Discussion
The bilateral IMAs are relatively superficial and in relatively fixed
positions. Because of the relatively high peripheral vascular resistance
before the operation, the blood flow of IMA demonstrated a triphasic
flow spectrum with high systolic velocity and low or no diastolic
velocity. One of the patients included in the postoperative follow-up
had intraoperative rupture of the initial segment of IMA, but the
remaining patients were able to undergo the measurements of luminal
diameter of the initial segment of IMA and flow spectrum. Previous
studies have shown that the success rate of exploration of initial and
thoracic segments (the second intercostal space) of IMA after CABG is
>99.5% [5], and the changes in the
hemodynamics of the vascular grafts may indicate abnormal function of
these vascular grafts, with sensitivity and specificity of 100% and
98.4%, respectively [6]. The spectrum of the left
IMA changes from a triphasic form to biphasic form to supply the
coronary vascular bed with low resistance after surgery, with a
significant increase in diastolic velocity and significant decrease in
systolic velocity and RI [7]. In addition, the
diastolic component of flow spectrum increases gradually from the
initial segment of IMA to vascular graft anastomosis[8, 9]. In this study, the postoperative PSV and
PDV increased significantly (P < 0.05). The differences
between the pre and postoperative PSV and PDV may be related to the
intraoperative treatment of IMA and postoperative assessment of the
location of IMA. In addition, the postoperative dVTI and dVTIF were
significantly increased. These results were consistent with the findings
of previous studies. Biceroglu et al. [10]conducted a three-year follow-up study of 38 patients who underwent CAGB
and found that approximately 18% of patients had collateral vessels in
the left IMA, with vascular diameters similar to the luminal diameter of
IMA and competitive flow between the collateral vessel and vascular
graft [10]. In addition, the ossification of IMA
alone affects the flow of vascular grafts [11]. In
this study, all patients had pedicled left IMA, ligation of all
collateral vessels, and in situ bypass of left IMA to left
anterior descending coronary artery. The surgical procedures were
performed by the same surgeon. Because of the free IMA and apparent
parasternal gas interference, we selected the supraclavicular region to
evaluate the initial segment of IMA postoperatively. An abnormal flow
spectrum strongly suggests abnormal vascular grafts and warrants
consideration of a timely clinical intervention.
In this study, indicators of left ventricular function (such as EDV and
SV) showed no statistically significant differences. A short period of
myocardial stunning, which is reversible, was found after ischemic
myocardial reperfusion. It takes a long time from reperfusion to full
recovery of myocardial function after CABG. Lin et
al.19] reported the critical point to be 6 months
postoperatively. The postoperative PDV, D/S, and dVTIF showed no
statistically significant effect on the flow of vascular grafts. Takagi
et al.[12] showed the postoperative D/S of initial
segment of IMA to be >0.6 and the dVTIF >0.5,
indicating good functions and flow of vascular grafts. Other studies
have shown that severe stenosis in anastomosis between left IMA and left
anterior descending branch occurs when the blood flow S/D
>1, with sensitivity and specificity of 100% and 85%,
respectively [13]. By comparing the results of
coronary artery angiography and ultrasound to evaluate the function of
vascular graft, researchers have found that when the postoperative PDV
is <35.95 cm s−1, the probability of
functional abnormality of the vascular graft increases by 34.19-fold[6]. The related parameters in this study had no
significant effect on the flow of vascular graft, suggesting that
although some patients had multiple coronary vascular lesions and
different degrees of stenosis at the distal end of the anterior
descending coronary artery, patients with a >50% degree of
stenosis had detectable changes in the ultrasonic spectrum. In addition,
their arteries had a certain flow reserve capacity, and their vascular
flow and flow spectrum of vascular grafts would change after exercise
and pressure load [14, 15]. Therefore, patients’
medications and exercise states may also have an impact on the study
results. This matter requires further investigation.
Wu et al. [16] conducted a flow distribution study
and showed that when the pressure in a Y-shaped connection pipe remained
unchanged, the flow distribution of gas-solid biphasic fluid was related
to the angle between the movable branch and main pipe and to the flow
rate of the main pipe. In addition, the changes in angle had a
pronounced impact on the flow distribution. However, these results have
not been confirmed in medical research. In this study, clear differences
in angle between the IMA and the subclavian artery (101–156°) were
found among individuals. Logistic regression analysis revealed that the
angle between the IMA and the subclavian artery was negatively
correlated with intraoperative flow rate, suggesting that the angle may
be an independent factor affecting the flow of vascular grafts. This
suggests that the angle between the IMA and the subclavian artery afterin situ bypass in the left breast may be an independent factor to
affect the flow after CABG.
In this study, the instantaneous volume blood flow and pulsatility index
of the vascular graft were intraoperatively measured using a coronary
blood flow instrument. The flow of the vascular graft at 1 week
postoperatively was calculated according to the luminal diameter of the
initial segment of vascular graft measured by ultrasound. Through the
paired t-test, we found the flow to be significantly increased at 1 week
postoperatively, and the luminal diameter of the vascular graft was
widened postoperatively (P = 0.002). Tagusari et al.[17] found that the luminal diameter of vascular
graft was widened by 1.43-fold, and the flow of vascular graft was
increased by 4.18-fold two weeks after CABG. In addition, the short-term
and low-term postoperative blood flow had no significant difference[14]. Nasu et al. [18]compared the results of coronary angiography and Doppler ultrasound
velocimetry and showed that mild to moderate stenosis in the proximal
coronary arteries of CABG patients was caused by the presence of
competitive flow. The flow of vascular grafts was lower than the blood
flow of patients with severe stenosis. Therefore, if abnormal flow of
the vascular graft is observed in patients during a postoperative
follow-up and competitive flow can be ruled out, functional abnormality
of the vascular graft should be considered and a timely clinical
treatment should be applied.