4. DISCUSSION
In the present study, we examined the predictors of post-operative AA,
including AF and AT occurring after mitral valve repair for treating MR.
We found that: 1) 9% of patients who underwent mitral valve repair for
treating MR met AA in the post-operative follow-up period, 2)
the aorto-mitral angle and the CPB
time were independent predictors of the post-operative AA and 3) the
patients with post-operative aorto-mitral angle ≤ 117 degrees had a
higher rate of post-operative AA than patients with the post-operative
aorto-mitral angle > 117 degrees.
New onset AA, including AF and AT, is the common arrhythmia after mitral
valve repair in patients with MR, and peri-operative and post-operative
AF have been reported to be associated with long-term all-cause
mortality.1,8 Therefore, the prediction and prevention
of AA after mitral valve repair in patients of MR can contribute to
improving their post-operative prognosis. Our results revealed that the
aorto-mitral angle is an independent predictor of post-operative AA. A
previous report indicated that a small aorto-mitral angle after mitral
valve repair for MR was an independent predictor of systolic anterior
motion leading to left ventricular outflow
obstruction.9 However, there are very few reports
about the relationship between the aorto-mitral angle and AA we present
in this study. A previous study has shown that post-operative
aorto-mitral angle is associated with the reduction of the LA diameter
after mitral valve repair, and LA enlargement is an important risk
factor for the incidence of late AF.3,10,11 The small
aorto-mitral angle represents the tilting of the mitral annular plane
and may functionally narrow the diastolic mitral orifice (Figure 4),
which is considered to act on a similar principle to that of functional
mitral stenosis after mitral valve repair for ischemic functional
MR.12,13
In the present study, high TMPG (> 3mmHg) was independently
associated with small aorto-mitral angle. Consequently, we hypothesized
that a small aorto-mitral angle functionally narrowed the diastolic
mitral orifice and increased the TMPG, resulting in the progression of
LA dilatation and the occurrence of the AA. However, neither
post-operative mean TMPG nor LA diameter was directly associated with an
AA event in our present results. Exercise stress tests can diagnose
patients with subclinical functional MS after mitral valve repair by
determining the effective orifice area index and pulmonary arterial
pressure that are associated with exercise
capacity.13,14 Otherwise, assessments of follow-up
echocardiographic data may be required to prove the association among
the aorto-mitral angle, TMPG, LA size and AA events. Further studies
will be needed to address this issue.
Various studies have reported that mitral valve repair with ring
annuloplasty affects the angle between the planes of the mitral valve
and the aortic valve. Some previous studies have reported that the
implantation of an undersized ring was associated with a small
aorto-mitral angle and a great TMPG.15,16 Avoidance of
the selection of an undersized ring may be useful in preventing a small
aorto-mitral angle leading to the occurrence of post-operative AA
events. A previous study has shown that mitral annuloplasty with a
C-shape band leads to a smaller aorto-mitral angle than those with a
full ring.3 The selection of the appropriate size and
type of an artificial ring can prevent a constrictive aorto-mitral angle
and the occurrence of AA events. Another previous study reported that
mitral valve repair with LA plication can also prevent the narrowing of
the post-operative mitral-septal angle, which is a parameter possessing
a pathological significance similar to that of the aorto-mitral
angle.17