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Hemolysis and thrombosis are among the most detrimental effects associated with mechanical heart valves. The strength and structure of the flows generated by the closure of mechanical heart valves can be correlated with the extent of blood damage. In this in vitro study, a tilting disk mechanical heart valve has been modified to measure the flow created within the valve housing during the closing phase. This is the first study to focus on the region just upstream of the mitral valve occluder during this part of the cardiac cycle, where cavitation is known to occur and blood damage is most severe. Closure of the tilting disk valve was studied in a "single shot" chamber driven by a pneumatic pump. Laser Doppler velocimetry was used to measure all three velocity components over a 30 ms period encompassing the initial valve impact and rebound. An acrylic window placed in the housing enabled us to make flow measurements as close as 200 microm away from the closed occluder. Velocity profiles reveal the development of an atrial vortex on the major orifice side of the valve shed off the tip of the leaflet. The vortex strength makes this region susceptible to cavitation. Mean and maximum axial velocities as high as 7 ms and 20 ms were recorded, respectively. At closure, peak wall shear rates of 80,000 s(-1) were calculated close to the valve tip. The region of the flow examined here has been identified as a likely location of hemolysis and thrombosis in tilting disk valves. The results of this first comprehensive study measuring the flow within the housing of a tilting disk valve may be helpful in minimizing the extent of blood damage through the combined efforts of experimental and computational fluid dynamics to improve mechanical heart valve designs.

Mechanical forces are known to affect the biomechanical properties of native and engineered cardiovascular tissue. In particular, shear stress that results from the relative motion of heart valve leaflets with respect to the blood flow is one important component of their mechanical environment in vivo. Although different types of bioreactors have been designed to subject cells to shear stress, devices to expose biological tissue are few. In an effort to address this issue, the aim of this study was to design an ex vivo tissue culture system to characterize the biological response of heart valve leaflets subjected to a well-defined steady or time-varying shear stress environment. The novel apparatus was designed based on a cone-and-plate viscometer. The device characteristics were defined to limit the secondary flow effects inherent to this particular geometry. The determination of the operating conditions producing the desired shear stress profile was streamlined using a computational fluid dynamic (CFD) model validated with laser Doppler velocimetry. The novel ex vivo tissue culture system was validated in terms of its capability to reproduce a desired cone rotation and to maintain sterile conditions. The CFD results demonstrated that a cone angle of 0.5 deg, a cone radius of 40 mm, and a gap of 0.2 mm between the cone apex and the plate could limit radial secondary flow effects. The novel cone-and-plate permits to expose nine tissue specimens to an identical shear stress waveform. The whole setup is capable of accommodating four cone-and-plate systems, thus concomitantly subjecting 36 tissue samples to desired shear stress condition. The innovative design enables the tissue specimens to be flush mounted in the plate in order to limit flow perturbations caused by the tissue thickness. The device is capable of producing shear stress rates of up to 650 dyn cm(-2) s(-1) (i.e., maximum shear stress rate experienced by the ventricular surface of an aortic valve leaflet) and was shown to maintain tissue under sterile conditions for 120 h. The novel ex vivo tissue culture system constitutes a valuable tool toward elucidating heart valve mechanobiology. Ultimately, this knowledge will permit the production of functional tissue engineered heart valves, and a better understanding of heart valve biology and disease progression.

Background: In vivo experimentation is the most realistic approach for exploring the vascular biological response to the hemodynamic stresses that are present in life. Post-mortem vascular casting has been used to define the in vivo geometry for hemodynamic simulation; however, this procedure damages or destroys the tissue and cells on which biological assays are to be performed., Method of Approach: Two statistical approaches, regional (RSH) and linear (LSH) statistical hemodynamics, are proposed and illustrated, in which flow simulations from one series of experiments are used to define a best estimate of the hemodynamic environment in a second series. As an illustration of the technique, RSH is used to compare the gene expression profiles of regions of the proximal external iliac arteries of swine exposed to different levels of time-average shear stress., Results: The results indicate that higher shears promote a more atheroprotective expression phenotype in porcine arterial endothelium., Conclusion: Statistical hemodynamics provides a realistic estimate of the hemodynamic stress on vascular tissue that can be correlated against biological response.

The long term patency of end-to-side peripheral artery bypasses are low due to failure of the graft generally at the distal end of the bypass. Both material mismatch between the graft and the host artery and junction hemodynamics are cited as being major factors in disease formation at the junction. This study uses experimental methods to investigate the major differences in fluid dynamics and wall mechanics at the proximal and distal ends for rigid and compliant bypass grafts. Injection moulding was used to produce idealized transparent and compliant models of the graft/ artery junction configuration. An ePTFE graft was then used to stiffen one of the models. These models were then investigated using two-dimensional video extensometry and one-dimensional laser Doppler anemometry to determine the junction deformations and fluid velocity profiles for the rigid and complaint graft anastomotic junctions. Junction strains were evaluated and generally found to be under 5% with a peak stain measured in the stiff graft model junction of 8.3% at 100 mmHg applied pressure. Hemodynamic results were found to yield up to 40% difference in fluid velocities for the stiff/compliant comparison but up to 80% for the proximal/distal end comparisons. Similar strain conditions were assumed for the proximal and distal models while significant differences were noted in their associated hemodynamic changes. In contrasting the fluid dynamics and wall mechanics for the proximal and distal anastomoses, it is evident from the results of this study, that junction hemodynamics are the more variable factor.

Blood viscosity changes with many pathologic conditions, but its importance has not been fully investigated because the current methods of measurement are poorly suited for clinical applications. The use of viscosity-sensitive fluorescent molecular rotors to determine fluid viscosity in a nonmechanical manner has been investigated recently, but it is unknown how the precision of the fluorescence-based method compares to established mechanical viscometry. Human blood plasma viscosity was modulated with high-viscosity plasma expanders, dextran, pentastarch, and hetastarch. The samples were divided into a calibration and a test set. The relationship between fluorescence emission and viscosity was established using the calibration set. Viscosity of the test set was determined by fluorescence and by cone-and-plate viscometer, and the precision of both methods compared. Molecular rotor fluorescence intensity showed a power law relationship with solution viscosity. Mechanical measurements deviated from the theoretical viscosity value by less than 7.6%, while fluorescence-based measurements deviated by less than 6%. The average coefficient of variation was 6.9% (mechanical measurement) and 3.4% to 3.8% (fluorescence-based measurement, depending on the molecular rotor used). Fluorescence-based viscometry exhibits comparable precision to mechanical viscometry. Fluorescence viscometry does not apply shear and is therefore more practical for biofluids which have apparent non-Newtonian properties. In addition, fluorescence instrumentation makes very fast serial measurements possible, thus promising new areas of application in laboratory and clinical settings.

In this work, we propose a simple method to simultaneously match the refractive index and kinematic viscosity of a circulating blood analog in hydraulic models for optical flow measurement techniques (PIV, PMFV, LDA, and LIF). The method is based on the determination of the volumetric proportions and temperature at which two transparent miscible liquids should be mixed to reproduce the targeted fluid characteristics. The temperature dependence models are a linear relation for the refractive index and an Arrhenius relation for the dynamic viscosity of each liquid. Then the dynamic viscosity of the mixture is represented with a Grunberg-Nissan model of type 1. Experimental tests for acrylic and blood viscosity were found to be in very good agreement with the targeted values (measured refractive index of 1.486 and kinematic viscosity of 3.454 milli-m2/s with targeted values of 1.47 and 3.300 milli-m2/s).

Nonlinear 3-D models with fluid-structure interactions (FSI) based on in vitro experiments are introduced and solved by ADINA to perform flow and stress/strain analysis for stenotic arteries with lipid cores. Navier-Stokes equations are used as the governing equations for the fluid. Hyperelastic Mooney-Rivlin models are used for both the arteries and lipid cores. Our results indicate that critical plaque stress/strain conditions are affected considerably by stenosis severity, eccentricity, lipid pool size, shape and position, plaque cap thickness, axial stretch, pressure, and fluid-structure interactions, and may be used for possible plaque rupture predictions.

The continuous flow ventricular assist device (VAD) is a miniature centrifugal pump, fully suspended by magnetic bearings, which is being developed for implantation in humans. The CF4 model is the first actual prototype of the final design product. The overall performances of blood flow in CF4 have been simulated using computational fluid dynamics (CFD) software: CFX, which is commercially available from ANSYS Inc. The flow regions modeled in CF4 include the inlet elbow, the five-blade impeller, the clearance gap below the impeller, and the exit volute. According to different needs from patients, a wide range of flow rates and revolutions per minute (RPM) have been studied. The flow rate-pressure curves are given. The streamlines in the flow field are drawn to detect stagnation points and vortices that could lead to thrombosis. The stress is calculated in the fluid field to estimate potential hemolysis. The stress is elevated to the decreased size of the blood flow paths through the smaller pump, but is still within the safe range. The thermal study on the pump, the blood and the surrounding tissue shows the temperature rise due to magnetoelectric heat sources and thermal dissipation is insignificant. CFD simulation proved valuable to demonstrate and to improve the performance of fluid flow in the design of a small size pump.

Based on the hypothesis that aggravating hemodynamic factors play a key role in the onset of arterial diseases, the methodology of "virtual prototyping" of branching blood vessels was applied to diseased external carotid artery (ECA) segments. The goals were to understand the underlying particle-hemodynamics and to provide various geometric design options for improved surgical reconstruction based on the minimization of critical hemodynamic wall parameters (HWPs). First, a representative carotid artery bifurcation (CAB) and then CABs with stenosed ECAs, i.e., a distally occluded ECA and an ECA stump, were analyzed based on transient three-dimensional blood flow solutions, employing a user-enhanced commercial finite volume code. Specifically, the HWPs, i.e., oscillatory shear index, wall shear stress angle gradient, near-wall residence time of monocytes, and near-wall helicity angle difference were evaluated to compare the merits of each design option, including a reconstructed near-optimal junction which generates the lowest HWP-values. The results provide physical insight to the biofluid dynamics of branching blood vessels and guide vascular surgeons as well as stent manufacturers towards interventions leading to high sustained patency rates.

This work introduces a method for the in vivo measurement and analysis of turbulence within the leakage of a mechanical heart valve. Several analysis techniques were applied to ultrasound measurements acquired within the atrium of a pig, and error associated with these techniques was analyzed. The technique chosen applies cyclic averaging to mean and maximum velocity measurements within small, normalized phase windows to calculate Reynolds normal stresses in the direction of the ultrasound beam. Maximum shear stresses are estimated from these normal stresses using an analytical technique. The stresses observed were smaller than those reported from previous in vitro simulations.

Blood flow in small curved tubes is modeled by the two-fluid model where a relatively cell-free fluid layer envelops a fluid core of higher viscosity. The parameters in the model are successfully curve fitted to experimental data for straight tubes. The curved tube equations are then solved by perturbation theory. It was found that curvature in general lowers the tube resistance, but increases the shear stress near the inside wall.

Recent in-vivo and in-vitro evidence indicates that fluid shear stress on the membrane of leukocytes has a powerful control over several aspects of their cell function. This evidence raises a question about the magnitude of the fluid shear stress on leukocytes in the circulation. The flow of plasma on the surface of a leukocyte at a very low Reynolds number is governed by the Stokes equation for the motion of a Newtonian fluid. We numerically estimated the distribution of fluid shear stress on a leukocyte membrane in a microvessel for the cases when the leukocyte is freely suspended, as well as rolling along or attached to a microvessel wall. The results indicate that the fluid shear stress distribution on the leukocyte membrane is nonuniform with a sharp increase when the leukocyte makes membrane attachment to the microvessel wall. In a microvessel (10 microns diameter), the fluid shear stress on the membrane of a freely suspended leukocyte (8 microns diameter) is estimated to be several times larger than the wall shear stress exerted by the undisturbed Poiseuille flow, and increases on an adherent leukocyte up to ten times. High temporal stress gradients are present in freely suspended leukocytes in shear flow due to cell rotation, which are proportional to the local shear rate. In comparison, the temporal stress gradients are reduced on the membrane of leukocytes that are rolling or firmly adhered to the endothelium. High temporal gradients of shear stress are also present on the endothelial wall. At a plasma viscosity of 1 cPoise, the peak shear stresses for suspended and adherent leukocytes are of the order of 10 dyn/cm2 and 100 dyn/cm2, respectively.

A numerical method is developed for simulating unsteady, 3-D, laminar flow through a bileaflet mechanical heart valve with the leaflets fixed. The method employs a dual-time-stepping artificial-compressibility approach together with overset (Chimera) grids and is second-order accurate in space and time. Calculations are carried out for the full 3-D valve geometry under steady inflow conditions on meshes with a total number of nodes ranging from 4 x 10(5) to 1.6 x 10(6). The computed results show that downstream of the leaflets the flow is dominated by two pairs of counter-rotating vortices, which originate on either side of the central orifice in the aortic sinus and rotate such that the common flow of each pair is directed away from the aortic wall. These vortices intensify with Reynolds number, and at a Reynolds number of approximately 1200 their complex interaction leads to the onset of unsteady flow and the break of symmetry with respect to both geometric planes of symmetry. Our results show the highly 3-D structure of the flow; question the validity of computationally expedient assumptions of flow symmetry; and demonstrate the need for highly resolved, fully 3-D simulations if computational fluid dynamics is to accurately predict the flow in prosthetic mechanical heart valves.

Research studies over the last three decades have established that hemodynamic interactions with the vascular surface as well as surgical injury are inciting mechanisms capable of eliciting distal anastomotic intimal hyperplasia (IH) and ultimate bypass graft failure. While abnormal wall shear stress (WSS) conditions have been widely shown to affect vascular biology and arterial wall self-regulation, the near-wall localization of critical blood particles by convection and diffusion may also play a significant role in IH development. It is hypothesized that locations of elevated platelet interactions with reactive or activated vascular surfaces, due to injury or endothelial dysfunction, are highly susceptible to IH initialization and progression. In an effort to assess the potential role of platelet-wall interactions, experimentally validated particle-hemodynamic simulations have been conducted for two commonly implemented end-to-side anastomotic configurations, with and without proximal outflow. Specifically, sites of significant particle interactions with the vascular surface have been identified by a novel near-wall residence time (NWRT) model for platelets, which includes shear stress-based factors for platelet activation as well as endothelial cell expression of thrombogenic and anti-thrombogenic compounds. Results indicate that the composite NWRT model for platelet-wall interactions effectively captures a reported shift in significant IH formation from the arterial floor of a relatively high-angle (30 deg) graft with no proximal outflow to the graft hood of a low-angle graft (10 deg) with 20% proximal outflow. In contrast, other WSS-based hemodynamic parameters did not identify the observed system-dependent shift in IH formation. However, large variations in WSS-vector magnitude and direction, as encapsulated by the WSS-gradient and WSS-angle-gradient parameters, were consistently observed along the IH-prone suture-line region. Of the multiple hemodynamic factors capable of eliciting a hyperplastic response at the cellular level, results of this study indicate the potential significance of platelet-wall interactions coinciding with regions of low WSS in the development of IH.

To investigate the effect of junction strands on microvessel permeability, we extend the previous analytical model developed by Fu et al. (1994, J. Biomech. Eng., 116, pp. 502-513), for the interendothelial cleft to include multiple junction strands in the cleft and an interface between the surface glycocalyx layer and the cleft entrance. Based on the electron microscopic observations by Adamson et al. (1998, Am. J. Physiol., 274(43), pp. H1885-H1894), that elevation of intracellular cAMP levels would increase number of tight junction strands, this two-junction-strand and two-pore model can successfully account for the experimental data for the decreased permeability to water, small and intermediate-sized solutes by cAMP.

We used an innovative, nonintrusive MRI technique called the two-dimensional (2D) Phase-Contrast (2DPC) velocity-imaging technique to investigate the effect of flow baffles on the dialysate-side flow distribution in two different hollow-fiber hemodialyzers (A and B); each with flow rates between 200 and 1000 mL/min (3.33 x 10(-6) and 1.67 x 10(-5) m3/s). Our experimental results show that (1) the dialysate-side flow distribution was nonuniform with channeling flow occurred at the peripheral cross section of these hollow-fiber hemodialyzers, and (2) the existing designs of flow baffles failed to promote uniform dialysate-side flow distribution for all flow rates studies.

Pulsatile turbulent flow in stenotic vessels has been numerically modeled using the Reynolds-averaged Navier-Stokes equation approach. The commercially available computational fluid dynamics code (CFD), FLUENT, has been used for these studies. Two different experiments were modeled involving pulsatile flow through axisymmetric stenoses. Four different turbulence models were employed to study their influence on the results. It was found that the low Reynolds number k-omega turbulence model was in much better agreement with previous experimental measurements than both the low and high Reynolds number versions of the RNG (renormalization-group theory) k-epsilon turbulence model and the standard k-epsilon model, with regard to predicting the mean flow distal to the stenosis including aspects of the vortex shedding process and the turbulent flow field. All models predicted a wall shear stress peak at the throat of the stenosis with minimum values observed distal to the stenosis where flow separation occurred.

The goal of this study was to quantify the effect of hemodynamic pressure, flow and waveform perturbations on the deposition of protein-sized particles in porcine carotid arteries ex vivo. An ex vivo perfusion system was used to control the pressure and flow environment for excised arterial tissue. Confocal laser microscopy images revealed that 200 nm particles were deposited intimally and that more spheres were evident along vessels perfused under oscillatory waveform conditions than all others. Under all pressure, flow and waveform conditions, particles were excluded from the media and adventitia of the vessel wall. The steady flow data support the use of Darcy's Law with pressure-dependent hydraulic permeability to model arterial tissue.

A new one-dimensional model is presented for the calculation of steady and unsteady flow through an indented two-dimensional channel with separation and reattachment. It is based on an interactive boundary layer approach, where the equations for the boundary layer flow near the channel walls and for an inviscid core flow are solved simultaneously. This approach requires no semi-empirical inputs, such as the location of separation and reattachment, which is an advantage over other existing one-dimensional models. Because of the need of an inviscid core alongside the boundary layers, the type of inflow as well as the length of the channel and the value of the Reynolds number poses some limitations on the use of the new model. Results have been obtained for steady flow through the indented channel of Ikeda and Matsuzaki. In further perspective, it is discussed how the present model, in contrast to other one-dimensional flow models, can be extended to calculate the flow in nonsymmetrical channels, by considering different boundary layers on each of the walls.

In the abdominal segment of the human aorta under a patient's average resting conditions, pulsatile blood flow exhibits complex laminar patterns with secondary flows induced by adjacent branches and irregular vessel geometries. The flow dynamics becomes more complex when there is a pathological condition that causes changes in the normal structural composition of the vessel wall, for example, in the presence of an aneurysm. This work examines the hemodynamics of pulsatile blood flow in hypothetical three-dimensional models of abdominal aortic aneurysms (AAAs). Numerical predictions of blood flow patterns and hemodynamic stresses in AAAs are performed in single-aneurysm, asymmetric, rigid wall models using the finite element method. We characterize pulsatile flow dynamics in AAAs for average resting conditions by means of identifying regions of disturbed flow and quantifying the disturbance by evaluating flow-induced stresses at the aneurysm wall, specifically wall pressure and wall shear stress. Physiologically realistic abdominal aortic blood flow is simulated under pulsatile conditions for the range of time-average Reynolds numbers 50 < or = Rem < or = 300, corresponding to a range of peak Reynolds numbers 262.5 < or = Repeak < or = 1575. The vortex dynamics induced by pulsatile flow in AAAs is depicted by a sequence of four different flow phases in one period of the cardiac pulse. Peak wall shear stress and peak wall pressure are reported as a function of the time-average Reynolds number and aneurysm asymmetry. The effect of asymmetry in hypothetically shaped AAAs is to increase the maximum wall shear stress at peak flow and to induce the appearance of secondary flows in late diastole.

The mass transfer behavior in the recirculation region downstream of an axisymmetric sudden expansion was examined. The Reynolds number, 500, and Schmidt number, 3200, were selected to model the mass transfer of molecules, such as ADP, in the arterial system. In a first step the transient mass transport applying zero diffusive flux at the wall was analyzed using experiments and two computational codes. The two codes were FLUENT, a commercially available finite volume method, and FTSP, a finite element code developed at Graz University of Technology. The comparison of the transient wall concentration values determined by the three methods was excellent and provides a measure of confidence for computational mass transfer calculations in convection dominated, separated flows. In a second step the effect of the flow separation on the stationary mass transport applying a permeability boundary condition at the water-permeable wall was analyzed using the finite element code FTSP. The results show an increase of luminal ADP surface concentration in the upstream and in the downstream tube of the sudden expansion geometry in the range of six and twelve percent of the bulk flow concentration. The effect of flow separation in the downstream tube on the wall concentration is a decrease of about ten percent of the difference between wall concentration and bulk concentration occurring at nearly fully developed flow at the downstream region at a distance of 66 downstream tube diameters from the expansion. The decrease of ADP flux into the wall is in the range of three percent of the flux at the downstream region.

Extensive flow studies are conducted in two carotid bifurcation flow phantoms. These phantoms exactly replicate the lumen of the plaque excised intact from two patients with severe carotid atherosclerosis. The input flow into the phantom's common carotid artery is steady. Novel scanning techniques for flow visualization and particle image velocimetry are used. In addition, a novel boundary treatment technique is employed in velocimetry to extract first order accurate velocity gradients at walls. The data show that the flow fields are highly three-dimensional. Numerous separation and recirculation zones dominate the flow domain, except at the lowest Reynolds numbers. The separation regions are often so severe that highly directed internal jets form. At high Reynolds numbers, the flows become unsteady and chaotic, even though the input flow is steady. Flow fields have large regions of energetic flow and almost stagnant recirculation zones. These recirculation zones range in size from the full size of the arteries to zones within crevasses smaller than 1 mm. Velocity field and streamline patterns conform well to the lumen geometry. The streamlines are highly tortuous. Stagnation points correlate well with the topological features of the stenosis. Vorticity maps confirm the highly complex and three dimensional nature of the flow. Wall shear stresses at the stenoses are estimated to be on the order of 10 Pa. These studies conclusively show that the nature of the flow in the diseased bifurcation is primarily dictated by the lumen geometry.

We present experimental and computational results that describe the level, distribution, and importance of velocity fluctuations within the venous anastomosis of an arteriovenous graft. The motivation of this work is to understand better the importance of biomechanical forces in the development of intimal hyperplasia within these grafts. Steady-flow in vitro studies (Re = 1060 and 1820) were conducted within a graft model that represents the venous anastomosis to measure velocity by means of laser Doppler anemometry. Numerical simulations with the same geometry and flow conditions were conducted by employing the spectral element technique. As flow enters the vein from the graft, the velocity field exhibits flow separation and coherent structures (weak turbulence) that originate from the separation shear layer. We also report results of a porcine animal study in which the distribution and magnitude of vein-wall vibration on the venous anastomosis were measured at the time of graft construction. Preliminary molecular biology studies indicate elevated activity levels of the extracellular regulatory kinase ERK1/2, a mitogen-activated protein kinase involved in mechanotransduction, at regions of increased vein-wall vibration. These findings suggest a potential relationship between the associated turbulence-induced vein-wall vibration and the development of intimal hyperplasia in arteriovenous grafts. Further research is necessary, however, in order to determine if a correlation exists and to differentiate the vibration effect from that of flow related effects.

We have developed an in vitro method for creating pulsatile flows to mimic coronary type flow patterns on a beat-to-beat basis. The flow is created by accelerating fluid loops about an axis, inducing relative wall motion. Using this technique, a variety of oscillating flow patterns can be generated and modulated. Such flow generation offers the potential to monitor sensitive, flow-dependent, biological parameters like thrombosis while minimizing background disturbances from pump action and circuit effects. We examined this potential by measuring the loop occlusion time for loops stented with stainless steel 7-9 NIR stents and stentless control loops.

A dual-pressure boundary condition has been developed for computational modelling of bifurcating conduits. The condition involves the imposition of a constant pressure on one branch while adjusting iteratively the pressure on the other branch until the desired flow division is obtained. The dual-pressure condition eliminates the need for specifying fully-developed flow conditions, which thereby enables significant reduction of the outlet branch lengths. The dual-pressure condition is suitable for both steady and time-periodic simulations of laminar or turbulent flows.

A novel macroscopic gas transport model, derived from fundamental engineering principles, is used to simulate the three-dimensional, unsteady respiration process within the alveolar region of the lungs. The simulations, mimicking the single-breath technique for measuring the lung diffusing capacity for carbon-monoxide (CO), allow the prediction of the red blood cell (RBC) distribution effects on the lung diffusing capacity. Results, obtained through numerical simulations, unveil a strong relationship between the type of distribution and the lung diffusing capacity. Several RBC distributions are considered, namely: normal (random), uniform, center-cluster, and corner-cluster red cell distributions. A nondimensional correlation is obtained in terms of a geometric parameter characterizing the RBC distribution, and presented as a useful tool for predicting the RBC distribution effect on the lung diffusing capacity. The effect of red cell movement is not considered in the present study because CO does not equilibrate with capillary blood within the time spent by blood in the capillary. Hence, blood flow effect on CO diffusion is expected to be only marginal.

In the circulation, flow-responsive endothelial cells (ECs) lining the lumen of blood vessels are continuously exposed to complex hemodynamic forces. To increase our understanding of EC response to these dynamic shearing forces, a novel in vitro flow model was developed to simulate pulsatile shear stress waveforms encountered by the endothelium in the arterial circulation. A modified waveform modeled after flow patterns in the human abdominal aorta was used to evaluate the biological responsiveness of human umbilical vein ECs to this new type of stimulus. Arterial pulsatile flow for 24 hours was compared to an equivalent time-average steady laminar shear stress, using no flow (static) culture conditions as a baseline. While both flow stimuli induced comparable changes in cell shape and alignment, distinct patterns of responses were observed in the distribution of actin stress fibers and vinculin-associated adhesion complexes, intrinsic migratory characteristics, and the expression of eNOS mRNA and protein. These results thus reveal a unique responsiveness of ECs to an arterial waveform and begin to elucidate the complex sensing capabilities of the endothelium to the dynamic characteristics of flows throughout the human vascular tree.

A microscopic velocimetry technique for evaluating the flow field over cultured endothelial cells was developed. Flow around a cell model scaled up by a factor of 100 was visualized by using an optical microscope and was quantified by using particle-tracking velocimetry. Wall shear stress on the model surface was determined from a two-dimensional velocity field interpolated from measured velocity vectors. Accuracy of the velocimetry was verified by measuring the flow over a sinusoidal cell model that had a wall shear stress profile analytically determined with linear perturbation theory. Comparison of the experimental results with the analytical solution revealed that the total error of the measured wall shear stress was 6 percent.

The study objectives were to quantify the time- and magnitude-dependence of flow-induced alignment in vascular smooth muscle cells (SMC) and to identify pathways related to the orientation process. Using an intensity gradient method, we demonstrated that SMC aligned in the direction perpendicular to applied shear stress, which contrasts with parallel alignment of endothelial cells under flow SMC alignment varied with the magnitude of and exposure time to shear stress and is a continuous process that is dependent on calcium and cycloskeleton based mechanisms. A clear understanding and control of flow-induced SMC alignment will have implications for vascular tissue engineering.

An automated three-dimensional particle tracking technique has been developed to study particle motion in modeled flow fields. A high speed video recording system. Kodak Ektapro 1000, with two cameras arranged relatively orthogonally is used for this technique. The particle tracking data are compared to theoretical Poiseuille flow and to laser Doppler data from an axisymmetric stenosis model. The particle tracking data are in good agreement with both theoretical and laser Doppler data, and at least 79 percent of the particle paths were determined successfully. Fluid dynamic properties derived by this technique are: 3-D particle paths, velocity, and particle residence time.

In order to simulate regions of flow separation observed in vivo, a conventional parallel plate flow chamber was modified to produce an asymmetric sudden expansion. The flow field was visualized using light reflecting particles and the size of the recirculation zone was measured by image analysis of the particles. Finite element numerical solutions of the two and three-dimensional forms of the Navier-Stokes equation were used to determine the wall shear stress distribution and predict the location of reattachment. For two different size expansions, numerical estimates of the reattachment point along the centerline of the flow chamber agreed well with experimental values for Reynolds numbers below 473. Even at a Reynolds number of 473, the flow could be approximated as two-dimensional for 80 percent of the chamber width. Peak shear stresses in the recirculation zone as high as 80 dyne/cm2 and shear stress gradients of 2500 (dyne/cm2)/cm were produced. As an application of this flow chamber, subconfluent bovine aortic endothelial cell shape and orientation were examined in the zone of recirculation during a 24 h exposure to flow at a Reynolds number of 267. After 24 h, gradients in cell orientation and shape were observed within the recirculation zone. At the location of reattachment, where the wall shear stress was zero but the shear stress gradients were large, cells plated at low density were still aligned with the direction of flow. No preferred orientation was observed at the gasket edge where the wall shear stress and shear stress gradients were zero. At higher cell densities, no alignment was observed at the separation point.(ABSTRACT TRUNCATED AT 250 WORDS)