Curt A. Wiederhielm

Reevaluation of Arterial Constitutive Relations: A FINITE-DEFORMATION APPROACH

The purpose of this investigation was to use the finite-deformation theory of elasticity to interpret pressure-diameter data for in situ canine aortas and other arterial response data reported in the literature. A meaningful mechanical property for arterial tissue was identified as ∂W1/∂I, the partial derivative of the strain-energy density function with respect to the first strain invariant. An exponential function was found to characterize the mechanical property ∂W1/∂I for all arteries considered. Thin-walled tube stress approximations were found to result in inaccurate values for arterial stresses and incremental elastic mechanical properties. Wave speeds calculated using ∂W1/∂I for these arterial tissues agreed well with experimental measurements of wave speeds reported in the literature. Elevated values for strain-energy density were found in the inner arterial tissue layers. These high values for strain energy may contribute to atherogenesis in relatively straight arteries (e.g., the abdominal aorta) subjected to hypertension.

Dynamics of capillary fluid exchange: a nonlinear computer simulation.

Abstract A nonlinear computer simulation program has been employed to analyze capillary fluid balance during a variety of perturbations, such as changes in arterial or venous pressures, plasma oncotic pressure, interstitial mucopolysaccharide content, and lymphatic obstruction. The results indicate that normally tissue colloid osmotic pressures of 9.3 mm Hg are required for an equilibrium of filtration and reabsorption, equivalent to an interstitial plasma protein concentration of 3.4%. Dilution of this extravascular protein pool forms a first line of defense against edema formation. Washout of interstitial protein during hypoproteinemia may serve as an effective form of autotransfusion. Simulation of five different types of edema revealed significant differences in interstitial fluid distribution and protein content. Comparison of the simulation program with a large number of physiological and clinical studies yielded good agreement, supporting the validity of the simulation program.

Characteristics of the servo-controlled micropipet pressure system

Abstract By use of an active nulling device, hydraulic pressures can be measured over a wide pressure and frequency range with glass micropipets having tip diameters smaller than 0.1 μm. Although pipets larger than 0.1 μm have the advantage of being relatively insensitive to external solution resistivity, to electro-osmotic phenomena, and to plugging, they may be too large for some applications. Small pipets, 0.1 μm and less, are sharp enough to pass through the toughest of tissue with minimal force but they require different electrical compensation from large pipets, have less intrinsic gain, are sensitive to resistivity and temperature changes in the external solution, are affected by electric current from contact potentials, and are more likely to become plugged. The behavior of the servo-controlled micropipet pressure system is discussed in terms of the static and dynamic relationship between electrical resistance and pressure in small and large pipets. A detailed description of the operating characteristics of the system is also given. Solutions to a set of equations expressing NaCl concentration and pipet resistance in terms of pipet geometry, pressure, and internal and external NaCl concentration were used to illustrate shifts in concentration profiles in the pipet tips for changes in pressure.

Effects of Upright Posture and Exercise on Pulmonary Hemodynamics in Patients with Central Cardiovascular Shunts

Previous hemodynamic studies of patients with septal defects, or patent ductus arteriosus, have been made with the patient recumbent in the horizontal posture. Our studies indicate unexpected changes in pulmonary blood flow with the upright posture, and often further increments in flow during walking that are different from those reported for exercise in recumbency. It is suggested that such defects reveal the complexity of factors regulating preferential flow under these circumstances.

Postnatal changes in hematology of the bat Antrozous pallidus

Blood oxygen capacity increases with growth as the result of increasing hemoglobin concentration, which is accompanied by increasing red blood cell count and hematocrit. Hematological profile of the postnatal bat approaches that of the adult by 42 days of age when the animal begins to fly. Hematological development in the bat is similar to that in other altricial small mammals.

DEFORMATION OF THE ARTERIAL VASA VASORUM AT NORMAL AND HYPERTENSIVE ARTERIAL PRESSURE

Abstract In a previous paper, a numerical procedure (the finite element method) was presented for the structural analysis of soft biological structures possessing complex geometry and mechanical properties and undergoing large deformations. This method was used here to model a vas vasorum in an arterial cross-section subjected to physiological and hypertensive mean arterial pressure levels. The deformation of the lumen of the model vas vasorum was determined for two cases: an arteriole and a venule. Estimates of hydraulic resistance and flow rate for these model vasa vasorum were calculated and compared with experimental total flow rates in the vasa vasorum reported in the literature. The results indicate that prolonged abnormally elevated arterial pressure could constrict venules of the vasa vasorum thereby impairing arterial wall tissue nourishment. If no compensatory mechanisms exist, tissue degeneration and/or atherogenesis could occur.

Pressure regulation in muscle of unanesthetized bats

In unanesthetized bats direct microvascular pressure measurements were made in the tensor plagiopatagii muscle without surgical intervention. Pressure recordings showed great variability due to vasomotion and other physiological stimuli, but were in the same order of magnitude as described in muscle of other species in the anesthetized state. A substantial part of the pressure gradient is dissipated at the level of the small arterioles. Arterial pressure was changed by depressurizing a box containing the body of the bat, while the muscle was under atmospheric condition. In all larger arterioles and in 12 of 19 smaller arterioles a linear relation existed between microvascular pressure and box pressure. Of 19 small arterioles, 7 exhibited a clear biphasic regulatory response to a decrease in arterial pressure. The data are discussed in relation to different models of regulation.

Structural response of relaxed and constricted arterioles

Abstract Analysis of viscoelastic properties of blood vessel walls presents difficult analytical problems in view of their non-homogeneity, anisotropy, and non-linear viscoelastic characteristics. The analytical technique used in this study is the numerical method of direct stiffness which has been successfully applied in preliminary studies. The direct stiffness method constitutes a finite element analysis, where the wall structure in an electron micrograph is represented by an assembly of triangular elements. Different material properties can be assigned to each triangular element; the method is thus ideally suited for computer analysis of non-homogeneous biological structures. Anatomical components considered in the analysis were two types of collagen fibers and smooth muscle cells. Typical circumferential stress-time and radius-time histories for selected points have been obtained in relaxed and constricted arterioles under static pressure loads. Plots of the stress distribution in cross sections of the vessel wall have been obtained in relaxed and constricted arterioles.

A digital system for studying interstitial transport of dye molecules

Abstract A method is presented for quantitatively describing the transport of absorbing dyes through the interstitial space in transilluminated preparations. A capillary bed is perfused with a vital dye, such as Patent Blue V, and the diffusion of the dye across the capillary wall and through the surrounding tissue is monitored by a microscope-closed-circuit television system. A slow-scan television camera, which scans a single line of the field 30 times per second, detects changes in optical density at various distances from the capillary. Changes in optical density are compared to those predicted by the equation for diffusion from a cylindrical source in an isotropic homogenous medium, and a value for the diffusion coefficient of the dye in the interstitium is obtained.

Amplifier for Linear Recording of Oxygen Saturation and Dye Dilution Curves

An amplifier system has been designed for use with commercially available oximeter earpiece or cuvette and a direct recording oscillograph. Linear response to changes in oxygen saturation and in vivo dye concentrations are obtained with a rise time less than 0.5 sec., noise level and drift less than 2.5 per cent and accuracy comparable to Van Slyke-Neill method. Standard error of estimate between Van Slyke-Neill manometric and cuvette oximeter amplifier-recorder systems is ±2 per cent.