冠脉支架虚拟植入过程的流固耦合动力学模型

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  摘 要:考虑了弯曲冠脉和血管支架的变形对腔内血流动力学特性的影响,构建了扩张冠脉-剪切细化血液流体的顺序流固耦合动力学模型。研究了血管支架连接筋几何结构(S型和N型)和血管狭窄率(24%、40%、50%)对血流动力学特性的影响,分析了血管支架介入引起的血管等效应力和血流脉动作用下的壁面剪切应力,进而评价了血管斑块的生物力学损伤。数值结果表明,血管斑块的高等效应力位于与血管支架连接筋接触的血管内最狭窄区域,血管斑块的高壁面剪切应力位于斑块组织径向下垂最显著区域。随着血管狭窄率的增加,血管斑块的等效应力和时间平均的壁面剪切应力显著上升,血管斑块面临的脆性断裂风险增强。与N型血管支架相比,S型血管支架具有更高的峰值壁面等效应力和壁面剪切应力,易于引起血管斑块壁面的脆性断裂。综上,所建立的冠脉支架虚拟植入过程的流固耦合动力学模型,对于评价血管损伤进而优化血管支架设计具有重要理论意义。
  关键词:冠脉支架;流固耦合;血管壁面损伤;血管狭窄率;连接筋结构
  DOI:10.15938/j.jhust.2021.03.003
  中图分类号: TP391.9;R318.01
  文献标志码: A
  文章编号: 1007-2683(2021)03-0019-07
  A Dynamic Fluid-solid Interaction Model for Virtual
  Implantation of Coronary Stents
  JIANG XU-dong1, XU Xin-bo1, TENG Xiao-yan2
  (1.School of Mechanical Power and Engineering, Harbin University of Science and Technology, Harbin 150080, China;
  2.School of Mechanical and Electrical Engineering, Harbin University of Engineering, Harbin 150001, China)
  Abstract:A sequential fluid-solid interaction model of expanded coronary-shear thinning blood is developed accounting for effect of both stent and vessel deformation on haemodynamics. The proposed computational model is employed to investigate the influence of stent link with different geometrical configurations (S-, N-shape) and vascular stenosis rates (24%, 40%, 50%) on haemodynamics. Both induced equivalent stress during stent implantation and wall shear stress by pulsating blood flow are obtained for biomechanical injury assessment within atherosclerosis plaque. The numerical results indicate that the developed high equivalent stress is located at the interface between the narrowest vascular lumen and stent link while high wall shear stress on the plaque with maximal radical tissue prolapse. With increasing vascular stenosis rates, the experienced equivalent stress and time-averaged wall shear stress remarkably increase leading to higher risk of plaque rupture. Compared with N-type stent, higher peak equivalent stress and wall shear stress are observed on the plaque for S-type stent implying a more vulnerable plaque prone to rupture. Consequently the developed fluid-interaction model will provide a theoretical significance for assessment of vascular injure and optimal design of coronary stent.
  Keywords:coronary stent; fluid-solid interaction; vascular wall injure; stenosis rate; stent link
  0 引 言
  血管支架介入術已成为动脉粥样硬化斑块导致冠脉狭窄问题的最有效治疗手段。但是,血管支架植入改变了狭窄血管的生物力学环境,在经历高等效应力和壁面剪应力作用下的损伤斑块可能衍生裂纹形成易脆性斑块[1-3]。通过临床手段难于鉴别易碎性斑块,而通过冠脉支架虚拟置入技术评估斑块的脆性断裂风险,对于抑制斑块破裂局部血栓形成和优化介入治疗方案具有重要临床价值。   2 擴张血管的血流动力学模型
  2.1 控制方程
  对于黏性不可压缩血液流体的Navier-Stokes方程表示为:
  ρfdVdt-·(-pI+μ(V+V))=0
  ·V=0(8)
  式中:ρf、μ为血液的密度和动力黏度;V、p为血液的速度场和压强场变量。
  鉴于人体血管在剪切速率γ·低于100s-1时所呈现的剪切稀化效应-黏度的率相关性,使用非牛顿Bird-Carreau模型描述血液的动力黏度,则有:
  μ=μ∞+(μ∞+μ0)[1+(λ-γ·)2]q-12(9)
  式中:μ代表血液流体动力黏度;μ0=0.056Pa·s、 μ∞=0.00345Pa·s分别表示低、高剪切黏度;λ-=3.31代表时间常数;q=0.357表示幂指数(上述模型参数参考文[25])。
  2.2 边界条件
  以扩张冠脉血管(含血管支架)内腔为刚性壁面,在内腔壁面施加无滑移运动约束。血管出口设置为零压强边界条件,入口速度指定为完全Hagen–Poiseuille速度轮廓。为了模拟人体冠脉的脉动流动条件,扩张血管入口中心的瞬时速度采用如图2所示的周期性曲线轮廓(心脏周期T=0.8s)。上述边界条件由式(10)表示为
  V=0 on Ωwall
  V=-V0n on Ωinlet
  p=0 on Ωoutlet(10)
  式中:V0是扩张血管入口中心沿切向矢量n的瞬时速度幅值。
  2.3 壁面剪切应力
  血流脉动引起的粘性应力-壁面剪切应力是导致冠脉血管损伤的血流动力学因素,过低的壁面剪切应力(<0.5Pa)有助于动脉粥样硬化斑块的形成,而过高的壁面剪切应力将有助于斑块的脆性断裂。壁面剪切应力τw定义为
  τw=-(-pI+μ(V+V))·nw(11)
  式中nw为单位壁面切向矢量。
  时间平均壁面剪切应力是壁面剪切应力在一个心脏脉动周期的平均值,其空间分布用于评价扩张血管内的血流动力学环境。时间平均壁面剪切应力〈τw〉表示为
  〈τw〉=1T∫T0|τw|dt(12)
  式中T为心脏循环周期(T=0.8s)。
  3 结果与讨论
  3.1 血管的等效应力
  血管斑块的高等效应力位于与血管支架连接筋接触的血管内最狭窄区域,随着血管狭窄率的增加,血管斑块的等效应力显著上升,血管斑块面临的脆性断裂风险增强,如图3所示。根据Schiavone[26],血管斑块的峰值等效应力为1.30MPa,本文对于S型支架引起的斑块峰值等效应力为1.25MPa(狭窄率为50%),数值模型与文[26]的计算误差为3.8%,验证了本文血管支架介入系统非线性有限元模型的有效性。与N型血管支架相比,S型血管支架具有更高的峰值壁面等效应力,易于引起血管斑块壁面的脆性断裂,如图4所示。
  3.2 血管的壁面剪切应力
  血管斑块的高壁面剪切应力位于斑块组织径向下垂最显著区域,如图5所示,随着血管狭窄率的增加,血管斑块的时间平均壁面剪切应力显著上升,血管斑块面临的脆性断裂风险增强。
  在入口血流速度达到峰值时,血管斑块的壁面剪切应力达到最大值,如图6所示。根据Timothy[16],血管壁面峰值剪切应力为13.5Pa,本文对于S型支架引起的壁面峰值剪切应力为14.5Pa(狭窄率为50%),数值模型与文[16]的计算误差为7.4%,验证了本文血管支架耦合系统血流动力学模型的有效性。
  此外,与N型血管支架相比,S型血管支架具有更高的峰值壁面剪切应力,易于引起血管斑块壁面的脆性断裂,如图7所示。
  4 结 论
  建立了扩张冠脉-剪切细化血液流体的顺序流固耦合动力学模型,以血管斑块的峰值等效应力和壁面剪切应力为指标,对比分析了血管支架连接筋结构和血管狭窄率对血管壁面损伤的影响,获得如下研究结论:
  1) 血管斑块的高等效应力位于与血管支架连接筋接触的血管内最狭窄区域,随着血管狭窄率的增加,血管斑块的壁面等效应力显著增长。
  2) 血管斑块的高剪切应力位于斑块组织径向下垂最显著区域,随着血管狭窄率的增加,血管斑块的时间平均壁面剪切应力显著增长。
  3)与N型血管支架相比,S型血管支架具有更高的峰值壁面等效应力和壁面剪切应力,易于引起血管斑块壁面的脆性断裂。
  综上,所建立的血管支架耦合系统血流动力学模型能够确定连接筋结构、狭窄率和斑块易脆性之间的联系,对于合理选择支架、设计支架以及冠状动脉狭窄介入性治疗手术规划具有科学的指导意义。
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  (编辑:温泽宇)
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