Discussion
The strategy of in situ LIMA grafting to the LAD (LIMA-LAD) is considered the “gold standard” of coronary revascularization[6].However, in some circumstances, such as the stenosis or occlusion of the LIMA and harvested damage, surgeons must choose other conduit materials to revascularize the LAD. SVG are still widely used because of their several advantages, including ease of access, ease of operation, sufficiency of length for transplantation, and short harvest time[7,8]. There is still no consensus on whether SVG grafted to the LAD could improve the flow compared with LIMA graft. Some studies have suggested that SVG grafted to the LAD has a higher blood flow (up to 35%) than IMA grafts, but other studies have suggested that there is no significant difference between SVGs and IMAs in terms blood flow[9,10]. This study similarly showed that usage of SVG to bypass to the LAD has the advantage of higher MGF and a lower value of the PI compared with LIMA during operation. The possible explanation may be that SVGs have larger diameters and are often not be affected by vasoactive drugs and neurohumoral fluids compared to arterial conduits. However, the explanation that SVG anastomosed directly to the ascending aorta with higher pressure and a higher low-gradient can cause a larger mean blood flow volume still lacks of assertive evidence. Previous studies have shown that the flow of the LIMA graft in Chinese people is lower than that in western people which may be related to the fact that the diameter of IMA graft in Chinese people is smaller[11]. Our previous study also found that the intraoperative blood flow of an in situ IMA graft was close to the blood flow in other related research results, but the flow of the LIMA would increase significantly one week postoperatively, which was considered to be related to the intraoperative use of vasoactive drugs and self-regulation[12].
Previous studies have demonstrated that studies on TTFM should consider arterial versus venous grafts and different coronary territories[5]. However, owing to the high patency, the use of the LIMA for LAD grafting has been a cornerstone of CABG surgery; thus, few studies have compared the TTFM parameters of different conduits used for LAD. Therefore, we aimed to compare the TTFM parameters of different conduits used for LAD revascularization.
The PI, calculated as (maximum flow volume-minimum flow volume)/(mean flow volume), is one of the TTFM measurements parameters that used for conduit evaluation during operation[13].The results of Di Giammarco et al study showed that the PI>5 may be an independent risk factors of graft dysfunction. Higher PI values indicate that there may be greater negative flow or lower average flow[14].The results of this study demonstrated that the PIs of the LIMA-LAD group are higher than those of the SVG-LAD group(P<0.001) both before and after PSM. We also found that in this study, the proportion of negative flow less than 10ml/min was larger in the LIMA-LAD group (P < 0.001), suggesting that there was more negative blood flow, i.e., competitive flow, in the early systolic in the arterial conduit group. In contrast, the venous conduits have less smooth muscle, low elasticity and small cyclical deformation of pipe diameter with pressure. Therefore, it is impossible to accommodate the reverse flow by adjusting the diameter of the conduits, and the probability of the occurrence of competitive flow may be relatively low[3]. Whether the state of perfusion of arterial conduits is delayed compared with the vein conduits,which caused the different pressure perfusion, need further studies to prove it.
In this study, 5 patients whose LAD was revascularized by both SVG and LIMA due to the multiple stenosis of LAD. The revascularization of LAD is of vital importance in the treatment of CAD, and the treatment of diffuse disease of LAD are still a major challenge for cardiologists and cardiac surgeons. The 5 cases that LAD was revascularized by two grafts including SVG and LIMA all had multiple diffuse lesions and the lesion length was over 2cm. Previous studies showed that the failure rate of the diffuse disease of LAD treated by PCI was about 40-50%。Traditional endarterectomy is complicated, time-consuming and involves many uncertain factors. Incomplete dissection caused by endarterectomy, especially incomplete distal dissection of diseased vessels, will seriously affect the blood flow of bridge vessels after bypass surgery and increase the incidence of postoperative complications [15]. There was no difference between SVG-LAD and LIMA-LAD in terms of TTFM parameters in the 5 cases. Two of the five cases, the myocardial injury markers exceeded the perioperative myocardial infarction criteria within 72 hours, which may be related to poor LAD conditions. However, the results of post-operative CTA showed that all of the grafts of the 5 cases were patent.
Previous studies have shown that competitive blood flow can still be found in both the grafts of LAD and DIAG even if both the LAD and DIAG have severe stenosis. There are 6 cases that received both SVG-LAD and LIMA-DIAG revascularization. In the 6 cases, no negative waveforms (competitive blood flow) were found in SVG-LAD grafts at the early stage of contraction and the PI value of SVG-LAD were all below 3.1. The reason that the LAD was revascularized with SVG was that the poor conditions of LAD(diffuse disease) and the DIAG was relatively large with more value of revascularization. However, there was no difference between the SVG-LAD and LIMA-DIAG in terms of TTFM parameters in the 6 cases.
Owing to some patients do not have symptoms or clinical signs of myocardial ischemia prior to discharge, few studies about the acute asymptomatic graft failure have been reported and graft failure rates remain unclear[16]. However, early asymptomatic graft failure may have negative impact on the patients’ short- and long-term outcomes and develop symptoms when exercise increase, because the relevant myocardial area are still unsupplied[17-19]. Cardiac CTA, as a low-invasive investigation method for the evaluation of the early grafts has been proved to be another choice besides coronary artery angiography in several studies[19, 20].CTA examination, as a part of the graft quality evaluation study in our center, was routinely used in patients who underwent CABG prior discharge. In our study, the early patency before discharge of SVG-LAD are comparable with that of the LIMA-LAD; however considering the long-term patency, the LIMA were recommended over SVG.