Session ML: Biofluids: Physiological Cardiovascular I

High resolution simulation of the left heart hemodynamics in patient-specific anatomies

Understanding left-ventricle (LV) hemodynamics is critical prerequisite for developing new methods for diagnosing, treating and managing left heart dysfunction diseases. We develop a high resolution computational model of the left heart based on data from MRI scan images from a healthy volunteer and develop a physiologic, cell-activation based model for calculating the kinematics of the LV chamber wall motion. At the mitral position, uniform pulsatile flow is specified while a bi-leaflet mechanical heart valve is placed at the aortic position. The CURVIB fluid-structure interaction methodology of Borazjani et al. (J. Comp. Physics, 2008) is extended to simulate the flow and ensuing motion of the aortic valve leaflets. The computed results show that the LV motion resulting from the model gives rise to global left heart parameters (e.g. heart rate, ejection fraction etc.) that are well within the human physiologic range. In addition the computed flow patterns during diastole are found to be in good agreement with previous in vitro and in vivo experimental observations. The model also provides the first insights into the flow patterns of the aortic mechanical valve leaflets in an anatomic left heart system. This work was supported by NIH Grant RO1-HL-07262 and the Minnesota Supercomputing Institute.   

Aortic Wave Dynamics and Its Influence on Left Ventricular Workload

Clinical and epidemiologic studies have shown that hypertension plays a key role in development of left ventricular (LV) hypertrophy and ultimately heart failure mostly due to increased LV workload. Therefore, it is crucial to diagnose and treat abnormal high LV workload at early stages. The pumping mechanism of the heart is pulsatile, thus it sends pressure and flow wave into the compliant aorta. The wave dynamics in the aorta is dominated by interplay of heart rate (HR), aortic rigidity, and location of reflection sites. We hypothesized that for a fixed cardiac output (CO) and peripheral resistance (PR), interplay of HR and aortic compliance can create conditions that minimize LV power requirement. We used a computational approach to test our hypothesis. Finite element method with direct coupling method of fluid-structure interaction (FSI) was used. Blood was assumed to be incompressible Newtonian fluid and aortic wall was considered elastic isotropic. Simulations were performed for various heart rates and aortic rigidities while inflow wave, CO, and PR were kept constant. For any aortic compliance, LV power requirement becomes minimal at a specific heart rate. The minimum shifts to higher heart rates as aortic rigidity increases.   

Leveraging theory from cosmodynamics to study the the effect of the pulsating heart on coronary arteries in a large-scale Lattice Boltzmann simulation

We present a computational method for the simulation of cardiovascular flows in realistic human geometries derived from computed tomography angiography (CTA) data. The simulation is based on the Lattice Boltzmann method to model the blood flow in large-scale arterial systems and extends previously published studies using static geometries to include the effect of the pulsating heart on the coronary arteries. We provide here the derivation for introducing the deformational forces exerted on the arterial flows from the movement of the heart by borrowing concepts from cosmodynamics. The deformational forces are then cast into the kinetic formalism by using a Gauss-Hermite projection procedure. In this presentation, we will discuss this method as well as provide an analysis of the impact of these additional forces on the endothelial shear stress, a quantity associated with the localization and progression of heart diseases like atherosclerosis.   

Analysis of velocity fluctuations downstream of a bileaflet mechanical heart valve

Bileaflet mechanical heart valves are widely used to replace diseased aortic heart valves. The stresses induced by the rich and unsteady non-physiological flow structures have been the focus to evaluate red blood cells damage and platelet activation, develop flow control strategies, or improve valve designs. In this study, we analyzed the flow fields obtained downstream of a bileaflet mechanical heart valve using time-resolved particle image velocimetry under pulsatile and steady flow conditions. Our study demonstrates the rich dynamics downstream of the valve and weighs the relevance of unsteady effects vs inertia effects on the different flow structures. Power spectrum analyses of the turbulent fluctuations highlight the highly anisotropic influence and the limited applicability of classical self-similar turbulence theory in describing the small-scale structures in the immediate vicinity of the valve.   

Mitigation of Shear-Induced Blood Damage by Mechanical Bileaflet Heart Valves

The strong transitory shear stress generated during the time-periodic closing of bileaflet mechanical heart valves that is associated with the formation of counter-rotating vortices near the leaflet edges may be damaging to blood elements and may result in platelet activation and therefore thrombosis and thromboembolism complications. These flow transients are investigated using fluorescent PIV in a new, low-volume test setup that reproduces the pulsatile physiological conditions associated with a 25 mm St. Jude Medical valve. The flow transients are partially suppressed and the platelet activation is minimized using miniature vortex generator arrays that are embedded on the surface of the leaflets. Measurements of the ensuing flow taken phase-locked to the leaflet motion demonstrate substantial modification of the transient vertical structures and concomitant reduction of Reynolds shear stresses. Human blood experiments validated the effectiveness of miniature vortex generators in reducing thrombus formation by over 42 percent.   

Visualization of Simulated Endothelial Shear Stress and Blood Flow in Coronary Arteries

Low endothelial shear stress (ESS) identifies areas of atherosclerotic disease lesion formation in the coronary arteries. However, it is impossible to directly measure ESS {\it in vivo} for an entire arterial tree. As part of the Multiscale Hemodynamics Project, computed tomography angiography (CTA) data is being used to obtain patient specific heart and coronary system geometries and then MUPHY, a multi-physics and multi-scale simulation code combining microscopic Molecular Dynamics (MD) with a hydro-kinetic Lattice Boltzmann (LB) method, is applied in order to simulate blood flow through the coronary arteries. Having effective visualizations of the simulation's multidimensional output, including ESS, is vital for the quick and thorough non-invasive evaluation of the patient. To this end, we have developed new visualization tools and techniques to make the simulation's output useful in a clinical diagnostic setting, examined the effectiveness of 2D versus 3D representations, and explored blood flow representations. The visualization methods developed are also applicable to other areas of fluid dynamics.   

Hemodynamic simulations in coronary aneurysms of a patient with Kawasaki Disease

Kawasaki Disease is the leading cause of acquired pediatric heart disease, and can cause large coronary artery aneurysms in untreated cases. A simulation case study has been performed for a 10-year-old male patient with coronary aneurysms. Specialized coronary boundary conditions along with a lumped parameter heart model mimic the interactions between the ventricles and the coronary arteries, achieving physiologic pressure and flow waveforms. Results show persistent low shear stress in the aneurismal regions, and abnormally high shear at the aneurysm neck. Correlation functions have been derived to compare wall shear stress and wall shear stress gradients with recirculation time with the idea of localizing zones of calcification and thrombosis. Results are compared with those of an artificially created normal coronary geometry for the same patient. The long-term goal of this work is to develop links between hemodynamics and thrombotic risk to assist in clinical decision-making.   

A Mechanical System to Reproduce Cardiovascular Flows

Within the framework of the ``Pumps{\&}Pipes'' collaboration between ExxonMobil Upstream Research Company and The DeBakey Heart and Vascular Center in Houston, a hydraulic control system was developed to accurately simulate general cardiovascular flows. The final goal of the development of the apparatus was the reproduction of the periodic flow of blood through the heart cavity with the capability of varying frequency and amplitude, as well as designing the systolic/diastolic volumetric profile over one period. The system consists of a computer-controlled linear actuator that drives hydraulic fluid in a closed loop to a secondary hydraulic cylinder. The test section of the apparatus is located inside a MRI machine, and the closed loop serves to physically separate all metal moving parts (control system and actuator cylinder) from the MRI-compatible pieces. The secondary cylinder is composed of nonmetallic elements and directly drives the test section circulatory flow loop. The circulatory loop consists of nonmetallic parts and several types of Newtonian and non-Newtonian fluids, which model the behavior of blood. This design allows for a periodic flow of blood-like fluid pushed through a modeled heart cavity capable of replicating any healthy heart condition as well as simulating anomalous conditions. The behavior of the flow inside the heart can thus be visualized by MRI techniques.   

Investigating the Flow and Biomechanics of the Embryonic Zebrafish Heart

Understanding flow and kinematic characteristics of the embryonic heart is a prerequisite to devise early intervention or detection methods in the context of congenital heart defects. In this study, the kinematics and fluid dynamics of the embryonic zebrafish heart were analyzed through the early stages of cardiac development (24-48 hours post-fertilization) in vivo using optical microscopy and high-speed video. Endocardial walls and individual blood cells were segmented from raw images and were tracked through the cardiac cycle. Particle tracking velocimetry analysis yielded quantitative blood cell velocity field, chamber volume, and flow rate information. It was seen that the pumping mechanism starts as a combined peristaltic and suction pump while the heart is in the tube configuration and transforms into a positive displacement pump after cardiac looping. Strong two-phase nature of the fluid is evident. This work provides us new understanding of the spatio-temporal characteristics of kinematics and blood cell velocity field inside the developing heart.