Session T14: Biological Fluid Dynamics: Medical Devices II

Vortex interactions and fluid mixing in the brain: targeted drug delivery for intraventricular hemorrhage patients

Approximately 65,000 people experience intracranial hemorrhage (ICH) in the United States per year. The mortality rate of ICH is more than 50%, where nearly half of the patients cannot survive the first 24 hours. The standard of patient care technology on ICH treatment has not seen significant development within the last 20 years, until a recent invention of a dual lumen catheter IRRAflow which utilizes active fluid exchange. In this study, we employed an immersed boundary-lattice Boltzmann method to evaluate the performance of the catheter in a patient specific 3-dimensional fluid-solid interaction model of the left lateral brain ventricle. We investigated the effectiveness of the active exchange mechanism by monitoring the irrigation of the stationary fluid inside the ventricle, vortex interactions, particle residence time, and dye simulation of the injected fluid. We discuss the chance of cloth formation using this delivery system, and compare it with clinical finding of IRRAflow in over 200 patients.   

An algebraic modeling of the modified assisted bidirectional Glenn

Infants born with a single ventricle require three stages of palliation. The long-term mortality rate for such patients is high, most associated with the unfavorable hemodynamics of the first and second stage operations. We previously proposed the assisted bidirectional Glenn (ABG) procedure and later the modified assisted bidirectional Glenn (mABG) procedure that offered several advantages over the conventional operations. However, the ABG could produce an SVC pressure that may be intolerable by patients. To avoid this complication, we relied on a specialized design of a nozzle in the mABG to improve the ejector pump effect in the SVC. Provided the variability in patient conditions, identifying the optimal nozzle design parameters requires expensive simulations that may not be feasible in a clinical setting. To overcome this challenge, in this study, we propose an algebraic model informed by CFD results that can accurately predict the key hemodynamics of the mABG without performing a CFD simulation. In this presentation, we will show the agreement between the proposed model and CFD as well as the behavior of the ABG circulation versus pulmonary vascular resistance and nozzle parameters. 

    

Visualization of partial occlusions and occlusions near bifurcations in coronary arteries and optimization of saline flushing for optical imaging

Diagnosis and treatment of partial or total occlusions within the coronary arteries can be improved by optical visualization of the occlusion and surrounding artery walls. We investigate, using CFD simulations and invitro experiments, the operation of a novel two-lumen saline suction-injection catheter to understand the saline-blood interactions that clears the volume between the imaging tip and the occlusions and arterial walls. The hemodynamics that control the mixing and evacuation of the opaque blood, and its substitution by transparent saline, are measured. The influence of artery curvature, as well as of arterial bifurcation are investigated since occlusions are commonly found after the left main coronary artery bifurcation. Operational parameters such as distance between the catheter tip and the blockage, as well as the suction-injection flow rates are analyzed to determine the optimum range of operation for saline-flushing catheters.   

Effect of Upper body venoarterial extracorporeal membrane oxygenation on end-organ oxygen delivery in respiratory failure patients: a computational study

Extracorporeal membrane oxygenation (ECMO) is a vital technology increasingly deployed to maintain patients with profound respiratory failure as a bridge to transplantation. Patients with end-stage lung disease complicated by right ventricle failure, e.g. patients with interstitial pulmonary fibrosis, are challenging to support as they are not amenable to traditional cannulation strategies. Use of upper body venoarterial ECMO has been reported promising despite limited understanding of its effects on end-organ perfusion and the oxygenated blood distribution within the systemic circulation. Leveraging computational tools, we developed a model of upper body VA ECMO to quantify oxygen transport and distribution in the aortic tree of a patient-specific anatomy. Clinical flow waveforms and dynamic boundary conditions, i.e. lumped parameter models, were applied to maximize the outcome accuracy. Results revealed direct relation between ECMO flow share, geometrical morphology, as well as systematic vascular resistances and oxygen distribution of different vital organs. Comprehensive computational modeling, similar to the one presented here, offers indispensable platforms with the potential of providing invaluable clinical insights to enhance therapy planning and improve patient outcomes.   

The dynamics of platelets in high residence time fluid elements in the left ventricle under LVAD support. A Numerical Hemodynamics Study of Lagrangian tracking in Intraventricular Flows.

Left Ventricular Assist Devices (LVAD) are auxiliary pumps that contribute to the pumping from failing left ventricle. As such, they provide life-sustaining treatment for end-stage HF. The most probable cause for mortality and morbidity is stroke associated with thrombosis. We study the potential for activation and aggregation of platelets within the left ventricle(LV) caused by altered hemodynamics, using Lagrangian tracking in a moving-wall ventricle flow simulation. A numerical model was created from patient-specific ventricle wall motion segmented from Cine-CT. Platelet-mimicking flow tracers were injected during mitral valve openings and tracked for over 15 cardiac cycles. Platelet trajectory statistics showed groups of platelets that stayed together for extremely long times, despite having their origins at several injection times. Platelets accumulate and stay, unmixed with other fluid elements until they leave the LV as a cohesive cluster. The extremely long residence time in the LV, together with the close proximity with other activated platelets, could identify the occurrence of LVAD-induced LV hemodynamics that cause the high risk of thrombosis.   

Hemodynamic Indicators of Cerebrovascular Accidents in Patients Implanted With a Left Ventricular Assist Device

Left Ventricular Assist Device (LVAD) is a mechanical circulatory support pump used for advanced Heart Failure (HF) treatment which operates by directing blood flow from a dysfunctional left ventricle directly into the aorta. Despite the emergence of LVAD as a primary treatment modality for HF, it is associated with serious complications such as stroke and thromboembolic events (referred to here as cerebrovascular accidents or CVA) after deployment. Such post-implant complications can reduce treatment efficacy and lead to fatalities as well. These complications have been found to be intimately linked to LVAD induced altered state of hemodynamics, particularly in the aorta. In this study we quantify several aortic hemodynamic features – namely wall shear stress, local normalized helicity, Q-criterion, and energy dissipation through virtual surgical in-silico models of patients treated with LVAD. We will illustrate these hemodynamic descriptors through patient-specific CFD simulations in two cohorts of patients – one with post-implant CVA and one without. Quantitative descriptors will be compared and contrasted across the two cohorts to identify hemodynamic indicators for CVA in patients with an LVAD.   

Predictive Embolic Trajectory Mapping of Ventricular Assist Device Outflow

A left ventricular assist device (LVAD) is a mechanical pump that provides circulatory support as a bridge-to-cardiac transplantation in patients with advanced heart failure. A potential adverse event of LVAD support is thrombus ingestion or formation, which may then travel through the device into the cerebral arteries, causing ischemic strokes. It has been previously demonstrated (mostly numerical in nature) that the graft geometrical parameters can affect the trajectories of particles. In this in-vitro study, we expand and verify such a hypothesis in a refractive-index-matched time-resolved particle tracking velocimetry (PTV) system. For this purpose, four patients with implanted LVADs are recruited. In addition to the existing cannula configurations, two other anastomosis angles with a comparable curvature and location are considered. Thin-wall phantoms of such models are 3D-printed with precision and placed in a flow loop providing physiological flow conditions. Precision fluorescent beads ranging from 0.02 to 1.0 mm are used to replicate emboli at two clinically relevant flow rates, spanning over 100 experimental cases combined. This systematic study identifies the optimal graft candidate that reduces the number of thrombi-replicating particles reaching cerebral vessels.   

Device for Simultaneous Pulse Pressure Waveform Acquisition and Analysis at the Carotid Arteries

Cardiovascular disease has become a more prominent threat as life expectancy continues to increase. To address this incremental need, noninvasive cardiovascular diagnostic methodologies are being explored to monitor cardiovascular health. Of particular interest is the pulse pressure waveform, the waveform generated by the superposition of propagating pressure waves in the cardiovascular system. Current methodologies analyze waveforms at a single point in space, utilizing waveform features to assess cardiovascular health. In this analysis the carotid artery has proven to be of greater clinical significance given its proximity to the heart. To this end, in the Gharib Lab we have developed a device for simultaneous acquisition of pressure waveforms at both carotid arteries. Simultaneous waveform acquisition adds a new dimension to current pulse wave analysis, allowing for feature comparison of the same pulse at different locations. This system could be a promising methodology to monitor and diagnose diseases that generate wave propagation asymmetries throughout the cardiovascular system.   

Hybrid RANS-LES Modeling of the FDA Round Robin Blood Pump

            Blood-contacting medical devices such as blood pumps possess an inherent need for accurate predictions of hemodynamic flow fields. Due to the lack of standardized validation methods for the use of computational fluid dynamics (CFD) simulations in medical devices, the Food and Drug Administration (FDA) developed an idealized centrifugal blood pump and established an inter-laboratory study to gather Particle Image Velocimetry (PIV) data for CFD Validation. This work focuses on benchmarking hybrid RANS-LES models (SBES, IDDES SST) and a standard RANS model (k-ω SST) against PIV results published online for three flow  conditions. Results show that the hybrid SBES and IDDES SST models more accurately predicted velocity fields. The SBES model slightly overpredicted pressure rise of the pump where the k-ω SST possessed a tighter fit to experimental data.

Hemodynamically-efficient graft design for endovascular repair in type B aortic dissection

Aortic dissection (AD) is a catastrophic cardiovascular event that is associated with considerable mortality. AD occurs as a result of a tear in the aortic wall, permitting blood to track within the aortic media layer under arterial pressure, separating the aorta into two separate lumens: the True Lumen (TL) and the False Lumen (FL). Current treatment for AD is medical anti-impluse therapy followed by thoracic endovascular aortic repair (TEVAR) to cover the proximal entry tear and induce FL thrombosis in order to decrease the risk of aneurysm formation and aortic rupture. However, only limited information on the fluid dynamics in dissected aortas after TEVAR has been reported in the literature. In this study, we used a computational approach employing a fluid-structure Interaction Lattice-Boltzmann method coupled with the Immersed Boundary technique to model various graft lengths and flow conditions. Results demonstrate a trend towards increased FL flow reversal as the graft length increases but also increased left ventricular workload, which can ultimately lead to heart failure. These finding suggest that there may exist an optimum graft length that can lead to improved long-term clinical outcomes.   

Residence time in intracranial aneurysms treated with coils: Experimental in-vitro analysis using PLIF.

Endovascular coiling is a common technique to treat cerebral aneurysms. The goal is to induce stagnation in the aneurysmal sac, with subsequent thrombosis and exclusion of the hemodynamics stresses on the aneurysmal wall, avoiding further remodeling and rupture. Hemodynamics in the coiled aneurysm is studied to characterize the treatment outcome. Hemodynamics metrics are often studied numerically due to the impossibility to visualize the coils in a clinical scan. In this study, we evaluate residence time (RT) in coiled aneurysms experimentally. Seven phantoms of arteries with aneurysm were used, reconstructed from clinical images. These models were coiled and rhodamine was injected in the aneurysmal sac. The parent vessel was filled with a transparent mixture (water, glycerol and sodium) with properties close to those of blood. Planar Laser Induced Fluorescence (PLIF) was used to measure the rhodamine wash-out time experimentally for each patient. RT was assessed through image analysis of the decay of the fluorescence intensity in the PLIF images. The results were compared with CFD simulations of the same coiled aneurysm. This study allows to validate the numerical model, but also to determine a surrogate for true RT in coiled aneurysm.   

Multi-Parametric Computational Investigation of Stent Design for Tortuous Coronary Artery Bifurcations

Our work aims to investigate the effect of individual branch tortuosities on the atherosclerotic susceptibility at the left main coronary bifurcation, and further investigate parameters related to the design of coronary stents in order to best suit the requirements of such arteries.

The bend angle has been used as the metric for tortuosity, being varied between a physiological range of 15 and 60 degrees for each branch, while keeping the other two branches unchanged. Idealized geometries of the left coronary artery have been analyzed, and the impact of the stent design parameters strut spacing and lumen protrusion on the hemodynamics of the vessel for both straight and tortuous arteries studied, using CFD.

Our results indicate that the LAD branch is most significantly impacted by a change in the curvature of either of the branches and may hence be said to be most susceptible to the formation of atherosclerotic lesions. The OSI is observed to be impacted only in the curved branch. In general, an increase in tortuosity is associated with an up to 13% increase in the average TAWSS.

An increase in the strut spacing is observed to be associated with improved hemodynamics at the coronary bifurcation. The introduction of a stent in the LM branch is observed to have an ameliorating impact on the hemodynamics of the LAD branch, with an up to 23% reduction in adverse TAWSS regions being observed with an increase in strut spacing. Additionally, greater lumen protrusion increases both adverse TAWSS (max. 15%) and adverse RRT (max. 18%) regions.