The effects of pulsatile versus nonpulsatile perfusion on blood viscoelasticity before and after deep hypothermic circulatory arrest in a neonatal piglet model.

Blood trauma increases blood viscoelasticity by increasing red cell aggregation and plasma viscosity and by decreasing cell deformability. During extracorporeal circulation, the mode of perfusion (pulsatile or nonpulsatile) may have a significant impact on blood trauma. In this study, a hydraulically driven dual chamber pulsatile pump system was compared to a standard nonpulsatile roller pump in terms of changes in the blood viscosity and elasticity during cardiopulmonary bypass (CPB) and pre and post deep hypothermic circulatory arrest (DHCA). Piglets, with an average weight of 3 kg, were used in the pulsatile (n = 5) or nonpulsatile group (n = 5). All animals were subjected to 25 min of hypothermia, 60 min of DHCA, 10 min of cold reperfusion, and 40 min of rewarming with a pump flow of 150 ml/kg/min. A pump rate of 150 bpm, pump ejection time of 120 ms, and stroke volume of 1 ml/kg were used during pulsatile CPB. Arterial blood samples were taken pre-CPB (36 degrees C), during normothermic CPB (35 degrees C), during hypothermic CPB (25 degrees C), pre-DHCA (18 degrees C), post-DHCA (19 degrees C), post-rewarming (35 degrees C), and post-CPB (36 degrees C). Viscosity and elasticity were measured at 2 Hz and 22 degrees C and at strains of 0.2, 1, and 5 using the Vilastic-3 Viscoelasticity Analyzer. Results suggest that the dual chamber neonate-infant pulsatile pump system produces less blood trauma than the standard nonpulsatile roller pump as indicated by lower values of both viscosity and elasticity during CPB support.

The effect of blood viscoelasticity on pulsatile flow in stationary and axially moving tubes.

An analytical solution for pulsatile flow of a generalized Maxwell fluid in straight rigid tubes, with and without axial vessel motion, has been used to calculate the effect of blood viscoelasticity on velocity profiles and shear stress in flows representative of those in the large arteries. Measured bulk flow rate Q waveforms were used as starting points in the calculations for the aorta and femoral arteries, from which axial pressure gradient delta P waves were derived that would reproduce the starting Q waves for viscoelastic flow. The delta P waves were then used to calculate velocity profiles for both viscoelastic and purely viscous flow. For the coronary artery, published delta P and axial vessel acceleration waveforms were used in a similar procedure to determine the separate and combined influences of viscoelasticity and vessel motion. Differences in local velocities, comparing viscous flow to viscoelastic flow, were in all cases less than about 2% of the peak local velocity. Differences in peak wall shear stress were less than about 3%. In the coronary artery, wall shear stress differences between viscous and viscoelastic flow were small, regardless of whether axial vessel motion was included. The shape of the wall shear stress waveform and its difference, however, changed dramatically between the stationary and moving vessel cases. The peaks in wall shear stress difference corresponded with large temporal gradients in the combined driving force for the flow.

Impedance of a fibrin clot in a cylindrical tube: relation to clot permeability and viscoelasticity.

By use of an impedance to quantify the pressure-to-flow relation for a clot-filled tube, a simple model is developed that encompasses both viscoelastic and porous properties of the clot. Measurements over a range of frequencies are used to separate the role of clot permeability from clot matrix elasticity. The theoretical impedance model consists of a series resistance and capacitance (representing structural flow) in parallel with a resistance (representing permeating flow). The viscoelasticity of the matrix, permeability and effective pore size are related to these three impedance elements. The validity of the model has been verified for a range of vessel sizes approximating small arteries (1 to 3 mm in diameter). The presence of dextran T40 in clotted fibrinogen solutions changes the clot impedance by increasing clot permeability and decreasing clot viscoelasticity. Because the flow contains two components, the behavior of a clot in vivo under pulsatile pressure cannot be predicted from the viscoelastic properties obtained from non-tube flow instruments nor from steady flow permeation measurements; a combination of the two as provided by oscillatory tube-flow measurements is required.

Non-Newtonian viscosity of human blood: flow-induced changes in microstructure.

Flow-induced changes in the red cell microstructure of human blood are identified from mechanical and optical evidence. On initiation of steady flow, a new microstructure develops as the shear strain increases through unit strain. This structure is identified with the formation of layers of red cells that slide on plasma layers (Thurston, 1989). At low shear rates, the cell layers are composed of aggregated cells, but at higher shear rates, the aggregates degrade to form thinner layers of oriented, compacted cells. The viscosity is determined by the hematocrit, the degree of compaction and viscosity within the cell layers, and the plasma viscosity. Degradation of cell aggregates is controlled by 1) the time required for the strain to increase by one unit (delta t1 = 1/shear rate) and 2) the dominant viscoelastic relaxation times of the red cell structures. Structures having relaxation times > delta t1 are degraded by cell disaggregation; when delta t1 is less than the shortest relaxation time of the layered cells, disaggregation and (cell and plasma) layer formation are nearly complete. Analyses of the non-Newtonian viscosity and cell layer characteristics are given for both normal and hardened cells.

Comparisons of the oscillatory shear viscoelasticity and composition of pathological synovial fluids.

Rheological and compositional properties of pathological synovial fluids were measured and compared in order to reveal differences between disease states. The cases include degenerative joint disease, rheumatoid arthritis, mixed connective tissue disease, and pseudogout. Using an oscillatory flow capillary instrument, measurements were made of both the frequency and shear rate dependence of the complex viscoelasticity. The fluid types differ most in their elasticity, with the degenerative joint disease fluids having the greatest average viscosity, elasticity and intrinsic viscosity, followed by the rheumatoid arthritis fluids, and the fluids from cases of mixed connective tissue disease. Differences in the hyaluronate and protein concentrations are not as great as those between rheological variables. The viscoelasticity of synovial fluid appears more strongly dependent on the degree of polymerization of hyaluronate than on its concentration. These synovial fluids conform well to a model of relaxation process truncation. Distinct types of elastic stress-strain behavior reveal the nature of the dynamic fluid structure.

The elastic yield stress of human blood.

The elastic yield stress is a key parameter in the way human blood flows, because after yielding the microstructure formed by the red cells undergoes a dramatic change and the blood is transformed to a superfluid state. This yielding of the structure is revealed in measurements of the progressive changes in viscoelasticity with increasing amplitude of oscillatory flow. Measurements of the magnitude and phase of the pressure differential along the tube length, and the volume flow through the tube, are performed with sinusoidal oscillatory flow in a cylindrical tube at a frequency near that of the normal pulse rate. This gives the shear rate, shear strain, and the viscous and elastic components of shear stress at the tube wall, and from these parameters the viscosity and elasticity of the blood are calculated. The instrumentation (Vilastic Scientific Inc.) used to perform these measurements produces graphs of the viscous and elastic shear stress versus the shear strain. Near unit strain the elastic stress reaches a maximum, marking the point at which the quiescent aggregated cell structure yields and reassembles into a layered structure. Examples are given to show how factors such as red cell aggregation and cell deformability affect the elastic yield stress.

A new method for the analysis of blood and plasma coagulation.

A new method of measuring and analyzing the clinically significant structural characteristics of blood clots is described, using an oscillatory flow instrument (Vilastic-3 Viscoelasticity Analyzer). The development of the viscous and elastic properties of the clot are monitored continuously at known levels of stress and strain in a cylindrical tube [1], and specific transitional events and their timing are obtained. In clinical practice, two types of clot analysis currently are in use. One measures the time required for the clot to reach some instrument-determined degree of development, which provides no information about the mechanical character of the clot. The second yields a continuous depiction of the way the forming clot affects the instrument [2] and gives numerical values that are inseparable from instrument characteristics. For greater universality, it is preferable to obtain the clot properties per se, as opposed to integrated clot-instrument parameters, and to document the timing of well-defined viscoelastic events. Measurements on clotting blood and plasma show that the dependence of clotting characteristics on measurements conditions and hence the need for precise control of stress or strain during the process.

Light transmission through blood in oscillatory flow.

Measurements were made of the intensity of light transmitted through various thicknesses of normal human blood confined between two parallel plane surfaces, one fixed and the other oscillating in its own plane. When the light propagation direction is perpendicular to the direction of shear flow the transmitted intensity contains a steady component and a dominant second harmonic of the oscillation frequency. For a thin layer of blood, the steady intensity is a minimum value when the oscillation amplitude produces unit strain. The second harmonic is very small at small strains, but increases rapidly near unit strain where it is approximately in phase with the strain. For thicker layers, the effects of viscoelastic shear waves reduce the size of the second harmonic and modify its phase. Changes in light transmission are interpreted by relating the optical density of the blood to the total amount of contact between red cells. In oscillatory flow at low strains (less than 1) cell-to-cell contact is reduced by disaggregation of cell groups, and light transmission decreases. Near unit strain, disaggregation becomes complete, cell alignment occurs, and light transmission is minimized. At higher strains cell-to-cell contact is increased by formation of aligned layers of compacted cells separated by parallel plasma layers, and light transmission increases.

Plasma release-cell layering theory for blood flow.

Significant internal structural changes occur in flowing blood when shear strain exceeds the critical value of 1 (unit strain), forcing alignment of the erythrocytes and releasing trapped plasma, which in turn leads to the formation of multiple layers of plasma on which oriented and compacted cells slide. These effects are identified in the inflections in the shear rate dependence of viscoelasticity of normal blood and in the viscous and elastic stress-to-strain relationships. Theoretical factors for plasma release and cell compaction allow calculation of the viscous and elastic properties of the cell layers from measured whole blood viscoelasticity and plasma viscosity. The new plasma release-cell layering theory encompasses, reinterprets and unifies many diverse previous observations relating to how blood flows, and provides a new understanding of the roles of red cell deformability and aggregation tendency.

Erythrocyte aggregate rheology by transmitted and reflected light.

Both the transmission of light through a confined layer of blood and the reflection from the surface of that layer have been utilized for studying the rheology of erythrocyte aggregates. The two methods do not necessarily provide the same information. The light reflected from the blood layer relates to the rheological behavior of erythrocytes near the blood surface, whereas the light transmitted relates more to the properties of blood in bulk. This investigation makes direct comparison between the transmitted and reflected light methods with regard to the kinetics of aggregation in thin and thick layers of blood as well as following shear flow excitation steps of different sizes. Also, the transmission and reflection for static blood layers of varying thicknesses were determined. The kinetics of aggregation from transmitted and reflected light measurements are compared both graphically and by equations containing multiple characteristic aggregation times. The number of characteristic times required for accurate description increases with the time over which the aggregation process is monitored. The first 40 seconds of the aggregation process are precisely described by two characteristic times. For normal blood the characteristic times from reflection measurements are shorter than those from transmission measurements.

Intrinsic viscoelasticity of blood cell suspensions: effects of erythrocyte deformability.

The intrinsic viscoelasticity of erythrocyte suspensions holds great potential for specifying the deformability of the individual, noninteracting cells in an oscillatory shear flow field. In order to extrapolate to zero cell concentration, the complex viscoelastic modulus was measured as a function of hematocrit using 2 Hertz oscillatory flow and a shear rate of 10/sec. This was done for both normal cells and cells with severely reduced deformability when hardened with glutaraldehyde. Suspension media were blood plasma, isotonic saline, and Dextran solutions. The real parts of the complex intrinsic visco-elasticities were obtained by an extrapolation using a regression fit to Huggins' equation. For normal cells in native plasma the values ranged from 1.7 to 2, increasing to the range 2.4 to 3.1 when the plasma was diluted with isotonic saline solution. For hardened cells the value obtained was near 3.5. These results are compared with theories for suspensions of both rigid and deformable particles. Several theories for deformable particles predict an increase in intrinsic viscoelasticity with increases in the ratio of the viscosity of the interior of the particle to that of the suspending medium. This ratio controls the balance between rotational and deformational response of the cell in the flow field. The trends of these theories were observed in the measurements.

Rheology of pharmaceutical systems: oscillatory and steady shear of non-Newtonian viscoelastic liquids.

A comparative analysis of oscillatory and steady shear rate measurements was made on carboxymethylcellulose solutions of two concentrations and two viscosity grades. In the oscillatory methods, the material is examined under nearly quiescent equilibrium conditions. Steady shear, conversely, produces large deformations and may yield false results, often interpreted as thixotropy, if the shear rate experiment is not conducted properly. Solutions of carboxymethylcellulose at concentrations ordinarily used in drug product formulations were examined by oscillatory and steady shear methods at low shear. Viscoelastic properties of pharmaceutical materials were measured using a newly developed oscillometric instrument. Mathematical expressions, formulated on the basis of a generalized Maxwell model for viscoelasticity and viscosity in steady shear, were correlated using these two rheological test methods. The results showed large increases in viscosity and relaxation time with increasing carboxymethylcellulose concentrations as well as with increasing molecular weights of the polymeric solute. The behavior of carboxymethylcellulose under both oscillatory and steady shear agreed with theory, linking the two methods of testing. Applications in pharmacy to this rheological analysis are presented. The present investigation attempted to bridge the gap between oscillatory and steady shear methods, demonstrating how both can find appropriate use in the analysis of non-Newtonian materials of pharmaceutical importance.