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Biomedical subjects

D P Giddens

Publications and source records attributed to D P Giddens.

At least 19 recordsLinked to original sources
PubMed1997

In-vivo measurements of blood flow velocity profiles in canine ilio-femoral anastomotic bypass grafts.

In-vivo velocity profiles were recorded with a 20 MHz 80-channel pulsed Doppler ultrasound velocimeter in canine end-to-side ilio-femoral anastomotic grafts. The geometries were obtained from casts of the anastomotic region, and flow rates were measured with electromagnetic flow probes. Three cases reported here include a "standard" geometry, which was similar to previously studied in vitro models, a stenosed geometry, and a case with below average flow rate. Observed flow features include separation at the hood and toe, movement of the floor stagnation point, and skewed profiles in the proximal outflow segment. Out-of-plane curvature and lateral displacement of the anastomosis inlet appear to have a strong effect on the flow fields. In addition, compliance affects the instantaneous flow rates within the proximal and distal branches.

Anastomosis, Surgical↗
PubMed1997

Measurements of velocity and wall shear stress inside a PTFE vascular graft model under steady flow conditions.

The flow field inside a model of a polytetrafluorethylene (PTFE) canine artery end-to-side bypass graft was studied under steady flow conditions using laser-Doppler anemometry. The anatomically realistic in vitro model was constructed to incorporate the major geometric features of the in vivo canine anastomosis geometry, most notably a larger graft than host artery diameter. The velocity measurements at Reynolds number 208, based on the host artery diameter, show the flow field to be three dimensional in nature. The wall shear stress distribution, computed from the near-wall velocity gradients, reveals a relatively low wall shear stress region on the wall opposite to the graft near the stagnation point approximately one artery diameter in axial length at the midplane. This low wall shear stress region extends to the sidewalls, suture lines, and along the PTFE graft where its axial length at the midplane is more than two artery diameters. The velocity distribution inside the graft model presented here provides a data set well suited for validation of numerical solutions on a model of this type.

Anastomosis, Surgical↗
PubMed1995

Effects of wall shear stress and fluid recirculation on the localization of circulating monocytes in a three-dimensional flow model.

There is a correlation between the location of early atherosclerotic lesions and the hemodynamic characteristics at those sites. Circulating monocytes are key cells in the pathogenesis of atherosclerotic plaques and localize at sites of atherogenesis. The hypothesis that the distribution of monocyte adhesion to the vascular wall is determined in part by hemodynamic factors was addressed by studying monocyte adhesion in an in vitro flow model in the absence of any biological activity in the model wall. Suspensions of U937 cells were perfused (Re = 200) through an axisymmetric silicone flow model with a stenosis followed by a reverse step. The model provided spatially varying wall shear stress, flow separation and reattachment, and a three-dimensional flow pattern. The cell rolling velocity and adhesion rates were determined by analysis of videomicrographs. Wall shear stress was obtained by numerical solution of the equations of fluid motion. Cell adhesion patterns were also studied in the presence of chemotactic peptide gradients. The cell rolling velocity varied linearly with wall shear stress. The adhesion rate tended to decrease with increasing local wall shear stress, but was also affected by the radial component of velocity and the dynamics of the recirculation region and flow reattachment. Adhesion was increased in the vicinity of chemotactic peptide sources downstream of the expansion site. Results with human monocytes were qualitatively similar to the U937 experiments. Differences in the adhesion rates of U937 cells occurring solely as a function of the fluid dynamic properties of the flow field were clearly demonstrated in the absence of any biological activity in the model wall.

Arterial Occlusive Diseases↗
PubMed1995

Pathobiology of plaque modeling and complication.

The artery wall adapts to changes in wall tension and wall shear stress by means of enlargement and changes in both thickness and composition. The intima may participate in these changes, and these compensatory adaptive-reactive modifications continue in the presence of atherogenesis. Further understanding of the interaction of the evolving plaque with the artery wall and the associated effects of the physical forces associated with the circulation should provide new insights into the nature of plaque instability and into the outcome of direct interventions.

Arteries↗
PubMed1995

An automated three-dimensional particle tracking technique for the study of modeled arterial flow fields.

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.

Algorithms↗
PubMed1994

Shear stress at a compliant model of the human carotid bifurcation.

To investigate the role of a compliant wall to the near wall hemodynamic flowfield, two models of the carotid bifurcation were constructed. Both were of identical internal geometries, however, one was made of compliant material which produced approximately the same degree of wall motion as that occurring in vivo while the other one was rigid. The inner geometries were formed from the same mold so that the configurations are directly comparable. Each model was placed in a pulsatile flow system that produced a physiologic flow waveform. Velocity was measured with a single component Laser system and wall shear rate was estimated from near wall data. Wall motion in the compliant model was measured by a wall motion transducer and the maximum diameter change varied between 4-7 percent in the model with the greatest change at the axis intersection. The mean shear stress in the compliant model was observed to be smaller by about 30 percent at most locations. The variation in peak shear stress was greater and occasionally reached as much as 100 percent with the compliant model consistently having smaller positive and negative peaks. The separation point was seen to move further upstream in the compliant cast. The modified flowfield in the presence of a compliant wall can then be important in the hemodynamic theory of atherogenesis.

Arteriosclerosis↗
PubMed1993

Effect of coarctation on matrix content of experimental aortic atherosclerosis: relation to location, plaque size and blood pressure.

Cynomolgus monkeys were fed an atherogenic diet for 6 months following surgically produced high-grade (n = 10) or mild (n = 16) mid-thoracic aortic coarctation. A diet-control (DC) group (n = 13) was fed the diet without coarctation. High-grade coarctation (HGC) resulted in 74.1% +/- 8.3% stenosis by aortography prior to sacrifice and was associated with systolic brachial blood pressures of 143.3 +/- 26.0 mmHg and gradients across the stenoses of 36.8 +/- 23.6 mmHg. Mild coarctation (MC) resulted in stenoses of 50.9% +/- 12.9%, brachial systolic pressures of 119.4 +/- 25.7 and gradients of 12.5 +/- 15.2 mm Hg (P < 0.01, P = 0.03 and P < 0.005, respectively, compared with HGC). When total plaque cross-sectional area exceeded 0.8 mm2, the entire arterial circumference was usually involved. HGC resulted in complete sparing or minimal plaque formation in sections distal to the stenoses compared with proximal sections (P < 0.001). There were no significant differences between MC and DC animals in plaque location or size. Matrix content increased with plaque area regardless of degree of stenosis or sampling level (P < 0.01), but lesions with more than 75% matrix content were more numerous in distal than in proximal sections despite their smaller size. The number of plaques with greater than 75% matrix content was increased proximal to HGC (P < 0.04). Thus, distal location and plaque size were independent determinants of plaque matrix content and matrix content was increased proximal to HGC regardless of plaque size. Attempts to evaluate effects of various regimens and interventions on plaque composition need to take location and plaque size, as well as blood pressure differences, into account.

Animals↗
PubMed1993

Hemodynamic patterns in two models of end-to-side vascular graft anastomoses: effects of pulsatility, flow division, Reynolds number, and hood length.

Flow behavior in models of end-to-side vascular graft anastomoses was studied under steady and pulsatile flow conditions. Models were constructed to simulate geometries employed in experimental studies on intimal thickening in a canine model. Reynolds numbers, division of flow in the outflow tracts and the pulsatile waveform employed were taken from measurements obtained in the canine model. Flows in the scaled-up, transparent models were visualized with white, neutrally buoyant particles which were photographed under laser illumination and also recorded on video tape under bright incandescent light. Strong, three-dimensional helical patterns which formed in the anastomotic junction were prominent features of the flow fields. Regions of low wall shear, oscillatory wall shear and long particle residence time were identified from the flow visualization experiments. Comparisons with the limited qualitative data available on intimal thickening in vascular graft anastomoses suggest a relation between localization of vascular intimal thickening and those surfaces experiencing low shear and long particle residence time.

Anastomosis, Surgical↗
PubMed1993

The role of fluid mechanics in the localization and detection of atherosclerosis.

Fluid dynamics research over the past twenty years has contributed immensely to our knowledge of atherosclerosis. The ability to detect localized atherosclerotic plaques using noninvasive ultrasonic methods was advanced significantly by investigations into the nature and occurrence of velocity disturbances created by arterial stenoses, and diagnosis of carotid bifurcation disease using a combination of ultrasonic imaging and Doppler measurement of blood velocity is now quite routine. Since atherosclerotic plaques tend to be localized at sites of branching and artery curvature and since these locations would be expected to harbor complex flow patterns, investigators postulated that fluid dynamics might play an initiating role in atherogenesis. Several fluid dynamic variables were proposed as initiating factors. Investigations were undertaken during the 1980s in which fluid dynamic model experiments with physiologic geometries and flow conditions were employed to simulate arterial flows and in which morphometric mapping of intimal thickness was performed in human arteries. Correlations between fluid dynamic variables and intimal thickness revealed that atherosclerotic plaques tended to occur at sites of low and oscillating wall shear stress; and these observations were reinforced by studies in a monkey model of atherosclerosis. Concomitantly, it was realized that arteries adapt their diameters so as to maintain wall shear stress in a narrow range of values around 15 dynes/cm2, findings which were based both on observations of normal arteries and on animal studies in which flow rates were manipulated and arterial diameter adaptation was measured.(ABSTRACT TRUNCATED AT 250 WORDS)

Arteriosclerosis↗
PubMed1993

Mechanical functional role of non-atherosclerotic intimal thickening.

Arteries adjust to alterations in wall shear stress or tensile stress by changes in diameter, wall thickness, structure and composition. The intima participates in these adaptive reactions, particularly when changes in mechanical stresses are imposed after physiologic stress levels have been established during growth. Decreased wall shear stress due to decreased flow, flow separation or complex flow patterns, or increases in tensile stress due to increases in pressure or radius stimulate non-atherosclerotic intimal proliferation. Intimal fibrocellular hypertrophy (IFH), in the form of compact fibrocellular layers resembling the media, stabilizes when the lumen diameter is reduced sufficiently or wall thickness is increased sufficiently to restore baseline wall shear or tensile stress. Reactive-adaptive intimal proliferation is not necessarily self-limiting and may continue in the form of intimal hyperplasia (IH) which is relatively matrix-free and poorly organized. If mural and intimal changes do not result in restoration of baseline wall shear and tensile stress, IH may proceed to further narrowing and stenosis. Identification of the cellular and molecular mechanisms which underly the responses which link flow to diameter, diameter and pressure to mural restructuring, and mural restructuring to intimal thickening should provide new insights into the nature of vessel adaptations in the absence or presence of atherogenesis.

Animals↗
PubMed1992

Simulation of Doppler ultrasound signals for a laminar, pulsatile, nonuniform flow.

A simulation for Doppler ultrasound quadrature signals from pulsatile, nonuniform flow is presented. It is an extension of an earlier simulation presented by Jones and Giddens (1990a) which was valid for laminar, uniform, steady flow and which included the stochastic characteristics introduced by scattering particles which enter and leave the sample volume at random times. Fourier transform and autoregressive spectral analysis techniques are used to compare the simulated signals to Doppler signals collected from an in vitro flow setup. Power spectra, Doppler frequency estimates and standard deviations of these estimates serve as standards of comparison. Results show that the simulation model generates realistic quadrature signals. The study improves the understanding of the physics of the Doppler process and shows that it can be modeled for complex flow conditions. The input parameters of the simulation are the Doppler instrument parameters and flow characteristics. This allows the simulation to be used for transducer design as well as in the study of the applicability of signal analysis techniques to Doppler ultrasound.

Blood Flow Velocity↗
PubMed1992

Anastomotic intimal hyperplasia: mechanical injury or flow induced.

All anastomotic intimal thickening may not be the same, and the underlying mechanism(s) regulating the different types may vary. We investigated the localization of experimental anastomotic intimal thickening in relation to known biomechanical and hemodynamic factors. Bilateral iliofemoral saphenous vein and polytetrafluoroethylene grafts were implanted in 13 mongrel dogs. The distal end-to-side anastomotic geometry was standardized, and the flow parameters were measured. After 8 weeks, seven of 10 animals (group I) with patent grafts were killed and the anastomoses fixed by perfusion. Histologic sections from each anastomosis were studied with light microscopy, and regions of intimal thickening were identified and quantitated with use of oculomicrometry. To characterize the anastomotic flow patterns, transparent silicone models were constructed from castings of the distal anastomosis of three animals (group II), and flow was visualized with use of helium-neon laser-illuminated particles under conditions simulating the in vivo pulsatile flow parameters. Histologic sections revealed two separate and distinct regions of anastomotic intimal thickening. The first, suture line intimal thickening, was greater in polytetrafluoroethylene anastomoses (0.35 +/- 0.23 microns) than in vein anastomoses (0.15 +/- 0.03 microns, p less than 0.05). The second distinct type of intimal thickening developed on the arterial floor and was the same in polytetrafluoroethylene (0.11 +/- 0.11 microns) and vein anastomoses (0.12 +/- 0.03 microns). Model flow visualization studies revealed a flow stagnation point along the arterial floor resulting in a region of low and oscillating shear where the second type of intimal thickening developed. High shear and short particle residence time were observed along the hood of the graft, an area devoid of intimal thickening.(ABSTRACT TRUNCATED AT 250 WORDS)

Anastomosis, Surgical↗
PubMed1992

Micro-architecture and composition of artery walls: relationship to location, diameter and the distribution of mechanical stress.

PURPOSE: We reviewed the structural basis of the mechanical properties of the arterial wall, in order to establish a coherent micro-anatomical basis for the differences in compliance among different arteries and a framework for assessing changes in the mechanical properties of specific individual arteries in relation to changing physical stresses. DATA IDENTIFICATION: The data and concepts presented here were derived from both earlier and ongoing work. Features that assure stability and integrity in relation to blood flow (wall shear stress) and pressure (mural tensile stress) were examined. Particular attention was paid to the morphogenetic and biosynthetic means by which arteries adapt to normal or abnormal modifications of these forces, particularly in relation to growth, location in the arterial tree and geometric configuration. RESULTS AND CONCLUSIONS: Thickness, composition and architecture of the artery wall, including thickness and composition of the intima, are normally determined by the stresses imposed by pressure and flow. Vessel radius is closely associated with flow, so that a normal baseline level of mean shear stress of about 15 dyn/cm2 is maintained or restored. Wall thickness and composition are determined by wall tension in relation to pressure and radius. Baseline levels of tensile stress differ with location but appear to be similar for homologous vessels. Changes in flow that modify the radius also modify wall tension. Changes in wall thickness and composition are likely to cause changes in compliance, due to altered flow and/or pressure patterns; these changes in compliance may be adaptive rather than destructive. Changes in the compliance of specific arteries over time may be used to evaluate the progression and severity of the conditions underlying these changes.

Animals↗
PubMed1991

Modern spectral analysis techniques for blood flow velocity and spectral measurements with pulsed Doppler ultrasound.

Four spectral analysis techniques were applied to pulsed Doppler ultrasonic quadrature signals to compare the relative merits of each technique for estimation of flow velocity and Doppler spectra. The four techniques were 1) the fast Fourier transform method, 2) the maximum likelihood method, 3) the Burg autoregressive algorithm, and 4) the modified covariance approach to autoregressive modeling. Both simulated signals and signals obtained from an in vitro flow system were studied. Optimal parameter values (e.g., model orders) were determined for each method, and the effects of signal-to-noise ratio and signal bandwidth were investigated. The modern spectral analysis techniques were shown to be superior to Fourier techniques in most circumstances, provided the model order was chosen appropriately. Robustness considerations tended to recommend the maximum likelihood method for both velocity and spectral estimation. Despite the restrictions of steady laminar flow, the results provide important basic information concerning the applicability of modern spectral analysis techniques to Doppler ultrasonic evaluation of arterial disease.

Algorithms↗
PubMed1990

Post-stenotic core flow behavior in pulsatile flow and its effects on wall shear stress.

Arteries of several species, including man, tend to adjust their diameters such that the mean wall shear stress is in the range of 10-20 dynes cm-2. Additionally, intimal thickening in the human carotid bifurcation correlates well with the reciprocal of wall shear stress as determined in model studies. The correlation indicates that wherever the local mean wall shear stress exceeds approximately 10 dynes cm-2, the artery tends to be spared from intimal thickening. However, it is not known whether mean shear stress, i.e. the time-averaged value, or the instantaneous shear stress is the appropriate correlative variable. Each of these variables suggests different mechanisms for the reaction of the artery wall to its hemodynamic environment. It is therefore important to devise means by which the effects of mean shear and pulsatile shear can be separated in the study of atherogenesis. The present investigation examines the post-stenotic flow field in Plexiglas models under pulsatile conditions approximating those in the aortas of the cynomolgus monkey, an animal often employed in atherogenesis research. Behavior of the core flow and its effects on wall shear stress are studied for stenoses of 75 and 90% area reductions using laser velocimetry. The results show that the post-stenotic field contains regions in which the mean wall shear stress is low, but the pulsatile excursions are large.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

A simulation of transit time effects in Doppler ultrasound signals.

A signal model is proposed which can be used to study frequency extraction techniques for Doppler ultrasound. The signal is based on the physics of the Doppler process and depends on a sliding window used to average a set of independent Gaussian random numbers. This window is related to the shape of the sample volume for the Doppler pulse and depends on the Doppler angle. Simulation results compare favorably with results from flow experiments in terms of the variance of the estimated Doppler shift, the shape of the power spectra and the behavior of the signals with respect to Burg autoregressive power spectra. A potential use of the signal in the study of spectral analysis techniques is presented.

Models, Theoretical↗