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