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Biomedical subjects

M H Friedman

Publications and source records attributed to M H Friedman.

At least 19 recordsLinked to original sources
PubMed1992

Effects of arterial compliance and non-Newtonian rheology on correlations between intimal thickness and wall shear.

A minimally diseased (mean intimal thickness = 56 microns) human aortic bifurcation was replicated in rigid and compliant flow-through casts. Both casts were perfused with physiological flow waves having the same Reynolds and unsteadiness numbers; the pulse pressure in the compliant cast produced radial strains similar to those expected from post-mortem measurements of the compliance of the original tissue. The compliant cast was perfused with a Newtonian fluid and one whose rheology was closer to that of blood. Wall shear rate histories were estimated from near-wall velocities obtained by laser Doppler velocimetry at identical sites in both casts. Intimal thickness was measured at corresponding sites in the original vessel and linear regressions were performed between these thicknesses and several normalized shear rate measures obtained from the histories. The correlations showed a positive slope--that is, the intima was thicker at sites exposed to higher shear rates--consistent with earlier results for relatively healthy vessels, but their significance was often poor. There was no significant effect of either model compliance or fluid rheology on the slopes of the correlations of intimal thickness against any normalized shear rate measure.

Aorta↗
PubMed1990

Some atherosclerosis may be a consequence of the normal adaptive vascular response to shear.

Estimates and measurements of fluid dynamic shear at the arterial wall suggest the presence of mechanisms to regulate this quantity. Arterial wall thickening prompted by chronic reductions in wall shear is one mechanism by which that regulation might be accomplished. Reduced wall shears that are a direct consequence of arterial geometry would be less susceptible to this mechanism, leading to exaggerated and focal intimal thickening, possibly leading to atherosclerosis, at affected sites. Thus some foci of vascular disease can be a natural, if undesirable, consequence of fluid shear regulation.

Adaptation, Physiological↗
PubMed1990

The effect of compliance on wall shear in casts of a human aortic bifurcation.

Rigid and compliant casts of a human aortic bifurcation were subjected to physiologically realistic pulsatile fluid flows. At a number of sites near the wall in the approximate median plane of the bifurcation of these models, fluid velocity was measured with a laser Doppler velocimeter, and wall motion (in the case of the compliant cast) was determined with a Reticon linescan camera. The velocity and wall motion data were combined to estimate the instantaneous shear rates at the cast wall. Analysis showed that at the outer walls the cast compliance reduced shear rates, while at the walls of the flow divider the shear rate was increased.

Aorta↗
PubMed1989

Variations in geometry and shear rate distribution in casts of human aortic bifurcations.

Shear rates measured at sites along the walls of ten casts of human aortic bifurcations are presented to provide an indication of the variability in the shear rate distribution that can occur among individuals. The geometric characteristics and flow parameters for each of the casts are also given. Much of the variability in wall shear distribution seen here results from variations in arterial geometry; variations in flow parameters can widen further the normal range of shear distributions.

Aorta, Abdominal↗
PubMed1988

Effect of flow partition on wall shear in a cast of a human coronary artery.

The effect of flow partition on wall shear was determined in a cast of a human coronary artery. A laser Doppler anemometer was used to measure the time varying velocities at selected sites close to the walls of the cast. Shear rates were derived by dividing the measured velocities at each site by the perpendicular distance from measurement site to the wall. At the bifurcation of the left main coronary artery into its anterior descending and circumflex branches the shear at sites near the outer walls of the two branches was more strongly affected by the flow partition than at sites on the flow divider and near the walls of the left main coronary artery.

Blood Flow Velocity↗
PubMed1988

TMJ disorders.

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Humans↗
PubMed1987

Correlation between wall shear and intimal thickness at a coronary artery branch.

Pulsatile velocities were measured by laser Doppler anemometry at fourteen sites near the walls of a cast of a minimally diseased human left coronary artery bifurcation. The flow wave used in the experiments was physiologically realistic. The sites selected for hemodynamic measurement were at the outer walls of the left main artery and its anterior descending and circumflex branches, and along the flow divider. The intimal and medial thicknesses at corresponding sites in the original branch were also measured. Wall shear rates were derived from the velocity data. The correlations between time-average or maximum instantaneous wall shear rate and intimal thickness had negative slopes (P less than 0.005); that is, the intima was generally thicker at sites exposed to lower shears. These results are consistent with those obtained earlier using other human arterial bifurcations.

Biomechanical Phenomena↗
PubMed1987

Correlation among shear rate measures in vascular flows.

A variety of shear rate measures have been calculated from hemodynamic data obtained by laser Doppler anemometry in flow-through casts of human aortic bifurcations. Included are measures sensitive to the mean and amplitude of the shear rate, its maximum rate of change, the duration of stasis and flow reversal near the wall, and the unidirectionality of the flow. Many of these measures are highly correlated with one another. This suggests that that it will be difficult to identify from in vivo measurements those aspects of the flow field to which the vessel wall is most sensitive. It may be possible to separate the effects of purely temporal factors (e.g., the duration of flow reversal) from those related to wall shear stress.

Aorta↗
PubMed1986

Shear-dependent thickening of the human arterial intima.

Hemodynamic data were obtained by laser Doppler anemometry at 163 sites in 10 flow-through casts of minimally diseased human aortic bifurcations, and the intimal thickness at each of these sites in the original vessels was measured (mean = 208 micron, range = 10-967 micron). Analysis of the results suggests that the intimal thickness at sites exposed to high shear stresses increases quickly to a modest value, growing slowly thereafter, while the thickness at sites exposed to low shears rises more slowly but, after time, reaches higher values. Thus the intima of younger vessels is thicker where shear is high, and that of older vessels is thicker where shear is low. This behavior is rationalized by a parsimonious model in which a substance from the lumen enters the wall and is catabolized or otherwise removed. The intimal permeability and removal rate both increase as the shear stress is raised. Intimal thickness is related to the amount of the substance in the wall. This model fits the experimental data with an overall standard error of estimate of 105 micron. Although the model is an extreme simplification of the actual thickening process, it shows that the observed results can be the consequence of competing shear-dependent processes.

Adult↗
PubMed1986

Measurement of wall motion and wall shear in a compliant arterial cast.

Initial measurements of the time-varying wall shear rate at two sites in a compliant cast of a human aortic bifurcation are presented. The shear rates were derived from flow velocities measured by laser Doppler velocimetry (LDV) near the moving walls of the cast. To derive these shear rate values, the distance from the velocimeter sampling volume to the cast wall must be known. The time variation of this distance was obtained from LDV measurements of the velocity of the wall itself.

Aorta↗