Vortex shedding in steady flow through a model of an arterial stenosis and its relevance to mural platelet deposition.

In this study, the development of unsteady vortical formations in the separated flow region distal to a stenosis throat is presented and compared with the platelet deposition measurements, to enhance our understanding of the mechanisms involved in platelet kinetics in flowing blood. Qualitative and quantitative flow visualization and numerical simulations were performed in a model of a streamlined axisymmetric stenosis with an area reduction of 84% at the throat of the stenosis. Measurements were performed at Reynolds numbers (Re), based on upstream diameter and average velocity, ranging from 300 to 1,800. Both the digital particle image visualization method employed and the numerical simulations were able to capture the motion of the vortices through the separated flow region. Periodic shedding of vortices began at approximately Re=375 and continued for the full range of Re studied. The locales at which these vortices are initiated, their size, and their life span, were a function of Re. The numerical simulations of turbulent flow through the stenosis model entailed a detailed depiction of the process of vortex shedding in the separated flow region downstream of the stenosis. These flow patterns were used to elucidate the mechanisms involved in blood platelet kinetics and deposition in the area in and around an arterial stenosis. The unsteady flow development in the recirculation region is hypothesized as the mechanism for observed changes in the distribution of mural platelet deposition between Re =300, 900, and 1,800, despite only a marginal variation in the size and shape of the recirculation zone under these flow conditions.

Fluid mechanics of arterial stenosis: relationship to the development of mural thrombus.

In this study, we analyzed blood flow through a model stenosis with Reynolds numbers ranging from 300 to 3,600 using both experimental and numerical methods. The jet produced at the throat was turbulent, leading to an axisymmetric region of slowly recirculating flow. For higher Reynolds numbers, this region became more disturbed and its length was reduced. The numerical predictions were confirmed by digital particle image velocimetry and used to describe the fluid dynamics mechanisms relevant to prior measurements of platelet deposition in canine blood flow (R.T. Schoephoerster et al., Atherosclerosis and Thrombosis 12:1806-1813, 1993). Actual deposition onto the wall was dependent on the wall shear stress distribution along the stenosis, increasing in areas of flow recirculation and reattachment. Platelet activation potential was analyzed under laminar and turbulent flow conditions in terms of the cumulative effect of the varying shear and elongational stresses, and the duration platelets are exposed to them along individual platelet paths. The cumulative product of shear rate and exposure time along a platelet path reached a value of 500, half the value needed for platelet activation under constant shear (J.M.. Ramstack et al., Journal of Biomechanics 12: 113-125, 1979).

Steady flow in an aneurysm model: correlation between fluid dynamics and blood platelet deposition.

Laminar and turbulent numerical simulations of steady flow in an aneurysm model were carried out over Reynolds numbers ranging from 300 to 3600. The numerical simulations are validated with Digital particle Image Velocimetry (DPIV) measurements, and used to study the fluid dynamic mechanisms that characterize aneurysm deterioration, by correlating them to in vitro blood platelet deposition results. It is shown that the recirculation zone formed inside the aneurysm cavity creates conditions that promote thrombus formation and the viability of rupture. Wall shear stress values in the recirculation zone are around one order of magnitude less than in the entrance zone. The point of reattachment at the distal end of the aneurysm is characterized by a pronounced wall shear stress peak. As the Reynolds number increases in laminar flow, the center of the recirculation region migrates toward the distal end of the aneurysm, increasing the pressure at the reattachment point. Under fully turbulent flow conditions (Re = 3600) the recirculation zone inside the aneurysm shrinks considerably. The wall shear stress values are almost one order of magnitude larger than those for the laminar cases. The fluid dynamics mechanisms inferred from the numerical simulation were correlated with measurements of blood platelet deposition, offering useful explanations for the different morphologies of the platelet deposition curves.

Performance assessment of prosthetic heart valves using the energy index method.

Traditional methods of characterizing valvular performance use some estimation of the effective opening area and the percent regurgitant volume. These methods are cumbersome because two parameters are used and their importance relative to one another is not revealed. The authors propose the use of a single parameter that is physically meaningful and accounts for characteristics of the valve throughout the cardiac cycle. The energy index, derived with use of a phase-by-phase analysis of the cardiac cycle, describes the energetic efficiency of the valve. The method's final form is: [Formula: see text] where Eps is the hydraulic energy available after systole, E+ is the energy dissipated in the valve while flow is positive, [Symbol: see text]_ and [Symbol: see text] are the regurgitant and forward volumes, respectively. Use of the El requires on-line measurement of valvular flow rate and pressure drop. The El was applied to a Medtronic-Hall (Minneapolis, MN), 25 mm prosthetic valve mounted in the aortic position of a cardiovascular simulator. Mild and severe degrees of valvular stenosis and regurgitance were simulated. Results indicate that the El is sensitive to either valvular condition and remains nearly constant, at 87%, for the normal valve tested over cardiac rates ranging from 50 to 100 beats per minute.

Effect of simulated valvular stenoticity on predicted flow area for a bileaflet valve using the Gorlin equation.

The possibility that the discharge coefficient (Cd) for a mechanical heart valve (MHV) is affected by valvular stenosis is addressed. A 29 mm bileaflet (Si. Jude Medical) mitral valve is tested on a cardiovascular duplicator (CVD) in its normal state and under two degrees of simulated stenosis. Stenoticity is simulated by bracing the occluders such that full opening is impossible. The pressure drop through the valve is described by a two-term second-order polynomial in flow, from which it is shown that the Cd should be a nonlinear function of the flow rate through the valve. The average difference between measured and calculated areas decreased from 74%, when a constant value of 0.7 was used for the Cd, to 3.7%, when the Cd was a nonlinear function of the flow rate.

A simulation of three-dimensional systolic flow dynamics in a spherical ventricle: effects of abnormal wall motion.

Alterations in left ventricle (LV) wall motion induced by ischemia will affect flow dynamics, and these altered flow fields can be used to evaluate LV pumping efficiency. LV chamber flow fields were obtained in this study by solving the discretized three-dimensional Navier-Stokes equations for viscous, incompressible unsteady flow by using the finite analytic method. Several cases of abnormal wall motion (AWM) were simulated by a manipulation of the boundary conditions to produce regions of hypokinesis, akinesis, and dyskinesis. These solutions were used to determine the central ejection region (CER), defined as the region of flow domain in which the obtained velocity field vectors are aligned +/- 3 degrees from the LV long axis. A CER coefficient was computed from information on the location and orientation of the CER within the LV cavity. Contraction of the spherical ventricle produced a vector field that was symmetric with respect to the long axis. For the simulations of AWM, an asymmetrical flow pattern developed, became more pronounced with increasing severity of AWM, and resulted in a shorter CER that shifted toward the ischemic region. The CER coefficients decreased monotonically with increased severity in AWM from 0.948 in the normal case to a low of 0.164 for the most severe case of AWM. The CER coefficient quantitatively displayed the sensitivity of the flow patterns to even moderate degrees of hypokinesis. In addition, visualization of the three-dimensional flow field reinforced the necessity of three-dimensional simulations to capture aspects of the flow that existing methods of two-dimensional flow imaging that use ultrasound may miss.

Evaluation of left ventricular function based on simulated systolic flow dynamics computed from regional wall motion.

Left ventricular (LV) chamber flow is undoubtedly influenced by the time-dependent regional motion of the LV wall. In an attempt to obtain diagnostic parameters based on LV chamber flow, we computed the LV chamber, two-dimensional systolic velocity and pressure distribution for two right anterior oblique (RAO) ventriculograms: one normal, one with ischemic coronary artery disease, and several simulations with prescribed abnormal wall motion. The flow fields are obtained by solving the discretized two-dimensional Navier-Stokes equations for viscous, incompressible unsteady flow using the finite analytic method. These solutions were used as a basis for two LV assessment parameters: (1) local pressure gradient near the LV wall, and (2) the central ejection region (CER), defined as the region of flow domain in which the obtained velocity field vectors are aligned +/- 5 degrees from the LV long axis. A CER coefficient, R, derived from the location and orientation of the CER within the LV cavity, is defined such that R = 0 for a heart which produces no CER, and R = 1 for a heart whose contraction is perfectly even along the entire RAO LV outline. The computed local pressure gradients in the ischemic heart near the apical wall region were reduced compared with those computed in the normal heart. An observable decrease in magnitude of the pressure gradients in the apical region for increasing severity of abnormal wall motion was also indicated. However, the prescribed abnormal wall motion simulations generated reduced pressure gradients in regions of abnormal wall motion and normal regions as well. Therefore, the local wall pressure gradient may not be suitable for localization of coronary occlusion but for presence of disease only. The time-averaged CER coefficient was 0.709 for the normal heart and 0.453 for the diseased heart. The CER shifted toward the region of LV wall which exhibits the abnormal motion, and the CER coefficient decreased with increasing severity of abnormal wall motion. The CER coefficient provides a qualitative and quantitative measure of global function that regional wall motion analysis cannot provide, and is a parameter which is sensitive to regional and temporal abnormalities and the resulting compensatory actions which cannot be detected by global parameters.

Effects of local geometry and fluid dynamics on regional platelet deposition on artificial surfaces.

An important aspect of blood-material interactions is the activation, adhesion, and subsequent aggregation of blood platelets on the artificial surface, all of which are directly affected by local fluid dynamics. The objective of this work was to directly correlate changing local fluid dynamic conditions produced by various vessel geometries, including stenosis, aneurysm, and separate contraction and expansion geometries, with quantitative in vitro measurements of regional platelet deposition. We directly measured platelet deposition as a function of axial position along four Lexan flow chambers with axisymmetric models of these geometries using 111In-labeled platelets. Platelet deposition was maximum in observed areas of flow recirculation and reattachment and minimum in locations of high shear and separation. For the stenosis geometry, two distinct regions of increased platelet deposition were apparent, one proximal to and one distal to the stenosis throat. An approximately linear increase in platelet densities was produced in the aneurysm region, increasing in the direction of flow. Through a comparison of platelet deposition with local fluid streamline orientation, we have shown that platelet deposition is increased in certain areas due to the enhanced convective transport of platelets and blood cells to the vessel wall along locally curved streamlines with velocity components perpendicular to the vessel wall.

Velocity and turbulence measurements past mitral valve prostheses in a model left ventricle.

Thrombogenesis and hemolysis have both been linked to the flow dynamics past heart valve prostheses. To learn more about the particular flow dynamics past mitral valve prostheses in the left ventricle under controlled experimental conditions, an in vitro study was performed. The experimental methods included velocity and turbulent shear stress measurements past caged-ball, tilting disc, bileaflet, and polyurethane trileaflet mitral valves in an acrylic rigid model of the left ventricle using laser Doppler anemometry. The results indicate that all four prosthetic heart valves studied create at least mildly disturbed flow fields. The effect of the left ventricular geometry on the flow development is to produce a stabilizing vortex which engulfs the entire left ventricular cavity, depending on the orientation of the valve. The measured turbulent shear stress magnitudes for all four valves did not exceed the reported value for hemolytic damage. However, the measured turbulent shear stresses were near or exceeded the critical shear stress reported in the literature for platelet lysis, a known precursor to thrombus formation.

Hemodynamic comparisons of polyurethane trileaflet and bioprosthetic heart valves.

Hemodynamic comparison of two polyurethane prosthetic heart valves with a bioprosthetic valve is presented. The valves were incorporated in a pulse duplicator simulating physiologic pulsatile flow, and comparisons between the valves were made on the transvalvular pressure drop, percent regurgitation, valve orifice area, rate of opening and closing as well as the performance index. The results showed that the functional characteristics of the polyurethane valve compared favorably with that of the bioprosthetic valve. The polyurethane valve can be a viable and inexpensive alternative, especially for short-term use in a total artificial heart as a bridge to transplant.

A survey of polar and non-polar lipids of mouse organs.

Total lipid was extracted from mouse (Mus musculus) heart, kidney, lung, liver, intestine, brain, stomach, dermis and epidermis and analyzed by quantitative thin-layer chromatography. All of the tissues contained phospholipids, triglycerides, sterols and free fatty acids. All tissues except brain contained small amounts of steryl esters, and all except stomach contained some glycosylceramides. Wax diesters were found in both the dermis and epidermis. Only epidermis contained a high proportion of ceramides. Acylglucosylceramides were uniquely present in epidermis.