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CFD analysis of venous CABG based on in-vivo CT datasets in patients …


Biology Articles » Bioengineering » Flow and wall shear stress in end-to-side and side-to-side anastomosis of venous coronary artery bypass grafts » Results

Results
- Flow and wall shear stress in end-to-side and side-to-side anastomosis of venous coronary artery bypass grafts

Mean heart rate during CT scanning was 47 ± 9 bpm for patient 1 and 51 ± 6 bpm for patient 2. The percent phase providing best image quality was found between 50 and 60% of the RR-interval for the RCA and between 60 and 70% for the left main artery (LMA), LAD, and LCX.

End-to-side anastomosis

Mass flow

The mean volumetric flow through the right coronary artery was 3.07 ml/sec. The mean volumetric flow through the CABG was 1.81 ml/sec (Figure 3; cut 5). Quantifications of mass flow through the proximal and distal part of the right coronary artery and the bypass close to the end-to-side anastomosis are demonstrated in Figure 3. The distribution of pulsatile flow could be determined by using the flow curves at each time-step of the cardiac cycle. There was a significant backflow into the proximal segment of the RCA (Figure 3; cut 2), reaching a maximum of 0.48 ml/sec. Due to this high backflow, the next proximal branch of the RCA (Figure 3; cut 4) was filled by blood coming from the bypass for the largest part of the cardiac cycle. The amount of backflow depends on the coronary mass flow and the bypass mass flow, which are characterized by different mass flow curves at their inlets, the angle between the bypass and the RCA, and the degree of the stenosis in the bypassed artery. The retrograde flow into the RCA reached a distance of 2.8 cm from the end-to-side anastomosis (Figure 4), leading to an area of stagnation between the two more proximal branches (Figure 3; cut 1 and cut 2).

Flow patterns and WSS

The WSS characterizes the tangential fluid forces that act on the vessel wall. The changes of WSS throughout the cardiac cycle showed a correlation with flow velocities, with WSS forces being high when blood flow was fast. Near the end-to-side anastomosis the maximum WSS spatial variation was approximately 1.5 Pa (Figures 4 and 5). The WSS ranged from 0.01 Pa at minimum inflow to approximately 2.0 Pa at maximum inflow. In the perianastomotic region, the time averaged WSS was 0.36 Pa. An area of high WSS was found during systole at the heel of the anastomosis. The lowest WSS was found in the RCA at the location where blood from the CABG hit the wall and was confronted by the flow coming from the native coronary artery. This formed a stagnation point at the impact site around which the bypass stream splits, as depicted by the flow streamlines in Figure 4. It is also shown here that backflow into the RCA persisted during most of the cardiac cycle. The highest WSS values were found during mid-systole (timestep 20) and were located at the heel of the anastomosis and at the side of a nearby clip that was placed during surgery (Figure 5). At the opposite side of the anastomosis the WSS values were lower, due to the presence of the stagnation zone.

Side-to-side anastomosis

Mass flow

The mean volumetric flow through the left coronary artery was 3.35 ml/sec. The mean volumetric flow through the CABG was 1.51 ml/sec. Quantifications of volumetric flow through the proximal and distal segment of the LAD and the proximal and distal bypass close to the side-to-side anastomosis are demonstrated in Figure 6. The distribution of pulsatile flow could be determined by using the flow curves at each time-step of the cardiac cycle. There was a small backflow into the proximal part of the left coronary artery (Figure 6; cut 3) lasting almost 40% of the cardiac cycle and reaching a maximum of 0.25 ml/sec. The maximum volumetric flux into the distal part of the artery (Figure 6; cut 4) was 1.65 ml/sec or 46.2 % of the entire flow. Throughout a cardiac cycle, 53% of the mass flow into the bypass remained in it and reached the next anastomosis (Figure 6; cut 2). In diastole additional blood circulated from the proximal into the distal part of the coronary artery (Figure 6; cut 3), partially compensating for the decrease in the amount of blood that came from the bypass (Figure 6; cut 4).

Flow patterns and WSS

Comparing side-to-side to end-to-side anastomosis, the mean WSS values were lower in the side-to-side anastomosis with a time-averaged value of 0.29 Pa in the perianastomotic region (Figure 7 and 8). The WSS values ranged from 0.02 Pa up to approximately 2.1 Pa at mid-systole. There were some areas of elevated WSS close to the anastomosis, but the highest WSS values were found in the atherosclerotic distal coronary artery (Figure 8). The elevated WSS in the distal coronary artery was mainly caused by increased bypass mass flow during systole. Furthermore, there was a high WSS region in the proximal part of the bypass due to a mild stenosis (Figure 7 and 8). In contrast, in the end-to-side anastomosis highest WSS values were located in the distal bypass graft close to the anastomosis. Lowest WSS values were found in the native coronary artery proximal to the side-to-side anastomosis. The maximum WSS spatial variation was approximately 1.0 Pa, being smaller than the WSS variation in end-to-side anastomosis. The streamline patterns presented in Figure 7 indicate absence of stagnation zones in the perianastomotic region of side-to-side anastomosis and a smoother stream division throughout the cardiac cycle as compared to the end-to-side anastomosis.


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