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.