Surgical closure of VSDs includes conventional surgery with CPB and minimally invasive closure of transthoracic VSDs under the guidance of TEE. Using standardized surgical techniques and postoperative care management, the outcomes of both surgeries are excellent, with mortality and morbidity rates approaching zero in almost all centres internationally. However, as one of the major complications, RSs are not as rare as we would like to believe in these two surgeries.
RSs, which are an important indicator for the surgical treatment of VSDs, have attracted increasing attention. The incidence of RSs after CPB reported by Deng was 31.2%, and an incidence of 35% was reported by Dodge-Khatami. The incidence of RSs after transthoracic device closure of VSDs was 15.8%, as reported by Ren C. In our series, the rates of RSs after CPB and periventricular device closure were 19.6% and 18.4%, respectively, which were similar to other reports.
According to the relevant literature and our experience, the reasons for postoperative residual shunts in the CPB group are as follows. First, during the operation, the margin of the VSD was not clearly exposed, and the suture was not tight and formed a gap. Second, the suture at the corner of the VSD was not well gap uniformity. Third, the patch was not perfectly clipped, and a wrinkle was formed. Fourth, the suture was shallow, the myocardium was avulsed, and an RS formed. In the MIC group, the main factor associated with RSs was whether the shape of the occluder fit the VSD. Most of the time, the margin of the VSD was bumpy and irregular, even though the tricuspid chordae tendineae was crossed, which would prevent the occluder from covering the entire VSD.
The Kaplan-Meier plots (Figure 1) showed that in the CPB group, for those patients with small RSs, the curve slumped faster than that of those with moderate RSs (P=0.034), but the groups tended to reach an approximate closure rate 2 years after discharge, which means that small RSs were more likely to undergo spontaneous closure, so there were no differences in the rate of spontaneous closure at the end of the follow-up (P=1.000). Another Kaplan-Meier plot (Figure 2) showed the same result in the MIC group, but one difference compared with the CPB group was that the moderate RS group tended to have difficulty undergoing spontaneous closure (p=0.045), which was hard to explain. In both groups, we found that factors including age, weight, sex, and VSD size showed no significant differences between small- and moderate-sized RSs, which indicates that RS closure was not connected to the above factors. An interesting finding of our study was that more small RSs occurred in the MIC group than in the CPB group because the occluder could be adjusted under the guidance of TEE (Table 4). As shown in Table 4, we found that factors including age, weight, sex, and VSD size were not significantly different between the two groups, which indicates that the size of the RS generated in the operation was not connected to the above factors. We also found that the spontaneous closure rate at 1 year and 2 years post-operation were not significantly different between the two groups, and the spontaneous closure rate at 1 year post-operation was approximately 70% in both groups and approximately 90% at two years (Figure 3).
The mechanism of VSD spontaneous closure is still not clear and may vary with the site of the defect, although many hypotheses have been proposed. Roberts speculated that closure of the RS resulted from endocardial proliferation, probably stimulated by turbulent blood flow through the RS, and RS would not closure without the superimposition of an active inflammatory process. Anderson suggested that spontaneous closure involved an adherent tricuspid valve leaflet, tricuspid leaflet tissue tags, subaortic tissue tags, prolapsed aortic valve leaflets, or a combination of these factors. Takaki et al demonstrated that the jet stream could create a tricuspid pouch, which was formed by the left-to-right shunt flow jet towards the tricuspid valve, followed by deposition of fibrin over the margins of the defect and adhesion of the septal leaflet of the tricuspid valve to the margin of the VSD.
We performed a Cox regression analysis to identify factors associated with spontaneous closure. The only variable we found to be significant was the size of the RS on initial postoperative echocardiography, which is supported by findings from other groups. Age, weight, sex, surgical technique, preoperative VSD size and shunt velocity were not significant factors. Although shunt velocity was reported to be a significant factor in RS spontaneous closure, it was not possible to draw this conclusion from our data. Some reports also reached this conclusion. Some reports[14, 15] proposed that age has a significant influence on the incidence of spontaneous VSD closure, while in our series, because of our criteria, the age span may not be large enough. Additional research should include patients of different ages, especially young patients.
In terms of limitations, this study was a retrospective review with all the inherent deficiencies of such studies. This was a retrospective single-centre study, and the sample size was small; thus, the results may not be applicable to the population as a whole. The type of VSD was limited to perimembranous VSDs, and the age span was not large enough.
In conclusion, similar to other procedures, MIC of transthoracic VSDs can also lead to RSs. We compared conventional surgery with CPB and MIC and found that the RS incidence and spontaneous closure rates were not significantly different. Small RSs were more likely to undergo spontaneous closure in both groups.