Poelma Christian, Watton Paul N, Ventikos Yiannis
Laboratory for Aero and Hydrodynamics, Delft University of Technology, Delft, The Netherlands
Department of Computer Science and INSIGNEO Institute of In Silico Medicine, University of Sheffield, Sheffield, UK.
J R Soc Interface. 2015 Apr 6;12(105). doi: 10.1098/rsif.2014.1394.
Haemodynamic forces appear to play an influential role in the evolution of aneurysms. This has led to numerous studies, usually based on computational fluid dynamics. Their focus is predominantly on the wall shear stress (WSS) and associated derived parameters, attempting to find correlations between particular patterns of haemodynamic indices and regions subjected to disease formation and progression. The indices are generally determined by integration of flow properties over a single cardiac cycle. In this study, we illustrate that in some cases the transitional flow in aneurysms can lead to significantly different WSS distributions in consecutive cardiac cycles. Accurate determination of time-averaged haemodynamic indices may thus require simulation of a large number of cycles, which contrasts with the common approach to determine parameters using data from a single cycle. To demonstrate the role of transitional flow, two exemplary cases are considered: flow in an abdominal aortic aneurysm and in an intracranial aneurysm. The key differences that are observed between these cases are explained in terms of the integral timescale of the transitional flows in comparison with the cardiac cycle duration: for relatively small geometries, transients will decay before the next cardiac cycle. In larger geometries, transients are still present when the systolic phase produces new instabilities. These residual fluctuations serve as random initial conditions and thus seed different flow patterns in each cycle. To judge whether statistics are converged, the derived indices from at least two successive cardiac cycles should be compared.
血流动力学力似乎在动脉瘤的发展过程中发挥着重要作用。这引发了众多研究，这些研究通常基于计算流体动力学。其主要关注点是壁面剪应力（WSS）及相关的派生参数，试图找出特定血流动力学指标模式与易发生疾病形成和进展的区域之间的相关性。这些指标通常通过在单个心动周期内对血流特性进行积分来确定。在本研究中，我们表明在某些情况下，动脉瘤内的过渡流可导致连续心动周期中壁面剪应力分布显著不同。因此，准确确定时间平均血流动力学指标可能需要模拟大量周期，这与使用单个周期数据来确定参数的常见方法形成对比。为了证明过渡流的作用，我们考虑了两个示例情况：腹主动脉瘤内的血流和颅内动脉瘤内的血流。根据过渡流的积分时间尺度与心动周期持续时间的比较来解释这些情况之间观察到的关键差异：对于相对较小的几何结构，瞬态将在下一个心动周期之前衰减。在较大的几何结构中，当收缩期产生新的不稳定性时，瞬态仍然存在。这些残余波动充当随机初始条件，从而在每个周期中引发不同的流型。为了判断统计数据是否收敛，应比较至少两个连续心动周期得出的指标。
