Discussion
This was a prospective observational study of the characteristics of the
ACZ in patients with AF using the databases from two centers. The main
findings were:
(1) the ACZ was observed in 95% patients, and all ACZ was linearly
distributed,
(2) the ACZ was most frequently observed in the anterior wall region
(77%),
(3) a longer ACZ was significantly associated with a larger LA size and
a prevalence of non-PAF,
(4) a larger LA size was associated with a higher overlapping rate of
LVA and ACZ, and
(5) a larger LA size was not associated with a higher overlapping rate
of CoAs and ACZ.
With the development of the 3-D
mapping system, there has been significant focus on the conduction
pattern of the LA. Recently, Yamaguchi et al reported that a difference
in bipolar voltage mapping between ring catheter and HD
Grid.14 They stated that the HD Grid can create a
voltage map that is independent of propagation direction and can assess
more precise LA conduction pattern. To date, several studies have
reported on the conduction abnormalities of the LA. However, the
conduction abnormalities varied between patients, and there were various
methods of mapping and different definitions of the ACZ in each study.
A limited number of studies have reported on conduction abnormalities at
the LA in humans. Markides et al studied the LA activation during SR in
19 AF patients with noncontact mapping system. They found that line of
conduction block running vertically in the LA posterior wall was
observed in all patients. Moreover, they reported that the anterior line
extending from the anterior mitral valve annuls toward the left atrial
appendage was observed in 9 patients (47%).15 Mouws
et al investigated the conduction abnormalities during sinus rhythm only
at the LA posterior wall using epicardial mapping in 268 patients who
underwent cardiac surgery.6 They showed that
conduction abnormalities occurred in 90% of patients. Moreover, in
their study, AF episode was associated with a higher incidence of
conduction abnormality, larger number of lines, and longer lines.
Roberts-Thomson et al reported the conduction properties of the
posterior LA during SR using epicardial mapping in 34 patients who
underwent cardiac surgery.16 In their study, 5
patients (15%) had an ACZ running vertically down the posterior LA. The
results of our study showed that the ACZ was observed in 95% patients,
and all ACZ was linearly distributed, similar to the results of previous
studies. On the other hand, the prevalence of the ACZ was higher in our
study than those in previous studies although our study did not include
patients with structural heart disease. This discrepancy in findings
might be due to different patient inclusion criteria, methods of mapping
(epicardial or endocardial mapping), mapping catheter, and definition of
the ACZ. However, very few studies have focused on the ACZ using the HD
Grid. Further investigation on the ACZ is warranted.
In our study, the ACZ was associated with a non-PAF, BMI, hypertension,
and LA diameter/BSA. These factors are well known predictors which
increase the prevalence of AF. It has also been established that non-PAF
increases predisposition to LA remodeling in a so-called “AF begets
AF” manner. A recent study showed
that patients with AF had a higher incidence of conduction abnormality,
larger number of lines, and longer lines than those without
AF.6 These findings support our results indicating
that existence of ACZ is associated with non-PAF, BMI, hypertension, and
LA diameter/BSA. Moreover, we found that a longer ACZ was significantly
associated with a larger LA size. The association between LA size and
ACZ yielded mixed results in previous studies. Roberts-Thomson et al
reported that the line of conduction delay was most marked in conditions
associated with greater LA enlargement.16 In contrast
to our results, the other study showed that LA dilation was not
predictor of long line of conduction abnormalities.6Furthermore, their patients had a larger LA size compared with our
patients because they included structural heart disease such as valvular
heart disease. This discrepancy in findings may be explained by the
different baseline LA size.
It is well-known that the ACZ plays an important role in atrial
tachyarrhythmias, however, the causes of the ACZ are still unclear. We
considered the following mechanisms of the ACZ. First, the ACZ might be
the result of an abrupt change in myocardial fiber direction. A previous
study showed that all hearts have subendocardial fibers, and the
longitudinal fibers were arranged in broad bands. Moreover, they found
that the most obvious broad band of longitudinal fibers was formed by
the “septopulmonary bundle”.17 Markides et al also
reported that abnormal conduction was associated with a change in these
myocardial fiber direction.15 In fact, the location of
the ACZ shown in Figure 1 matches the septopulmonary bundle at
the anterior wall. On the other hand, the incidence of the ACZ in the
posterior wall in our study was lower than in the previous study. This
discrepancy in the incidence of the ACZ may be explained by the
different activation flow. In our study, the activation in the posterior
wall was activated vertically from the roof and bottom because mapping
was performed during high right atrial pacing (Supplementary
video ). Therefore, the incidence
of the ACZ in the posterior wall was low in our study, and the pacing
from different site might have led to the different distribution of the
ACZ. Second, the compression from the external organs might induce the
ACZ. Mayyas et al found that endothelin-1, which promote myocyte
hypertrophy and interstitial fibrosis, is an important factor in AF
development and persistence.18 They also mentioned
that greater wall stress may produce higher levels of endothelin-1
because this factor is stretch
mediated. Based on this report, we
hypothesized that externally-mediated wall stress leads to regional
differences in the ACZ. In our study, the overlapping rate of LVA (mild
and moderate) and ACZ was higher than that of CoAs and ACZ. Therefore,
the ACZ is associated with an LVA rather than a CoAs. These findings
indirectly suggest that the ACZ might be a precursor to LVA.