4.3 The mechanisms of the CST phenomenon
The two distinct time differences required for the successful isolation of the superior and ipsilateral inferior PVs in this study would lead to the possibility of two mechanisms for the CST phenomenon. As shown in Table 2, the isolated cases were divided into two groups. In one group, an almost simultaneous isolation of both PVs was observed in cases 10, 14, and 15 (“simultaneous” group), whereas in another group, the superior PV was isolated with a non-simultaneous but rather different timing from that of the ipsilateral inferior PV, as in cases 7, 9, 11, and 16 (“non-simultaneous” group). Based on these findings, two mechanisms could be proposed, as delineated in Figure 5. An epicardial connection intervening between the ipsilateral PVs (epicardial conduction) may exist in the “simultaneous” group. On the other hand, the thick carina tissue, which could interrupt a complete isolation of the superior PV by CB application to the superior PV until additional freezing has been applied to the inferior PV (carina conduction), may connect the superior and inferior PVs in the “non-simultaneous” group.
The prevalence of an epicardial conduction was reported by previous studies to be 13.5%-20%.16-18 To date, various epicardial conduction paths between the PVs and other atrial structures have been reported: LSPV-posterior LA, LSPV-vein of Marshall (VOM) area, LSPV-LA roof, left common PV (LCPV)-VOM area, LCPV-posterior LA, RSPV-posterior LA, and RSPV-LA roof.16-21 Takahashi et al. reported that out of 49 cases in which the LSPV was difficult to isolate by radiofrequency catheter ablation, 7 cases had an earliest activation site in the LIPV during intra-LSPV pacing, which suggested an epicardial conduction between the ipsilateral PVs.20
The durability of acute PV isolation and the long-term AF survival rate have been reported to be lower in cases with epicardial conduction than in cases without such conduction.16-17 In our study, no acute electrical reconnections of the PVs were documented in the CST success group.
Anatomical reports have described that the thickest myocardial sleeves around the PVs are found in the carina region.22,23Cabrera et al. reported these connections from an anatomical aspect.23,24 In histologic sections of 15 hearts, 27%, 53%, and 20% of myocardial strands between the superior and inferior PVs were epicardial, subendocardial, and from both aspects of the PV wall, respectively. The strands on the epicardial PV wall are likely to correspond to electrical epicardial intervening connections. That anatomical aspect of the PVs may account for the two conduction patterns observed during CB applications and eventual CST phenomenon.