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.     

管径比对全阻塞动脉旁路移植流场影响的数值研究

目的 研究管径比对全阻塞动脉旁路移植流场的影响,为指导动脉旁路移植手术,减少术后再狭窄提供理论依据。方法 采用数值方法研究5种不同移植管与主血管的管径比对全阻塞情况下动脉旁路移植流场的影响,分析速度、二次流、壁面切应力和壁面切应力梯度等血流动力学参数的分布及其随管径比增大的改变。同时,为表明本文所采用模型的合理性,针对目前常使用的两类模型,比较在管径比1.0情况下全阻塞完整模型（Model A）、全阻塞局部模型（Model B）和75％狭窄完整模型（Model C）之间的血流动力学差异。结果 Model A和Model C的血流动力学特性是完全不同的;移植管顶部截面内的速度分布对下游吻合处的主血管底部壁面切应力的影响是显著的,最大相差达79％。大管径比时,主血管底部的壁面低切应力区较大,但壁面切应力分布均匀,壁面切应力梯度较小。而小管径比时,主血管底部的壁面低切应力区较小,但壁面切应力梯度较大。结论 采用整体模型单独研究全阻塞情况下的管径比对流场的影响是有必要的。管径比对全阻塞动脉旁路移植的流场具有显著影响,采用大管径比进行动脉旁路移植将有助于缓解吻合口处由于再狭窄而产生的阻塞。     

单路和双路CABG中血流动力学的比较

为了改善冠状动脉搭桥术后的血流动力学，提出了对称双路搭桥的改进措施。利用有限元分析方法，对冠状动脉搭桥术中单路移植管和对称双路移植管内的生理流动进行了数值模拟，并对两种情况下的血流动力学计算结果进行了比较。计算结果分析了缝合区附近的流场、壁面剪应力等血流动力学因素在心动周期内的时空分布情况。研究结果表明，对称双路搭桥比单路搭桥具有更合理的血流动力学，可以避免动脉粥样硬化的危险性血流动力学因素，从而减少手术再狭窄的发生。     

小直径血管移植物的研究进展

小直径血管移植物的研究目前主要集中在人工材料血管和组织工程血管两大方面.本文重点围绕提高人工血管抗血栓能力、减少内膜增生进行总结;对组织工程血管可降解支架材料中如何减少组织工程血管血栓形成、增强其力学强度的研究进展作一综述.     

Effect of pulsatile arterial diameter variations on blood flow estimated by Doppler ultrasound

High-resolution measurements of common carotid and femoral arterial diameters have been performed by ultrasound echo devices. When combined with pulsed Doppler measurements of cross-sectional averaged velocity in the same vessels, exact calculations of flow were made possible. The median peak-to-peak pulsatile diameter variations were 0.19 mm (2.8 per cent) in the femoral artery and 0.49mm (6.7 per cent) in the common carotid artery. Flow values were calculated either by taking the time-averaged diameter as a constant value, or by taking into account the dynamic variations in diameter. In comparing the two values, a quantification of the magnitude of error introduced by the averaging of the diameter was made possible. An error in the range 1.5–3.8 per cent was found for the femoral artery, whereas the error in the common carotid artery was in the range 0.4–3.6 per cent despite the larger amplitude of the pulsations in this vessel.     

The course of flavivirus infections in mice in the presence of the graft versus host reaction under conditions of 2-way incompatibility

The graft versus host reaction (GVHR) under conditions of two-way incompatibility was shown to increase average longevity (AL) of mice with experimental tick-borne encephalitis (TBE) and to activate asymptomatic infection caused by Langat virus. Experiments with TBE virus demonstrated participation of viable T lymphocytes in GVHR. Inhibition of function of both humoral and cellular immunity was shown to occur in GVHR. The assumption that the increase in the average longevity in TBE is associated with the inhibition of the damaging function of cellular immunity is experimentally substantiated.     

Deterministic nonlinear characteristics of in vivo blood flow velocity and arteriolar diameter fluctuations

We have performed a nonlinear analysis of fluctuations in red cell velocity and arteriolar calibre in the mesenteric bed of the anaesthetized rat. Measurements were obtained under control conditions and during local superfusion with NG-nitro-L-arginine (L-NNA, 30 microM) and tetrabutylammonium (TBA, 0.1 mM), which suppress NO synthesis and block Ca2+ activated K+ channels (KCa), respectively. Time series were analysed by calculating correlation dimensions and largest Lyapunov exponents. Both statistics were higher for red cell velocity than diameter fluctuations, thereby potentially differentiating between global and local mechanisms that regulate microvascular flow. Evidence for underlying nonlinear structure was provided by analysis of surrogate time series generated from the experimental data following randomization of Fourier phase. Complexity indices characterizing time series under control conditions were in general higher than those derived from data obtained during superfusion with L-NNA and TBA.     

The possible role of platelets in bypassing the contact phase of blood coagulation

Data presented herein and previously support an active role for platelets in promoting the interaction and activation of the coagulation proteins of the contact phase of intrinsic coagulation. The platelet membrane, activated by ADP collagen or thrombin, can promote the proteolytic activation of factor XII to factor XIIa in the presence of kallikrein and high molecular weight kininogen. The zymogen factor XI associates with high molecular weight kininogen in plasma and becomes bound to a site on the membrane of thrombin or collagen activated platelets. Thereafter, platelet bound factor XI can be proteolytically activated to factor XIa either in the presence of factor XIIa or in the presence of kallikrein. These observations could explain the absence of bleeding complications in patients with factor XII deficiency. In addition, platelets contain a molecule which has a higher molecular weight than plasma factor XI and possibly consists of a tetramer of four identical subunits of 52000 daltons each of which is functionally and immunologically similar to plasma factor XI. Since this molecule is present in the platelets of patients with severe plasma factor XI deficiency and no evidence of bleeding, we postulate that platelet factor XI can substitute for plasma factor XI in hemostasis and possibly account for the considerable variability in clinical severity observed in patients with factor XI deficiency.     

双路搭桥术的移植管直径对血液流动的影响

移植管直径对搭桥术的长期通畅率具有至关重要的作用.为了弄清楚移植管直径对双路搭桥术中血液流动的具体影响,利用数值仿真的方法对两个股动脉双路搭桥模型中的生理脉动流进行了研究.为便于比较,除移植管直径不同以外,两个模型均采用相同的几何参数来建模.我们将研究直径一小和一大两个模型.两个模型的流动仿真边界条件相同.峰值雷诺数为832.8,Wcmersley数为6.14.重点分析了下游缝合区流场.对速度场、上下游足尖的压力差、壁面切应力及其梯度、应力波动系数等血流动力学参数的时空分布进行了比较.研究表明,大直径移植管模型下游缝合区的纵向速度和二次流速度较为紊乱,而其足尖下游远端的轴向流动则具有较大而均匀的纵向速度.同时,大移植管模型具有较小的回流、较均匀的壁面切应力、较低的压力、较小的壁面切应力梯度等特点.然而在二次流分布和应力波动系数方面却没有优势.总之,大移植管模型在近壁部分具有更好、更均匀的血流动力学现象,可以有效地避免术后内膜增生和再狭窄的发生和发展.     

A numerical study of blood flow in coronary artery bypass graft side-to-side anastomoses

Purpose: When sequential grafts are used in multivessel coronary artery bypass grafting, the graft first supplies blood to one or more coronary arteries via a side-to-side anastomosis. We studied hemodynamics in idealized models of parallel and diamond side-to-side anastomoses, identifying features that might promote restenosis. Methods: Blood flow was computed in three representative anastomosis configurations: parallel side-to-side, diamond side-to-side, and end-to-side. We compared configurations and the effect of host-graft diameter ratio. Results:Hemodynamic patterns depended strongly on anastomosis geometry and graft/host diameter ratio. In the distal graft, the diamond configuration had large areas of low wall shear stress (WSS) and high spatial WSS gradients. In the proximal graft the unfavorable WSS patterns were comparable for all models, while the distal portion of the host artery the diamond model was best. Models with smaller host arteries had smaller regions of low WSS. Conclusions: The parallel configuration was preferred over the diamond for maintaining graft patency, while the diamond configuration appeared best for maintaining host artery patency. Since graft patency is critical, parallel configurations seem hemodynamically advantageous. Larger graft/host ratios have better hemodynamic performance than smaller ones. © 2002 Biomedical Engineering Society. PAC2002: 8719Uv, 8710+e     

Stem compensation of loop sensors for electromagnetic blood flow and vascular diameter measurement

Accuracy of electromagnetic blood flow —and vascular diameter — measurements by means of intravascular loop probes in an extracorporeal alternating magnetic field can be impaired by an error signal induced in the probe stem. The standard method of removing this artifact by twisting the lead wires is difficult to implement in this case because the wires are bifilar and are made of highly resilient metals. The solution described here makes use of a hair pin loop incorporated within the probe stem in which an e.m.f. is induced that can be used to cancel the unwanted e.m.f. induced in the lead wires of the loop of the sensor. This scheme is applicable to simple loop probes as well as to orthogonal double-loop sensors.     

冠状动脉旁路移植术后竞争血流对桥血管血流动力学的影响

目的 研究冠状动脉旁路移植术后,不同程度竞争血流对左侧胸廓内动脉管壁壁面切应力(wall shear stress, WSS)的影响。方法 基于阈值分割方法,通过采用SimVascular软件从CT图像提取并重建左侧胸廓内动脉(left internal mammary artery, LIMA) 左前降支（left anterior descending, LAD）吻合口模型。根据LAD的狭窄程度将竞争血流分为无竞争（100%狭窄）、轻度竞争（50%、75%狭窄）和高度竞争血流（0%、30%狭窄）3组。通过数值模拟方法观察上述情况下吻合口模型的血流动力学表现。结果 随着竞争血流的增加,LIMA桥血管管壁WSS逐渐降低,但是震荡程度逐渐增加。高度竞争血流组LIMA管壁平均WSS（0%狭窄：2.73 Pa,30%狭窄：2.85 Pa）明显低于轻度竞争血流组（50%狭窄：4.77 Pa,75%狭窄：6.01 Pa）和无竞争血流组(100%狭窄：8.64 Pa);而其震荡剪切指数(oscillatory shear index, OSI)（0%狭窄：0.206,30%狭窄：0.085）却明显高于轻度竞争血流组（50%狭窄：0.014,75%狭窄：0.013）和无竞争血流组(100%狭窄：0.006)。结论 当LAD狭窄程度小于50%时,大量的竞争血流会使LIMA管壁WSS明显降低,并且呈剧烈震荡的状态,这种不利的WSS表现会影响LIMA的远期通畅率,并造成手术远期疗效的下降。     

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。     

Metformin, but not glimepiride, improves carotid artery diameter and blood flow in patients with type 2 diabetes mellitus

OBJECTIVE:

To compare the effects of glimepiride and metformin on vascular reactivity, hemostatic factors and glucose and lipid profiles in patients with type 2 diabetes.

METHODS:

A prospective study was performed in 16 uncontrolled patients with diabetes previously treated with dietary intervention. The participants were randomized into metformin or glimepiride therapy groups. After four months, the patients were crossed over with no washout period to the alternative treatment for an additional four-month period on similar dosage schedules. The following variables were assessed before and after four months of each treatment: 1) fasting glycemia, insulin, catecholamines, lipid profiles and HbA₁ levels; 2) t-PA and PAI-1 (antigen and activity), platelet aggregation and fibrinogen and plasminogen levels; and 3) the flow indices of the carotid and brachial arteries. In addition, at the end of each period, a 12-hour metabolic profile was obtained after fasting and every 2 hours thereafter.

RESULTS:

Both therapies resulted in similar decreases in fasting glucose, triglyceride and norepinephrine levels, and they increased the fibrinolytic factor plasminogen but decreased t-PA activity. Metformin caused lower insulin and pro-insulin levels and higher glucagon levels and increased systolic carotid diameter and blood flow. Neither metformin nor glimepiride affected endothelial-dependent or endothelial-independent vasodilation of the brachial artery.

CONCLUSIONS:

Glimepiride and metformin were effective in improving glucose and lipid profiles and norepinephrine levels. Metformin afforded more protection against macrovascular diabetes complications, increased systolic carotid artery diameter and total and systolic blood flow, and decreased insulin levels. As both therapies increased plasminogen levels but reduced t-PA activity, a coagulation process was likely still ongoing.     

Preoperative coagulation studies to predict blood component usage in coronary artery bypass graft surgery

Bleeding remains a serious complication of cardiac surgery. Studies indicate that preoperative fibrinogen concentration is an independent predictor of blood loss during coronary artery bypass graft (CABG) surgery. This study evaluates whether fibrinogen concentration is a better predictor of blood usage than the prothrombin time (PT) and activated partial thromboplastin time (aPTT) tests. Patients not taking clopidogrel bisulfate who underwent CABG surgery during a 3-month period at a 350-bed community hospital were included in this prospective study. The parameters evaluated included patient’s age, preoperative coagulation test results (PT, international normalized ratio [INR], aPTT, fibrinogen), and number of blood components transfused. A probability value of less than 0.05 was deemed significant. Thirty-five patients were included in this study. Mean blood usage was 6 units. Patient’s age approached significance as a predictor of blood usage, and fibrinogen levels trended toward significance more than the other coagulation parameters. In this study, the increased age of the patient and low plasma concentrations of fibrinogen were associated with increased blood usage. Although no indicators clearly demonstrated statistical significance, the vast difference in the probability values for patients’ ages and fibrinogen levels indicated that there was a trend toward significance in blood usage for CABG patients. Further studies with larger patient populations are indicated.     

Unsteady simulation of distal blood flow in an end-to-side anastomosed coronary bypass graft with stenosis

In this paper, we report on the unsteady state modeling of blood flow in an end-to-side anastomosed bypass graft, which has a stenosis upstream from the junction. In coronary artery bypass grafting/surgery (CABG), new arteries are created in order to provide blood to the heart using other blood vessels as conduits to bypass the blocked section in the patient's coronary arteries. The failure of coronary artery bypass procedures has been attributed to both intimal hyperplasia (IH) and atherosclerosis. It is believed that these two phenomena are, in turn, related to the local hemodynamic factors. In this work, a three-dimensional computational fluid dynamics analysis is used to simulate the physiological blood flow through a model of a stenosed coronary bypass graft with the realistic assumption of non-Newtonian flow for human blood. For different flow repartitions and at different times of the cycle, both the recirculating areas and wall shear stress (WSS) are studied. Based on the different distribution of flow rates in the bypass graft and the host artery, the flow features are investigated and the influence of non-Newtonian behavior is discussed in terms of separation points, reattachment points, and the wall shear stresses. Various differences are observed based on the assumption of non-Newtonian behavior of blood, which have not been reported before when a simplified Newtonian approach is utilized.     

Photoacoustic determination of blood vessel diameter

A double-ring sensor was applied in photoacoustic tomographic imaging of artificial blood vessels as well as blood vessels in a rabbit ear. The peak-to-peak time (tau(pp)) of the laser (1064 nm) induced pressure transient was used to estimate the axial vessel diameter. Comparison with the actual vessel diameter showed that the diameter could be approximated by 2ctau(pp), with c the speed of sound in blood. Using this relation, the lateral diameter could also precisely be determined. In vivo imaging and monitoring of changes in vessel diameters was feasible. Finally, acoustic time traces were recorded while flushing a vessel in the rabbit ear with saline, which proved that the main contribution to the laser-induced pressure transient is caused by blood inside the vessel and that the vessel wall gives only a minor contribution.     

Thermographic visualisation of coronary artery blood flow during by-pass surgery.

A technique is described for visualising coronary artery blood flow and assessing myocardial perfusion during aorta to coronary artery bypass surgery. The technique involves an injection of cold fluid into the graft which is then viewed thermographically using an infra-red camera.