Lipid deposition in rat aortas with intraluminal hemispherical plug stenosis. A morphological and biophysical study.

A new method was devised to create a stenosis in the rat abdominal aorta. To restrict blood flow, a hemispherical plug was inserted into the aorta through a renal artery. This type of intrinsic (intraluminal) stenosis minimizes possible intramural effects associated with external compression or ligation which severely deform the arterial wall. In the aorta of hypercholesterolemic rats, lipid deposits were distributed in crescent-shaped patches proximal and distal to the plug, whereas lipid deposition in the opposite aortic wall was inhibited. Based on enlarged physical scale models used to study the flow field, the regions of lipid deposition were found to coincide with regions of low shear stress, stagnation, and recirculation. Shear stress was elevated at the wall opposite the plug. These results show that when confounding mural effects are minimized, lipid deposition is promoted in regions of low shear stress with recirculation and inhibited in regions of elevated shear stress.

Variation of arterial compliance within the cardiac pressure pulse.

Past investigations of in vivo arterial behavior have concentrated on determining material properties based upon the maximum and minimum pressure and diameter measured over a pulse cycle. A new in vivo technique, based upon continuous measurement of pressure and flow, has been developed to study arterial compliance throughout the pulse cycle. Compliance in the abdominal aorta of rats showed different behavior during the rising and falling portion of the pressure pulse. Previous investigations of canine arteries which used different methods are consistent with these findings. This study demonstrates the utility of a new measurement technique and shows some trends in compliance within the pulse cycle which have neither been revealed by static tests nor by dynamic tests which focused on pulse averaged values.

Lipid deposition and intimal stress and strain. A study in rats with aortic stenosis.

These experiments were designed to study the topography of lipid deposition in the stenotic aorta of hypercholesterolemic rats, and to correlate it with flow conditions and intimal stresses and strains studied in a scale biophysical model and in a computer model. A 69% +/- 5% stenosis was produced with a U-shaped metal clip. One month to 8 months later, the aorta was studied en face by light microscopy after fixation and lipid staining. The intima in the throat of the stenosis was almost completely free of lipid, whereas symmetric lipid deposits occurred as bands just above and especially just below the stenosis; elsewhere lipid deposits appeared to be random. The flow data obtained from the scale model showed that the intima in the throat of the stenosis was subjected to an increase of as much as 20 times in shear stress, whereas the lipid deposits just above and just below the stenosis were associated with asymmetric flow conditions: the proximal area corresponded to a region of rapidly increasing shear stress, the distal area to a region of low to normal shear stress and separated flow. A finite element computer model based on the aortic deformations indicated that the endothelium at the inlet and outlet of the stenosis is subjected to a symmetric pattern of elevated stresses and strains. These results indicate that 1) the pattern of lipid deposition can not be adequately explained by a hypothesis based solely on flow conditions, and 2) lipid deposits can develop in areas of increased fluid shear stress, decreased fluid shear stress, and increased intimal strains.

Periodic flow at airway bifurcations. III. Energy dissipation.

We measured the energy dissipation associated with large-amplitude periodic flow through airway bifurcation models. Each model consisted of a single asymmetric bifurcation with a different branching angle and area ratio, with each branch terminated into an identical elastic load. Sinusoidal volumetric oscillations were applied at the parent duct so that the upstream Reynolds number (Re) varied from 30 to 77,000 and the Womersley parameter (alpha) from 4 to 30. Pressures were measured continuously at the parent duct and at both terminals, and instantaneous branch flow rates were calculated. Time-averaged energy dissipation in the bifurcation was computed from an energy budget over a control volume integrated over a cycle and was expressed as a friction factor, F. We found that when tidal volume was small [ratio of tidal volume to resident (dead space) volume, VT/VD less than 1], F was independent of branching angle and fell with increasing alpha and VT/VD. When tidal volume was large (VT/VD greater than 1), F increased with increasing branching angle and varied less strongly with alpha and VT/VD. No simple benchmark flow represented the data well over the entire experimental range. This study demonstrates that only two nondimensional parameters, alpha and VT/VD, are necessary and are sufficient to describe time-averaged energy dissipation in a given bifurcation geometry during sinusoidal flow.

Endothelial adaptations in aortic stenosis. Correlation with flow parameters.

A 69 +/- 5% stenosis was produced in the rat aorta, with the purpose of correlating endothelial changes with local flow patterns and with levels of shear stress; the hydrodynamic data were obtained from a scaled-up model of the stenosed aorta. In the throat of the stenosis, where shear stress values were 15-25 times normal, the endothelium was stripped off within 1 hour. It regenerated at half the rate of controls but modulated into a cell type that could withstand the increased shear stress. Adaptations included changes in cell orientation, number, length, width, thickness, stress fibers, and anchoring structures, as well as changes in the length, argyrophilia, and permeability of the junctions. Areas of either elongated or "polygonal" cells consistently developed at the same sites in relation to the stenosis, but the hydrodynamic data showed that they did not always correspond (as had been anticipated) to high and low shear, respectively. It is concluded that endothelial cell shape in the living artery must be determined by some other factor(s) in addition to shear stress.

Viral hepatitis and hydraulic parameters: an alternative hypothesis.

Point source outbreaks of viral hepatitis have been associated with the consumption of contaminated food and water, yet epidemiologic analysis of the disease in the general population usually provides strong correlations with sub-standard socioeconomic conditions. However, statistical analysis of municipal public health records in Worcester, Massachusetts showed that significant correlations existed between the incidence of viral hepatitis and certain water and sewer parameters. A 5-yr period spanning the epidemic outbreak of 1969 to 1970 was analyzed. Significantly higher incidences of the disease occurred in those areas of the city served by old water and sewer pipes, combined sewers and the low pressure water distribution system. Based on this epidemiologic evidence, the authors propose that an alternate hypothesis, based on hydraulic arguments, can be made which will explain the observations. Although neither hypothesis, socioeconomic or hydraulic, can be excluded solely on the basis of epidemiologic results, this new hypothesis may prove useful in the design of methods for environmental control of viral hepatitis.