Yong Seng Tan

Computational model of blood flow in the aorto-coronary bypass graft

BackgroundCoronary 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.MethodsThe 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.ResultsThe 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.ConclusionThe 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.

Numerical investigation and identification of susceptible sites of atherosclerotic lesion formation in a complete coronary artery bypass model

As hemodynamics is widely believed to correlate with anastomotic stenosis in coronary bypass surgery, this paper investigates the flow characteristics and distributions of the hemodynamic parameters (HPs) in a coronary bypass model (which includes both proximal and distal anastomoses), under physiological flow conditions. Disturbed flows (flow separation/reattachment, vortical and secondary flows) as well as regions of high oscillatory shear index (OSI) with low wall shear stress (WSS), i.e., high-OSI-and-low-WSS and low-OSI-and-high-WSS were found in the proximal and distal anastomoses, especially at the toe and heel regions of distal anastomosis, which indicate highly suspected sites for the onset of the atherosclerotic lesions. The flow patterns found in the graft and distal anastomoses of our model at deceleration phases are different from those of the isolated distal anastomosis model. In addition, a huge significant difference in segmental averages of HPs was found between the distal and proximal anastomoses. These findings further suggest that intimal hyperplasia would be more prone to form in the distal anastomosis than in the proximal anastomosis, particularly along the suture line at the toe and heel of distal anastomosis.

A Novel Coronary Artery Bypass Graft Design of Sequential Anastomoses

In this paper, the hemodynamics in a three-dimensional out-of-plane sequential bypass graft model is first investigated. Based on the advantageous flow characteristics observed within the side-to-side (STS) anastomosis in the sequential bypass graft simulation, a new CABG coupled-sequential anastomosis configuration is designed, entailing coupled STS and end-to-side (ETS) anastomotic components. In this new CABG design, the flow fields and distributions of various wall shear stress parameters within the STS and ETS anastomotic regions are studied, and compared to those of the conventional distal anastomosis, by means of computational fluid dynamics simulation of pulsatile Newtonian blood flow. Simulation results demonstrate that the new sequential anastomoses model provides: (i) a more uniform and smooth flow at the ETS anastomosis, without any stagnation point on the artery bed and vortex formation in the heel region of the ETS anastomosis within the coronary artery; (ii) a spare route for the blood flow to the coronary artery, to avoid re-operation in case of re-stenosis in either of the anastomoses; and (iii) improved distribution of hemodynamic parameters at the coronary artery bed and in the heel region of the ETS anastomosis, with more moderate shear stress indices. These advantages of the new design over the conventional ETS anastomosis are influenced by the occlusion ratio of the native coronary artery, and are most prominent when the proximal segment of the coronary artery is fully occluded. By varying the design parameters of the anastomotic angle and distance between the two anastomoses, the superior coupled STS–ETS anastomoses design is found to have the anastomotic angle of 30° and 30 mm distance between the two (STS and ETS) components.

CABG MODELS FLOW SIMULATION STUDY ON THE EFFECTS OF VALVE REMNANTS IN THE VENOUS GRAFT

Venous valves and sinuses are frequently observed in vein grafts in the coronary artery bypass grafts (CABG). However, from the biomedical engineering viewpoint, vein grafts are always assumed as smooth tubes in the existing simulations, and no effort has been made to investigate the effects of jaggedness of the graft inner wall due to the valve cusps remnants and valve sinus (in case of valve-stripped saphenous vein (SV) grafts) on the blood flow patterns and hemodynamic parameters (HPs). In this paper, the effects of the inner surface irregularities of a vein graft on the blood flow is investigated in the graft as well as in the distal anastomotic region, with a more realistic geometry of valve-stripped SV, by means of numerical simulation of pulsatile, Newtonian blood flow. The simulation results demonstrate that the valve remnants and sinuses cause disturbances in the flow field within the graft (due to vortices formation within the valve sinuses) and undesirable distribution of HPs, which can result ...

A New Coronary Artery Bypass Graft (CABG) Distal Anastomosis Design

This study is motivated by the requirement for a CABG distal anastomosis configuration which can bring about considerable improvements in the flow field and hemodynamic parameter (HP) distributions. A new CABG configuration is designed and compared to the conventional end-to-side (ETS) distal anastomosis by means of numerical simulation of pulsatile, Newtonian blood flow. There are two components in the design: a side-to-side (STS) anastomosis located distal to the coronary stenosis, after which the graft end is anastomosed further distal to the same coronary artery in an ETS fashion. Simulation results demonstrate that the new model has improved the flow fields and HP distributions as compared to the conventional ETS anastomosis, especially at the coronary artery bed and the heel region. Furthermore, this configuration provides a spare route for the blood flow to the coronary artery in case of re-stenosis in either of the anastomoses to avoid re-operation. Since this design involves one additional anastomosis and prolonged operation time, it is difficult to explicitly judge whether this configuration favors the CABG life span. It thus requires further concrete evidences from in-vivo experiments to demonstrate the outcome practically.

Effect of Irregularities of Graft Inner Wall at the Anastomosis of a Coronary Artery Bypass Graft

It is well-established in medical literature that venous valves play a role in late segmental stenosis of vein grafts in the coronary artery bypass grafts (CABG). However, from the engineering point of view, vein grafts are always assumed as smooth tubes in the existing simulations, and no efforts have been put on the effects of vein valves and jaggedness of the graft inner wall due to the competent valves (in case of reversed saphenous vein (SV) grafts) or the valve cusps remnants and valve sinus (in case of valve-stripped SV grfats) on the blood flow.

Determination of the Left Ventricular Myofiber Angle Analytically and Its Significance

It is known that the tremendous internal pressure build-up in the left ventricle (LV) cavity during isovolumic contraction is due to the contraction of the spirally woven myocardial fibers. In this paper, a biomathematical model is developed to investigate the fiber angle using the theory of elasticity. Simultaneously, another simplified model in order to reduce the mathematical complexity was also developed to determine the fiber angle. The results of these two models showed that both the myocardial fiber angles are in same magnitude

In-vitro Experimental Validation Of Hemodynamics Study For Proximal Anastomosis Models

Hemodynamics is widely believed to correlate with the stenosis of coronary artery bypass graft (CABG). Although some researchers had investigated distal anastomosis, further studies upon the proximal anastomosis are still necessary as the initiation and growth of the stenosis process may be influenced by the nature of the flow at the proximal anastomosis. Therefore in this project, flow characteristics of proximal anastomosis models were studied numerically in order to enhance the understanding of the stenosis pathophysiological process and particle image velocimetry (PIV) measurements were also carried out to validate the numerical simulation results. Two models (viz. 90deg and 135deg anastomotic models) were firstly designed to mimic the proximal anastomosis of CABG for left and right coronary arteries respectively. A fair match between numerical and experimental data was observed in terms of flow characteristics, velocity profiles and WSS distributions under physiological flow conditions. The overall difference between their velocity profiles ranged from 8% to 54%. It is evident from the findings that PFV measurement can obtain quantitative results as accurate as LDA and numerical simulation is able to provide much detailed information with enough mesh density. In addition, the 135deg model would result in better patency rate based on hemodynamic analysis