Pacing site, EC, and ablation outcome
In the present study, all patients underwent LA electroanatomical mapping during both RAA and RAPW pacing. As shown in Figure 3, in the EC group, breakthroughs of the RPV carina and the LA anteroseptal region were detected only during RAPW pacing, although RAA pacing showed a breakthrough at the LA anteroseptal region only. Prior to the present study, Yoshida and colleagues identified 6 of the 34 (18%) patients with right-sided carina breakthrough by electroanatomical mapping during sinus rhythm.6 Compared with their study, our study had a higher proportion of patients with right-sided carina breakthrough (12/41, 29%). These results are attributed to the fact that the site of impulse origin affects the choice of the propagation route. The sinus node is located in the anterior or anterolateral quarter of the superior vena cava and the RA junction,13 and the RAA is located anterior and medial to the RA. If the distance between the impulse origin and RAPW, where the intercaval bundle is assumed to be attached, is long, LA activation may be predominated by the Bachmann bundle at the LA anteroseptum in patients with right-sided carina breakthrough. Accordingly, the presence of EC may be underestimated by electroanatomical mapping during sinus rhythm or RAA pacing.
The majority of patients in the EC group required additive carina ablation to achieve PV isolation, compared to almost no patient in the non-EC group. However, two cases required carina ablation even in the non-EC group, suggesting that not only EC but also factors such as a thick PV carina can prevent PV isolation by circumferential ablation alone. Another possibility is that that EC could not be accurately detected in these cases. The exact location of the epicardial fiber attachment on the RAPW was difficult to determine; therefore, inappropriate RAPW pacing could not depict EC, and carina ablation was indispensable. To solve such a problem, Hasebe et al. performed a simple pacing maneuver in patients in whom the right pulmonary vein was not isolated by circumferential ablation, and clearly separated EC from the gap of circumferential lesion. 14 Their method may provide an alternative solution in cases which ECs are present but difficult to depict before ablation.
Furthermore, even in cases with EC, PV isolation was achieved by circumferential ablation only. This is probably because the circumferential lesions were proximal enough to PV to achieve isolation by ablating near the LA insertion of the intercaval bundle.
Relationship between EAT, aging, and EC
Our study suggests two potentially important contributors to the absence of a right-sided EC of the PV. The accumulation of periatrial/intercaval EAT and advancing age were significantly associated with the absence of an EC. There are several reasons why these factors were involved. The EAT is a metabolically active organ that secretes adipocytokines such as adiponectin, tumor necrosis factor-α, interleukin 6, monocyte chemoattractant protein-1,15 and Activin A.9 As there are no fascial boundaries between the EAT and the myocardium, these adipokines may have local pro-fibrotic and pro-inflammatory effects that facilitate fibrosis on the adjacent atrial myocardium. Indeed, the secretome from human EAT has been shown to induce fibrosis of the atrium in an organo-culture model of rat atria.9
Furthermore, in an experimental animal study conducted by Mahajan et al.,11 persistent obesity was associated with significant myocardial fatty infiltration, EAT accumulation, and interstitial fibrosis with subsequent slowing of atrial conduction velocity. Such direct fatty infiltration and fibrosis separating myocytes could directly result in the slowing of regional conduction in a manner similar to microfibrosis.16 The conduction of the intercaval bundle, which connects the RA to the right PV on the epicardial side, is assumed to be blocked by infiltration and the local effect of adjacent EAT. A clinical study conducted by Hanaki et al. demonstrated that shorter interatrial distance between the posterior wall of the RA and the right PVs was associated with the necessity for carina ablation.17In their study, the amount of EAT was not measured, but the fact that the interatrial distance was short could be assumed to mean that the EAT was small. This result seems to support the present study, which showed that a small amount of EAT can affect the presence of ECs.
Aging has also been shown to change electrical properties. A clinical study demonstrated aging to be associated with increased atrial conduction time.18 A human study conducted by Spach and Dolber also demonstrated the age-related development of extensive collagenous connective tissue that separates small groups of fibers and subsequently decreases atrial conduction.19 Hence, the age-related development of atrial fibrosis is an important contributor to the slowing of epicardial atrial conduction in elderly patients.