Möglichkeiten und Grenzen der intraoperativen Bypassevaluation mittels Transit-Time-Dopplerflussmessung

The transit time flow measurement (TTFM) with the MediStim VeriQ System is a well-established, easily reproducible method that has been used for over 20 years for intraoperative quality control of bypass grafts. In the meantime, there are attempts to increase the predictive power of TTFM to 100% with complementary methods such as epicardial ultrasound (ECUS) (Taggart D, 2020). All in all, this method represents a great help for decision making for experienced coronary surgeons. TTFM showed a correlation of better bypass flow and lower pulsatility index (PI) with higher grade of native coronary stenosis of same coronary territories. This can also be expected in patients, where the indication for bypasses was done after morphological quantification of the stenosis by the coronary angiography only. A statistical significance of the comparison between stenosed and occluded coronary vessels was only found for the PI in LIMA–LAD grafts (stenosed 2,34±1,02 ml/min und occluded 1,67±0,49 ml/min, p=0,036) and in vein to RIVP bypasses (stenosed 3,08±1,66 ml/min und occluded 1,45±0,79 ml/min, p=0,01), but not in vein to RCA bypasses. The mean blood flow in the LIMA-LAD bypass was lower in stenosed 54,2± 29,2 ml/min vs. occluded 67,2± 25,4 ml/min native vessel, but did not reach statistical significance (p=0,08). A significant dependence of the MBF (p=0,019) and the PI (p=0,022) from the degree of native LAD stenosis was observed for the LIMA-LAD bypass. The TTFM data from our patients showed no gender difference in hemodynamics, both in the total MBF (female 155±56 ml/min, male 179±79 ml/min, p=0,202), LIMA-LAD bypass flow (female 49±32 ml/min, male 57±28 ml/min, p=0,098) and PI (female 2,28±0,87, male 2,31±1,02, p=0,756). Similarly, LVEF and BMI have no significant influence on TTFM parameters. Underlying degree of coronary complexity was the only preoperative characteristic that significantly influenced the total bypass flow (p=0,005). We could not prove the hypothesis that a higher total bypass blood flow in the ischemic areas decreased postoperative troponin or CK-MB levels nor increased postoperative blood clearance. For that we took the blood values from the first postoperative laboratory and the laboratory values of the first, second, third and fourth postoperative day as measuring points. For this purpose, the data were additionally divided into 2 groups, once in 2 groups of patients where the flow per anastomosis was 25 ml/min and >25 ml/min, and in 2 groups of patients where the flow per anastomosis was 40 ml/min and >40 ml/min. As limit values for this division, we took the "cut-off" values for the MBF for the LIMA-LAD bypasses and for the venous bypasses. The statistical evaluation was performed as a general linear model with repeated measurements. The threshold values were taken as inter-subject factor, the total bypass flow and flow per anastomosis as covariates. In the whole collective, statistically no significant dependence of the postoperative CK-MB course on the total bypass flow (p=0.693) and on the flow per anastomosis (p=0.863) was found. Similar findings were seen for the troponin course (p=0.325 and p=0.754, respectively). There was no statistical difference between the "25 ml/min" groups and the "40 ml/min" groups in the group division for both marker courses. Also, the maximum levels of troponin (singular bypass group 8229 pg/ml, sequential bypass group 9459 pg/ml, p=0,62) and CKMB (singular bypass group 54,3±78,8 U/l, sequential bypass group 50,5±61,7 U/l, p=0,75) were not affected by bypass flow and coronary revascularization technique. The comparison of coronary revascularization techniques showed no superiority of singular (SBG) versus sequential bypass construction (MBG) and vice versa. In our patient collective we had more distal anastomoses in the sequential bypass group (SBG 2,72±0,59, MBG 3,97±0,83, p<0,001), more proximal anastomoses in the singular bypass group (SBG 1,67±0,61, MBG 1,39±0,69, p=0,05) and more use of the right mammary artery in the sequential group (p=0,002). The bypass flow was higher but without statistical significance in the MBG group (MBG 182±79 ml/min, SBG 163±70 ml/min, p=0,14), the flow pro anastomosis was statistically higher in SBG group (SBG 61±25,7 ml/min, MBG 46,4±19 ml/min, p<0,001), the flow was strongly dependent on the number of distal anastomoses (SBG R2=0,12, p=0,004, effect size by Cohen f=0,37 und MBG R2=0,095, p=0,02, effect size by Cohen f=0,31), the maximum cardiac enzymes were not influenced by the total bypass flow (SBG R2=0,002, p=0,68 and MBG R2=0,005, p=0,57), the flow pro anastomosis (SBG R2=0,005, p=0,58 and MBG R2=0,004, p=0,603) or by the number of distal anastomoses (SBG R2=0,008, p=0,47 and MBG R2=0,002, p=0,77). This makes both techniques equal from the TTFM point of view. According to our TTFM data, the resistance in the bypasses turned out to be a variable that has a very strong influence on the bypass flow, regardless of the preoperative patient characteristics. This applies to all grafts (LIMA-LAD R2=0, 606, p<0,001, effect size by Cohen f=1,25; left radial artery (LRA)–RCX R2=0,461, p=0,011, effect size by Cohen f=0,92, vein-RCX R2=0,549, p<0,001, effect size by Cohen f=1,11; vein–RCA R2=0,77, p<0,001, effect size by Cohen f=1,83). The multiple linear regression showed no influence of the gender, age, BMI, comorbidities, symptoms or medication on the resistance in the bypasses (p = 0,683). With the ROC-curve analysis we try to determine the "cut-off" value for the resistance in the bypasses on the basis of the “cut-off” values for bypass flow and 88 PI in the LIMA-LAD and vein bypasses given by others. According to the “cut off” value for the bypass flow, for LIMA-LAD bypass is the “cut off” value 6,54W (AUC 0,869, sensitivity 85,7%, specificity 85,7%, p=0,002) and for the vein bypasses 5,78W (AUC 0,94, sensitivity 80,8%, specificity 100%, p<0,001). According to the PI “cut-off” values, for LIMA-LAD bypass is the “cut off” value 5,57W (AUC 0,82, sensitivity 81%, specificity 78,6%, p<0,001) and for the vein bypasses 8,45W (AUC 0,953, sensitivity 81,3%, specificity 100%, p=0,001). Further research is needed to determine the exact "cut-off" value. The effect of the mean arterial pressure on the LIMA-LAD bypasses flow was very strong (R2=0,124, p=0,008, effect size by Cohen f=0,37), which was not observed in the other coronary territories (vein to RCX R2=0,04, p=0,272, LRA to RCX R2=0,023, p=0,625 and vein to RCA R2=0,02 und p=0,461). The comparison of the supply areas showed higher diastolic flow in the LAD supply area (compared to vein to RCX p=0,004, LRA to RCX p=0,017, vein to RCA p<0,001, vein to RD p=0,661) and more resistance in the RD territory (compared to LIMA–LAD p=0,008, vein to RCX p=0,044, LRA to RCX p=0,046, vein to RCA p=0,021. There were no statistically significant differences in the MBF and PI supply areas. The "T-graft anastomosis" poses a special challenge for TTFM. We`ve measured the MBF on 3 different points, on the LIMA trunk before the “T-graft” and on LIMA and RIMA after the “T-graft”. We found a significant difference between the MBF of the LIMA trunk and the MBF of the sum of LIMA+RIMA after the "T-graft" (LIMA trunk 91,4± 59,4 ml/min, LIMA+RIMA 115±95 ml/min, p<0,001), something that questions the accuracy of TTFM. Furthermore, the flow of the "T-graft" was not influenced by the number of distal anastomoses (R2=0,01, p=0,08). As expected, the PI of the LIMA after the "T- anastomosis" was larger than that of the other "T-graft" branches (LIMA trunk 1,30±0,69, LIMA after “T-graft” 1,84±1,26, RIMA after “T-graft” 1,39±1,11, p<0,001). We found no difference in MBF and PI between the patients that had postoperative myocardial infarction or needed postoperative coronary angiography, postoperative revascularization or died after the operation and all other patients. In summary, even for this single construct TTFM failed to be predictive for major adverse events. With our studies we here could show that the options of TTFM are very different. The limitations are mainly in the predictive function of TTFM, which needs future research to make TTFM even more reliable and allow to give valid cut-off-points