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.