血管生物反应器流场的数值分析

目的 研究一种新型小口径血管生物反应器在不同工况下的流场分布。方法 采用数值方法模拟反应器外筒单独旋转、内筒单独旋转和内外筒同向等速旋转情况下其内部的流场分布,对流速和切应力等参数进行比较分析。结果 血管生物反应器工作时,培养液随内外筒的旋转而旋转流动,速度分布均匀;可为培养液内的细胞提供非破坏性的低切应力环境;并且反应器内部的切应力大小与转速呈二次函数关系。结论 该反应器能为血管培养提供良好的培养环境,本研究可为其实验研究提供指导意见和理论基础。     

混流式血泵的数值模拟及性能分析

轴流式血泵转速过高、离心式血泵容易产生流动死区是造成血液损伤的重要原因,而混流式血泵能有效缓解轴流式血泵的转速过高以及离心式血泵的流动死区问题。基于此,本研究旨在探究闭式叶轮混流式血泵的性能效果。通过数值模拟的方法对闭式叶轮混流式血泵进行数值模拟,分析该类型血泵的流场特性及压力分布情况,探讨其水力性能以及可能对红细胞造成的损伤程度,并与半开式叶轮结构混流式血泵的数值模拟结果进行性能对比。结果表明:本研究中的闭式叶轮混流式血泵具有良好的性能,能够安全高效运行。该泵在5 L/min下能够达到100 mm Hg的扬程,血泵内流动均匀,没有明显的涡流、回流以及流动停滞现象,压力分布均匀合理,可有效地避免血栓;溶血指数平均值(HI)为4.99×10^-4,具有良好的血液相容性;与半开式叶轮混流式血泵相比,闭式叶轮混流式血泵扬程和效率更高、溶血指数平均值更小,且具有更好的水力性能及避免血液损伤的能力。通过本文研究结果,或能为闭式叶轮混流式血泵的性能评价提供依据。     

Numerical analysis of the flow characteristics of rotary blood pump

Thrombus formation and hemolysis have been linked to the dynamics of blood flow in rotary blood pumps and ventricular assist devices. Hemolysis occurs as the blood passes through the pump housing, and thrombi develop in stagnation and low-velocity regions. The predicted velocities, pressure, and turbulence quantities from the numerical simulation are used to identify regions of high shear stress and internal recirculation. A nimerical technique is described that simulates the hydrodynamic characteristics of a rotary blood pump with a flow rate of 6 l/min at a rotational speed of 3000 RPM. A computational fluid dynamics (CFD) code, CFX 4, is used to solve the time-dependent incompressible Navier-Stokes equations using a transient finite volume method and three-dimensional structured grids. The simulation utilized the sliding mesh capabilities of this numerical code to model the rotating impeller and examine the effect of blade shape on the hydrodynamic performance of the blood pump in terms of pressure rise, flow rates, and energy losses. The first impeller model has six straight channels; the second impeller has six backward-curved channels. The results for two impeller configurations are presented and discussed. The curvedpump design resulted in higher pressure rise and maximum shear stresses than the straight-channel one. In general the paper demonstrates that CFD is an essential numerical tool for optimizing pump performance with the aim of reducing trauma to the blood cells.     

Splenic blood flow and intrasplenic flow distribution in rats

Summary In 75 rats, anesthetized with pentobarbital and breathing spontaneously, regional splenic blood flow (rSBF) was measured by means of the⁸⁵Kr(β)-clearance technique after an intraaortic slug injection of the dissolved indicator. In the normal and undisturbed spleen in situ rSBF is linearly related to the mean arterial blood pressure (MABP) within the range of 30–140 mm Hg. Mean rSBF is 0.71 ml/g/min, the mean arterial blood pressure being 105 mm Hg. In normal rats rSBF decreases significantly with increasing body weight or age. After total obstruction of the open circulation by application of rigid spherocytes, mean rSBF is reduced to 0.26 ml/g/min and is independent of the mean arterial blood pressure within the same range. In splenomegaly, due to enhanced reticuloendothelial activity and intensified immunological responses after tumor implantation, an increase in total splenic blood flow is found. However, related to 1 g of splenic wet weight, rSBF is diminished. In splenomegaly, rSBF also linearly depends on MABP within a wide range. Mean rSBF is 0.51 ml/g/min, the mean arterial blood pressure being 91 mm Hg. The distribution of intrasplenic blood flow between open and closed circulation depends on the size of the mean arterial blood pressure. The perfusion rate of the open circulation, compared with rSBF amounts to 72–93% (MABP=80–130 mm Hg).     

Numerical analysis for the blood flow in a patient-specific ophthalmic artery

In this paper we investigate blood flow in the ophthalmic artery (OA) which is the major artery supplying blood to the eyes. An OA and several other cerebral arteries are digitized from a computed tomography angiography (CTA) image of an aneurysm patient. Utilizing a reduced version (1D) of the governing Navier–Stokes equations we solve the transient flow in these arteries. The flow waveform of the patient–OA is compared with that in a healthy vascular tree, and also with published ultrasonic measurements. We found that hyperemia rather than ischemia occurred in the OA, and we suggest that this was unlikely to be the cause of impaired vision in the patient. A more likely explanation is the compression of the optic nerves caused by the mass of the aneurysm.     

Experimental analysis of the hemodynamics in punctured vascular access grafts

The hemodynamics in the vascular access graft are influenced by the flow aspirated and injected through the two needles during hemodialysis. For the first time, the impact of needle flow on vascular access performance, measured in an in vitro set up, is reported. A vascular access model, consisting of a loop polytetrafluoroethylene graft sewn to a compliant artery and vein, simulated the patient. The extracorporeal circuit was connected to the model. Three mean access flow rates (QG; 500, 1,000, and 1,500 ml/min) and five roller pump flow rates (Q(R); 0, 200, 300, 400, and 500 ml/min) were studied. Mean, systolic, and diastolic pressure and according pressure drops were derived at 14 loci. Systolic, diastolic, and mean pressures drop along the graft decreased with increasing Q(R) and decreasing Q(G). At Q(R) = 500 ml/min and Q(G) = 500 ml/min, the mean pressure drop over the graft was negative (-10 mm Hg), indicating a reversed pressure profile, originating at the puncture site of the venous needle. Mean pressure in the venous outlet segment was about 100 mm Hg compared with only 75 mm Hg without needle flow. The combination of a low Q(G) (500 ml/min) and high Q(R) (> 300 ml/min) must be avoided because venous pressures can rise to 100 mm Hg and load the venous system. The results of this in vitro setup indicate that high Q(R) (> 400 ml/min) should be avoided at Q(G) up to 1,000 ml/min; however, in vivo tests have to be performed to prove this thesis. This study demonstrates the need for a well-functioning vascular access (Q(G) > 600 ml/ min) to perform adequate dialysis and to avoid venous system loading.     

Endothelialization of PTFE vascular grafts under flow induces significant cell changes

BACKGROUND: To minimize thrombogeneity of small diameter PTFE grafts they are usually coated in vitro with endothelial cells under static culture conditions. The disadvantage of this technique is that a cell layer is formed that fails to withstand shear stress typical in normal blood flow. METHOD: Since the in vivo functional and structural status of endothelial cells correlates with the applied shear stress, we developed a computer-controlled perfusion system to seed and culture cells on PTFE-grafts up to a confluent monolayer under the influence of increasing shear stress. The confluence of endothelial coating was defined by immunohistological staining of cross sections, and by upper light microscopy of flattened graft samples. In addition, the expression of fibronectin as an important adhesion molecule was estimated. RESULTS AND DISCUSSION: The application of pulsatile shear stress (6.6 dyn/cm2, 5 min) to grafts endothelialized under perfusion (n = 7) did not lead to a disruption of the confluent cell layer. In contrast, a 5 min long shear stress of 3 dyn/cm2 was sufficient to wash more than 50% of cells off the PTFE-graft cultured under static conditions (n = 6). The perfusion cultures showed a significantly higher proliferation rate in comparison with static cultures. This effect was reproducibile in both serum-containing and serum-free culture media. The expression of fibronectin by endothelial cells was significantly higher in the perfused graft compared to the static one. These results suggest the practicability of endothelialized PTFE vascular grafts, preconditioned to shear rates similar to the in vivo situation, as an alternative bypass material in cardiac surgery.     

基于三维医学影像的脑动脉瘤内血液流动的数值模拟

目的 利用数值模拟研究具有病人特异性的脑动脉瘤内的血液流动,为脑动脉瘤的破裂风险的评价和动脉瘤介入栓塞后复发风险的评价提供帮助。方法 从两例脑动脉瘤病人的3D-RA数据中重建动脉瘤几何模型,血液流变学模型选择假塑性非牛顿流体模型,利用商用CFD软件Fluent对两例动脉瘤内的血液流动进行数值模拟。结果 数值模拟给出了动脉瘤内的流线图、重要截面上的速度分布图、壁面上的切应力分布和压力分布图。并且绘制了在收缩期时刻动脉瘤颈部和瘤顶部各20个点上的壁面切应力和压力的变化情况。结论 血流动力学因素如流速、压力、壁面切应力、流动对壁面的冲击状况等因素与动脉瘤的生长和破裂密切相关,而由于脑动脉瘤形态各异、载瘤动脉与动脉瘤体的几何关系复杂,所以具有病人特异性的数值模拟对于研究动脉瘤破裂和复发风险具有重要价值。动脉瘤颈部的壁面切应力和壁面切应力的波动的变化规律并不相同,需要进一步研究壁面切应力的波动与脑动脉瘤生长与破裂之间的定量关系。     

Freeze-dissection analysis of 133Xe distribution to measure regional renal blood flow

To verify the freeze-dissection technique for measuring renal blood flow (RBF) distribution in anesthetized dogs we compared the sum of all compartment flow rates with total RBF, compared compartment flow distribution with intercepts, and determined recirculated 133Xe. By dissection, we found that the cortex, delineated by its granular brownish red appearance, comprised 69%, whereas the outer medulla (the reddish portion) was 18% of the total kidney weight. Average cortical flow was 3.77 ml X g-1 X min-1 and the y-intercept for 133Xe washout was 85% of the initial radioactivity distribution. Outer medullary flow was 2.01 ml X g-1 X min-1 with a y-intercept of 13%. In a 100-g kidney, total cortical flow would be 259.8 ml/min (3.77 ml X g-1 X min-1 X 68.9 g) and total outer medullary flow would be 36.0 ml/min. These calculations indicate that 86% of the total flow is distributed to the cortex and 13% to the outer medulla, as the intercept-calculated percentages indicated. Summing whole cortical and medullary flows results in a flow that agrees with the electromagnetically measured flow of 292 ml/min for a 100-g kidney.     

In vivo validation of numerical prediction of blood flow in arterial bypass grafts

In planning operations for patients with cardiovascular disease, vascular surgeons rely on their training, past experiences with patients with similar conditions, and diagnostic imaging data. However, variability in patient anatomy and physiology makes it difficult to quantitatively predict the surgical outcome for a specific patient a priori. We have developed a simulation-based medical planning system that utilizes three-dimensional finite-element analysis methods and patient-specific anatomic and physiologic information to predict changes in blood flow resulting from surgical bypass procedures. In order to apply these computational methods, they must be validated against direct experimental measurements. In this study, we compared in vivo flow measurements obtained using magnetic resonance imaging techniques to calculated flow values predicted using our analysis methods in thoraco–thoraco aortic bypass procedures in eight pigs. Predicted average flow rates and flow rate waveforms were compared for two locations. The predicted and measured waveforms had similar shapes and amplitudes, while flow distribution predictions were within 10.6% of the experimental data. The average absolute difference in the bypass-to-inlet blood flow ratio was 5.4±2.8%. For the aorta-to-inlet blood flow ratio, the average absolute difference was 6.0±3.3%. © 2002 Biomedical Engineering Society. PAC2002: 8761Lh, 8719Uv     

Influence of blood rheology on intrarenal blood flow distribution.

This study examined the effects of hematocrit and colloid-induced changes in blood rheology on renal blood flow (RBF) distribution. Blood viscosity, measured by coneplate viscometry, was raised 65% in anesthetized dogs either by increasing hematocrit or giving isoncotic dextran 500. Blood volume was kept constant and arterial pressure was unchanged. An increase in hematocrit caused either no change in RBF or a slight increase at each cortical level (zones 1-4). However, dextran hyperviscosity caused decreases in all four zones, the most pronounced in zone 1 (8.03-5.19 ml min(-1) g(-1)) and the least in zone 4 (1.63-1.46 ml min(-1) g(-1)). These data suggest either that increased hematocrit has less effect on "apparent viscosity" of blood within the kidney than does plasma colloid or that increased hematocrit causes renal vasodilatation while colloid hyperviscosity does not cause it. Since GFR remained constant in both types of hyperviscosity, it is possible that GFR is the autoregulated variable and the observed changes in RBF distribution and vascular resistance resulted from the changes in afferent and efferent arteriolar resistance required to preserve GFR.