following ablation when using RF or laser balloon energy, while no increase in wall thickness was observed when using the cryoballoon. While acute development of tissue oedema is well known after RF, 9,19,20, limited data are available regarding acute tissue changes after laser energy delivery13,21,22. Apart from the lack of direct contact of the energy source with the tissue (the optical fiber delivering arc of laser energy is in a balloon), laser energy as with RF produces tissue damage through heating and is delivered in a point-by-point fashion. Thus, it is not surprising that the two energy modalities might share similar mechanisms of tissue injury, including acute wall thickness increase from oedema. In the study by Mangrum et al13, a significantly more pronounced wall thickening was observed after RF ablation than after laser ablation, however a lower RF power was used.
In another IVUS study11, tissue oedema was reported in 90% of the PVs after cryoablation (and similar number of freezes per vein). Of note, in this study dissection-like changes were also observed, together with oedema, in most of the PVs, while in our study dissection-like changes were observed only in one vein after cryoballoon ablation and, interestingly, this occurred in the context of acute wall tissue thickening. It could be hypothesized that in these veins the oedema was due to the mechanical injury associated with dissection, rather than being a direct consequence of cryoenergy delivery. In the sequential process of tissue injury produced by cryoenergy23,24, tissue oedema is thought to occur only at a late stage, once the tissue has thawed, following freezing, and has become hyperaemic, and to gradually progress over subsequent hours25. Concordantly, early PV imaging in our study showed no acute wall thickening suggestive of development of oedema.
The different morphological changes produced by the different ablation energies could suggest different mechanisms of lesion failure. Recent data suggest that the adjustment of the ablation settings based on baseline LA wall thickness can improve the procedure outcome and reduce the risk of collateral injury12. A further adjustment based on the acute wall thickening produced by energy delivery for ablation could also be beneficial when using RF or laser energy and could potentially highlight gaps between lesions.
Apart from acute wall thickening, we observed a reduction of the thickness of the PV muscular sleeve after PVI catheter ablation. Myocardial sleeves are known to extend from the left atrium into the PVs walls and to be a source of focal activity triggering AF26. The thickness reduction after ablation could indicate damage, translating to elimination of the PV potentials and acute electrical isolation of the vein and may also explain why it is often impossible to get local capture during pacing to demonstrate exit-block. Whether durable PV isolation correlates with a certain degree of wall thickness reduction or complete disappearance of the muscular sleeve after catheter ablation is unclear. We did not observe a correlation between degree of wall thickness increase or muscular sleeve thickness reduction after the first catheter ablation procedure and evidence of PV reconnection at the second catheter ablation procedure, however only a small number of PVs were checked with a second ablation procedure in our study.