Influences of graft diameter on the blood flow in 2-way bypassing surgery

The graft diameter plays a critically important role in the long-term patency rates of bypass surgery. To clarify the influence of graft diameter on the blood flows in the femoral 2-way bypass surgery, the physiologically pulsatile flows in two femoral bypass models were simulated with numerical methods. For the sake of comparison, the models were constructed with identical geometry parameters except the different diameters of grafts. Two models with small and large grafts were studied. The boundary conditions for the simulation of blood flow were constant for both models. The maximum Reynolds number was 832.8, and the Womersley number was 6.14. The emphases of results were on the analysis of flow fields in the vicinity of the distal anastomosis. The temporal-spatial distributions of velocity vectors, pressure drop between the proximal and distal toe, wall shear stresses, wall shear stress gradients and oscillating shear index were compared. The present study indicated that femoral artery bypassed with a large graft demonstrated disturbed axial flow and secondary flow at the distal anastomosis while the axial flow at its downstream of toe was featured with larger and more uniform longitudinal velocities. Meanwhile, the large model exhibits less refluences, relatively uniform wall shear stresses, lower pressure and smaller wall shear stress gradients, whereas it does not have any advantages in the distributions of secondary flow and the oscillating shear index. In general, the large model exhibits better and more uniform hemodynamic phenomena near the vessel wall and may be effective in preventing the initiation and development of postoperative intimal hyperplasia and restenosis.     

Accelerated phase contrast MRI using hybrid one‐ and two‐sided flow encodings only (HOTFEO)

The aim of this work was to develop and evaluate a fast phase contrast magnetic resonance imaging (PC‐MRI) technique with hybrid one‐ and two‐sided flow encodings only (HOTFEO) for accurate blood flow and velocity measurements of three‐directional velocity encoding PC‐MRI. Four‐dimensional (4D) PC‐MRI acquires flow‐compensated (FC) and three‐directional flow‐encoded (FE) echoes in an interleaved fashion. We hypothesize that the blood flow velocity direction (not magnitude) has minimal change between two consecutive cardiac phases. This assumption provides a velocity direction constraint that can achieve 4/3‐fold acceleration using three‐directional FE data to calculate FC data instead of acquiring them. The HOTFEO acquisition pattern can address the ill‐conditioned constraint and improve the calculation accuracy. HOTFEO was evaluated in healthy volunteers and compared with conventional two‐dimensional (2D) and 4D flow imaging techniques with FC and three‐directional FE acquisitions (FC/3FE). Compared with FC/3FE, Bland–Altman tests showed that the 4/3‐fold accelerated HOTFEO technique resulted in relatively small bias error for total volumetric flow (0.89% for prospective 2D data, –1.19% for retrospective 4D data and –3.40% for prospective 4D data) and maximum peak velocity (0.50% for prospective 2D data, –0.17% for retrospective 4D data and –2.00% for prospective 4D data) measurements in common carotid arteries. HOTFEO can accelerate three‐directional velocity encoding PC‐MRI whilst maintaining the measurement accuracy of the total volumetric flow and maximum peak velocity.     

Numerical simulations of stenosed femoral artery with symmetric 2-way bypass graft

The numerical simulation of the conventional 1-way bypass graft and the presented 2-way bypass graft used in the stenosed femoral arteries were completed. Purpose of the present study was to ascertain whether the modified geometry configuration of 2-way bypass graft could improve the flow conditions of femoral bypass graft. The physiological flows in 1-way and 2-way models were simulated with finite element method under the same calculation conditions. Comparisons were made between the hemodynamics of 1-way and 2-way models. The results suggested that 2-way bypass graft could result in more favorable and regular flow conditions than 1-way bypass graft, and could decrease the probability of intimal hyperplasia, restenosis and surgery failure in femoral bypass graft.     

Computational Investigation of Hemodynamics in Fully Stenosed CABG

Coronary Artery Bypass Graft (CABG) is an important surgical treatment for critically stenosed arteries. Unfortunately restenosis always occurs after CABG surgery, which bring about surgery failure, lntimal thickening in the CABG distal anastomosis has been implicated as the major cause of restenosis and long-term graft failure. The nonuniform hemodynamics including disturbed flows, recirculation zones, oscillating wall shear stress, and long particle residence time were thought to be the possible etiologies. Numerical simulation was proved to be of great help and guidance meaning for the biofluid mechanics research and the CABG surgical plan. The present study was based on the hypothesis that the geometry configuration of CABG could greatly influence the hemodynamics in the vicinity of anastomosis. The hemodynamic features of two geometry models of end-to-side CABG were studied and compared. One simulated a conventional CABG with 1-way bypass graft, and the other simulated a modified CABG with symmetric 2-way bypass graft. The numerical investigations of hemodynamics in these two models with fully stenosed coronary arteries were accomplished using finite element method. The temporal and spatial distributions of hemodynamics were analyzed and compared. Results showed that the presence of symmetric 2-way bypass graft was of reasonable and favorable hemodynamics than 1-way bypass graft. The modified CABG model created a more hemodynamically efficient streamlined environment with higher mean and maximum axial velocities and lower radial velocities than the conventional 1-way model. Meanwhile, the symmetric 2-way bypass graft was featured with low pressure near the wall, high and uniform WSS in the host artery. All of these were favorable for inhibiting the development of intimal thickening, restenosis, and ultimate failure of the CABG, and it could considerably improve the flow conditions and decrease the probability of intimal hyperplasia and restenosis of CABG.     

股动脉双路搭桥的血流动力学仿真

在传统股动脉搭桥术中，由于下游缝合区采用“端对侧”的“单路搭桥”缝合方式，移植管中血流会对缝合区底面产生很大冲击，引起局部血流动力学急剧变化，并可能因此而导致血管再狭窄和手术失败。由于缝合区几何结构的不对称必然导致流场的不均匀，本研究提出了“对称双路搭桥”的构想以图改善血流动力学。本研究对“单路”和“对称双路”两种连接模型中的血液流动进行了数值模拟。两种模型利用相同的几何参数进行建模，并采用了相同的边界条件。数值模拟结果包括流场、壁面切应力及其梯度等血流动力学参数。研究表明，“对称双路”模型比“单路”模型具有较大的纵向速度、较小的二次流、较均匀的壁面切应力等。因此，“对称双路”搭桥模型具有更好的血流动力学，可以减少股动脉搭桥术后内膜增生和再狭窄的可能性。     

Alteration of hemodynamics in aneurysm models by stenting: Influence of stent porosity

Recent developments in minimally invasive approach to cerebrovascular diseases include the placement of stents in arteries for treatment of aneurysms. Preliminary clinical observations and experimental studies have shown that intravascular stents traversing the orifice may lead to thrombosis and subsequent occlusion of the aneurysm. The alterations in vessel local hemodynamics due to the introduction of a stent are not yet well understood. We investigated changes in local hemodynamics resulting from stent implantation. Pulsatile flow patterns in an experimental flow appraratus were visualized using laser-induced fluorescence of rhodamine dye. The test cells were constructed in a rectangular shape to facilitate an undisturbed longitudinal view of flow patterns in parent vessel and aneurysm models with and without porous stents. Woven nitinol stents of various porosities (76%, 80%, 82%, and 85%) were investigated. The selected fluid dynamic similarity parameters (Reynolds and Womersley numbers) represented conditions usually found in high-flow, larger arteries in humans (such as the carotid artery) and low-flow, smaller arteries (such as the vertebral artery). The mean Reynolds number for the larger arteries was 180, with maximum/minimum values of 490/−30 and the Womersley number was 5.3. The mean Reynolds number for the smaller arteries was 90, with maximum/minimum values of 230/2, and the Womersley number was 2.7. For the larger arteries modeled, placement of a stent of the lowest porosity across the aneurysm orifice resulted in reduction of aneurysmal vortex speed and decreased interaction with parent vessel flow. For smaller arteries, a stent of the same porosity led to a substantial reduction of parent vessel/aneurysmal flow interaction and the appearance of a nonrecirculating crescent of fluid rich in rhodamine dye in the aneurysm dome. Our results can help explainin vivo thrombus formation within an aneurysm after placement of a stent that is compatible with local hemodynamics.     

Haemodynamic model of a unilateral iliac stenosis with an aortoiliac bypass

A hydrodynamic model for the part of the human arterial network below the renal arteries has been constructed using specially fabricated distensible tubes and a pulsatile pump to simulate an aortoiliac bypass. The experiments and the computer model indicated that no ‘steal’ occurred due to the insertion of the bypass graft. Also, the results showed that the length of the stenosis had a non-systematic apparent effect on the physiological significance of the obstruction and that the kinetic power represented only a small percentage of the total power. The total power efficiency of the bypass graft was unaffected by its elastic properties. The experimental investigation also indicated that the pressure drop across the stenosis was considerably larger than the drop calculated using the Poiseuille flow relationship when the stenosis was severe. Therefore, a critical arterial stenosis value cannot be defined as an obstruction of a constant percentage reduction of luminal area. It varies directly with the effective cross-sectional area and inversely with the flow rate. The value of angiography in assessing the functional significance of any arterial stenosis is there-fore limited. A better method for evaluation requires quantitative measurements of local blood pressure and blood flow, not only at rest, but also under conditions creating augmented flows due to exercise.     

A steady flow analysis on the stented and non-stented sidewall aneurysm models.

As part of a general investigation on the effects of blood flow patterns in sidewall aneurysm, in vitro steady flow studies on rigid aneurysm models have been conducted using Particle Image Velocimetry over a range of Reynolds number from 200 to 1600. Above Reynolds number 700, one large recirculating vortex would be formed, occupying the entire aneurysmal pouch. The centre of the vortex is located at region near to the distal neck. A pair of counter rotating vortices would however be formed at Reynolds numbers below 700. For all the aneurysm models considered, the vortex strength, in general, is stronger at higher Reynolds numbers but lower at larger aneurysm size. Maximum strength of the vortex is about 15% of the bulk mean velocity in the upstream parent tube. Estimates of the wall shear stresses are derived from the near wall velocity measurements. Highest level of wall shear stresses always appears at the distal neck of the aneurysmal pouch. Stents and springs of different porosity have been used to dampen the flow movement inside the aneurysm so as to induce the possible formation of thrombosis. It is found that the flow movement inside the aneurysmal pouch can be suppressed to less than 5% of the bulk mean velocity by both devices. Furthermore, regions of high wall shear stresses at the distal neck could also be suppressed by almost 90%. The present results would be useful for further improvements in stent (or spring) technology.     

弯曲颈总动脉内流场的模拟实验研究

目的模拟研究弯曲颈总动脉定常流动速度剖面的不对称性。方法用粒子图像测速技术(PIV)测量并分析曲率分别为1:60和1:20的90°弯管模型在颈总动脉生理平均雷诺数、峰值雷诺数和极限雷诺数条件下,在不同位置的弯曲平面及其垂直方向的轴向速度剖面。结果当颈总动脉的曲率等于1:60时,在人生理平均流动雷诺数条件下,颈总动脉的速度剖面可用偏斜的抛物面来近似;当颈总动脉曲率增大或流动雷诺数增大时,弯曲平面上的速度剖面中部出现下凹趋势,垂直方向上出现M型速度剖面;沿管轴不同位置上的速度剖面形状不同。结论为确定颈动脉分叉定常流动不对称入口速度剖面条件,认识速度剖面的形状如何随流动雷诺数、弯管曲率和截面位置变化提供实验依据。     

Changes in turbulence with rotation of the omnicarbon prosthesis

This study was performed to determine whether annular plane orientation of the Omnicarbon aortic valve influences forward flow turbulence. The Omnicarbon prostheses was modified to allow in situ manual rotation of the valve when implanted in the aortic position of eight 90 kg pigs. Pulsed Doppler ultrasound was used to acquire velocity measurements at 17 locations within the cross-sectional area of the ascending aorta. In each animal, 12 valve rotations were tested in this manner. Reynolds normal stresses were estimated from the velocity measurements. High Reynolds normal stresses were concentrated between left and posterior-right sides of the aortic wall for all orientations studied. No trends in mean or maximum Reynolds normal stresses with respect to valve rotation were consistent in the experiments. Unlike previous experiments with the Medtronic-Hall tilting disc valve, these experiments showed no notable changes in Reynolds normal stress with respect to orientation of the Omnicarbon valve. This suggests that the tendency of turbulent stresses to change with tilting disc valve orientation may be dependent on valve design.