Wall shear stress distribution along the cusp of a tri-leaflet prosthetic valve

Shear Stress Related Blood Damage along the Cusp of a Tri-Leaflet Prosthetic Valve

Blood flowing through a prosthetic heart valve can be damaged by flow-induced shear forces. Fluid dynamics variables and geometric factors play an important role in the evaluation of shear-stress-related blood damage. Central-flow prosthetic valves have been considered as an optimal replacement for mechanical and biological valves. Recently it was shown that shear stress distribution along the surface of a polyurethane cusp reaches values that can damage the blood elements. A mathematical model correlating the effects of shear stresses on blood corpuscles with clinical findings was employed in vitro. The model can be applied to the effects of blood-surface interaction and is of clinical relevance

Computational methods for the aortic heart valve and its replacements

INTRODUCTION

Replacement with a prosthetic device remains a major treatment option for the patients suffering from heart valve disease, with prevalence growing resulting from an ageing population. While the most popular replacement heart valve continues to be the bioprosthetic heart valve (BHV), its durability remains limited. There is thus a continued need to develop a general understanding of the underlying mechanisms limiting BHV durability to facilitate development of a more durable prosthesis. In this regard, computational models can play a pivotal role as they can evaluate our understanding of the underlying mechanisms and be used to optimize designs that may not always be intuitive. Areas covered: This review covers recent progress in computational models for the simulation of BHV, with a focus on aortic valve (AV) replacement. Recent contributions in valve geometry, leaflet material models, novel methods for numerical simulation, and applications to BHV optimization are discussed. This information should serve not only to infer reliable and dependable BHV function, but also to establish guidelines and insight for the design of future prosthetic valves by analyzing the influence of design, hemodynamics and tissue mechanics. Expert commentary: The paradigm of predictive modeling of heart valve prosthesis are becoming a reality which can simultaneously improve clinical outcomes and reduce costs. It can also lead to patient-specific valve design.

Aortic valve: mechanical environment and mechanobiology

The aortic valve (AV) experiences a complex mechanical environment, which includes tension, flexure, pressure, and shear stress forces due to blood flow during each cardiac cycle. This mechanical environment regulates AV tissue structure by constantly renewing and remodeling the phenotype. In vitro, ex vivo and in vivo studies have shown that pathological states such as hypertension and congenital defect like bicuspid AV (BAV) can potentially alter the AV's mechanical environment, triggering a cascade of remodeling, inflammation, and calcification activities in AV tissue. Alteration in mechanical environment is first sensed by the endothelium, which in turn induces changes in the extracellular matrix, and triggers cell differentiation and activation. However, the molecular mechanism of this process is not understood very well. Understanding these mechanisms is critical for advancing the development of effective medical based therapies. Recently, there have been some interesting studies on characterizing the hemodynamics associated with AV, especially in pathologies like BAV, using different experimental and numerical methods. Here, we review the current knowledge of the local AV mechanical environment and its effect on valve biology, focusing on in vitro and ex vivo approaches.

Hemodynamic environments from opposing sides of human aortic valve leaflets evoke distinct endothelial phenotypes in vitro

The regulation of valvular endothelial phenotypes by the hemodynamic environments of the human aortic valve is poorly understood. The nodular lesions of calcific aortic stenosis (CAS) develop predominantly beneath the aortic surface of the valve leaflets in the valvular fibrosa layer. However, the mechanisms of this regional localization remain poorly characterized. In this study, we combine numerical simulation with in vitro experimentation to investigate the hypothesis that the previously documented differences between valve endothelial phenotypes are linked to distinct hemodynamic environments characteristic of these individual anatomical locations. A finite-element model of the aortic valve was created, describing the dynamic motion of the valve cusps and blood in the valve throughout the cardiac cycle. A fluid mesh with high resolution on the fluid boundary was used to allow accurate computation of the wall shear stresses. This model was used to compute two distinct shear stress waveforms, one for the ventricular surface and one for the aortic surface. These waveforms were then applied experimentally to cultured human endothelial cells and the expression of several pathophysiological relevant genes was assessed. Compared to endothelial cells subjected to shear stress waveforms representative of the aortic face, the endothelial cells subjected to the ventricular waveform showed significantly increased expression of the "atheroprotective" transcription factor Kruppel-like factor 2 (KLF2) and the matricellular protein Nephroblastoma overexpressed (NOV), and suppressed expression of chemokine Monocyte-chemotactic protein-1 (MCP-1). Our observations suggest that the difference in shear stress waveforms between the two sides of the aortic valve leaflet may contribute to the documented differential side-specific gene expression, and may be relevant for the development and progression of CAS and the potential role of endothelial mechanotransduction in this disease.

Valvular endothelial cells and the mechanoregulation of valvular pathology

Endothelial cells are critical mediators of haemodynamic forces and as such are important foci for initiation of vascular pathology. Valvular leaflets are also lined with endothelial cells, though a similar role in mechanosensing has not been demonstrated. Recent evidence has shown that valvular endothelial cells respond morphologically to shear stress, and several studies have implicated valvular endothelial dysfunction in the pathogenesis of disease. This review seeks to combine what is known about vascular and valvular haemodynamics, endothelial response to mechanical stimuli and the pathogenesis of valvular diseases to form a hypothesis as to how mechanical stimuli can initiate valvular endothelial dysfunction and disease progression. From this analysis, it appears that inflow surface-related bacterial/thrombotic vegetative endocarditis is a high shear-driven endothelial denudation phenomenon, while the outflow surface with its related calcific/atherosclerotic degeneration is a low/oscillatory shear-driven endothelial activation phenomenon. Further understanding of these mechanisms may help lead to earlier diagnostic tools and therapeutic strategies.

In vivo wall shear stress measured by magnetic resonance velocity mapping in the normal human abdominal aorta

OBJECTIVE

To apply a new non-invasive method for quantification of in vivo wall shear stress (WSS) by magnetic resonance (MR) FAcE velocity mapping and measure WSS in the human abdominal aorta.

DESIGN

Prospective, open study.

MATERIAL

Six volunteers.

METHODS

MR FAcE velocity method was developed for measurements of mean, maximum, minimum WSS and oscillating shear index (OSI) values at the anterior and posterior walls of suprarenal and infrarenal abdominal aorta.

RESULTS

The mean, maximum and minimum WSS values were 0.63/0.28, 4.07/2.72 and -0.71/-1.00 N/m2, respectively, in the suprarenal/infrarenal aorta. The mean WSS was 0.35 N/m2 (p < 0.001) and the maximum WSS was 1.36 N/m2 (p < 0.0001) lower in the infrarenal aorta than in the suprarenal aorta. Mean, maximum minimum WSS and OSI values in the infrarenal position differed (p < 0.01) between the anterior and posterior walls.

CONCLUSION

WSS can be determined in vivo by MR FAcE velocity technique. Since the lowest WSS values were measured in the infrarenal, posterior blood-to-wall interface, the theory of more pronounced atherosclerosis development in low and oscillating WSS domains was not contradicted by the results of the present study.

Monitoring combined antithrombotic treatments in patients with prosthetic heart valves using transcranial Doppler and coagulation markers

BACKGROUND AND PURPOSE

The combined use of coumarin and low-dose aspirin appears to reduce the risk of systemic embolism at a low risk of bleeding. The remaining incidence of embolism of approximately 2%/y is still high. Methods for real-time detection of embolic events have not been used thus far to monitor the efficacy of therapeutic strategies. They might permit individually tailored, effective treatments.

METHODS

The frequency of embolic signals in both middle cerebral arteries was monitored using a two-channel 2-MHz transcranial Doppler system. We examined five patients with mechanical prosthetic heart valves suffering from recurrent cerebral ischemic symptoms despite adequate anticoagulant therapy (international normalized ratio, 3.0 to 4.3). Measurements were performed on coumarin alone (four baseline values) and subsequent to the addition of intravenous (500 mg bolus) and oral (100 mg/d for 10 days) aspirin or intravenous (5000 IU bolus) heparin. The prothrombotic markers thrombin-antithrombin III complex, fibrinopeptide A, D-dimer, and platelet beta-thromboglobulin were measured simultaneously.

RESULTS

None of the combined drug regimens led to a significant reduction of the emboli count. The values of thrombin-antithrombin III complex, fibrinopeptide A, and D-dimer were already within normal limits with coumarin alone. The beta-thromboglobulin levels, however, were increased, and additional aspirin or heparin did not reduce them. There was no correlation between the emboli count and the prothrombotic markers or between the prothrombotic markers and the different drug regimens.

CONCLUSIONS

The rate of cerebral emboli measured with transcranial Doppler in the group of high-risk patients studied was not influenced by additional antiplatelet therapy. The emboli are likely to be composed at least in part of platelets, and their generation seems not dependent on thrombin or cyclooxygenase. There is an apparent discrepancy between the unchanged rate of emboli during Doppler monitoring found in this and other studies and the partial efficacy of combined treatment with coumarin and aspirin in clinical long-term studies. This may be explained by differences in the composition or size of the emboli.

Errors in the estimation of arterial wall shear rates that result from curve fitting of velocity profiles

An analysis was performed to determine the error that results from the estimation of the wall shear rates based on linear and quadratic curve-fittings of the measured velocity profiles. For steady, fully developed flow in a straight vessel, the error for the linear method is linearly related to the distance between the probe and the wall, dr1, and the error for the quadratic method is zero. With pulsatile flow, especially a physiological pulsatile flow in a large artery, the thickness of the velocity boundary layer, delta is small, and the error in the estimation of wall shear based on curve fitting is much higher than that with steady flow. In addition, there is a phase lag between the actual shear rate and the measured one. In oscillatory flow, the error increases with the distance ratio dr1/delta and, for a quadratic method, also with the distance ratio dr2/dr1, where dr2 is the distance of the second probe from the wall. The quadratic method has a distinct advantage in accuracy over the linear method when dr1/delta << 1, i.e. when the first velocity point is well within the boundary layer. The use of this analysis in arterial flow involves many simplifications, including Newtonian fluid, rigid walls, and the linear summation of the harmonic components, and can provide more qualitative than quantitative guidance.

Differential accumulation of intimal monocyte-macrophages relative to lipoproteins and lipofuscin corresponds to hemodynamic forces on cardiac valves in mice

We have studied the distribution of monocyte-macrophages, lipids, and lipoproteins in sections of aorta and aortic valves from mice fed an atherogenic diet. By immunocytochemical analysis with Mac-1 and F4/80 antibodies, apolipoprotein B antibody, and oil red O staining, three discrete regions were identified: 1) the aortic wall of the sinus of Valsalva, which contained deposits of lipid that colocalized with lipoproteins and monocyte-macrophages; 2) the sides of the aortic valve leaflets facing the ventricle, which did not contain lipids or lipoproteins but which were lined with macrophages that colocalized with lipofuscin; and 3) the sides of the leaflets facing the aorta, which did not contain lipids, lipoproteins, monocyte-macrophages, or lipofuscin deposits. This pattern of distribution resembles the expected distribution of mechanical forces, especially those of systolic blood flow, which in the three areas are predominantly 1) low-shear disturbed flow, 2) high-shear laminar flow, and 3) low-shear laminar flow, respectively. These findings suggest that lesions in the mouse closely resemble early atherosclerotic lesions in humans and other primates with respect to monocyte-macrophage and lipoprotein accumulation. The results also strikingly demonstrate that the accumulation of monocyte-macrophages and lipoproteins can occur independently, with spatial differences corresponding to the distribution of hemodynamic forces.

Stress distribution on the cusps of a polyurethane trileaflet heart valve prosthesis in the closed position

In this paper, a finite element analysis of the stress distribution on the cusps of a polyurethane trileaflet heart valve prosthesis in the closed position is presented. The geometry of the valve was modified from a relationship proposed by Ghista and Reul (J. Biomechanics 10, 313-324, 1977). The effects of variations in stent height, leaflet thickness and coaptation area on the stress distribution were also analyzed. Analyses were performed with both rigid and flexible stents for the trileaflet valve in order to delineate the effect of stent flexibility on the leaflet stress distribution. The results showed that regions of stress concentration were present near the commissural attachment similar to those predicted with the bioprostheses. The stresses on the leaflets were reduced by increasing the stent height with both rigid and flexible stents. Selectively increasing the leaflet thickness near the commissures and also increasing the coaptation area did not prove to reduce the leaflet stresses when the stent flexibility was taken into account. The possible effect of high stresses on the structural integrity of polyurethane leaflets and its relationship with calcification is yet to be investigated.