Turbulent blood flow in the ascending aorta of humans with normal and diseased aortic valves

Wall Shear Stress Estimation of Thoracic Aortic Aneurysm Using Computational Fluid Dynamics

An attempt has been made to evaluate the effects of wall shear stress (WSS) on thoracic aortic aneurysm (TAA) using Computational Fluid Dynamics (CFD). Aneurysm is an excessive localized swelling of the arterial wall due to many physiological factors and it may rupture causing shock or sudden death. The existing imaging modalities such as MRI and CT assist in the visualization of anomalies in internal organs. However, the expected dynamic behaviour of arterial bulge under stressed condition can only be effectively evaluated through mathematical modelling. In this work, a 3D aneurysm model is reconstructed from the CT scan slices and eventually the model is imported to Star CCM+ (Siemens, USA) for intensive CFD analysis. The domain is discretized using polyhedral mesh with prism layers to capture the weakening boundary more accurately. When there is flow reversal in TAA as seen in the velocity vector plot, there is a chance of cell damage causing clots. This is because of the shear created in the system due to the flow pattern. It is observed from the proposed mathematical modelling that the deteriorating WSS is an indicator for possible rupture and its value oscillates over a cardiac cycle as well as over different stress conditions. In this model, the vortex formation pattern and flow reversals are also captured. The non-Newtonian model, including a pulsatile flow instead of a steady average flow, does not overpredict the WSS (15.29 Pa compared to 16 Pa for the Newtonian model). Although in a cycle the flow behaviour is laminar-turbulent-laminar (LTL), utilizing the non-Newtonian model along with LTL model also overpredicted the WSS with a value of 20.1 Pa. The numerical study presented here provides good insight of TAA using a systematic approach to numerical modelling and analysis.

Age-Related Vascular Changes Affect Turbulence in Aortic Blood Flow

Turbulent blood flow is implicated in the pathogenesis of several aortic diseases but the extent and degree of turbulent blood flow in the normal aorta is unknown. We aimed to quantify the extent and degree of turbulece in the normal aorta and to assess whether age impacts the degree of turbulence. 22 young normal males (23.7 ± 3.0 y.o.) and 20 old normal males (70.9 ± 3.5 y.o.) were examined using four dimensional flow magnetic resonance imaging (4D Flow MRI) to quantify the turbulent kinetic energy (TKE), a measure of the intensity of turbulence, in the aorta. All healthy subjects developed turbulent flow in the aorta, with total TKE of 3–19 mJ. The overall degree of turbulence in the entire aorta was similar between the groups, although the old subjects had about 73% more total TKE in the ascending aorta compared to the young subjects (young = 3.7 ± 1.8 mJ, old = 6.4 ± 2.4 mJ, p < 0.001). This increase in ascending aorta TKE in old subjects was associated with age-related dilation of the ascending aorta which increases the volume available for turbulence development. Conversely, age-related dilation of the descending and abdominal aorta decreased the average flow velocity and suppressed the development of turbulence. In conclusion, turbulent blood flow develops in the aorta of normal subjects and is impacted by age-related geometric changes. Non-invasive assessment enables the determination of normal levels of turbulent flow in the aorta which is a prerequisite for understanding the role of turbulence in the pathophysiology of cardiovascular disease.

Biomechanics of the Circulating Tumor Cell Microenvironment

Circulating tumor cells (CTCs) exist in a microenvironment quite different from the solid tumor tissue microenvironment. They are detached from matrix and exposed to the immune system and hemodynamic forces leading to the conclusion that life as a CTC is "nasty, brutish, and short." While there is much evidence to support this assertion, the mechanisms underlying this are much less clear. In this chapter we will specifically focus on biomechanical influences on CTCs in the circulation and examine in detail the question of whether CTCs are mechanically fragile, a commonly held idea that is lacking in direct evidence. We will review multiple lines of evidence indicating, perhaps counterintuitively, that viable cancer cells are mechanically robust in the face of exposures to physiologic shear stresses that would be encountered by CTCs during their passage through the circulation. Finally, we present emerging evidence that malignant epithelial cells, as opposed to their benign counterparts, possess specific mechanisms that enable them to endure these mechanical stresses.

Investigation of Atrial Vortices Using a Novel Right Heart Model and Possible Implications for Atrial Thrombus Formation

The main aim of this paper is to characterize vortical flow structures in the healthy human right atrium, their impact on wall shear stresses and possible implications for atrial thrombus formation. 3D Particle Tracking Velocimetry is applied to a novel anatomically accurate compliant silicone right heart model to study the phase averaged and fluctuating flow velocity within the right atrium, inferior vena cava and superior vena cava under physiological conditions. We identify the development of two vortex rings in the bulk of the right atrium during the atrial filling phase leading to a rinsing effect at the atrial wall which break down during ventricular filling. We show that the vortex ring formation affects the hemodynamics of the atrial flow by a strong correlation (ρ = 0.7) between the vortical structures and local wall shear stresses. Low wall shear stress regions are associated with absence of the coherent vortical structures which might be potential risk regions for atrial thrombus formation. We discuss possible implications for atrial thrombus formation in different regions of the right atrium.

Characterization of Transition to Turbulence for Blood in a Straight Pipe Under Steady Flow Conditions

Blood is a complex fluid that, among other things, has been established to behave as a shear thinning, non-Newtonian fluid when exposed to low shear rates (SR). Many hemodynamic investigations use a Newtonian fluid to represent blood when the flow field of study has relatively high SR (>200 s-1). Shear thinning fluids have been shown to exhibit differences in transition to turbulence (TT) compared to that of Newtonian fluids. Incorrect prediction of the transition point in a simulation could result in erroneous hemodynamic force predictions. The goal of the present study was to compare velocity profiles near TT of whole blood and Newtonian blood analogs in a straight rigid pipe with a diameter 6.35 mm under steady flow conditions. Rheology was measured for six samples of whole porcine blood and three samples of a Newtonian fluid, and the results show blood acts as a shear thinning non-Newtonian fluid. Measurements also revealed that blood viscosity at SR = 200 s-1 is significantly larger than at SR = 1000 s-1 (13.8%, p < 0.001). Doppler ultrasound (DUS) was used to measure velocity profiles for blood and Newtonian samples at different flow rates to produce Reynolds numbers (Re) ranging from 1000 to 3300 (based on viscosity at SR = 1000 s-1). Two mathematically defined methods, based on the velocity profile shape change and turbulent kinetic energy (TKE), were used to detect TT. Results show similar parabolic velocity profiles for both blood and the Newtonian fluid for Re < 2200. However, differences were observed between blood and Newtonian fluid velocity profiles for larger Re. The Newtonian fluid had blunt-like velocity profiles starting at Re = 2403 ± 8 which indicated transition. In contrast, blood did not show this velocity profile change until Re = 2871 ± 104. The Newtonian fluid had large velocity fluctuations (root mean square (RMS) > 20%) with a maximum TKE near the pipe center at Re = 2316 ± 34 which indicated transition. In contrast, blood results showed the maximum TKE at Re = 2806 ± 109. Overall, the critical Re was delayed by ∼20% (p < 0.001) for blood compared to the Newtonian fluid. Thus, a Newtonian assumption for blood at flow conditions near transition could lead to large errors in velocity prediction for steady flow in a straight pipe. However, these results are specific to this pipe diameter and not generalizable since SR is highly dependent on pipe diameter. Further research is necessary to understand this relation in different pipe sizes, more complex geometries, and under pulsatile flow conditions.

The effects of tapering and artery wall stiffness on treatments for Coarctation of the Aorta

Coarctation of the Aorta is a congenital narrowing of the aorta. Two commonly used treatments are resection and end-to-end anastomosis, and stent placements. We simulate blood flow through one-dimensional models of aortas. Different artery stiffnesses, due to treatments, are included in our model, and used to compare blood flow properties in the treated aortas. We expand our previously published model to include the natural tapering of aortas. We look at change in aorta wall radius, blood pressure and blood flow velocity, and find that, of the two treatments, the resection and end-to-end anastomosis treatment more closely matches healthy aortas.

Assessment of turbulent viscous stress using ICOSA 4D Flow MRI for prediction of hemodynamic blood damage

Flow-induced blood damage plays an important role in determining the hemodynamic impact of abnormal blood flow, but quantifying of these effects, which are dominated by shear stresses in highly fluctuating turbulent flow, has not been feasible. This study evaluated the novel application of turbulence tensor measurements using simulated 4D Flow MRI data with six-directional velocity encoding for assessing hemodynamic stresses and corresponding blood damage index (BDI) in stenotic turbulent blood flow. The results showed that 4D Flow MRI underestimates the maximum principal shear stress of laminar viscous stress (PLVS), and overestimates the maximum principal shear stress of Reynolds stress (PRSS) with increasing voxel size. PLVS and PRSS were also overestimated by about 1.2 and 4.6 times at medium signal to noise ratio (SNR) = 20. In contrast, the square sum of the turbulent viscous shear stress (TVSS), which is used for blood damage index (BDI) estimation, was not severely affected by SNR and voxel size. The square sum of TVSS and the BDI at SNR >20 were underestimated by less than 1% and 10%, respectively. In conclusion, this study demonstrated the feasibility of 4D Flow MRI based quantification of TVSS and BDI which are closely linked to blood damage.

The influence of the aortic valve angle on the hemodynamic features of the thoracic aorta

Since the first observation of a helical flow pattern in aortic blood flow, the existence of helical blood flow has been found to be associated with various pathological conditions such as bicuspid aortic valve, aortic stenosis, and aortic dilatation. However, an understanding of the development of helical blood flow and its clinical implications are still lacking. In our present study, we hypothesized that the direction and angle of aortic inflow can influence helical flow patterns and related hemodynamic features in the thoracic aorta. Therefore, we investigated the hemodynamic features in the thoracic aorta and various aortic inflow angles using patient-specific vascular phantoms that were generated using a 3D printer and time-resolved, 3D, phase-contrast magnetic resonance imaging (PC-MRI). The results show that the rotational direction and strength of helical blood flow in the thoracic aorta largely vary according to the inflow direction of the aorta, and a higher helical velocity results in higher wall shear stress distributions. In addition, right-handed rotational flow conditions with higher rotational velocities imply a larger total kinetic energy than left-handed rotational flow conditions with lower rotational velocities.

A generalized optimization principle for asymmetric branching in fluidic networks

When applied to a branching network, Murray’s law states that the optimal branching of vascular networks is achieved when the cube of the parent channel radius is equal to the sum of the cubes of the daughter channel radii. It is considered integral to understanding biological networks and for the biomimetic design of artificial fluidic systems. However, despite its ubiquity, we demonstrate that Murray’s law is only optimal (i.e. maximizes flow conductance per unit volume) for symmetric branching, where the local optimization of each individual channel corresponds to the global optimum of the network as a whole. In this paper, we present a generalized law that is valid for asymmetric branching, for any cross-sectional shape, and for a range of fluidic models. We verify our analytical solutions with the numerical optimization of a bifurcating fluidic network for the examples of laminar, turbulent and non-Newtonian fluid flows.

Hemodynamics through the congenitally bicuspid aortic valve: a computational fluid dynamics comparison of opening orifice area and leaflet orientation

A computational fluid dynamics model of a bicuspid aortic valve has been developed using idealised three-dimensional geometry. The aim was to compare how the orifice area and leaflet orientation affect the hemodynamics of a pure bicuspid valve. By applying physiologic material properties and boundary conditions, blood flow shear stresses were predicted during peak systole. A reduced orifice area altered blood velocity, the pressure drop across the valve and the wall shear stress through the valve. Bicuspid models predicted impaired blood flow similar to a stenotic valve, but the flow patterns were specific to leaflet orientation. Flow patterns developed in bicuspid aortic valves, such as helical flow, were sensitive to cusp orientation. In conclusion, the reduced opening area of a bicuspid aortic valve amplifies any impaired hemodynamics, but cusp orientation determines subsequent flow patterns which may determine the specific regions downstream from the valve most at risk of clinical complications.

Turbulent Kinetic Energy Measurement Using Phase Contrast MRI for Estimating the Post-Stenotic Pressure Drop: In Vitro Validation and Clinical Application

BACKGROUND

Although the measurement of turbulence kinetic energy (TKE) by using magnetic resonance imaging (MRI) has been introduced as an alternative index for quantifying energy loss through the cardiac valve, experimental verification and clinical application of this parameter are still required.

OBJECTIVES

The goal of this study is to verify MRI measurements of TKE by using a phantom stenosis with particle image velocimetry (PIV) as the reference standard. In addition, the feasibility of measuring TKE with MRI is explored.

METHODS

MRI measurements of TKE through a phantom stenosis was performed by using clinical 3T MRI scanner. The MRI measurements were verified experimentally by using PIV as the reference standard. In vivo application of MRI-driven TKE was explored in seven patients with aortic valve disease and one healthy volunteer. Transvalvular gradients measured by MRI and echocardiography were compared.

RESULTS

MRI and PIV measurements of TKE are consistent for turbulent flow (0.666 < R2 < 0.738) with a mean difference of -11.13 J/m3 (SD = 4.34 J/m3). Results of MRI and PIV measurements differ by 2.76 ± 0.82 cm/s (velocity) and -11.13 ± 4.34 J/m3 (TKE) for turbulent flow (Re > 400). The turbulence pressure drop correlates strongly with total TKE (R2 = 0.986). However, in vivo measurements of TKE are not consistent with the transvalvular pressure gradient estimated by echocardiography.

CONCLUSIONS

These results suggest that TKE measurement via MRI may provide a potential benefit as an energy-loss index to characterize blood flow through the aortic valve. However, further clinical studies are necessary to reach definitive conclusions regarding this technique.

The association between platelet-to-lymphocyte ratio and inflammatory markers with the severity of aortic stenosis

AIM

Platelet-to-lymphocyte ratio (PLR) was proposed as a novel indirect marker of inflammation. We aimed to evaluate the relationship between PLR and aortic stenosis (AS).

PATIENTS & METHODS

The study population included 453 patients of which 98 patients with severe AS, 206 patients with mild-to-moderate AS and 149 patients as control.

RESULTS

PLR was significantly increased in parallel to the severity of AS (p < 0.001). There was significant positive correlation between PLR and maximum, and mean systolic transaortic gradient (all p < 0.001). Also, PLR was correlated with C-reactive protein and neutrophil-to-lymphocyte ratio (all p < 0.001) and PLR was independently associated with the presence of AS (p < 0.001).

CONCLUSION

We found a significant relationship between PLR and AS.

Fluid-structure interaction modeling of calcific aortic valve disease using patient-specific three-dimensional calcification scans

Calcific aortic valve disease (CAVD) is characterized by calcification accumulation and thickening of the aortic valve cusps, leading to stenosis. The importance of fluid flow shear stress in the initiation and regulation of CAVD progression is well known and has been studied recently using fluid-structure interaction (FSI) models. While cusp calcifications are three-dimensional (3D) masses, previously published FSI models have represented them as either stiffened or thickened two-dimensional (2D) cusps. This study investigates the hemodynamic effect of these calcifications employing FSI models using 3D patient-specific calcification masses. A new reverse calcification technique (RCT) is used for modeling different stages of calcification growth based on the spatial distribution of calcification density. The RCT is applied to generate the 3D calcification deposits reconstructed from a patient-specific CT scans. Our results showed that consideration of 3D calcification deposits led to both higher fluid shear stresses and unique fluid shear stress distribution on the aortic side of the cusps that may have an impact on the calcification growth rate. However, the flow did not seem to affect the geometry of the calcification during the growth phase.

Aortic flow patterns before and after personalised external aortic root support implantation in Marfan patients

Implantation of a personalised external aortic root support (PEARS) in the Marfan aorta is a new procedure that has emerged recently, but its haemodynamic implication has not been investigated. The objective of this study was to compare the flow characteristics and hemodynamic indices in the aorta before and after insertion of PEARS, using combined cardiovascular magnetic resonance imaging (CMR) and computational fluid dynamics (CFD). Pre- and post-PEARS MR images were acquired from 3 patients and used to build patient-specific models and upstream flow conditions, which were incorporated into the CFD simulations. The results revealed that while the qualitative patterns of the haemodynamics were similar before and after PEARS implantation, the post-PEARS aortas had slightly less disturbed flow at the sinuses, as a result of reduced diameters in the post-PEARS aortic roots. Quantitative differences were observed between the pre- and post-PEARS aortas, in that the mean values of helicity flow index (HFI) varied by -10%, 35% and 20% in post-PEARS aortas of Patients 1, 2 and 3, respectively, but all values were within the range reported for normal aortas. Comparisons with MR measured velocities in the descending aorta of Patient 2 demonstrated that the computational models were able to reproduce the important flow features observed in vivo.

On the accuracy of viscous and turbulent loss quantification in stenotic aortic flow using phase-contrast MRI

PURPOSE

To investigate the limits of phase contrast MRI (PC-MRI)-based measurements of viscous losses and turbulent kinetic energy (TKE) pertaining to spatial resolution, signal-to-noise ratio (SNR), and non-Gaussian intravoxel velocity distributions.

THEORY AND METHODS

High-resolution particle tracking velocimetry data obtained in a realistic aortic phantom with stenotic flow were used to simulate PC-MRI measurements at different resolutions and noise levels. Laminar viscous losses were computed using the spatial gradients of the mean velocity vector field, and TKE levels were derived based on the intravoxel phase dispersion of flow-sensitized PC-MRI measurements.

RESULTS

Increasing the voxel size from 0.625 to 2.5 mm resulted in an underestimation of viscous losses of up to 83%, whereas total TKE values showed errors of <15% and reduced sensitivity to voxel size. Relative errors in viscous loss quantification were found to be less dependent on noise levels when compared with TKE values. In general, a SNR of 20-30 is required for both methods.

CONCLUSION

At spatial resolutions feasible in clinical three-dimensional PC-MRI measurements, viscous losses of stenotic flows are significantly underestimated, whereas TKE shows smaller errors and reduced sensitivity to spatial resolution. Magn Reson Med 76:191-196, 2016. © 2015 Wiley Periodicals, Inc.

Effects of aortic irregularities on blood flow

Anatomic aortic anomalies are seen in many medical conditions and are known to cause disturbances in blood flow. Turner syndrome (TS) is a genetic disorder occurring only in females where cardiovascular anomalies, particularly of the aorta, are frequently encountered. In this study, numerical simulations are applied to investigate the flow characteristics in four TS patient- related aortic arches (a normal geometry, dilatation, coarctation and elongation of the transverse aorta). The Quemada viscosity model was applied to account for the non-Newtonian behavior of blood. The blood is treated as a mixture consisting of water and red blood cells (RBC) where the RBCs are modeled as a convected scalar. The results show clear geometry effects where the flow structures and RBC distribution are significantly different between the aortas. Transitional flow is observed as a jet is formed due to a constriction in the descending aorta for the coarctation case. RBC dilution is found to vary between the aortas, influencing the WSS. Moreover, the local variations in RBC volume fraction may induce large viscosity variations, stressing the importance of accounting for the non-Newtonian effects.

Animal models of surgically manipulated flow velocities to study shear stress-induced atherosclerosis

Atherosclerosis is a chronic inflammatory disease of the arterial tree that develops at predisposed sites, coinciding with locations that are exposed to low or oscillating shear stress. Manipulating flow velocity, and concomitantly shear stress, has proven adequate to promote endothelial activation and subsequent plaque formation in animals. In this article, we will give an overview of the animal models that have been designed to study the causal relationship between shear stress and atherosclerosis by surgically manipulating blood flow velocity profiles. These surgically manipulated models include arteriovenous fistulas, vascular grafts, arterial ligation, and perivascular devices. We review these models of manipulated blood flow velocity from an engineering and biological perspective, focusing on the shear stress profiles they induce and the vascular pathology that is observed.

An immersogeometric variational framework for fluid-structure interaction: application to bioprosthetic heart valves

In this paper, we develop a geometrically flexible technique for computational fluid-structure interaction (FSI). The motivating application is the simulation of tri-leaflet bioprosthetic heart valve function over the complete cardiac cycle. Due to the complex motion of the heart valve leaflets, the fluid domain undergoes large deformations, including changes of topology. The proposed method directly analyzes a spline-based surface representation of the structure by immersing it into a non-boundary-fitted discretization of the surrounding fluid domain. This places our method within an emerging class of computational techniques that aim to capture geometry on non-boundary-fitted analysis meshes. We introduce the term "immersogeometric analysis" to identify this paradigm. The framework starts with an augmented Lagrangian formulation for FSI that enforces kinematic constraints with a combination of Lagrange multipliers and penalty forces. For immersed volumetric objects, we formally eliminate the multiplier field by substituting a fluid-structure interface traction, arriving at Nitsche's method for enforcing Dirichlet boundary conditions on object surfaces. For immersed thin shell structures modeled geometrically as surfaces, the tractions from opposite sides cancel due to the continuity of the background fluid solution space, leaving a penalty method. Application to a bioprosthetic heart valve, where there is a large pressure jump across the leaflets, reveals shortcomings of the penalty approach. To counteract steep pressure gradients through the structure without the conditioning problems that accompany strong penalty forces, we resurrect the Lagrange multiplier field. Further, since the fluid discretization is not tailored to the structure geometry, there is a significant error in the approximation of pressure discontinuities across the shell. This error becomes especially troublesome in residual-based stabilized methods for incompressible flow, leading to problematic compressibility at practical levels of refinement. We modify existing stabilized methods to improve performance. To evaluate the accuracy of the proposed methods, we test them on benchmark problems and compare the results with those of established boundary-fitted techniques. Finally, we simulate the coupling of the bioprosthetic heart valve and the surrounding blood flow under physiological conditions, demonstrating the effectiveness of the proposed techniques in practical computations.

The primary cilium as sensor of fluid flow: new building blocks to the model. A review in the theme: cell signaling: proteins, pathways and mechanisms

The primary cilium is an extraordinary organelle. For many years, it had the full attention of only a few dedicated scientists fascinated by its uniqueness. Unexpectedly, after decades of obscurity, it has moved very quickly into the limelight with the increasing evidence of its central role in the many genetic variations that lead to what are now known as ciliopathies. These studies implicated unique biological functions of the primary cilium, which are not completely straightforward. In parallel, and initially completely unrelated to the ciliopathies, the primary cilium was characterized functionally as an organelle that makes cells more susceptible to changes in fluid flow. Thus the primary cilium was suggested to function as a flow-sensing device. This characterization has been substantiated for many epithelial cell types over the years. Nevertheless, part of the central mechanism of signal transduction has not been explained, largely because of the substantial technical challenges of working with this delicate organelle. The current review considers the recent advances that allow us to fill some of the holes in the model of signal transduction in cilium-mediated responses to fluid flow and to pursue the physiological implications of this peculiar organelle.

Human heart conjugate cooling simulation: unsteady thermo-fluid-stress analysis

The main objective of this work was to demonstrate computationally that realistic human hearts can be cooled much faster by performing conjugate heat transfer consisting of pumping a cold liquid through the cardiac chambers and major veins while keeping the heart submerged in cold gelatin filling a cooling container. The human heart geometry used for simulations was obtained from three-dimensional, high resolution CT-angio scans. Two fluid flow domains for the right (pulmonic) and left (systemic) heart circulations, and two solid domains for the heart tissue and gelatin solution were defined for multi-domain numerical simulation. Detailed unsteady temperature fields within the heart tissue were calculated during the conjugate cooling process. A linear thermoelasticity analysis was performed to assess the stresses applied on the heart due to the coolant fluid shear and normal forces and to examine the thermal stress caused by temperature variation inside the heart. It was demonstrated that a conjugate cooling effort with coolant temperature at +4°C is capable of reducing the average heart temperature from +37°C to +8°C in 25 minutes for cases in which the coolant was steadily pumped only through major heart inlet veins and cavities.

Sinus of Valsalva: a converging nozzle that contributes to stable flow in the coronary arteries

The anatomy of the sinuses of Valsalva has not been considered from the viewpoint of a converging nozzle. Converging nozzles reduce turbulence. We reviewed computed tomographic images of the left and right sinuses of Valsalva in 20 consecutive patients. The sinuses of Valsalva were shown to have a shape in the axial projection that approximates a cubic equation nozzle, although the sinuses of Valsalva are not axisymmetric. The ratios of the cross-sectional area of the inlet to cross-sectional areas of the outlet, assuming the sinuses are axisymmetric, were 14 and 17 in the left and right sinuses, respectively. Calculations by others show that turbulent kinetic energy at the exit (at the coronary ostia) of such axisymmetric nozzles would be reduced by 97%. We conclude that the sinuses of Valsalva have the configuration of a converging nozzle and prevent or reduce turbulent flow in the proximal portions of the coronary arteries.

Drug delivery interfaces in the 21st century: from science fiction ideas to viable technologies

Early science fiction envisioned the future of drug delivery as targeted micrometer-scale submarines and "cyborg" body parts. Here we describe the progression of the field toward technologies that are now beginning to capture aspects of this early vision. Specifically, we focus on the two most prominent types of systems in drug delivery: the intravascular micro/nano drug carriers for delivery to the site of pathology and drug-loaded implantable devices that facilitate release with the predefined kinetics or in response to a specific cue. We discuss the unmet clinical needs that inspire these designs, the physiological factors that pose difficult challenges for their realization, and viable technologies that promise robust solutions. We also offer a perspective on where drug delivery may be in the next 50 years based on expected advances in material engineering and in the context of future diagnostics.

Mapping mean and fluctuating velocities by Bayesian multipoint MR velocity encoding-validation against 3D particle tracking velocimetry

PURPOSE

To validate Bayesian multipoint MR velocity encoding against particle tracking velocimetry for measuring velocity vector fields and fluctuating velocities in a realistic aortic model.

METHODS

An elastic cast of a human aortic arch equipped with an 80 or 64% stenotic section was driven by a pulsatile pump. Peak velocities and peak turbulent kinetic energies of more than 3 m/s and 1000 J/m(3) could be generated. Velocity vector fields and fluctuating velocities were assessed using Bayesian multipoint MR velocity encoding with varying numbers of velocity encoding points and particle tracking velocimetry in the ascending aorta.

RESULTS

Velocities and turbulent kinetic energies measured with 5-fold k-t undersampled 10-point MR velocity encoding and particle tracking velocimetry were found to reveal good correlation with mean differences of -4.8 ± 13.3 cm/s and r(2) = 0.98 for velocities and -21.8 ± 53.9 J/m(3) and r(2) = 0.98 for turbulent kinetic energies, respectively. Three-dimensional velocity patterns of fast flow downstream of the stenoses and regions of elevated velocity fluctuations were found to agree well.

CONCLUSION

Accelerated Bayesian multipoint MR velocity encoding has been demonstrated to be accurate for assessing mean and fluctuating velocities against the reference standard particle tracking velocimetry. The MR method holds considerable potential to map velocity vector fields and turbulent kinetic energies in clinically feasible exam times of <15 min.

A Numerical Study of Aortic Flow Stability and Comparison With In Vivo Flow Measurements

The development of an engineering transitional turbulence model and its subsequent evaluation and validation for some diseased cardiovascular flows have been suggestive of its likely utility in normal aortas. The existence of experimental data from human aortas, acquired in the early 1970s with catheter-mounted hot film velocimeters, provided the opportunity to compare the performance of the model on such flows. A generic human aorta, derived from magnetic resonance anatomical and velocity images of a young volunteer, was used as the basis for varying both Reynolds number (Re) and Womersley parameter (α) to match four experimental data points from human ascending aortas, comprising two with disturbed flow and two with apparently undisturbed flow. Trials were made with three different levels of inflow turbulence intensity (Tu) to find if a single level could represent the four different cases with 4000 < Re < 10,000 and 17 < α < 26. A necessary boundary condition includes the inflow “turbulence” level, and convincing results were obtained for all four cases with inflow Tu = 1.0%, providing additional confidence in the application of the transitional model in flows in larger arteries. The Reynolds-averaged Navier–Stokes (RANS)-based shear stress transport (SST) transitional model is capable of capturing the correct flow state in the human aorta when low inflow turbulence intensity (1.0%) is specified.

Including aortic valve morphology in computational fluid dynamics simulations: initial findings and application to aortic coarctation

Computational fluid dynamics (CFD) simulations quantifying thoracic aortic flow patterns have not included disturbances from the aortic valve (AoV). 80% of patients with aortic coarctation (CoA) have a bicuspid aortic valve (BAV) which may cause adverse flow patterns contributing to morbidity. Our objectives were to develop a method to account for the AoV in CFD simulations, and quantify its impact on local hemodynamics. The method developed facilitates segmentation of the AoV, spatiotemporal interpolation of segments, and anatomic positioning of segments at the CFD model inlet. The AoV was included in CFD model examples of a normal (tricuspid AoV) and a post-surgical CoA patient (BAV). Velocity, turbulent kinetic energy (TKE), time-averaged wall shear stress (TAWSS), and oscillatory shear index (OSI) results were compared to equivalent simulations using a plug inlet profile. The plug inlet greatly underestimated TKE for both examples. TAWSS differences extended throughout the thoracic aorta for the CoA BAV, but were limited to the arch for the normal example. OSI differences existed mainly in the ascending aorta for both cases. The impact of AoV can now be included with CFD simulations to identify regions of deleterious hemodynamics thereby advancing simulations of the thoracic aorta one step closer to reality.

Acoustic source separation for the detection of coronary artery sounds

Coronary artery disease (CAD) is the leading cause of death in the United States, being responsible for more than 20% of all deaths in the country. This is in large part due to the difficulty of diagnostic screening for CAD. Phonoangiography seeks to detect CAD via the acoustic signature associated with turbulent flow near an abnormally constricted, or stenosed, region. However, the usefulness of the technique is severely hindered by the low strength of the CAD signal compared to the background noise within the chest. In this work, acoustic finite element analysis (FEA) was performed on physiologically accurate chest geometries to demonstrate the feasibility of an original acoustic source separation methodology for isolating coronary sounds. This approach is based upon pseudoinversion of mixing matrices determined through a combination of experiment and computation. This allows calculation of the sound emitted by the coronary arteries based upon measurements of the acoustic velocity on the chest surface. This work demonstrates the feasibility of such a technique computationally and examines the vulnerability of the proposed approach to measurement errors.

Detection of innocent systolic murmurs by auscultation and their relation to hematologic and echocardiographic findings in clinically normal Whippets

OBJECTIVE

To determine murmur prevalence by auscultation of 105 apparently healthy Whippets without signs of cardiac disease, to determine the origin of these murmurs, and to evaluate the influence of sex, type of pedigree (ie, bred for showing or racing), and training on these murmurs.

DESIGN

Cross-sectional study.

ANIMALS

105 client-owned Whippets.

PROCEDURES

All dogs were auscultated by the first author and underwent a complete physical and cardiological examination, together with a hematologic assessment. Several RBC variables and echocardiographic variables were compared between dogs with or without a murmur at the level of the aortic valve.

RESULTS

44 of 105 (41.9%) dogs had no murmur. A soft systolic murmur was present with point of maximal intensity at the level of the aortic valve in 50 (47.6%) dogs, at the level of the pulmonic valve in 8 (7.6%) dogs, and at the level of the mitral valve in 3 (2.9%) dogs. No significant differences were found in heart rate, rhythm, murmur presence, point of maximal intensity, and murmur grade between males and females, between dogs with race- and show-type pedigrees, or between dogs in training and not in training. Dogs with a murmur at the level of the aortic valve had a significantly higher aortic and pulmonic blood flow velocity and cardiac output, compared with dogs without a murmur.

CONCLUSIONS AND CLINICAL RELEVANCE

Whippets have a high prevalence of soft systolic murmurs in the absence of any structural abnormalities, which fit the description of innocent murmurs. No influence of sex, pedigree type, or training was found on the occurrence of these murmurs in Whippets.

The Acute Impact of Smoking One Cigarette on Cardiac Hemodynamic Parameters

Background

The acute impact of tobacco smoking on the cardiac hemodynamic parameters and its pathological implication in the process of arterial atherosclerosis need further exploration. This investigation was purposed to assess the acute impact of tobacco smoke on blood pressure and cardiac hemodynamic parameters.

Methods

Using an Ultrasonic Cardiac Output Monitor, and DINAMAP Pro 400 Series V2 blood pressure monitor, several cardiac hemodynamic parameters and the blood pressure were assessed in 14 smokers, 11 females and 3 males, at 2 time points, before and after smoking of one cigarette. Data, in terms of ratio of the means and 95% confidence interval were analyzed using ANOVA.

Results

Single-subject design in which the subject has served as his/her own control has been used. Tobacco smoking led to statistically significant acute increase in the means of all hemodynamic parameters, except for heart rate in female subjects, as compared to the means obtained before smoking.

Conclusions

Cigarette smoking induces acute non-physiologic alteration in cardiac outflow forces, exposing the aortic valve and arch to mechanical injury that might be implicated in initiating and promoting the process of aortic arch atherosclerosis and associated pathological lesions.

AORTIC COARCTATION: RECENT DEVELOPMENTS IN EXPERIMENTAL AND COMPUTATIONAL METHODS TO ASSESS TREATMENTS FOR THIS SIMPLE CONDITION

Coarctation of the aorta (CoA) is often considered a relatively simple disease, but long-term outcomes suggest otherwise as life expectancies are decades less than in the average population and substantial morbidity often exists. What follows is an expanded version of collective work conducted by the authors' and numerous collaborators that was presented at the 1st International Conference on Computational Simulation in Congenital Heart Disease pertaining to recent advances for CoA. The work begins by focusing on what is known about blood flow, pressure and indices of wall shear stress (WSS) in patients with normal vascular anatomy from both clinical imaging and the use of computational fluid dynamics (CFD) techniques. Hemodynamic alterations observed in CFD studies from untreated CoA patients and those undergoing surgical or interventional treatment are subsequently discussed. The impact of surgical approach, stent design and valve morphology are also presented for these patient populations. Finally, recent work from a representative experimental animal model of CoA that may offer insight into proposed mechanisms of long-term morbidity in CoA is presented.

Incidence and natural history of coronary artery aneurysm developing after drug-eluting stent implantation

AIMS

There is a growing concern about the occurrence of coronary artery aneurysms (CAAs) after drug-eluting stent (DES) implantation and their long-term course. We assessed the occurrence and the factors affecting the long-term outcome of DES-associated CAA.

METHODS AND RESULTS

We analyzed 3,612 consecutive patients (4,419 lesions) who underwent follow-up angiography after DES implantation. All 34 CAAs (0.76% per lesion) in 29 patients (0.8% per patient) were detected at follow-up, and the mean elapsed time from DES implantation to CAA diagnosis was 414 ± 213 days. Angiographically, CAAs developed almost exclusively in complex (type B2/C) de novo lesions (30 [88.2%] of 34 lesions), and lesion length was significantly greater in patients with CAA than without CAA (26.9 ± 9.03 vs 23.1 ± 7.14 mm; P = .004). Myocardial infarction with stent thrombosis occurred in 5 patients with CAA (17.2%), 4 of whom were on aspirin only without clopidogrel.

CONCLUSION

Although CAAs rarely develop after DES implantation and show mostly favorable clinical courses, long-term maintenance of clopidogrel therapy might be required to minimize occurrence of adverse clinical events resulting from stent thrombosis.

Increased Mean Platelet Volume in Patients With Aortic Stenosis

Background: Systemic thromboembolism is a serious complication in patients with aortic stenosis (AS). Previous studies have demonstrated that platelet activation occurs in patients with AS. The aim of this study was to assess the mean platelet volume (MPV), an indicator of platelet activation in patients with AS. Patients and Methods: The study group consisted of 43 patients with AS. An age-, gender-, and body mass index−matched control group consisted of 35 healthy volunteers. All patients and control participants underwent echocardiographic examination. We measured the serum MPV values in patients and control participants. Results: Mean platelet volume was significantly higher among patients with AS when compared with the control group (8.7 ± 1.0 vs 7.9 ± 0.9 fL, respectively; P = .001). Conclusion: We have shown that MPV was significantly elevated in patients with AS compared to control participant.

Outcome of Primary Percutaneous Intervention in Patients With Infarct-Related Coronary Artery Ectasia

Data related to the incidence and clinical outcome of acute myocardial infarction (AMI) in patients with preexisting coronary artery ectasia (CAE) are limited. We assessed whether infarct-related artery ectasia (EIRA) indicates an untoward clinical outcome in patients with AMI undergoing primary percutaneous coronary intervention (pPCI). Consecutive patients (n = 643) who presented with AMI and were treated with pPCI were analyzed retrospectively; 31 patients (4.8%) had EIRA. Patients who had EIRA were significantly younger and had higher incidence of hypertension, previous stroke, smoking, inferior wall AMI, and Killip score >1. Infarct-related artery ectasia was more frequent in the right coronary artery (RCA). Impaired epicardial arterial flow, thrombus burden score of infarct-related artery (IRA), impaired Thrombolysis in Myocardial Infarction (TIMI) Myocardial Perfusion Grade, and distal embolization were significantly higher whereas ST-segment resolution and collateral vascular development were significantly lower in patients with EIRA. Infarct-related artery ectasia was an independent predictor of adverse outcome (odds ratio: 0.197; 95% confidence interval [CI]: 0.062-0.633; P = .006).

Numerical simulation of pulsatile turbulent flow in tapering stenosed arteries

Purpose

The purpose of this paper is to investigate the geometric effects and pulsatile characteristics during the stenotic flows in tapering arteries.

Design/methodology/approach

The low Reynolds number k − ω turbulence model is applied to describe the stenotic flows in the tapering arteries in this paper. The results are divided into two sections. The first section characterizes the geometric effects on the turbulent flow under steady condition. The second section illustrates the key physiological parameters including the pressure drop and wall stress during the periodic cycle of the pulsatile flow in the arteries.

Findings

The tapering and stenoses severity intensify the turbulent flow and stretch the recirculation zones in the turbulent arterial flow. The wall shear stress, pressure drop and velocity vary most intensively at the peak phase during the periodic cycle of the pulsatile turbulent flow.

Originality/value

This paper provides a comprehensive understanding of the spatial‐temporal fluid dynamics involved in turbulent and transitional arterial flow with stenoses. The low Reynolds number k − ω turbulence model method is applied for the analyses of the geometric effects on the arterial flow and fluid feature during the periodic cycle.

Severity of aortic stenosis assessed by carotid pulse recordings and phonocardiography

The external carotid pulse, the PCG, and the ECG were studied in 26 adult patients with valvular aortic stenosis whose systolic peak pressure gradients ranged from 18 to 165 mmHg. A significant correlation was found between the rapidity of the pulse upstroke, as measured by the T-time, and the location of the peak of the systolic murmur during ventricular ejection, on the one hand, and the gradient, on the other. The left ventricular ejection time (LVET) related directly and the pre-ejection period (PEP) indirectly with the gradient. There was a signficant inverse relationship between the PEP/LVET quotient and the pressure gradient but this quotient did not classify the patients according to the severity of the stenosis as well as the T-time and the location of the peak of murmur. When a combination of the T-time, the PEP/LVET, and the location of the peak of the murmur was used in each patient, a good discrimination between the patients was achieved. When the pressure gradient was above 50 mmHg, at lease one of the measurements was abnormal and when it exceeded 100 mmHg, at least two measurements were abnormal. The study further showed that it is possible to separate patients with valvular aortic stenosis from those with hypertrophic obstructive cardiomyopathy lidiopathic hypertrophic subaortic stenosis) or mitral insufficiency on the basis of carotid pulse tracings and PCGs.

Assessment of fluctuating velocities in disturbed cardiovascular blood flow: in vivo feasibility of generalized phase-contrast MRI

PURPOSE

To evaluate the feasibility of generalized phase-contrast magnetic resonance imaging (PC-MRI) for the noninvasive assessment of fluctuating velocities in cardiovascular blood flow.

MATERIALS AND METHODS

Multidimensional PC-MRI was used in a generalized manner to map mean flow velocities and intravoxel velocity standard deviation (IVSD) values in one healthy aorta and in three patients with different cardiovascular diseases. The acquired data were used to assess the kinetic energy of both the mean (MKE) and the fluctuating (TKE) velocity field.

RESULTS

In all of the subjects, both mean and fluctuating flow data were successfully acquired. The highest TKE values in the patients were found at sites characterized by abnormal flow conditions. No regional increase in TKE was found in the normal aorta.

CONCLUSION

PC-MRI IVSD mapping is able to detect flow abnormalities in a variety of human cardiovascular conditions and shows promise for the quantitative assessment of turbulence. This approach may assist in clarifying the role of disturbed hemodynamics in cardiovascular diseases.

Neonatal Aortic Arch Hemodynamics and Perfusion During Cardiopulmonary Bypass

The objective of this study is to quantify the detailed three-dimensional (3D) pulsatile hemodynamics, mechanical loading, and perfusion characteristics of a patient-specific neonatal aortic arch during cardiopulmonary bypass (CPB). The 3D cardiac magnetic resonance imaging (MRI) reconstruction of a pediatric patient with a normal aortic arch is modified based on clinical literature to represent the neonatal morphology and flow conditions. The anatomical dimensions are verified from several literature sources. The CPB is created virtually in the computer by clamping the ascending aorta and inserting the computer-aided design model of the 10 Fr tapered generic cannula. Pulsatile (130 bpm) 3D blood flow velocities and pressures are computed using the commercial computational fluid dynamics (CFD) software. Second order accurate CFD settings are validated against particle image velocimetry experiments in an earlier study with a complex cardiovascular unsteady benchmark. CFD results in this manuscript are further compared with the in vivo physiological CPB pressure waveforms and demonstrated excellent agreement. Cannula inlet flow waveforms are measured from in vivo PC-MRI and 3 kg piglet neonatal animal model physiological experiments, distributed equally between the head-neck vessels and the descending aorta. Neonatal 3D aortic hemodynamics is also compared with that of the pediatric and fetal aortic stages. Detailed 3D flow fields, blood damage, wall shear stress (WSS), pressure drop, perfusion, and hemodynamic parameters describing the pulsatile energetics are calculated for both the physiological neonatal aorta and for the CPB aorta assembly. The primary flow structure is the high-speed canulla jet flow (∼3.0 m/s at peak flow), which eventually stagnates at the anterior aortic arch wall and low velocity flow in the cross-clamp pouch. These structures contributed to the reduced flow pulsatility (85%), increased WSS (50%), power loss (28%), and blood damage (288%), compared with normal neonatal aortic physiology. These drastic hemodynamic differences and associated intense biophysical loading of the pathological CPB configuration necessitate urgent bioengineering improvements—in hardware design, perfusion flow waveform, and configuration. This study serves to document the baseline condition, while the methodology presented can be utilized in preliminary CPB cannula design and in optimization studies reducing animal experiments. Coupled to a lumped-parameter model the 3D hemodynamic characteristics will aid the surgical decision making process of the perfusion strategies in complex congenital heart surgeries.

Analysis of Flow Disturbance in a Stenosed Carotid Artery Bifurcation Using Two-Equation Transitional and Turbulence Models

In this study, newly developed two-equation turbulence models and transitional variants are employed for the prediction of blood flow patterns in a diseased carotid artery where the growth, progression, and structure of the plaque at rupture are closely linked to low and oscillating wall shear stresses. Moreover, the laminar-turbulent transition in the poststenotic zone can alter the separation zone length, wall shear stress, and pressure distribution over the plaque, with potential implications for stresses within the plaque. Following the validation with well established experimental measurements and numerical studies, a magnetic-resonance (MR) image-based model of the carotid bifurcation with 70% stenosis was reconstructed and simulated using realistic patient-specific conditions. Laminar flow, a correlation-based transitional version of Menter’s hybrid k‐ϵ∕k‐ω shear stress transport (SST) model and its “scale adaptive simulation” (SAS) variant were implemented in pulsatile simulations from which analyses of velocity profiles, wall shear stress, and turbulence intensity were conducted. In general, the transitional version of SST and its SAS variant are shown to give a better overall agreement than their standard counterparts with experimental data for pulsatile flow in an axisymmetric stenosed tube. For the patient-specific case reported, the wall shear stress analysis showed discernable differences between the laminar flow and SST transitional models but virtually no difference between the SST transitional model and its SAS variant.

Use of an ultrasound blood-mimicking fluid for Doppler investigations of turbulence in vitro

Turbulence is an important factor in the assessment of stenotic disease and a possible causative mechanism for thromboembolism. Previous Doppler studies of turbulence have typically used whole-blood preparations or suspensions of erythrocytes. Recently, a water-glycerol based blood-mimicking fluid (BMF) has been developed for use in Doppler ultrasound studies. This fluid has desirable ultrasound properties but it has not previously been described during in vitro investigations of turbulence intensity. We report on investigations of grid-generated and constrained-jet turbulence in an in vitro test system. The BMF was found to generate significant levels of turbulence during steady flow at physiological flow rates, producing turbulent patterns in the distal region that were consistent with previous studies. Turbulence intensity increased significantly with flow rate (p < 0.005) for both the constrained jet and the constrained grid. Based on our observations, we conclude that a water-glycerol based BMF provides a suitable working fluid during in vitro investigations of turbulence using Doppler ultrasound.

In vitro hemodynamic investigation of the embryonic aortic arch at late gestation

This study focuses on the dynamic flow through the fetal aortic arch driven by the concurrent action of right and left ventricles. We created a parametric pulsatile computational fluid dynamics (CFD) model of the fetal aortic junction with physiologic vessel geometries. To gain a better biophysical understanding, an in vitro experimental fetal flow loop for flow visualization was constructed for identical CFD conditions. CFD and in vitro experimental results were comparable. Swirling flow during the acceleration phase of the cardiac cycle and unidirectional flow following mid-deceleration phase were observed in pulmonary arteries (PA), head-neck vessels, and descending aorta. Right-to-left (oxygenated) blood flowed through the ductus arteriosus (DA) posterior relative to the antegrade left ventricular outflow tract (LVOT) stream and resembled jet flow. LVOT and right ventricular outflow tract flow mixing had not completed until approximately 3.5 descending aorta diameters downstream of the DA insertion into the aortic arch. Normal arch model flow patterns were then compared to flow patterns of four common congenital heart malformations that include aortic arch anomalies. Weak oscillatory reversing flow through the DA junction was observed only for the Tetralogy of Fallot configuration. PA and hypoplastic left heart syndrome configurations demonstrated complex, abnormal flow patterns in the PAs and head-neck vessels. Aortic coarctation resulted in large-scale recirculating flow in the aortic arch proximal to the DA. Intravascular flow patterns spatially correlated with abnormal vascular structures consistent with the paradigm that abnormal intravascular flow patterns associated with congenital heart disease influence vascular growth and function.