Coupled fluid-wall modelling of steady flow in stenotic carotid arteries

Arterial stenoses may cause critical blood flow and wall conditions leading to clinical complications. In this paper computational models of stenotic carotid arteries are proposed and the vessel wall collapse phenomenon is studied. The models are based on fluid-structure interactions (FSI) between blood and the arterial walls. Coupled finite element and computational fluid dynamics methods are used to simultaneously solve for stress and displacement in the solid, and for pressure, velocity and shear stress in the fluid domain. Results show high wall shear stress at the stenosis throat and low (negative) values accompanied by disturbed flow patterns downstream of the stenosis. The wall circumferential stress varies abruptly from tensile to compressive along the stenosis with high stress concentration on the plaque shoulders showing regions of possible plaque rupture. Wall compression and collapse are observed for severe cases. Post-stenotic collapse of the arterial wall occurs for stenotic severity as low as 50%, with the assumption that a given amount of blood flow needs to pass the stenotic artery; whereas if constant pressure drop should be maintained across a constriction, then collapse happens at severity of 75% and above. The former assumption is based on the requirement of adequate blood supply to the downstream organs/tissue, while the latter stems from the fact that the pumping mechanism of the body has a limited capacity in regulating blood pressure, in case a stenosis appears in the vasculature.     

Simulation of steady cardiovascular flow in the presence of stenosis using a finite element method

Stenosis could affect one or more segments of the human cardiovascular system. It is a problem capable of causing grave effects. In the present study, the finite element method has been utilised to construct a computer simulation model for the human cardiovascular system in which one or more blood carrying elastic segments are affected by stenosis. Computational effects on the effects of stenosis in aorta arch, carotid, and coronary arteries on parameters of steady flow through the system are presented. It is found that when the total flow rate through the heart is maintained constant, the most notable effect is a very marked increase in pressure drop occurring over the length of the vessel affected with stenosis. Pressure drop in many other segments also increases but by a much smaller extent. On the other hand, when the pressure at the inlet of the ascending aorta and the outlet of the vena cava are maintained constant, the most marked effect is a decrease of flow rate through the stenosed vessel. Stenosis not only causes a pressure drop in the affected segments but it also changes pressures at points distal from the site of stenosis. It also causes a redistribution of flow through the cardiovascular system.     

Self-expanding stents in the treatment of carotid artery subtotal occlusion: a clinical study on the patients of Hubei and Sichuan in China

Stroke is a major cause of death and disability throughout the world. Subtotal occlusion of the carotid artery is an important risk factor for stroke. Carotid artery stenting (CAS) is increasingly being used for treatment of carotid artery stenosis. There are many reports on treating carotid artery stenosis by CAS; however, there are few case reports on treating symptomatic carotid artery subtotal occlusion by CAS, especially in China. To test the clinical applicability, technical results, and morbidity of carotid artery stenting (CAS) in the treatment of symptomatic carotid artery subtotal occlusion. The clinical data of 27 symptomatic carotid artery subtotal occlusion patients treated with carotid stenting from January 2005 to December 2009 at the University Hospital of Hubei University for Nationalities and the West China Hospital of Sichuan University were systematically reviewed and analysed. Our results suggest that after balloon pre-dilation of the stenotic lesion, with the protection of a distal filter protection device, carotid stenting can be performed to treat symptomatic carotid artery subtotal occlusion patients with acceptable safety. However, the widespread use of this technique still requires large-scale clinical investigation.     

Dopplersonographie der extracraniellen hirnversorgenden und der peripheren Arterien bei Arterioskleroserisikofaktoren und bei KHE

Summary Doppler examination of the neck and limbs of 160 patients with a predisposition to arterial vascular disease in the presence of hypertension, hyperlipoproteinaemia, cigarette smoking and diabetes mellitus, 66% of whom had a combination of two or more of these risk factors, showed in 48 cases single or combined lesions in the arteries of the neck. One patient had unilateral stenosis of the common carotid artery; 20 patients showed unilateral and seven bilateral stenosis of the internal cartid artery; nine patients had unilateral and two patients bilateral stenosis of the external cartid artery; 28 patients showed unilateral flow reduction in the vertebral artery. There was a combined lesion of two or three vessels of the neck in 12 patients. Stenotic peripheral arteries at different levels of the leg with an ankle pressure index decrease of up to 22.5% were detected in 54 patients. These peripheral arterial diseases were associated with stenotic carotid arteries in 28%. Peripheral vascular lesions were present in 40% of patients with stenotic carotid arteries. Of the 160 patients studied 67 had proven coronary heart disease; 22% of these showed stenotic carotid arteries and 29% showed stenotic peripheral arteries. Combined lesions of the coronary, carotid and peripheral arteries were found in 13%.

     

Semi-automated carotid lumen segmentation in computed tomography angiography images

Carotid artery stenosis causes narrowing of carotid lumens and may lead to brain infarction. The purpose of this study was to develop a semi-automated method of segmenting vessel walls, surrounding tissues, and more importantly, the carotid artery lumen by contrast computed tomography angiography (CTA) images and to define the severity of stenosis and present a three-dimensional model of the carotid for visual inspection. In vivo contrast CTA images of 14 patients (7 normal subjects and 7 patients undergoing endarterectomy) were analyzed using a multi-step segmentation algorithm. This method uses graph cut followed by watershed and Hessian based shortest path method in order to extract lumen boundary correctly without being corrupted in the presence of surrounding tissues. Quantitative measurements of the proposed method were compared with those of manual delineation by independent board-certified radiologists. The results were quantitatively evaluated using spatial overlap surface distance indices. A slightly strong match was shown in terms of dice similarity coefficient (DSC) = 0.87_x005f
0.08; mean surface distance (Dmsd) = 0.32
0.32; root mean squared surface distance (Drmssd) = 0.49
0.54 and maximum surface distance (Dmax) = 2.14
2.08 between manual and automated segmentation of common, internal and external carotid arteries, carotid bifurcation and stenotic artery, respectively. Quantitative measurements showed that the proposed method has high potential to segment the carotid lumen and is robust to the changes of the lumen diameter and the shape of the stenosis area at the bifurcation site. The proposed method for CTA images provides a fast and reliable tool to quantify the severity of carotid artery stenosis.     

颈内动脉狭窄对体循环系统影响的集中参数模型研究

研究颈内动脉病变时对整个颈动脉系统的影响。建立了一个带颈动脉分岔血管分支的体循环系统集中参数模型,基于一组可较好地模拟正常生理曲线的参数,通过改变颈内动脉段的集总参数来模拟颈内动脉狭窄,研究了其对颈动脉系统的影响。结果表明:颈内动脉发生狭窄时,会使其自身的流量、压力和颈总动脉的流量发生显著变化。这些变化主要是由液阻和液容的改变引起的,其中液阻的影响更显著。此外,颈内动脉发生狭窄对于颈外动脉的影响较小,对颈总动脉入口压力则不产生影响。由此说明:利用三维计算模型对颈动脉分岔血管进行血流动力学数值模拟时,入口边界条件用入口压力比用入口流量更合适。     

Fatigue behaviour of Nitinol peripheral stents: The role of plaque shape studied with computational structural analyses

Fatigue resistance of Nitinol stents implanted into femoro-popliteal arteries is a critical issue for the particular biomechanical environment of this district. Hip and knee joint movements due to the cyclic daily activity expose the superficial femoral artery (SFA), and therefore the implanted stents, to quite large and cyclic deformations influencing stent fatigue resistance. Objective of this work is to provide a tool based on finite element analysis able to evaluate the biomechanical effect of SFA on stent fatigue resistance. Computer simulations of the treatment of stenotic vessel by angioplasty and stenting and of the subsequent in vivo loading conditions (axial compression and bending) were carried out. Three different stenotic vessel models were defined, by keeping a constant stenosis rate and changing the plaque sharpness and number of stenoses. The fatigue behaviour was analysed comparing the amplitude and mean value distribution of the first principal strain in the whole stent for the different simulated conditions. Results showed that the maximum mean strain is similar in all the models, while the alternating strain is related to both plaque shape and loading conditions. In conclusion, this study confirms the requisite of replicating in vivo loading conditions. It also reveals the importance of taking into account the thickness variation of the vessel in the stenotic zone in the assessment of the stent fatigue resistance.     

The spasm of resistance vessel concept of ischemic heart disease and other ischemic diseases

This communication will discuss the spasm of resistance vessel concept of ischemic heart disease and other ischemic diseases, and will focus on ischemic heart disease. The hypothesis is regarded as a separate model or paradigm, and is based on the principle that spasm of resistance vessels directly induces symptoms in these conditions. Resistance vessels help maintain vascular homeostasis through autoregulatory mechanisms, and spasm of resistance vessels is considered to represent 'inappropriate' activation of these mechanisms by disease states, which are equated with risk factors. For ischemic heart disease, the most important risk factor is stenotic coronary artery disease, and the concept asserts that severe ischemia secondary to coronary artery disease causes sufficient tissue injury to incite injury-spasm of resistance vessels. While it is universally accepted that occlusions of epicardial arteries by stenotic coronary artery disease, spasm, and thromboses directly induce clinical symptoms, the hypothesis suggests that these occlusions have other roles. The concept accepts all current treatments of ischemic heart disease, but as this disorder is viewed differently, a significantly different direction of research is proposed for improving its treatment and prevention.     

颈动脉斑块的流固耦合数值模拟

目的 提出一种基于真实颈动脉狭窄患者的双向流固耦合计算方法,用于分析不同类型颈动脉斑块在病变处的血流动力学参数以及斑块自身的变形和应力变化。方法 对颈动脉中度狭窄病人CT血管造影数据进行3D建模,分离出颈动脉血管壁模型和斑块模型,并进行瞬态流固耦合计算。模拟颈动脉粥样硬化的早期到形成斑块,将斑块类型分为增厚斑块、脂质斑块、混合斑块和钙化斑块,其中增厚斑块视为无斑块的情况,代表血管内膜-中膜的增厚,并对不同斑块类型的狭窄颈动脉进行对比分析。结果 不同类型斑块对整体血流流动影响不大,但是脂质斑块病变处的壁面剪切应力低于其余斑块;以增厚斑块作为对照,斑块出现会抑制动脉的扩张,其中脂质斑块最为明显;钙化斑块有最高的平均结构应力,相反脂质斑块平均结构应力则是最低。结论 本文所提出的计算方法能够同时对流体和固体区域进行分析。研究结果有助于更好理解不同斑块类型对于颈动脉病变处的影响。     

Platelet deposition in rabbit common carotid arteries promoted by arterial stenosis and spasm

Coronary arteriograhy in patients with ischemic heart disease often shows spasm of the coronary arteries. The question is whether spasm is a triggering factor for thrombosis in a stenotic artery. If so, what are the mechanisms for this? A stenosing teflon ring was applied to the right common carotid artery of anesthetized rabbits and l‐nor‐epinephrine was dripped over the outer surface of both carotid arteries, causing spasm. In control animals an indifferent solution did not cause spasm. Nineteen rabbits were killed 30 min or 24 h after treatment. Microscopically, arteries with stenosis and spasm contained thrombi nearby the stenosis significantly more often than arteries in control animals. In another 14 rabbits, killed at 30 min, the number of platelets on the intimal surface away from the stenosis was quantified. In arteries with both stenosis and spasm the counts were significantly greater than in arteries with no treatment. The intimal surface in stenotic and spastic arteries showed assumed imprints of eddying flow and endothelial injury downstream and upstream of the stenosis. Spastic arteries showed increased folding of the internal elastic membrane, altered endothelial cells, and adhering platelets. Spasm in a rabbit artery with a preformed stenosis facilitates thrombosis probably by creating increased flow disturbances. Spasm may induce endothelial injury, causing adherence of platelets.     

Changes in the cardiac output and general peripheral resistance after obstruction of a coronary artery

Summary The left subclavicular, both carotid and femoral arteries were shunted; then the brachiocephalic artery and aorta were obstructed, distal to the origin of the left subclavicular artery. The output of the left ventricle, excluding that portion of it which flows into the coronary arteries, entered the rotameter through theleft subclavicular artery and flowed into the distal sections of carotid arteries and the proximal sections of the femoral artery. The average blood pressure in the system was measured by an electric manometer. From these data, general peripheral resistance could be estimated according to the data obtained. An average blood pressure of 80–100 mm Hg is the best at which to study the changes of hemodynamic indices occurring after obstruction of the coronary artery.(Presented by V.V. Parin, Active Member of the AMN SSSR) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 50, No. 7, pp. 27–33, July, 1960     

弹性动脉狭窄血管内血液流动的ALE方法分析

针对弹性动脉狭窄血管中血液的流动状况,采用任意拉格朗日-欧拉方法(ALE),在给定相同的边界条件下模拟出了弹性血管和刚性血管中血流速度和压力的变化情况,分析了不同狭窄程度模型的血流状态.结果表明:随着动脉的狭窄程度增加,其中心流速越来越大,窄前压力逐渐升高,窄后压力逐渐降低,狭窄前后的压差单调增加;刚性血管无法对人体正常生理状态进行比较好的模拟,其计算所得结论与实际情况差距较大.说明ALE方法对血液流动的数值研究是可行的.     

Dynamic pressure-flow diagrams and impedance in experimental arterial stenosis and in stenotic atherosclerosis

Dynamic pressure-flow diagrams were generated by plotting instantaneous pulsatile pressure versus pulsatile flow measured 2 to 3 cm proximal to a graded arterial stenosis in the dog femoral artery. The slope of the major axis of the diagram was defined as the pulse impedance (Z _P).Z _p under steady-state and vasodilated conditions was linearly related to the large vessel impedance (Z _l) calculated independently as the average vascular input impedance modulus from 3 to 19 Hz, and linearly related to the hydraulic resistance of the stenosis. In a separate study,Z _p andZ _l each were shown to be greater in patients with severe femoral artery stenotic atherosclerosis than in control patents, and unchanged during vasodilation. In addition,Z _p andZ _l were reduced following femoral-to-popliteal bypass procedures. In summary, dynamic pressure-flow diagrams provide an easy-to-obtain index of the effective vascular hydraulic input impedance in stenotic arteries, and an index of the severity of distal arterial stenosis. This research has been supported in part by the North Carolina Heart Association (Grant-in-Aid 1974-75-A-62) and the Environmental Protection Agency, Chapel Hill, North Carolina.     

High-molecular-weight polyethylene oxide improves blood flow through stenosed carotid artery

Intravenous injection of 0.1 mg/kg high-molecular-weight polyethylene oxide (molecular weight 5.8×10⁶) to narcotized cats with experimental stenosis of the right carotid artery considerably improved blood flow and reduce blood pressure in stenosed vessel. This normalizes cerebral blood supply via both carotid arteries despite a decrease in systemic blood pressure. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 126, No. 7, pp. 60–62, July, 1998     

镍钛合金材料自膨胀式Wingspan支架置入症状性颅内动脉狭窄血管13例1年随访

背景：Gateway-WingSpan支架系统是2005年通过美国FDA认证,用于治疗症状性颅内动脉狭窄的镍钛合金自膨胀式支架,2007年在中国获得批准正式应用。 目的：进一步验证镍钛合金材料自膨胀式Wingspan支架置入治疗症状性脑动脉狭窄的近期疗效及安全性。 方法：对13例症状性脑动脉狭窄患者在Gateway球囊预扩下行Wingspan支架置入,狭窄病变18处,其中颈内动脉海绵窦段狭窄4处,大脑中动脉水平段狭窄11处,椎动脉末段狭窄2处,基底动脉下段狭窄1处。狭窄长度5~15 mm,平均狭窄(85.3±10.0)%。 结果与结论：支架置入成功率100%,置入后即刻造影显示残余狭窄在(30.5±12.0)%;随访12个月,未发生新发卒中或短暂性脑缺血发作,影像学检查显示均无再狭窄发生。13例均未出现与支架有关的动脉夹层、动脉内膜撕裂及急性支架内血栓形成等。均无支架的移位、断裂。支架置入后3 d内出现低血压 1例,皮下血肿1例,经治疗后症状均消失。结果显示镍钛合金材料自膨胀式Wingspan支架置入治疗症状性脑动脉狭窄比较安全,短期疗效较好。     

Development of anti-atherosclerotic tissue-engineered blood vessel by A20-regulated endothelial progenitor cells seeding decellularized vascular matrix

To investigate whether decellularized vascular tissues and A20-regulated endothelial progenitor cells can be used for constructing a transgenic tissue-engineered blood vessel with anti-atherosclerotic vascular stenotic properties. A20 gene-transfected endothelial progenitor cells differentiated endothelial cells and smooth muscle cells attached to and migrated into the decellularized porcine vascular scaffolding in a bioreactor. The histology of the conduits revealed viable and layered tissue. Scanning electron microscopy showed confluent, homogeneous tissue surfaces. The mechanical strength of the pulsed constructs was similar to that of the human artery. In vivo, the A20 gene-transfected tissue-engineered blood vessels were transplanted into the carotid artery of a rat for 6 months. Blood vessel xenotransplantation caused hyperacute rejection; all transplanted control blood vessels were completely rejected, but A20-transfected tissue-engineered blood vessels demonstrated good flow on implantation, and remained open for 6 months postoperatively, as assessed by Doppler. The HE stain demonstrated that the vessels were patent, without evidence of stenosis or dilatation after 6 months. These results demonstrate that transgenic tissue-engineered blood vessels have long-term patency and unique anti-stenotic properties.     

Persistent carotid-vertebral anastomosis associated with contralateral accessory middle cerebral artery

An exceptional case is reported of a complex anomaly of the brain arteries including low left carotid bifurcation, persistent carotid-vertebral anastomosis on the left and accessory middle cerebral artery on the right; the whole posterior circulation was only filled through the anomalous anastomotic vessel which joined with the vertebral artery because of a contralateral hypoplasic vertebral artery and the absence of both posterior communicating arteries. This association has not been previously reported in the literature. The embryological processes leading to these arterial anomalies are discussed. Risks related to a severe carotid stenosis or occlusion and to surgical or endovascular procedures in patients harboring these arterial anomalies are emphasized.     

血管壁弹性模量对颈动脉狭窄血流储备分数的影响

目的探索将血流储备分数(fractional flow reserve,FFR)引入颈动脉狭窄评估的可行性,并且分析血管壁弹性模量对颈动脉狭窄中血液动力学参数和FFR计算结果的影响。方法利用计算机辅助设计软件建立颈动脉分叉标准模型并获得不同狭窄率的模型。假设血管壁为线弹性材料,血液为不可压缩牛顿流体,在脉动流条件下,利用有限元分析软件进行颈动脉狭窄模型中血液流动的流固耦数值模拟,获得各种血液动力学参数,并计算相应的FFR值。结果当弹性模量固定时,随着狭窄率增加,模型中狭窄部位的FFR逐渐减小,且此时其弹性壁与刚性壁的FFR相对差异随着狭窄率的增加而增加;当狭窄率固定为70%时,随着弹性模量增加,FFR会逐渐减小。结论采用FFR对颈动脉狭窄程度进行功能性评估需要考虑血管壁弹性的影响;狭窄率越大,血管壁弹性模量对FFR的影响越大。     

Influence of carotid baroreceptors on different components of the vascular system

1. Reflex changes in wall tension of the lateral saphenous vein of one hind limb, the splenic veins and capsule, and the resistance vessels of the other hind limb caused by changes in baroreceptor activity were measured in vagotomized dogs under thiopentone-chloralose anaesthesia.2. Three different methods were used to alter pressure in one or both carotid sinuses. (1) Both carotid sinuses were vascularly isolated and filled with fully oxygenated Krebs-Ringer bicarbonate solution (pH 7.4) from a reservoir in which the pressure could be altered at will. (2) One sinus was denervated, and the contralateral sinus was perfused with arterial blood at different flow rates. (3) One sinus was denervated, and the innervated sinus was perfused with arterial blood at constant flow, the pressure being altered by changing the outflow resistance.3. The left saphenous vein was perfused at constant flow with autologous blood; changes in perfusion pressure were used as a measure of changes in veno-motor activity. The right common iliac artery was perfused at constant flow to measure changes in resistance vessel activity. Blood flow through the spleen was temporarily arrested, trapping a fixed volume of blood in the organ. Under these conditions, changes in splenic vein pressure were a measure of changes in smooth-muscle tension in the splenic capsule and veins.4. In order to assess the responses to baroreceptor stimulation in terms of alterations in sympathetic nerve traffic to different components of the peripheral vascular system, ;frequency-response curves' were constructed for spleen, saphenous vein, and limb resistance vessels by electrical stimulation of the splenic nerves and lumbar sympathetic chains.5. The saphenous vein showed no consistent response to changes in baroreceptor activity. Reduction in carotid sinus pressure from 180 to 100 mm Hg caused an increase in venous pressure in the isovolumetric spleen and in the iliac artery perfusion pressure. These results were confirmed by electrical stimulation of the carotid sinus nerve. Whereas the peak responses of the limb resistance vessels corresponded to an increase in lumbar sympathetic nerve traffic of 6-10 c/s, the maximal splenic responses were equivalent to an increase in splenic nerve traffic of 1-4 c/s. These results are consistent with selective autonomic nervous control of different components of the peripheral vascular system.     

Carotid ultrasonography: evaluation of carotid ultrasonography

Carotid ultrasonography is a laboratory procedure showing how arteriosclerosis screening can diagnose carotid artery occlusion and high-grade stenosis. It is useful for inspection in general practice because of its non-invasiveness, development of sonography equipment, laboratory methods and the establishment of an evaluation method. We generally use a linear array probe of around 7-8MHz for carotid observation and combine the B mode method, color and power Doppler method, and pulsed Doppler method for inspection. At vessel analysis, the intima-media thickness, the property of plaque and stenotic ratio are evaluated. We observe the direction of bloodstream and presence of stricture by Doppler color flow imaging and measure flow velocity and the wave pattern by pulsed Doppler method and, with the B mode method, evaluate the extent or degree of stricture. This can be depicted well using a convex array probe and sector array probe when the mental change caused by disease is deep, and depiction is difficult by linear array probe.