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Biomedical subjects

G B Thurston

Publications and source records attributed to G B Thurston.

At least 19 recordsLinked to original sources
PubMed1999

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.

Animals↗
PubMed1993

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.

Blood Physiological Phenomena↗
PubMed1993

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.

Blood Coagulation↗
PubMed1990

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.

Blood Physiological Phenomena↗
PubMed1989

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.

Blood Viscosity↗
PubMed1988

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.

Erythrocyte Aggregation↗
PubMed1987

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.

Blood Viscosity↗
PubMed1978

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.

Carboxymethylcellulose Sodium↗