Quantitative Measurements of Competitive Flow in Left Internal Mammary Artery Bypass Grafts

OBJECTIVE: A low flow rate with diminished wall shear stress in arteries can cause intima hyperplasia and diameter reduction. In coronary artery bypass grafting (CABG) a low flow rate through the graft can cause graft failure (i.e. the string sign effect and occlusion). Arterial grafts bypassing an intermediate or non-significant stenosed coronary artery show higher incidence of graft failure, implicating the importance of competitive flow from the coronary artery. Therefore this experimental study quantifies flow rates, with and without flow competition, for the left internal mammary artery (LIMA). METHODS: 30 Patients undergoing off-pump CABG using LIMA-grafts to bypass left anterior descending (LAD) arteries were included. The LIMA-graft flow rate was measured intra-operatively with Transit Time Flow Measurements (TTFM). Competitive flow was assessed by measuring graft flow rate with and without closing of the LAD. To research the effect of the severity of coronary artery stenosis, patients were divided into three groups based on percentage of LAD-stenosis; Group 1: 70%. RESULTS: There were significant differences in mean flow rate, pulsatility index, back flow, peak systolic flow and peak diastolic flow (Table 1), between LIMA-graft flow measurements with the LAD open and closed. 12 patients (40%) had a LIMA-graft flow rate of 20 ml/min after closing of the LAD in 9 patients. Group 3 had a significantly higher LIMA-graft mean flow rate compared to group 1 (40 ± 29 ml/min vs. 22 ± 10 ml/min, p = 0.01) and group 2 (40 ± 29 ml/min vs. 20 ± 8 ml/min, p = 0.02). CONCLUSIONS: LIMA-graft conditions, as previously described in literature, were more favourable when flow competition was absent. Competitive flow from the target coronary artery decreases LIMA-graft flow rate and can be a cause of a low flow rate, possibly resulting in graft failure. Patients with an intermediate or non-significant coronary artery stenosis show more flow competition and significantly lower graft flow rate