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R F Mabon

Publications and source records attributed to R F Mabon.

7 recordsLinked to original sources
PubMed1986

Poststenotic flow disturbance in the dog aorta as measured with pulsed Doppler ultrasound.

Blood flow velocity was measured in the dog aorta distal to mechanically induced constrictions of various degrees of severity employing an 8-MHz pulsed Doppler ultrasound velocimeter and a phase-lock loop frequency tracking method for extracting velocity from the Doppler quadrature signals. The data were analyzed to construct ensemble average velocity waveforms and random velocity disturbances. In any individual animal the effect of increasing the degree of stenosis beyond approximately 25 percent area reduction was to produce increasing levels of random velocity disturbance. However, variability among animals was such that the sensitivity of random behavior to the degree of stenosis was degraded to the point that it appears difficult to employ Doppler ultrasound measurements of random disturbances to discriminate among stenoses with area reductions less than approximately 75 percent. On the other hand, coherent vortex structures in velocity waveforms consistently occurred distal to mild constrictions (25-50 percent area reduction). Comparison of the phase-lock loop Doppler ultrasound data with simultaneous measurements using invasive hot-film anemometry, which possesses excellent frequency response, demonstrates that the ultrasound method can reliably detect those flow phenomena in such cases. Thus, the identification of coherent, rather than random, flow disturbances may offer improved diagnostic capability for noninvasively detecting arteriosclerotic plaques at relatively early stages of development.

Animals↗
PubMed1983

Carotid bifurcation atherosclerosis. Quantitative correlation of plaque localization with flow velocity profiles and wall shear stress.

The distribution of nonstenosing, asymptomatic intimal plaques in 12 adult human carotid bifurcations obtained at autopsy was compared with the distribution of flow streamline patterns, flow velocity profiles, and shear stresses in corresponding scale models. The postmortem specimens were fixed while distended to restore normal in vivo length, diameter, and configuration. Angiograms were used to measure branch angles and diameters, and transverse histological sections were studied at five standard sampling levels. Intimal thickness was determined at 15 degrees intervals around the circumference of the vessel sections from contour tracings of images projected onto a digitizing plate. In the models, laser-Doppler anemometry was used to determine flow velocity profiles and shear stresses at levels corresponding to the standard specimen sampling sites under conditions of steady flow at Reynolds numbers of 400, 800, and 1200, and flow patterns were visualized by hydrogen bubble and dye-washout techniques. Intimal thickening was greatest and consistently eccentric in the carotid sinus. With the center of the flow divider as the 0 degree index point, mid-sinus sections showed minimum intimal thickness (0.05 +/- 0.02 mm) within 15 degrees of the index point, while maximum thickness (0.9 +/- 0.1 mm) occurred at 161 +/- 16 degrees, i.e., on the outer wall opposite the flow divider. Where the intima was thinnest, along the inner wall, flow streamlines in the model remain axially aligned and unidirectional, with velocity maxima shifted toward the flow divider apex. Wall shear stress along the inner wall ranged from 31 to 600 dynes/cm2 depending on the Reynolds number. Where the intima was thickest, along the outer wall opposite the flow divider apex, the pattern of flow was complex and included a region of separation and reversal of axial flow as well as the development of counter-rotating helical trajectories. Wall shear stress along the outer wall ranged from 0 to -6 dynes/cm2. Intimal thickening at the common carotid and distal internal carotid levels of section was minimal and was distributed uniformly about the circumference. We conclude that in the human carotid bifurcation, regions of moderate to high shear stress, where flow remains unidirectional and axially aligned, are relatively spared of intimal thickening. Intimal thickening and atherosclerosis develop largely in regions of relatively low wall shear stress, flow separation, and departure from axially aligned, unidirectional flow. Similar quantitative evaluations of other atherosclerosis-prone locations and corresponding flow profile studies in geometrically accurate models may reveal which of these hemodynamic conditions are most consistently associated with the development of intimal disease.

Adult↗
PubMed1982

Steady flow in a model of the human carotid bifurcation. Part I--flow visualization.

The geometry of a typical adult human carotid bifurcation, complete with the sinus, was established from a study of a large number of angiograms. A rigid model was constructed from glass and investigations were performed under steady flow conditions using flow visualization techniques over a range of upstream Reynolds numbers and flow division ratios through the branches representative of physiologic conditions expected in the human vasculature. The study reveals a complex flow field in which secondary flows play an important role. The separation regions occurring at the outer corners of the branching are also subjected to much higher shear stress. Comparison with pathologic data on localization of atherosclerotic lesions indicates that zones susceptible to disease experience low or oscillatory shear stress while regions subject to higher shear are free of deposits.

Adolescent↗
PubMed1982

Steady flow in a model of the human carotid bifurcation. Part II--laser-Doppler anemometer measurements.

The evidence for hypothesizing a relationship between hemodynamics and atherogenesis as well as the motivation for selecting the carotid bifurcation for extensive fluid dynamic studies has been discussed in Part I of this two-paper sequence. Part II deals with velocity measurements within the bifurcation model described by Fig. 1 and Table 1 of the previous paper. A plexiglass model conforming to the dimensions of the average carotid bifurcation was machined and employed for velocity measurements with a laser-Doppler anemometer (LDA). The objective of this phase of the study was to obtain quantitative information on the velocity field and to estimate levels and directions of wall shear stress in the region of the bifurcation.

Adult↗
PubMed1978

Fluid dynamics in cerebral angiography.

Flow field characteristics are described so that the physician may have a better understanding of the mechanical effects of rapid intravascular injection of a highly ionized hypertonic solution. Theoretical and experimental results which illustrate the fluid dynamics involved are presented. This knowledge may be helpful in diminishing the overall complication rate in cerebral angiography and in outlining the carotid arterial complex, which is frequently the site of dense atheromatous plaque deposition.

Arteries↗
PubMed1976

Measurements of disordered flows distal to subtotal vascular stenoses in the thoracic aortas of dogs.

Instantaneous blood velocity measurements employing a constant temperature hot film anemometer were obtained in the region distal to externally enforced, subtotal vascular stenoses in the descending thoracic aortas of anesthetized dogs. Our objectives were to determine alterations in velocity waveforms and energy spectra as the degree of stenosis was increased. We paid particular attention to distinguishing features of the flow which were characteristic of turbulence. Our results indicate that, for the vessels studied, disturbances in the velocity waveforms occur for very minor localized constrictions. The energy spectra follow certain similarity parameters within a restricted region of the distal velocity field. For severe stenoses relatively high levels of energy exist in frequency ranges which previously have been found to produce poststenotic dilation. The measurements suggest that velocity waveforms and energy spectra provide a very early clue to the existence of localized occlusive vascular disease in larger vessels and that, within a limited region distal to a stenosis, the degree of constriction may be estimated by similarity analysis of the energy spectra.

Animals↗