Discussion
BAV related aortopathy has been a controversial issue in the past few years. Most of the researches focused on BAV with dilated ascending aorta, but there were limited documents of patients with normal-sized proximal aorta. In this current study, the following main findings were described: 1) ascending aortic dimensions at baseline were larger in BAV patients than in TAV patients, but growth rates of aortic size in a follow-up of 7 years were comparable between two groups after AVR ; 2) the incidence of adverse aortic events were very low in BAV patients with normal-sized proximal aorta after AVR, which was not different from TAV patients; 3) ascending aortic dimension at baseline was a significant predictor of progression of ascending aorta .
Although the exact pathogenesis of BAV related aortopathy is not yet well elucidated, there were two main hypotheses being widely accepted: hemodynamic and genetic causal factors. Robicsek and colleagues first described the patterns of transvalvular flow in BAV patients.10 After that, many clinical and basic researches focused on the role of valve related hemodynamics and found consistently eccentric turbulent transvalvular flow in non-stenotic or regurgitated BAV patients.11-14 BAV related hemodynamic factors, such as tensile and shear stresses, play an important role in dilation of ascending aorta. In an ex-vivo model, Juraszek and colleagues found significant pressure differences in various locations of ascending aorta in BAV compared with TAV, which may contribute to BAV related aortopathy.15 On the other hand, some studies have found the ascending aortic wall of BAV patients is intrinsically different from that of TAV patients.16-19 Due to significant lack of fibrillin-1, vascular smooth muscular cell would detach from elastin and collagen, leading to apoptosis and loss of structural integrity.3 Several genes mutations have been reported in BAV patients, including that involved in cell growth, differentiation, and matrix deposition and apoptosis.20, 21 There is still no consensus on the roles of these two factors. The two different hypothesis lead to different surgical strategies for the ascending aorta in BAV patients: the former may lead to a relatively conservative strategy, it’s believed that the ascending aorta will not further progress after disappearance of hemodynamic factors due to AVR; however, the latter may bring out a more aggressive strategy.
In the present study, we found a larger ascending aorta at baseline in BAV patients compared to TAV patients. However, there was a comparable progression of ascending aortic dimension in both groups in a relatively long term follow up period. Most of the patients in our study groups didn’t experience severe ascending aortic dilatation. In accordance with previous studies, there was a considerable low risk of adverse aortic events in our population.9, 22, 23 Mayo published a large series of cases with an average follow up of 12 years after AVR, there was a very low rate (1.9%) of aortic events in BAV patients.24 Meanwhile, many clinical basic studies have found that the transvalvular flow patterns are significantly abnormal in BAV patients, which is obviously related to ascending aortic dilation.11-14 According to the present study and other literatures, we thought that the hemodynamic factor may be the main pathogenesis of BAV related aortopathy.
We didn’t evaluate per year growth rate of the ascending aorta in BAV group, but it seems that there was no significant aortic dilation during follow-up in patients with normal ascending aorta at baseline after AVR. Valentina Agnese and colleagues found a growth rate of 0.5mm per year of ascending aorta in BAV patients, and the growth rate per year was high in the first two years then decreased steadily.22However, all included patients in their study didn’t undergo AVR. A correlation between artery hypertension and aortic dilation was observed in some research,9, 25 but the correlation was insignificant in our study, that perhaps due to a more precise control of artery hypertension after AVR in our patients. Notably, we found ascending aortic dimension at baseline was a significant predictor of progression in ascending aorta. Further prospective study should be conducted to confirm this finding.