Effects of aggregation on the blood flow velocity field measured by a μPIV based technique

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.The flow of red blood cells is investigated by means of a micro-PIV based technique at physiological hematocrit levels and in the presence of aggregation. The technique developed differs from typical micro-PIV as the RBCs are used as tracer particles and illumination is provided by a simple halogen light source. Changes in the microstructure of blood caused by aggregation were observed to affect the RBC flow characteristics in a narrow-gap plate-plate geometry. At low shear rates, high aggregation caused the RBC motion to become essentially two-dimensional and network formation lead to the flow deviating from the expected radial profile. The accuracy of the micro-PIV technique was shown to be dependent on aggregation, illustrating the need to take aggregation into account in future RBC flow studies.This work was supported in part by the EPSRC Life Sciences Interface program (EP/F007736/1) and by the Leverhulme Trust(F/07 040/X)

Flow field characterisation of aggregating human blood in bifurcating microchannels

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Erythrocyte aggregation is a shear dependent physiological phenomenon that modifies local properties of blood flow. Blood flow characteristics in microvascular bifurcations are dependent on many parameters; however the influence of erythrocyte aggregation has not been investigated previously in vitro. In the present study, micro-PIV is used to provide high spatial resolution velocity data for both erythrocytes and suspending medium for aggregating and non-aggregating human blood samples in a microchannel with a T-bifurcation geometry on the scale of the microcirculation. Simultaneous hematocrit distributions are inferred from brightfield images. Full field shear distributions are described for an evenly split flow and single flow rate. Velocity profiles of cells upstream of the bifurcation are found to be less blunt than those of the suspended particles. Daughter branch velocity profiles downstream of the bifurcation are skewed towards the wall closest to the parent branch, and non-aggregating cell velocities are significantly less blunted than those of the aggregating case. The local hematocrit is increased at the channel wall opposite the parent branch and a cell-depleted layer is observed near the channel wall closest to the parent branch. Thus, it is shown that aggregation influences both hematocrit and velocity distributions around and downstream of a bifurcation

Quantifying local characteristics of velocity, aggregation and hematocrit of human erythrocytes in a microchannel flow

The effect of erythrocyte aggregation on blood viscosity and microcirculatory flow is a poorly understood area of haemodynamics, especially with relevance to serious pathological conditions. Advances in microfluidics have made it possible to study the details of blood flow in the microscale, however, important issues such as the relationship between the local microstructure and local flow characteristics have not been investigated extensively. In the present study an experimental system involving simple brightfield microscopy has been successfully developed for simultaneous, time-resolved quantification of velocity fields and local aggregation of human red blood cells (RBC) in microchannels. RBCs were suspended in Dextran and phosphate buffer saline solutions for the control of aggregation. Local aggregation characteristics were investigated at bulk and local levels using statistical and edge-detection image processing techniques. A special case of aggregating flow in a microchannel, in which hematocrit gradients were present, was studied as a function of flowrate and time. The level of aggregation was found to strongly correlate with local variations in velocity in both the bulk flow and wall regions. The edge detection based analysis showed that near the side wall large aggregates are associated with regions corresponding to high local velocities and low local shear. On the contrary, in the bulk flow region large aggregates occurred in regions of low velocity and high erythrocyte concentration suggesting a combined effect of haematocrit and velocity distributions on local aggregation characteristics. The results of this study showed that using multiple methods for aggregation quantification, albeit empirical, could help towards a robust characterisation of the structural properties of the fluid

Quantifying local characteristics of velocity, aggregation and hematocrit of human erythrocytes in a microchannel flow

The effect of erythrocyte aggregation on blood viscosity and microcirculatory flow is a poorly understood area of haemodynamics, especially with relevance to serious pathological conditions. Advances in microfluidics have made it possible to study the details of blood flow in the microscale, however, important issues such as the relationship between the local microstructure and local flow characteristics have not been investigated extensively. In the present study an experimental system involving simple brightfield microscopy has been successfully developed for simultaneous, time-resolved quantification of velocity fields and local aggregation of human red blood cells (RBC) in microchannels. RBCs were suspended in Dextran and phosphate buffer saline solutions for the control of aggregation. Local aggregation characteristics were investigated at bulk and local levels using statistical and edge-detection image processing techniques. A special case of aggregating flow in a microchannel, in which hematocrit gradients were present, was studied as a function of flowrate and time. The level of aggregation was found to strongly correlate with local variations in velocity in both the bulk flow and wall regions. The edge detection based analysis showed that near the side wall large aggregates are associated with regions corresponding to high local velocities and low local shear. On the contrary, in the bulk flow region large aggregates occurred in regions of low velocity and high erythrocyte concentration suggesting a combined effect of hematocrit and velocity distributions on local aggregation characteristics. The results of this study showed that using multiple methods for aggregation quantification, albeit empirical, could help towards a robust characterisation of the structural properties of the fluid

Numerical and in vitro experimental study of arterial deformation and buckling under hypertension and atherosclerotic conditions

Cardiovascular diseases remain the major cause of mortality worldwide. Pathologies of the vasculature such as atherosclerosis are often related to biochemical and genetic factors as well as mechanical effects that strongly change the function and shape of arteries. The present work is part of a general research project which aims to better understand the mechanical mechanisms responsible for atherosclerotic plaque formation and rupture. The chosen approach is to use numerical fluidstructure interaction (FSI) methods to study the relative influence of hemodynamic parameters on the structural stresses generated on plaques. To this aim, a numerical study of a simplified straight vessel exposed to lumen pressure was investigated under quiescent and steady flow conditions. As the internal pressure or the steady velocity increases, the vessel buckles lead-ing to a non-linear large deformation behaviour. The results have been validated using theoretical predictions for the buckling thresholds. Further studies on idealised cardiovascular conditions such as stenosis (i.e., lumen constriction) or aneurysm like (i.e., arterial wall expansion) formation have also been performed

Haemodynamical stress in mouse aortic arch with atherosclerotic plaques: Preliminary study of plaque progression

Atherosclerotic plaques develop at particular sites in the arterial tree, and this regional localisation depends largely on haemodynamic parameters (such as wall shear stress; WSS) as described in the literature. Plaque rupture can result in heart attack or stroke and hence understanding the development and vulnerability of atherosclerotic plaques is critically important. The purpose of this study is to characterise the haemodynamics of blood flow in the mouse aortic arch using numerical modelling. The geometries are digitalised from synchrotron imaging and realistic pulsatile blood flow is considered under rigid wall assumptions. Two cases are considered; arteries with and without plaque. Mice that are fed under fat diet present plaques in the aortic arch whose size is dependent on the number of weeks under the diet. The plaque distribution in the region is however relatively constant through the different samples. This result underlines the influence of the geometry and consequently of the wall shear stresses for plaque formation with plaques growing in region of relative low shear stresses. A discussion of the flow field in real geometry in the presence and absence of plaques is conducted. The presence of plaques was shown to alter the blood flow and hence WSS distribution, with regions of localised high WSS, mainly on the wall of the brachiocephalic artery where luminal narrowing is most pronounced. In addition, arch plaques are shown to induce recirculation in the blood flow, a phenomenon with potential influence on the progression of the plaques. The oscillatory shear index and the relative residence time have been calculated on the geometry with plaques to show the presence of this recirculation in the arch, an approach that may be useful for future studies on plaque progression

Three-dimensional numerical simulation of blood flow in mouse aortic arch around atherosclerotic plaques

Atherosclerosis is a progressive disease, involving the build-up of lipid streaks in artery walls, leading to plaques. Understanding the development of atherosclerosis and plaque vulnerability is critically important since plaque rupture can result in heart attack or stroke. Plaques can be divided into two distinct types: those likely to rupture (vulnerable) or less likely to rupture (stable). In the last decade, researchers have been interested in studying the influence of the mechanical effects (blood shear stress, pressure forces and structural stress) on the plaque formation, progression and rupture processes but no general agreement has been found. The purpose of the present work is to include more realistic conditions for the numerical calculations of the blood flow by implementing real geometries with plaques in the numerical model. Hemodynamical parameters are studied in both diseased and healthy configurations. The healthy configuration is obtained by removing numerically the plaques from three dimensional geometries obtained by micro-computed tomography. A new hemodynamical parameter is also introduced to relate the location of plaques to the characteristics of the flow in the healthy configuration

Extreme beginnings: the effect of cyclones on the development of Innisfail Far North Queensland

From the earliest days of white settlement in North Queensland, cyclones have shaped the way settlers have responded to living with destructive winds, torrential rains, storm surges and flooding. These extreme events have inflicted a heavy toll on the built environment and brought about structural changes in many towns to counter the effects of nature. This paper explores the development of the Innisfail area in an extreme tropical environment. By following the relationships between cyclones and the evolution of construction methods, a more enduring built environment has emerged. In particular these structural changes have come under scrutiny provided by the recent severe Cyclone Larry. The destructive impact on significant heritage places is high and appropriate conservation remains a challenge for those charged with protecting it

Are We Over Oxidized? Oxidative Stress, Cardiovascular Disease, and the Future of Intervention Studies with Antioxidants

A number of recent clinical trials with antioxidants, notably vitamin C and E, have provided no support for the commonly held view that increasing our intake of antioxidants will offset the ravages of cardiovascular disease as well as other diseases (for extensive critical reviews see: Kritharides and Stocker 2002; Antoniades et al 2003; Touyz 2004). Is this conclusion justified? The role of antioxidant dietary adjuncts and therapy in prevention and treatment remains a highly important clinical question. In this opinion article we address the question: Is there a future for antioxidant therapy in the treatment and prevention of cardiovascular disease? We conclude that there is a need for better-designed studies as well as a re-thinking of the choice of antioxidants