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.