Effect of exercise on blood flow through the aortic valve: a combined clinical and numerical study

The aim of this study was to measure the cardiac output and stroke volume for a healthy subject by coupling an echocardiogram Doppler (echo-Doppler) method with a fluid–structure interaction (FSI) simulation at rest and during exercise. Blood flow through aortic valve was measured by Doppler flow echocardiography. Aortic valve geometry was calculated by echocardiographic imaging. An FSI simulation was performed, using an arbitrary Lagrangian–Eulerian mesh. Boundary conditions were defined by pressure loads on ventricular and aortic sides. Pressure loads applied brachial pressures with (stage 1) and without (stage 2) differences between brachial, central and left ventricular pressures. FSI results for cardiac output were 15.4% lower than Doppler results for stage 1 (r = 0.999). This difference increased to 22.3% for stage 2. FSI results for stroke volume were undervalued by 15.3% when compared to Doppler results at stage 1 and 26.2% at stage 2 (r = 0.94). The predicted mean backflow of blood was 4.6%. Our results show that numerical methods can be combined with clinical measurements to provide good estimates of patient-specific cardiac output and stroke volume at different heart rates.     

A numerical study of flow in curved tubes simulating coronary arteries

Numerical simulations of pulsatile flow in coronary arteries which take into account the curvature associated with the bending of arteries over the surface of the heart are presented for resting, excited and drug induced states. The study was motivated by reported observations of atherosclerotic plaque localization on the inner curvature of coronary arteries. The simulated flow field appears quasi-steady under resting conditions with wall shear stress always highest on the outside wall and only a single secondary flow vortex in the half tube. However, reversal of wall shear stress direction at the inside wall does occur under resting flow conditions and this is not a quasi-steady characteristic. The flow field is markedly unsteady under excited conditions with wall shear stress sometimes peaking on the inside wall and an increase in the magnitude of wall shear stress reversal on the inside wall. However, only a single secondary flow vortex in the half tube is observed. Implications of the simulations for the role of fluid mechanics in coronary artery atherosclerosis are also discussed.     

Direct numerical simulation of a 2D-stented aortic heart valve at physiological flow rates

We study the nonlinear interaction of an aortic heart valve, composed of hyperelastic corrugated leaflets of finite density attached to a stented vessel under physiological flow conditions. In our numerical simulations, we use a 2D idealised representation of this arrangement. Blood flow is caused by a time-varying pressure gradient that mimics that of the aortic valve and corresponds to a peak Reynolds number equal to 4050. Here, we fully account for the shear-thinning behaviour of the blood and large deformations and contact between the leaflets by solving the momentum and mass balances for blood and leaflets. The mixed finite element/Galerkin method along with linear discontinuous Lagrange multipliers for coupling the fluid and elastic domains is adopted. Moreover, a series of challenging numerical issues such as the finite length of the computational domain and the conditions that should be imposed on its inflow/outflow boundaries, the accurate time integration of the parabolic and hyperbolic momentum equations, the contact between the leaflets and the non-conforming mesh refinement in part of the domain are successfully resolved. Calculations for the velocity and the shear stress fields of the blood reveal that boundary layers appear on both sides of a leaflet. The one along the ventricular side transfers blood with high momentum from the core region of the vessel to the annulus or the sinusoidal expansion, causing the continuous development of flow instabilities. At peak systole, vortices are convected in the flow direction along the annulus of the vessel, whereas during the closure stage of the valve, an extremely large vortex develops in each half of the flow domain.     

Initial hydrodynamic study on a new intraaortic axial flow pump: Dynamic aortic valve

Rotary blood pumps have been researched as implantable ventricular assist devices for years. To further reduce the complex of implanted axial pumps, the authors proposed a new concept of intraaortic axial pump, termed previously as “dynamic aortic valve (DAV)”. Instead of being driven by an intraaortic micro-electric motor, it was powered by a magnetic field from outside of body. To ensure the perfusion of coronary artery, the axial flow pump is to be implanted in the position of aortic valve. It could serve as either a blood pump or a mechanical valve depending on the power input. This research tested the feasibility of the new concept in model study. A column, made from permanent magnet, is jointed to an impeller in a concentric way to form a “rotor-impeller”. Supported by a hanging shaft cantilevered in the center of a rigid cage, the rotor-impeller can be turned by the magnetic field in the surrounding space. In the present prototype, the rotor is 8 mm in diameter and 15 mm in length, the impeller has 3 vanes with an outer diameter of 18 mm. The supporting cage is 22 mm in outer diameter and 20 mm in length. When tested, the DAV prototype is inserted into the tube of a mock circuit. The alternative magnetic field is produced by a rotating magnet placed side by side with the rotor-impeller at a distance of 30 mm. Once the alternative magnetic field is presented in the surrounding space, the DAV starts to turn, leading to a pressure difference and liquid flow in the tube. The flow rate or pressure difference is proportioned to rotary speed. At the maximal output of hydraulic power, the flow rate reached 5 L/min against an afterload of 100 mmHg. The maximal pressure difference generated by DAV at a rotation rate of 12600 r/min was 147 mmHg. The preliminary results demonstrated the feasibility of “DAV”, further research on this concept is justifiable.     

Initial hydrodynamic study on a new intraaortic axial flow pump: Dynamic aortic valve

Rotary blood pumps have been researched as implantable ventricular assist devices for years. To further reduce the complex of implanted axial pumps, the authors proposed a new concept of intraaortic axial pump, termed previously as "dynamic aortic valve (DAV)". Instead of being driven by an intraaortic micro-electric motor, it was powered by a magnetic field from outside of body. To ensure the perfusion of coronary artery, the axial flow pump is to be implanted in the position of aortic valve. It could serve as either a blood pump or a mechanical valve depending on the power input. This research tested the feasibility of the new concept in model study. A column, made from permanent magnet, is jointed to an impeller in a concentric way to form a "rotor-impeller". Supported by a hanging shaft cantilevered in the center of a rigid cage, the rotor-impeller can be turned by the magnetic field in the surrounding space. In the present prototype, the rotor is 8 mm in diameter and 15 mm in length, the impeller ha     

Risk evaluation of adverse aortic events in patients with non-circular aortic annulus after transcatheter aortic valve implantation: a numerical study

Biomechanics and Modeling in Mechanobiology - Transcatheter aortic valve implantation (TAVI) is a micro-invasive surgery used to treat patients with aortic stenosis (AS) efficiently. However, the...     

A case study on implantation strategies to mitigate coronary obstruction in a patient receiving transcatheter aortic valve replacement

Coronary obstruction is a life threatening complication during and post-transcatheter aortic valve replacement (TAVR). The objective of this preliminary work is to investigate the mechanisms underlying coronary obstruction in a patient after TAVR, in whom coronary obstruction was confirmed in addition to highlighting the importance of pre-procedural planning. The aortic root of an 80-year old male patient with coronary obstruction during TAVR–where a 29 mm SAPIEN 3 was deployed-was segmented from Computed Tomography scans and 3D-printed with compliant material. Flow and pressure data were acquired in this 3D-printed model in-vitro using a pulse duplicator under physiological conditions for the cases: a 29 mm SAPIEN 3, a 26 mm SAPIEN 3 expanded with a 29 mm balloon, and a 31 mm Medtronic-CoreValve deployed annularly, supra and sub-annularly respectively. Only the CoreValve in sub-annular axial position and the 29 mm SAPIEN 3 yielded pressure gradients (PG) lower than 10 mmHg (6.76 ± 0.52 and 5.72 ± 0.13 mmHg respectively) while the 26 mm SAPIEN 3, CoreValve in normal and supra-annular positions yielded higher PGs (15.5 ± 0.48, 12.2 ± 0.15 and 10.8 ± 0.24 mmHg respectively). 29 mm SAPIEN 3 implantation yielded an FFR value of 45.7 ± 0.6%. However, 31 mm CoreValve in any of the three different annular positions yielded FFR values going from 89.6 ± 1.1% in supra-annular position to 98.3 ± 1.1% in sub-annular position. Implantation with a 26 mm SAPIEN 3 expanded with a 29 mm balloon also yielded an FFR of 92.1 ± 1.2%. Coronary obstruction in this patient could have been prevented through usage of different valve types and/or through usage of a different combination of valve size-balloon sizes.     

A numerical analysis of the aortic blood flow pattern during pulsed cardiopulmonary bypass

In the modern era, stroke remains a main cause of morbidity after cardiac surgery despite continuing improvements in the cardiopulmonary bypass (CPB) techniques. The aim of the current work was to numerically investigate the blood flow in aorta and epiaortic vessels during standard and pulsed CPB, obtained with the intra-aortic balloon pump (IABP). A multi-scale model, realized coupling a 3D computational fluid dynamics study with a 0D model, was developed and validated with in vivo data. The presence of IABP improved the flow pattern directed towards the epiaortic vessels with a mean flow increase of 6.3% and reduced flow vorticity.     

The effect of elastin damage on the mechanics of the aortic valve

Porcine bioprosthetic heart valves degenerate and fail mechanically through a mechanism that is currently not well understood. It has been suggested that damage to the elastin component of prosthetic valve cusps could be responsible for changes in the mechanical function of the valve that would predispose it to increased damage and ultimate failure. To determine whether damage to elastin can produce the structural and mechanical changes that could initiate the process of bioprosthetic valve degeneration, we developed an elastase treatment protocol that fragments elastin and negates its mechanical contribution to the valve tissue. Valve cusps were mechanically tested before and after digestion to measure the mechanical changes resulting from elastin damage. Elastin damage produced a decrease in radial and circumferential extensibility (from 43 to 18% strain radially and 12 to 7% strain circumferentially), with a slight increase in stiffness (1.3-2.6kN/m for radial and 10.6-11.9kN/m for circumferential directions). Digestions with trypsin, which does not cleave elastin, confirmed that the changes in mechanics of the circumferential samples were likely due to the nonspecific removal of proteoglycans by elastase, while the changes in the radial samples were indeed due to elastin damage. Removing the mechanical contribution of elastin alters the mechanical behavior of the aortic valve cusp, primarily in the radial direction. This finding implies that damage to elastin will distend the cusps, reduce their extensibility, and increase their stiffness. Damage to elastin may therefore contribute to the degeneration and failure of prosthetic valves.     

Effect of wedging on the flow characteristics past tilting disc aortic valve prosthesis

To evaluate the efficacy of implanting a tilting disc aortic valve prosthesis in an angulated (wedged) supra-annular position, an in vitro experimental study was performed. The aortic valve prosthesis was mounted in an axi-symmetric valve chamber in a wedged position and incorporated in a mock circulatory system. Measurements were obtained on the transvalvular pressure gradient, percent regurgitation as well as velocity profiles and turbulent normal stresses distal to the valve. Our results showed that there was no significant reduction in the pressure gradient in mounting a larger sized valve in the wedged supra-annular position. On the other hand, the percent regurgitation increased with increase in heart rate and wedge angle. The valve failed to function properly above 110 beats min-1 at any wedge angle with the normal flow rate. The velocity profiles also showed significant changes with an increase in the turbulent normal stress with increase in wedge angle. Hence our study suggests that implanting the tilting disc prosthesis in a wedged supra-annular position in the aorta is not advisable.     

Multiphase hemodynamic simulation of pulsatile flow in a coronary artery

A multiphase transient non-Newtonian three-dimensional (3-D) computational fluid dynamics (CFD) simulation has been performed for pulsatile hemodynamics in an idealized curved section of a human coronary artery. We present the first prediction, to the authors' knowledge, of particulate buildup on the inside curvature using the multiphase theory of dense suspension hemodynamics. In this study, the particulates are red blood cells (RBCs). The location of RBC buildup on the inside curvature correlates with lower wall shear stress (WSS) relative to the outside curvature. These predictions provide insight into how blood-borne particulates interact with artery walls and hence, have relevance for understanding atherogenesis since clinical observations show that atherosclerotic plaques generally form on the inside curvatures of arteries. The buildup of RBCs on the inside curvature is driven by the secondary flow and higher residence times. The higher viscosity in the central portion of the curved vessel tends to block their flow, causing them to migrate preferentially through the boundary layer. The reason for this is the nearly neutrally buoyant nature of the dense two-phase hemodynamic flow. The two-phase non-Newtonian viscosity model predicts greater shear thinning than the single-phase non-Newtonian model. Consequently, the secondary flow induced in the curvature is weaker. The waveforms for computed hemodynamic parameters, such as hematocrit, WSS, and viscosity, follow the prescribed inlet velocity waveforms. The lower oscillatory WSS produced on the inside curvature has implications for understanding thickening of the intimal layer.     

Impact of coronary tortuosity on coronary pressure: numerical simulation study

Background

Coronary tortuosity (CT) is a common coronary angiographic finding. Whether CT leads to an apparent reduction in coronary pressure distal to the tortuous segment of the coronary artery is still unknown. The purpose of this study is to determine the impact of CT on coronary pressure distribution by numerical simulation.

Methods

21 idealized models were created to investigate the influence of coronary tortuosity angle (CTA) and coronary tortuosity number (CTN) on coronary pressure distribution. A 2D incompressible Newtonian flow was assumed and the computational simulation was performed using finite volume method. CTA of 30°, 60°, 90°, 120° and CTN of 0, 1, 2, 3, 4, 5 were discussed under both steady and pulsatile conditions, and the changes of outlet pressure and inlet velocity during the cardiac cycle were considered.

Results

Coronary pressure distribution was affected both by CTA and CTN. We found that the pressure drop between the start and the end of the CT segment decreased with CTA, and the length of the CT segment also declined with CTA. An increase in CTN resulted in an increase in the pressure drop.

Conclusions

Compared to no-CT, CT can results in more decrease of coronary blood pressure in dependence on the severity of tortuosity and severe CT may cause myocardial ischemia.     

Effects of bifurcation-specific and conventional stents on coronary bifurcation flow. An experimental and numerical study

Our paper builds on existing research into conventional bare metal stents in order to assess new devices specifically designed for coronary bifurcation angioplasty. The first aim is to validate the numerical model against data from in vitro experiments on stented coronary phantoms. A surface mesh was built in accordance with micro-computed tomography images obtained from coronary stents implanted in silicone models and used for numerical analysis. Computational simulations for steady and unsteady cases generally agreed with their experimental counterparts. A second objective is to compare the hemodynamic performance of one of these new devices (Stentys) to that of conventional devices and stenting techniques in a simplified coronary bifurcation model. Four different coronary bifurcation stenting techniques were analyzed. We have focused on factors contributing to restenosis, such as wall shear stress (WSS), oscillatory shear index (OSI), pressure loss, and local normalized helicity (LNH). It was found that bifurcation-specific stents implanted in the side branch led to increased malapposition. This effect has proved to be more important than stent specific design characteristics such as strut size (different for conventional and Stentys stent). This conclusion is confirmed by means of drop in pressure and mechanical energy loss rate calculation; for the latter, the increase ranged from 9% to 17%, depending on the stenting technique, when dedicated stents were implanted in the side branch. The behavior patterns presented in this study should be double-checked against those obtained in more realistic geometries.     

Laser anemometry measurements of pulsatile flow past aortic valve prostheses

Experimental results are presented on physiological pulsatile flow past caged ball and tilting disc aortic valve prostheses mounted in an axisymmetric chamber incorporated in a mock circulatory system. The measurements of velocity profiles and turbulent normal stresses during several times in a cardiac cycle were obtained using laser-Doppler anemometry. Our results show that with increased angle of opening for the tilting disc valves, a large but locally confined vortex is observed along the wall in the minor flow region throughout most of the cardiac cycle. The turbulent normal stresses measured downstream to the tilting disc in the minor flow region parallel to the tilt axis were found to be larger than those measured downstream to the caged ball valves. Comparison of measurements with steady flow at flow rates comparable to peak pulsatile flow rate show that the turbulent normal stresses are larger by a factor of two in pulsatile flow with a frequency of 1.2 Hz.     

A simple coronary blood flow model to study the collateral flow index

Biomechanics and Modeling in Mechanobiology - In this work, we present a novel modeling framework to investigate the effects of collateral circulation into the coronary blood flow physiology. A...     

Management of a protruding right coronary artery stent during aortic valve replacement for aortic stenosis

ABSTRACT: We describe the successful management of a stent protruding from the right coronary ostium into the aortic root in the setting of aortic valve replacement for aortic stenosis. Due to advances in medical care more elderly patients present for aortic valve surgery after percutaneous coronary intervention. Therefore, with an aging population due to advances in medical care, more patients will present for aortic valve surgery after percutaneous coronary intervention. We suggest a degree of caution before valve deployment in transcatheter aortic valve intervention or during annular manipulation in patients undergoing traditional aortic valve replacement with coexisting patent proximal stents.     

Modeling the effect of flow heterogeneity on coronary permeability-surface area

In 11 anesthetized pigs, the left anterior descending coronary artery (LAD) was cannulated and pump perfused with blood before and during maximum adenosine vasodilation. For LAD plasma flows (F) ranging from 0.42 to 3.6 ml.min-1.g perfused tissue-1, we injected radiolabeled microspheres to measure heterogeneity and used the multiple indicator-dilution method to measure permeability-surface area product (PS) for EDTA. Heterogeneity of flow from the LAD was expressed as relative dispersion (RD) = standard deviation of flow/mean flow. Values of RD, corrected for tissue sample size using fractal theory, ranged from 13 to 87%, approaching 16-35% at high F. We developed a "variable-recruitment model" of regional heterogeneous capillary transport to correct PS for flow heterogeneity and capillary surface area recruitment. Values of PS ranged from 0.14 to 0.96 ml.min-1.g-1. Accounting for heterogeneity increased PS values by 0-18% compared with homogeneous values. Results revealed PS to be proportional to flow up to F = 1.5-2.1 ml.min-1.g-1 and then was constant at higher flows. The initial increase of PS with F may be due to capillary recruitment. When full recruitment is reached, PS becomes independent of F. We conclude that flow heterogeneity is significant but not readily predictable in the pig myocardium and that the use of microspheres to correct indicator-dilution data for flow heterogeneity improves the interpretation of multiple-tracer studies, particularly when tracers are used to study interventions that may alter flow distribution.