Design and optimization of coronary arteries bypass graft using numerical method

It is well documented in literature that the Coronary Artery Bypass Graft (CABG) fails after a short period of time, due to the development of plaque within the bypass graft junction known as intimal hyperplasia. Various in vivo and in vitro studies have linked the development of intimal hyperplasia of the bypass graft to the abnormal hemodynamics and compliance mismatch. Thus, it is necessary to analyse and understand the hemodynamic forces inside the coronary artery bypass and its mechanical structure and geometrical characteristics under the correct physiological conditions. However, this information is difficult to obtain from in vivo and as a result, in vitro experimental and numerical investigations of idealistic and realistic models of CABG surgery have been extensively used to obtain the hemodynamics and mechanical forces within the bypass. The obtained results can be used to design and optimise the bypass graft configurations so that the development of the artery diseases reduces and the patency of the bypass increases. In recent years, computational fluid dynamics (CFD) has been extensively used as an effective numerical tool to investigate physical and geometrical factors that influence the hemodynamics of various configurations of CABG. With CFD the pressure and flow characteristics, as well as wall shear stress (WSS) for steady and cyclic flows can be accurately and effectively analysed. Utilization of CFD in biomechanics and hemodynamic researches is now widely accepted as a better alternative to in vitro and in vivo measurements which can be very expensive and time consuming. Still, there is always a need to obtain a good quantitative experimental data for validation and verification of the numerical predictions. Combining CFD with experimental and imaging techniques has been used widely to analyse the hemodynamics of various bypass and graft junctions. Employment of these two complementary techniques allows effective determination of various factors such as blood flow fields, wall shear stress and gradients, deformation of the artery and graft junction as well as the degree of compliance mismatch for the assessment of cardiovascular diseases. Although there are a numerous publications on the hemodynamic investigation of various coronary arteries bypass graft designs, literature does not provide still definitive clarification on the optimum anastomosis geometry, graft length and transitional curvature to achieve hemodynamic characteristics that promote failure-free bypass conduits. In addition, in order to model and simulate the hemodynamics of blood flow and arterial tissue, various assumptions have to be carried out to reduce the computational complexities and unknown parameter data which may affect the predicted results. Therefore, there is still a need for further work to analyse the hemodynamics of effective anastomosis configurations using realistic patient-specific boundary conditions. Moreover, the knowledge of fluid structure interaction (FSI) in coronary artery bypass is not fully investigated yet, due to some difficulties and complexities in setting up and solving the model, particularly for highly deformable analyses. In this thesis, various numerical techniques and surgical plan of coronary artery bypass grafts have been reviewed and discussed to determine the most suitable geometries that provide better hemodynamics, reduce intimal thickening, and extend the patency rates. In addition, the numerical method is identified as the most affordable technique to analyse cardiovascular systems and provide adequate hemodynamic data and structure deformation. However, various parameters that affect the numerical predictions, are identified and determined. In the next part of the thesis, to build up some degree of confidence and validate the numerical prediction, rigid and flexible models have been carried out to mimic the hemodynamic characteristics and structure deformation of CABG. The flow visualization and structure deformation data, which were obtained from Particle Image Velocimetry and High Speed Camera, are then compared with computational fluid dynamics results. Although some mismatches between two sets of data have been detected, various factors can be blamed for the discrepancies between numerical simulations and experimental data. However, the numerical simulation is considered to be enough to determine and analyse the hemodynamics forces, WSS distributions as well as structure deformation of the cardiovascular system, which are almost impossible to measure in in vivo. The numerical simulation on Consequence grafting and Y grafting are then carried out to examine and validate the hemodynamics of these configurations with particular emphasize on the parameters that could initiate the development of intimal hyperplasia. Although the results of our numerical study indicate that each surgical plan demonstrates its own hemodynamic risks and some areas of elevated WSS close to the anastomosis, the conclusion and comparison of clinical evidence of restenosis on the side-to-side and end-to-side configurations are still missing. Nonetheless, the computational methods presented here can assist in understanding the local hemodynamic environment in order to study the vascular bypass graft patency. Future studies on validation and comparison of the circulation flow and Dean flow vortex to the development of intimal hyperplasia are warranted. Moreover, the determination of abnormal hemodynamics caused by the compliance mismatch between the graft conduits and the host arteries, either natural or synthetic is critical in terms of design and optimisation of graft configurations. Hence, fully couple FSI simulations have been performed to determine the interaction between fluid forces and structure deformation on a selected end-to-side configurations under corrected operating conditions. The contributions of this Doctoral dissertation are highlighted below: · Determined the natured concerns of the CABG from hemodynamics and structure deformation points of view. · Reviewed possible surgical planning of clinical application in terms of patency rate and long term survival so that the best configurations can be determined. · Developed and selected the correct boundary conditions and sufficient investigation method to analyse the hemodynamics and fluid structure interaction of the coronary artery bypass graft. · Developed experiment setups to compare and validate with numerical simulations, in order to build up the confidence on the numerical method. · Investigated and determined the hemodynamic characteristics of Y grafting and Consequence grafting so that the best configurations can be decided. · Investigated and determined the effect of the degree of compliance by fluid structure interaction tool in order to relate to clinical implications.