Eric Poon

Numerical investigations of the haemodynamic changes associated with stent malapposition in an idealised coronary artery

The deployment of a coronary stent near complex lesions can sometimes lead to incomplete stent apposition (ISA), an undesirable side effect of coronary stent implantation. Three-dimensional computational fluid dynamics (CFD) calculations are performed on simplified stent models (with either square or circular cross-section struts) inside an idealised coronary artery to analyse the effect of different levels of ISA to the change in haemodynamics inside the artery. The clinical significance of ISA is reported using haemodynamic metrics like wall shear stress (WSS) and wall shear stress gradient (WSSG). A coronary stent with square cross-sectional strut shows different levels of reverse flow for malapposition distance (MD) between 0mm and 0.12 mm. Chaotic blood flow is usually observed at late diastole and early systole for MD=0mm and 0.12 mm but are suppressed for MD=0.06 mm. The struts with circular cross section delay the flow chaotic process as compared to square cross-sectional struts at the same MD and also reduce the level of fluctuations found in the flow field. However, further increase in MD can lead to chaotic flow not only at late diastole and early systole, but it also leads to chaotic flow at the end of systole. In all cases, WSS increases above the threshold value (0.5 Pa) as MD increases due to the diminishing reverse flow near the artery wall. Increasing MD also results in an elevated WSSG as flow becomes more chaotic, except for square struts at MD=0.06 mm.

Advances in three-dimensional coronary imaging and computational fluid dynamics: is virtual fractional flow reserve more than just a pretty picture?

Percutaneous coronary intervention (PCI) has shown a high success rate in the treatment of coronary artery disease. The decision to perform PCI often relies on the cardiologist’s visual interpretation of coronary lesions during angiography. This has inherent limitations, particularly due to the low resolution and two-dimensional nature of angiography. State-of-the-art modalities such as three-dimensional quantitative coronary angiography, optical coherence tomography and invasive fractional flow reserve (FFR) may improve clinicians’ understanding of both the anatomical and physiological importance of coronary lesions. While invasive FFR is the gold standard technique for assessment of the haemodynamic significance of coronary lesions, recent studies have explored a surrogate for FFR derived solely from three-dimensional reconstruction of the invasive angiogram, and therefore eliminating need for a pressure wire. Utilizing advanced computational fluid dynamics research, this virtual fractional flow reserve (vFFR) has demonstrated reasonable correlation with invasive measurements and remains an intense area of ongoing study. However, at present, several limitations and computational fluid dynamic assumptions may preclude vFFR from widespread clinical use. This review demonstrates the tight integration of advanced three-dimensional imaging techniques and vFFR in assessing coronary artery disease, reviews the advantages and disadvantages of such techniques and attempts to provide a glimpse of how such advances may benefit future clinical decision-making during PCI.

Endothelial shear stress 5 years after implantation of a coronary bioresorbable scaffold

Aims As a sine qua non for arterial wall physiology, local hemodynamic forces such as endothelial shear stress (ESS) may influence long-term vessel changes as bioabsorbable scaffolds dissolve. The aim of this study was to perform serial computational fluid dynamic (CFD) simulations to examine immediate and long-term haemodynamic and vascular changes following bioresorbable scaffold placement. Methods and results Coronary arterial models with long-term serial assessment (baseline and 5 years) were reconstructed through fusion of intravascular optical coherence tomography and angiography. Pulsatile non-Newtonian CFD simulations were performed to calculate the ESS and relative blood viscosity. Time-averaged, systolic, and diastolic results were compared between follow-ups. Seven patients (seven lesions) were included in this analysis. A marked heterogeneity in ESS and localised regions of high blood viscosity were observed post-implantation. Percent vessel area exposed to low averaged ESS (<1 Pa) significantly decreased over 5 years (15.92% vs. 4.99%, P < 0.0001) whereas moderate (1-7 Pa) and high ESS (>7 Pa) did not significantly change (moderate ESS: 76.93% vs. 80.7%, P = 0.546; high ESS: 7.15% vs. 14.31%, P = 0.281), leading to higher ESS at follow-up. A positive correlation was observed between baseline ESS and change in lumen area at 5 years (P < 0.0001). Maximum blood viscosity significantly decreased over 5 years (4.30 ± 1.54 vs. 3.21± 0.57, P = 0.028). Conclusion Immediately after scaffold implantation, coronary arteries demonstrate an alternans of extremely low and high ESS values and localized areas of high blood viscosity. These initial local haemodynamic disturbances may trigger fibrin deposition and thrombosis. Also, low ESS can promote neointimal hyperplasia, but may also contribute to appropriate scaffold healing with normalisation of ESS and reduction in peak blood viscosity by 5 years.

Numerical and experimental investigations of the flow–pressure relation in multiple sequential stenoses coronary artery

Virtual fractional flow reserve (vFFR) has been evaluated as an adjunct to invasive fractional flow reserve (FFR) in the light of its operational and economic benefits. The accuracy of vFFR and the complexity of hyperemic flow simulation are still not clearly understood. This study investigates the flow–pressure relation in an idealised multiple sequential stenoses coronary artery model via numerical and experimental approaches. Pressure drop is linearly correlated with flow rate irrespective of the number of stenosis. Computational fluid dynamics results are in good agreement with the experimental data, demonstrating reasonable accuracy of vFFR. It was also found that the difference between data obtained with steady and pulsatile flows is negligible, indicating the steady flow may be used instead of pulsatile flow conditions in vFFR computation. This study adds to the current understanding of vFFR and may improve its clinical applicability as an adjunct to invasively determined FFR.

Five-year follow-up of underexpanded and overexpanded bioresorbable scaffolds: self-correction and impact on shear stress

textabstractUnderexpansion and overexpansion have been incriminated as causative factors of adverse cardiac events. However, dynamic biological interaction between vessel wall and scaffold may attenuate the adverse haemodynamic impact of overexpansion or underexpansion.

Computational fluid dynamics study of common stent models inside idealised curved coronary arteries

Abstract The haemodynamic behaviour of blood inside a coronary artery after stenting is greatly affected by individual stent features as well as complex geometrical properties of the artery including tortuosity and curvature. Regions at higher risk of restenosis, as measured by low wall shear stress (WSS < 0.5 Pa), have not yet been studied in detail in curved stented arteries. In this study, three-dimensional computational modelling and computational fluid dynamics methodologies were used to analyse the haemodynamic characteristics in curved stented arteries using several common stent models. Results in this study showed that stent strut thickness was one major factor influencing the distribution of WSS in curved arteries. Regions of low WSS were found behind struts, particularly those oriented at a large angle relative to the streamwise flow direction. These findings were similar to those obtained in studies of straight arteries. An uneven distribution of WSS at the inner and outer bends of curved arteries was observed where the WSS was lower at the inner bend. In this study, it was also shown that stents with a helical configuration generated an extra swirling component of the flow based on the helical direction; however, this extra swirl in the flow field did not cause significant changes on the distribution of WSS under the current setup.

Effects of streamwise rotation on the dynamics of a droplet

An initially streamwise rotating droplet released into a uniform cross flow is studied numerically. The computations are performed using a finite volume Navier–Stokes solver which employs a moving mesh interface tracking scheme to locate the interface. With a large initial Weber number (Wei = 40) the streamwise rotating droplet flattens along the free stream direction more quickly as rotation rate (Ω*) increases, and leads to a dramatic increase in the dynamic drag coefficient (CD/A*, where A* is the dimensionless frontal area). On the other hand, for Wei = 4 and 0.4 at Ω*≥0.6, the flattening of the droplet is less pronounced and the droplet even restores to spherical shape, hence, CD/A* decreases sharply. The dynamic drag coefficient even becomes negative for Wei = 4 and 0.4 at Ω*=1. At the largest deformation, the droplet can be classified into three major shapes: biconvex, convex-concave, and biconcave. One dominant feature of the wake downstream of the droplet is the formation and convection of vortex...

Haemodynamic effects of incomplete stent apposition in curved coronary arteries

Incomplete stent apposition (ISA, also known as malapposition) is a complication that affects day-to-day coronary stenting procedures. ISA is more prominent in complex arterial geometries, such as curvature, asa result of the limited conformability of coronary stents. These malapposed struts disturb the otherwise near-wall laminar blood flow and form a micro-recirculation environment. The micro-recirculation environment is often associated with low wall shear stress (WSS) and upsets the delicate balance of vascular biology, providing possible nidus for thrombosis and restenosis. In this study, a three-dimensional (3D) stent model was virtually deployed into an idealised curved coronary artery. Computational fluid dynamics (CFD) simulations were carried out to systematically analyse the haemodynamic effects of increasing maximum ISA distances, ranging from 180 (moderate), 400 (intermediate) to 910μm (severe) in an artery with decreasing radius of curvature (ROC). Micro-recirculations around both proximal and distal malapposed struts become more pronounced as compared to fully-apposed struts. The accompanying areas of low temporally-averaged WSS (AL-TAWSS) can increase twofold compared to the fully-apposed condition. Furthermore, substantial regions (∼5.2% and 9.0%) of AL-TAWSS are detached from the distal end of the malapposed struts in both moderate and intermediate cases respectively. Malapposed stents also induce more variation of TAWSS at the inner bend of the artery. At the stent surface, maximum WSS increases threefold from the fully-apposed case to intermediate ISA. High WSS on the strut surface is known to activate platelets which when exposed to the micro-recirculation environment may lead to their deposition and thrombosis.

Improvement in local haemodynamics 5 years after implantation of a coronary bioresorbable scaffold: A pulsatile non-Newtonian shear stress analysis

A 3.0 × 18 mm Absorb bioresorbable vascular scaffold (Abbott Vascular, Santa Clara, CA, USA) was implanted in the left circumflex coronary artery of patient with stable angina pectoris. Optical coherence tomography (OCT) revealed a well-expanded and apposed scaffold (pullback speed: 18 mm/s, acquisition rate: 180 frames/s). A patient-specific 3D geometry of the scaffolded lumen was generated by fusing OCT with coronary angiography. Pulsatile computational fluid dynamic (CFD) simulations were carried out by solving the Navier–Stokes equations. Blood was modelled as non-Newtonian fluid. Endothelial shear stress (ESS) at lumen and scaffold surfaces was calculated as product of local blood viscosity and near-wall velocity gradient. During low-coronary flow in systole, low-ESS predominates (PanelsA and C) whereas at peak diastolic flow the vessel is exposed to high-ESS (PanelsB and D). Irrespective of systolic or diastolic phase, the post-implantation vessel appears highly corrugated with an alternans of high-ESS on top of the struts and low-ESS between struts (PanelsA and B), that has been associated with blood micro-recirculation, fibrin deposition, and platelet aggregation. By 5 years, heterogeneity in ESS has largely dissipated (PanelsC and D, see Supplementary data online, Videos S1) while ESS distribution narrows and becomes more homogenous in both systole and diastole (PanelE). Histograms depicting the overall percent area of the simulated vessel exposed to varying levels of ESS during each of the studied flow conditions and time-points (PanelE) demonstrate that vessel exposure to very low and very high ESS decreases, leading to an overall increase in mid-range values of ESS generally considered to be more physiologic.

Coronary optical coherence tomography-derived virtual fractional flow reserve (FFR): anatomy and physiology all-in-one

Eric K.W. Poon, Vikas Thondapu, Eve Revalor, Andrew Ooi, and Peter Barlis* Department of Mechanical Engineering, Melbourne School of Engineering, The University of Melbourne, Melbourne, VIC 3010, Australia; Department of Medicine, Melbourne Medical School, Faculty of Medicine, Dentistry & Health Sciences, The University of Melbourne, Melbourne, VIC 3010, Australia; and Department of Biomedical Engineering, Melbourne School of Engineering, The University of Melbourne, Melbourne, VIC 3010, Australia * Corresponding author. Tel: 161 3 9928 6370, Fax; 161 3 9928 6372, Email: pbarlis@unimelb.edu.au