Limitations 
This study has the following limitations: first of all, there is no etiological classification of AVD during the analysis (rheumatism, degeneration, congenital, ischemic, infection and trauma, etc). Although in our center, the vast majority of patients underwent mitral valve surgery because of rheumatic changes, it is difficult to determine the pathological type of secondary aortic valve disease. Previous studies have shown that pathological types have different effects on the progression of the disease. Bahler RC [32] thought that the progression of AVD was positively correlated with the degree of valve calcification, for example. Wagner S [33] also pointed out that, in degenerative-calcific AS, valve stiffness and calcification are the primary factors affecting the progression of valve disease, and observed that the progression of congenital and rheumatic AVD is slower than that of degenerative diseases.
Another limitation is that 84 patients have been excluded from the statistical process because they were followed up for less than 2 years, and the outcome (death or other causes) of these patients could not be determined. Baria K et al [7] found that it would take about 14.1 years to undergo a secondary operation for patients with severe AVD after mitral valve surgery. However, the follow-up time of this study was just 4.3±2.1 years. Data that may contribute to the results have not been observed.
The above reasons may lead to some deviations in the research results. To obtain more accurate results, it is necessary to further expand the data content and research cohort for follow-up research.
Conclusion  
It has become a common practice to perform aortic valve replacement in symptomatic patients with severe AVD identified by transthoracic echocardiography. The guideline [34] states that it is reasonable for patients with moderate AR or AS (aortic velocity 3.0-3.9m/s) in stage B to perform aortic valve replacement in conjunction with other cardiac surgery (class IIA of recommendation, level C of evidence). However, considering that double-valve surgery will prolong the duration of cardiopulmonary bypass and increase the risk of complications, prophylactic aortic valve replacement may not be recommended for mild AVD. Vaturi M et al also pointed out that mild AVD during mitral valve surgery rarely progresses to moderate or severe, even after a longer follow-up period [6, 35]. Combined with the results of this study, it is suggested that patients with mild to moderate AVD, high peak velocity of the aortic valve (>1.7m/s) or small size left ventricular (LVD<35mm) should be considered as potential candidates for rapid progression of AVD, which need cardiotonic and diuresis treatment to reduce the load for a long time after the operation and closely followed up. Although this option may increase morbidity and mortality in a very small number of patients, it avoids the risk of double valve surgery for most patients. At the same time, it is suggested to relax the indications for simultaneous tricuspid valve repair during mitral valve surgery. In contrast, the selection of valve size should be as accurate and conservative as possible according to the patient’s condition to avoid valve mismatch. Furthermore, in order to prove the accuracy of the above results, it is necessary to expand the cohort for further study.
Statement of Ethics The study was approved by the Ethics Committee of the Zhujiang Hospital of Southern Medical University, with waiver of informed consent.
Conflict of Interest Statement
The authors have no conflicts of interest to disclose.
Funding Sources
There were no sources of funding.
Author Contributions
All authors fulfil the ICMJE criteria for authorship. ZY.S., YS.Y., and H.L designed the study. ZY.S., YN.L., and YY.Y were responsible for acquisition and analysis of data and drafting the paper. SB.L., YS.Y., and H.L were responsible for interpretation of data. YS.Y., and XM.Z critically revised the paper. All authors have read and approved the final version of the paper and take responsibility for the work.