Donald L. Fry

Acute Vascular Endothelial Changes Associated with Increased Blood Velocity Gradients

The purpose of this study is to quantify the acute changes in endothelial histology that are associated with an induced increase in blood velocity. A nontraumatic intra-aortic device was designed to produce a rapid convergence of the aortic blood stream into a narrow channel along the ventral aspect of the thoracic aorta in dogs. The endothelial surface overlying this channel was exposed to a broad range of surface shearing stress by the accelerated blood flow. Techniques were developed to quantify the resulting distribution of shearing stress so that the stress to which the endothelial surface was exposed at every point along the channel could be determined. Special histologic techniques were developed using formalin fixation and gelatin embedding of the tissue so that endothelial cytology could be studied and criteria for normal cells established. Using these criteria, cell counts were done to establish the “normal” endothelial cell population density as a function of stress exposure. The stress corresponding to the mode of these cell density distribution curves was defined as the acute yield stress (τc). The acute yield stress for endothelial cells was found to be 379±85 (SD) dynes/cm2. Exposure to stress in excess of this value for periods as short as one hour resulted in marked deterioration of the endothelial surface consisting of endothelial cytoplasmic swelling, cell deformation, cell disintegration, and finally dissolution and erosion of cell substance. The relationship of these events to cellular rheology and interfacial chemistry is discussed.

The Elastic Symmetry of Arterial Segments in Dogs

Elastic symmetry was studied in the middle descending thoracic aorta, abdominal aorta, and left common carotid artery under physiologic ranges of loading in ten dogs. A segment of the blood vessel was isolated and hung vertically. As the segment was pressurized, the radius, length, and the rotation of the lower end of the vessel were measured with respect to the fixed upper end. In addition, the angular displacement of a glass whisker initially placed perpendicularly through the wall was measured. From these data it was possible to calculate the values of shearing strains and elongating strains associated with pressurization and various imposed longitudinal stresses. The values of shearing strain varied from 0.003 to 0.115 over pressure ranges of 3 to 270 cm H2O. In all instances the values of shearing strain were much smaller than the corresponding elongating strains. It was concluded that the vessel has elastic properties that are nearly symmetrical about the planes perpendicular to principal stresses under physiologic loading, i.e., the vessel may be treated as a cylindrically orthotropic tube.

Certain histological and chemical responses of the vascular interface to acutely induced mechanical stress in the aorta of the dog.

The purpose of this study is to quantify certain histologic and chemical responses of the intimal tissues in vivo to acutely induced mechanical stresses. Evans blue dye was given to tag serum albumin and an artificial fat emulsion was infused so that altered fluxes of either serum proteins or the artificial chylomicrons across the vascular interface into the intimal region could be detected. Special histologic and photodensimetric techniques were developed to estimate these fluxes as well as the architectural changes in the endothelial cell population. Architectural changes were quantified by doing endothelial cell counts to quantify the “normal” and “abnormal” endothelial cell population density as a function of stress exposure. The stress corresponding to the greatest rate of change of normal to abnormal cell forms is defined as the acute critical yield stress (τc) and was found to average < 420 dynes/cm2. Similarly the stress at which the greatest number of cells are being eroded is defined as the erosion stress (τe). The flux of Evans blue dye into the intima increased with pressure or wall strain, with shearing stress, and with increased turbulence. The flux of artificial chylomicrons into the intimal region never occurred in the presence of a normal endothelial cell population and was found to be most heavy in areas of total cellular erosion.

Endothelial Nuclear Patterns in the Canine Arterial Tree with Particular Reference to Hemodynamic Events

The objective of the study was (1) to measure systematically the orientation, morphology, and population density of endothelial nuclei of the canine thoracic aorta and its major branches and (2) to obtain evidence in a chronic in vivo preparation that altered flow patterns do indeed change patterns of nuclear orientation. For this purpose, a segment of the descending thoracic aorta was removed, opened longitudinally, and reclosed to form a tube with a new longitudinal axis 90° from the original vessel axis. The new segment was then sutured back in place. The animals were killed at suitable postoperative periods. Endothelial nuclear patterns were studied from en face photomicrographs of preparations stained with Evans blue dye. Results indicated: (1) In uniform vessel segments, e.g., middle and lower descending thoracic aorta, the nuclei were oriented parallel to the axis of the blood vessel, and the ratio of major to minor axes of the nucleus was large. The flow in these regions is known to be stable. (2) Nonaxial, less-ordered nuclear orientation with smaller ratios of major to minor axes were found in entrance regions of many major arteries and in the ascending aorta. (3) In chronic studies in which the flow pattern was altered, the nuclear pattern realigned in the direction of flow within 10 days after surgery.

Application of Heated-Film Velocity and Shear Probes to Hemodynamic Studies

A constant-temperature heated-film anemometer system has been adapted for the detailed study of in-vivo aortic velocity fields. Two types of sensing probes were developed: a velocity probe and a velocity-gradient or fluid shear stress probe. These probes were evaluated for steady and pulsatile flow in rigid circular tubes using both a glycerin-water mixture and blood. Measurements using both devices agreed closely with the values predicted by well established theory. Moreover, the integrated velocity profiles that were measured correlated well with the simultaneously recorded flow values using orifice meter and electromagnetic flowmeter techniques. In-vivo studies were made along the thoracic aortas of anesthetized dogs and pigs. Velocity measurements along the aorta indicated that the velocity profiles are blunt. The flow-pulse forms obtained by the heated-film technique in vivo were also similar in magnitude and contour to those obtained simultaneously from an electromagnetic flowmeter. Fully developed turbulent flow was not observed; however, occasional “eddy” turbulence occurred in the aortic arch of dogs weighing less than 30 kg. Preliminary measurements indicate that peak wall-shear stresses reach values that are approximately one-third that of the endothelial yield stress.

Longitudinal Tethering of Arteries in Dogs

The magnitude and properties of longitudinal vascular tethering were studied in the thoracic aorta, abdominal aorta, and femoral artery of 23 dogs. The tethering was found to consist of a dominant viscoelastic restraining element that demonstrated a moderate degree of stress relaxation and a significant inertial component that was related to the mass of the vessel and its surrounding tissues (the “added mass”). Static and dynamic studies of these properties showed them to be surprisingly linear. The simplest linear model that could simulate these experimental data consisted of two elastic components, two frictional components and a lumped inertial component. Within the physiologic range of frequencies (1 to 20 cycles/sec), the two elastic components became dynamically coupled so that they appeared to act as a single spring, the stiffness of which could be characterized by a “dynamic spring constant.” The mean value of the dynamic spring constant was 12 g/cm3 for the thoracic aorta; it increased along the aorta toward the femoral artery to almost 70 times that value. Contrary to the simple tethering model assumed by Womersley, these data showed that the mechanical behavior of the system is strongly influenced by frictional components.

Certain Chemorheologic Considerations Regarding the Blood Vascular Interface with Particular Reference to Coronary Artery Disease

The locations of coronary atheroma in a hyperlipemic dog are shown to occur at points in the arterial tree where one would expect the most intense exposure to physical stress. Studies are presented that were designed to quantify certain aspects of the histological and physicochemical response of the arterial intima to a variety of acutely imposed mechanical stresses. These results indicate that the endothelial-cell population has an acute yield stress to shear of about 400 dynes/cm2; that exposure to stress in excess of this is associated with cytological and chemical changes, suggesting that physical processes are occurring, such as “yielding,” “melting,” “dissolving,” “imbibition,” and “permeation” of the interfacial tissues; and that even in the absence of cytological changes, the permeability of the interface for albumin is increased by increasing the stress acting on the interface whether in shear, compression, or tension. The relations of these events to tissue rheology, the strain energy density of the interfacial region, and the “critical energy” of molecular separation in the interfacial substance are discussed in terms of the energetics of molecular interactions. Classical rate process theory is used in an effort to express these ideas in more conventional chemical terminology. Arguments are presented in support of the postulate that mechanical energy added to the vascular interface may be used either as the driving force for a chemical process or can be used to increase the rate constant for such processes as interfacial yielding, melting, dissolving, imbibition, or permeation by serum proteins. The added mechanical energy is assumed to increase the rate constant by increasing the number of defects in the array of interfacial molecules or by increasing the average energy level of the molecules, thus decreasing the activation energy for the particular process being considered. As a corollary to this, it follows that the chemical “barrier function” of the vascular interface is enhanced by maintaining its energy density as low as possible and will be degraded in regions of increased energy density. These notions lead to the suggestion that the topography of increased serum protein flux as well as atheromatous plaques is determined among other things by the distribution of mechanically induced increased energy density in the vascular interface.

Theoretical Considerations of the Bronchial Pressure-Flow-Volume Relationships with Particular Reference to the Maximum Expiratory Flow Volume Curve

It is generally accepted that abnormalities of air flow mechanics associated with pulmonary diseases such as chronic pulmonary emphysema reside in the airways and lung parenchyma below the larynx. It was the purpose of this paper to explore the theoretical and experimental aspects of flow through a collapsible tube such as a bronchial segment and relate the inferences of these theoretical considerations to the pressure-flow-volume relationships found in normal and diseased states. The pressure gradient at a point along the bronchial segment was related to the physical properties of the gas, the flow, and the dimension of the tube. The radius of the bronchial segment at a point along its length was related to the difference between the lateral intra-luminal pressure and the stress acting on the outside of the segment by certain physical constants describing the properties of the tube. The stress on the outside of the segment was related to the physical properties, the deformation and rate of deformation of the peribronchial tissues. Thus, the pressure gradient along the bronchial segment could be expressed in terms of the flow, transmural stress and system constants. By integration the relationship of total pressure drop along the segment to flow was established. Pressure-flow curves from this relationship have flow maxima for certain sets of constants. Differentiation of this pressure-flow equation partially with respect to pressure and setting the derivatives of flow equal to zero yielded a second equation that satisfied only the maxima of the pressure-flow curve. Since the pressure-flow equation satisfied all points on the curve, these two equations could be solved simultaneously to eliminate pressure between them. This resulted in a third expression which related the maximum flow that can be achieved in the bronchial segment to the transmural stress and the system constants describing the physical properties of the segment. Experimental data from a rubber model of a bronchial segment were presented in support of this third equation.

A preliminary lung model for simulating the aerodynamics of the bronchial tree

Abstract A numerical method for modeling the mechanical behavior of complicated multi-phase, nonlinear, distributed, biological flow-systems is suggested with particular reference to the study of pulmonary mechanics. The major purpose of the model presented is to provide a computational tool for estimation of certain variables along the conduit system which are of physiologic and pathologic interest but cannot be measured directly. Results are presented which illustrate the distribution of pressure, conduit area, gas velocity, Reynolds number, and wall shearing stress along the bronchial tree. With proper modifications these may be extended to include distributions of other variables of interest such as temperature, “turbulent intensity,” etc. Although the numerical results that are presented simulate the known mechanical behavior of the lung, any detailed interpretation must be approached cautiously at this time since many of the components and parameters of the model necessarily had to be chosen arbitrarily for lack of good experimental data.

Histochemical detection and differentiation of free and esterified cholesterol in swine atherosclerosis using filipin

The fluorescent probe, filipin, and the lipid-soluble dye, oil red O, have been used to simultaneously detect and differentiate free and esterified cholesterol, respectively, in tissue sections prepared from spontaneous atherosclerotic lesions of swine. This was possible because filipin stains free cholesterol but does not stain cholesteryl ester and because oil red O stains cholesteryl ester but does not stain free cholesterol. Oil red O-stained lipid accumulated intra- and extracellularly but separate from filipin-stained lipid. Spherical filipin-stained particles and elongated filipin-stained crystals accumulated in the extracellular space. Interestingly, some cells appeared to accumulate these filipin-stained particles exclusively. The spherical filipin-stained particles have not been previously recognized because they are not stained by oil red O. This and the fact that extensive compartmentalization of filipin and oil red O-stained lipid occurs in atherosclerotic lesions are new observations to be considered in the pathogenesis of vascular cholesterol accumulation.