[Show abstract][Hide abstract] ABSTRACT: Early researches on the artery bypass graft (ABG) generally took the assumption of rigid vessel wall which ignored the wall compliancy. To obtain more realistic and physiological hemodynamic parameters, a fluid structure interaction (FSI) study on a complete ABG was carried out. It was concluded: 1) a compliant vessel is able to expand its vessel diameter and decrease its anastomosis angle to achieve a buffer for the blood, thereby help to reduce endothelial cell injury. 2) The vessel walls experienced their maximum deformation at the time of peak pressure while the deformation could be ignored during diastole. However, the consideration of wall compliance didn't quantitatively change the flow characters compared to those of rigid walls. 3) Generally speaking, the hemodynamic priority of helical-type ABG over a conventional one was further strengthened by adopting compliant vessel wall. 4) The consideration of the wall deformation revealed a hidden fact by the rigid wall assumption: helical ABG aggravated the risk of intimal hyperplasia (IH) at its toe region due to its geometry and flow asymmetry. The current study may be useful for surgeons and graft designers to optimize the current and future ABG configurations and selection of materials.
Full-text · Article · May 2014 · ASAIO journal (American Society for Artificial Internal Organs: 1992)
[Show abstract][Hide abstract] ABSTRACT: The classic single-phase Newtonian blood flow model ignores the motion of red blood cells (RBCs) and their interaction with plasma. To address these issues, we adopted a multiphase non-Newtonian model to carry out a comparative study between a helical artery bypass graft (ABG) and a conventional ABG in which the blood flow is composed of plasma and RBCs. The investigation focused on the mechanism of RBC buildup in an ABG but the haemodynamic parameters obtained by single-phase and multiphase models were also compared. The aggregation of RBCs along the inside wall of a conventional ABG and at the heel of its distal anastomosis was predicted while a poor aggregation was observed along the helical ABG. In addition, RBCs were observed to gradually sediment along the gravity direction. However, the computed haemodynamic parameters by multiphase model qualitatively agreed well with those by single-phase model. It was concluded that (1) the single-phase computational fluid dynamics (CFD) is reasonable to do the computation of haemodynamic parameters in ABGs; (2) secondary flow does not definitely produce buildup of RBCs in the inside curvature, its configuration played an important role in the movement of RBCs and the dominating one-way rotating flow in a helical ABG guaranteed no buildup of RBCs on its inside wall and (3) gravity direction is important for the movement of RBCs which may help to explain why doing exercise is good for human health. This study helps to shed light on the migration of RBCs in ABGs, which cannot be explored by single-phase CFD models, and provides more understanding of the underlying flow mechanism for ABG failure.
Full-text · Article · Oct 2013 · Computer Methods in Biomechanics and Biomedical Engineering
[Show abstract][Hide abstract] ABSTRACT: Local oxygen lack in arterial walls (hypoxia) plays a very important role in the initiation, progression and development of intimal hyperplasia (IH) and thrombosis. Aiming to find out whether a helical-type artery bypass graft (ABG) is hypoxia beneficial, a numerical study was carried out to compare oxygen transport between a helical-type ABG and a conventional-type ABG. The dimensionless mass transfer coefficient (Sherwood number) was introduced to evaluate the oxygen mass transfer distribution and detailed oxygen wall flux was computed. The results show that the intrinsic geometry of a helical-type ABG resulted in improved hypoxia and the oxygen-depleted fluid located proximally to the occluded section as compared with that of a conventional-type ABG. However, benefits aside, distinct double low regions (low wall shear stress (WSS) and hypoxia) which might be most prone to IH and more localised and thicker boundary layer of oxygen-depleted fluid were observed at the helical-type ABG. This may explain why the helical flow plays a detrimental role at some locations in the human body. In addition, it was observed that although low WSS region was always accompanied with low oxygen supply, the oxygen transport rate did not adjust simultaneously with flow. The change in oxygen distribution usually lagged behind the flow change. A physiological WSS region may be associated with hypoxia condition. This study captured the qualitative trend of oxygen distribution in ABGs and the effect of helical geometry on reducing hypoxia, which is useful in the structural design of swirling flow vascular devices.
Full-text · Article · Jul 2012 · Computer Methods in Biomechanics and Biomedical Engineering
[Show abstract][Hide abstract] ABSTRACT: Preliminary studies on an individual helical graft indicated that its hemodynamics might be improved while pressure drop increased compared with a traditional graft. Aiming to investigate whether the benefits of a helical graft to hemodynamics dominate its deficits, this study numerically carried out comparative study of helical-type artery bypass graft (ABG) and traditional-type ABG under both steady and pulsatile flow conditions. The results showed that a helical-type ABG resulted in reduced oscillating shear index, improved wall shear stress, enhanced flow mixing and three-dimensionality, and improved flow behavior at the distal anastomosis and occluded section of the host vessel compared with traditional-type ABG. More important, although a helical-type ABG did increase the pressure drop compared with a traditional one, its maximum percentage increase during a cardiac cycle was <21% which is still within physiological sense. Therefore, we believe that the adoption of helical bypass could help to prevent intimal hyperplasia (IH) and thrombosis at the distal anastomosis and improve the graft patency while keeping clinical maintenance. This investigation provided an important basis for the clinical applications and theory support of helical graft.
Full-text · Article · Sep 2011 · ASAIO journal (American Society for Artificial Internal Organs: 1992)