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Published on: December 6, 2024
Computational model of blood flow in the aorto-coronary bypass graft
Meena Sankaranarayanan1, Leok Poh Chua, Dhanjoo N Ghista
1School of Mechanical and Production Engineering, Nanyang Technological University, 63 97 98, Singapore. msmeena@ntu.edu.sg <msmeena@ntu.edu.sg>
Insights
Saphenous vein grafts used in coronary artery bypass grafting surgery show high failure rates due to intimal hyperplasia. This study used computational fluid dynamics to analyze flow dynamics and wall shear stress at anastomoses, identifying mid-diastole as peak perfusion time.
Area of Science:
- Cardiovascular Surgery
- Biomedical Engineering
- Fluid Dynamics
Background:
- Coronary artery bypass grafting (CABG) uses arterial and venous grafts, with saphenous vein grafts (SVGs) having lower long-term patency rates compared to arterial grafts.
- Over 50% of SVGs occlude within 10 years, often due to intimal hyperplasia at anastomotic sites, a condition linked to non-modifiable and modifiable factors.
- Modifiable factors, including graft flow dynamics and wall shear stress (WSS) at anastomoses, are key research interests for understanding SVG failure.
Purpose of the Study:
- To investigate the complex flow patterns and wall shear stress distributions within realistic three-dimensional coronary bypass graft models.
- To identify how hemodynamic factors at proximal and distal anastomoses contribute to the failure of saphenous vein grafts in coronary artery bypass grafting.
- To correlate flow disturbances and WSS variations with the sites of intimal hyperplasia and long-term graft patency.
Main Methods:
- Construction of three-dimensional computational fluid dynamics (CFD) models simulating aorto-right and aorto-left coronary bypass graft systems.
- Incorporation of real-life surgical dimensions for the aorta, saphenous vein, and coronary artery, including cross-sectional shape changes of the vein conduit.
- Quasi-steady flow simulations using a finite-volume approach, with physiological flow-rate data input at critical points of the cardiovascular system.
Main Results:
- Detailed visualization of flow fields and wall shear stress distributions at proximal and distal anastomotic sites during systole and mid-diastole.
- Demonstration of how the occluded coronary artery is perfused throughout the cardiac cycle under simulated bypass conditions.
- Identification of specific flow patterns and WSS variations associated with potential sites of intimal hyperplasia and arterial disease progression.
Conclusions:
- Maximum perfusion of the occluded coronary artery via the bypass graft occurs during mid-diastole.
- Significant variations in wall shear stress are predominantly observed around the distal anastomotic region.
- These findings offer insights into vein graft disease mechanisms, potentially aiding in strategies to alleviate or delay graft failure.
Background:
Coronary artery bypass grafting surgery is an effective treatment modality for patients with severe coronary artery disease. The conduits used during the surgery include both the arterial and venous conduits. Long- term graft patency rate for the internal mammary arterial graft is superior, but the same is not true for the saphenous vein grafts. At 10 years, more than 50% of the vein grafts would have occluded and many of them are diseased. Why do the saphenous vein grafts fail the test of time? Many causes have been proposed for saphenous graft failure. Some are non-modifiable and the rest are modifiable. Non-modifiable causes include different histological structure of the vein compared to artery, size disparity between coronary artery and saphenous vein. However, researches are more interested in the modifiable causes, such as graft flow dynamics and wall shear stress distribution at the anastomotic sites. Formation of intimal hyperplasia at the anastomotic junction has been implicated as the root cause of long- term graft failure. Many researchers have analyzed the complex flow patterns in the distal sapheno-coronary anastomotic region, using various simulated model in an attempt to explain the site of preferential intimal hyperplasia based on the flow disturbances and differential wall stress distribution. In this paper, the geometrical bypass models (aorto-left coronary bypass graft model and aorto-right coronary bypass graft model) are based on real-life situations. In our models, the dimensions of the aorta, saphenous vein and the coronary artery simulate the actual dimensions at surgery. Both the proximal and distal anastomoses are considered at the same time, and we also take into the consideration the cross-sectional shape change of the venous conduit from circular to elliptical. Contrary to previous works, we have carried out computational fluid dynamics (CFD) study in the entire aorta-graft-perfused artery domain. The results reported here focus on (i) the complex flow patterns both at the proximal and distal anastomotic sites, and (ii) the wall shear stress distribution, which is an important factor that contributes to graft patency.
Methods:
The three-dimensional coronary bypass models of the aorto-right coronary bypass and the aorto-left coronary bypass systems are constructed using computational fluid-dynamics software (Fluent 6.0.1). To have a better understanding of the flow dynamics at specific time instants of the cardiac cycle, quasi-steady flow simulations are performed, using a finite-volume approach. The data input to the models are the physiological measurements of flow-rates at (i) the aortic entrance, (ii) the ascending aorta, (iii) the left coronary artery, and (iv) the right coronary artery.
Results:
The flow field and the wall shear stress are calculated throughout the cycle, but reported in this paper at two different instants of the cardiac cycle, one at the onset of ejection and the other during mid-diastole for both the right and left aorto-coronary bypass graft models. Plots of velocity-vector and the wall shear stress distributions are displayed in the aorto-graft-coronary arterial flow-field domain. We have shown (i) how the blocked coronary artery is being perfused in systole and diastole, (ii) the flow patterns at the two anastomotic junctions, proximal and distal anastomotic sites, and (iii) the shear stress distributions and their associations with arterial disease.
Conclusion:
The computed results have revealed that (i) maximum perfusion of the occluded artery occurs during mid-diastole, and (ii) the maximum wall shear-stress variation is observed around the distal anastomotic region. These results can enable the clinicians to have a better understanding of vein graft disease, and hopefully we can offer a solution to alleviate or delay the occurrence of vein graft disease.
