Differing effects of duration of illness on CT findings in alcoholism and schizophrenia.
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Adult↗
Biomedical subjects
K O Lim
Publications and source records attributed to K O Lim.
The effect of temperature and glutaraldehyde fixation on the mechanical properties of bovine pericardial tissues.
A comparison is made of the elastic response of fresh pericardial tissues at 25, 37, and 42 degrees C, and also of fresh and glutaraldehyde-fixed tissues. Strips of bovine pericardial tissues cut perpendicular to the base-apex axis of the heart were used. An Instron machine was used for uniaxial tensile tests, and the strain-rate used was 666.7% X min-1. No significant differences in tissue mechanical properties were observed for temperature values of 25, 37, and 42 degrees C. However tissues fixed in a glutaraldehyde solution were more extensible than fresh tissues. The elastic responses of tissue preserved for 1 day in glutaraldehyde are not very different from those preserved for up to 10 days.
Animals↗
Anisotropic elasticity of bovine pericardial tissues.
Artificial heart valves fashioned from bovine pericardial tissues appear to perform favourably when compared to those fashioned from other biological materials. Tissue mechanical properties have been implicated as a possible factor in determining the success or failure of the materials used for constructing tissue valve substitutes. In this study, strips of bovine pericardial tissues, cut in three different directions (along the base-apex axis of the heart, perpendicular to this axis and at an angle of 45 degrees to this axis), were subjected to uniaxial tensile tests using an Instron machine, at strain rates of 666.7, 166.7, 66.7, and 6.7% X min-1. The pericardial tissues were found to have non-linear stress-strain curves. Anisotropy was also observed, in that the tissue was most extensible when stretched along the vertical direction and least extensible when stained along the horizontal direction. The tissues also exhibited viscoelastic properties, as the extensibility was found to decrease with the increasing strain-rate. The strain at rupture for these tissues, however, were found to be independent of the strain rates and the direction in which they were stretched. On the other hand the stress at rupture was largest when stretched in the horizontal direction and least when stretched in the vertical direction. Stresses at rupture also increase with the strain rate. The implications of the observed tissue properties, when these are used in the fashioning of tissue valve substitutes, will be discussed.
Animals↗
A simple acoustic displacement transducer system.
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Acoustics↗
Entry flow in a circular tube of aortic arch dimensions.
Steady flow within a uniform circular curved tube formed by two 90-deg elbows was studied as a function of psi, the angle between the planes of curvature of the two elbows. Boundary layer separation was found at two locations. The sites of these separation zones were observed to be essentially independent of psi while the Reynolds number at which separation was first detected was found to decrease as psi increased. The relation between separation and the pathogenesis of atherosclerosis is discussed. Secondary flow pattern was found to depend on psi and in some instances on Reynolds number as well.
Aorta, Thoracic↗
Frequency analysis of the first and second heart sounds of humans.
Each sound signal of the first and the second heart sounds of humans was divided into three equal segments. The frequency spectrum of each segment was analyzed using the fast-Fourier-transform technique. Theory predicts that if the first and second heart sounds are caused principally by the respective vibrations of the atrioventricular and semilunar valves, then the frequency of these sounds should increase with an increase in the pressure difference across these valves. In other words, the frequency of the heart sounds should increase with time, as the pressure difference across these valves increases with time for most of the duration of the sounds. Our frequency analysis, however, showed that there is no systematic relation between the predominant frequency of successive segments of the sound signals. This therefore appears to indicate that it is quite unlikely that the vibrations of the heart valves are the principal contributor to the origin of the first and second heart sounds.
Adult↗
Ultrastructure and mechanical properties of chordae tendineae from a myxomatous tricuspid valve.
The ultrastructure and mechanical properties of chordae from a surgically removed myxomatous tricuspid valve were examined. Transmission and scanning electron microscopy showed the internal structure of the abnormal chordae to consist of a loose and relatively sparse network of disorganized collagen fibrils, with no well defined collagen bundles present. The abnormal tissue was found to be more extensible than normal and it ruptured at a stress that was only about 6% of the breaking stress for normal chordae. Chordal rupture, a common complication in this disorder, can be attributed to this decrease in chordal strength. At high strains, a decreased elastic modulus was also observed, which may be attributed to tissue changes and an alteration in collagen fibril structure and/or composition.
Aged↗
Microprocessor control of a ventricular volume servo-pump.
Measurement of the instantaneous pressure-volume relationship of the left ventricle is fundamental to the study of ventricular mechanics. In order to effectively investigate this relationship, it is necessary to vary and control the time course of ventricular volume change in a variety of prescribed manners. In the past, we used an analog circuit to generate command signals for a servo-pump system which controlled ventricular volume. The use of analog control limited the variety of volume waveforms which could be generated. To overcome this limitation, we developed a new system in which the servo-pump is controlled by an inexpensive microprocessor based computer, capable of generating an unlimited repertoire of volume waveforms. The computer system also made possible the use of adaptive control to increase the system fidelity. Finally, such a system provides for ease of adjustment to new hardware, should future research require it.
Animals↗
Impedance loading servo pump system for excised canine ventricle.
To investigate ventricular-arterial system interaction, we have developed a hybrid-computer-controlled impedance loading servo pump system that enables us to impose a simulated arterial hydraulic impedance on an excised canine ventricle. An analog computer programmed to simulate a three-element Windkessel model of the arterial system computes instantaneous aortic flow from the instantaneous ventricular pressure. The time integral of this flow is used to command a volume servo pump system that controls the instantaneous ventricular volume. All parameter values in the loading system are controlled by a digital computer. The actual impedance spectrum generated by the system was reasonably close to that expected from the arterial model. The unique features of this system are the following. 1) The instantaneous volume of the ventricle, which is crucial information, can be measured. 2) If needed, the arterial impedance model can easily be reprogrammed to generate more complex impedance spectra. 3) The vascular parameters can be made nonlinear or time varying through the digital computer control.
Animals↗
Analysis of mitral and aortic valve vibrations and their role in the production of the first and second heart sounds.
Using a theoretical approach and stress and vibration analysis, formulae have been derived for the frequencies that would be generated by considering the mitral and aortic valve leaflets as vibrating membranes. In the analysis the physical characteristics of the valves, such as loading, valve geometry and valve tissue mechanical properties, were taken into consideration. The analysis showed that the fundamental frequencies for both the vibrating valves increase with the pressure difference across the valve membranes. This suggests that for a better understanding of the role played by the vibrating heart valves in the production of heart sounds, data are needed on the relation between these frequencies and the pressure difference across the valves. Unfortunately such data do not seem to be available.
Aortic Valve↗
Elasticity of ligated carotid arteries of rabbits with and without a cholesterol-rich diet.
Elasticity of carotid arterial segments of rabbits proximal and distal to an induced ligation was examined. In the presence of both ligation and hypercholesterolemia, longitudinal distensibility was decreased while circumferential distensibility increased. The decrease in longitudinal distensibility was attributed to a decrease in the slack of collagen while the increase in circumferential distensibility was attributed to an increase in the slack of collagen as well as a decrease in the elastances of elastin and collagen. Cholesterol alone was found not to affect the elasticity, while ligation acting alone slightly decreased the circumferential distensibility while leaving the longitudinal distensibility unchanged. High serum cholesterol was also found to accelerate intimal proliferation in regions near the ligature. Some intimal proliferation was seen near the ligature, but this could not be quantitated as the sections were too short for pressure fixation. If intimal proliferation is a precursor of atherosclerotic lesions, then formation of these lesions might be accelerated near occluded regions and maybe also in regions of disturbed blood flow when high serum cholesterol is present.
Animals↗
Mechanical properties and ultrastructure of normal human tricuspid valve chordae tendineae.
The mechanical properties and ultrastructure of normal human tricuspid valve chordae tendineae were examined. Mechanical studies showed that these chordae exhibited less extensibility than normal mitral valve chordae of comparable size. The non-linear stress-strain curve of the tricuspid chordae, however, can be attributed to their microstructure. Under scanning electron microscopy these chordae were found to consist of fairly straight collagen bundles that were made up of networks of collagen fibrils. Transmission electron microscopical studies showed that the distribution of collagen fibril size was bimodal with an average fibril diameter of 441 A (S.E. = 2A) while the average fibril density was 198.4 (S.E. = 8.4) per 10(-8) cm2. These data data from the transmission electron micrographs were found to be different from those already reported for the mitral valve chordae. Thus, fine structural differences exist between chordae from both the mitral and tricuspid valves. These differences in fine structure could be due to the fact that these valves are under different in vivo mechanical stresses and they may account for the less extensible nature exhibited by the tricuspid chordae tendineae.
Aged↗
Dynamic elasticity of human mitral valve chorade tendinease.
Chordae tendineae are under constant dynamic stress and, therefore, measurements of static properties alone cannot provide a complete analysis of their mechanical response under stress. This study investigated the dynamic viscoelastic properties of human mitral valve chordae tendineae. The tissue was subjected to sinusoidal strain variations over a frequency range of 0.42 to 6.68 Hz. At a fixed strain, the storage modulus, E', was found to be independent of applied frequency but varied inversely with chordal thickness. Values in the order of 10(8) dyn cm-2 (1 dyn=10 muN) were found. E' also increased with strain level. The phase lag, phi, between stressing function and response was found to be small (0.2-0.058 rad) and decreased with frequency. Values of phi at each frequency were found to be independent of chordal size and strain level. This property would enable a smooth and even closure of the valve and provide a more rapid response at elevated heart rates. The loss modulus, E'', was found to be 18 to 50 times smaller than E'. This implied an almost complete recovery on removal of any stress on the tissue. E'', which also decreased with frequency, was found to be smaller for the larger chordae.
Adult↗
Scanning electron microscopical study of human mitral valve chordae tendineae.
Scanning electron microscopy was used to examine the surface and internal structure of adult human mitral valve chordae tendineae. The endocardial cell surface of the tissue was found to be relatively smooth and even. Just beneath the endocardial surface, widely spaced collagen fibers were found. These were observed to be aligned at an angle to the chordal long axis and to wrap themselves around the central collagen core. The chordal core consisted of relatively straight collagen fibers that were arranged with their axis parallel to the long axis of the chordae. At the microstructural level, these collagen fibers were observed to consist of a network of collagen fibrils. This architectural arrangement is an advantageous feature for a tissue that is under constant dynamic stress. The scanning observations were confirmed by transmission electron microscopy.
Chordae Tendineae↗
Cardiac valve replacement--a brief review.
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Heart Valve Prosthesis↗
Low frequency dynamic viscoelastic properties of human mitral valve tissue.
The dynamic viscoelasticity of the anterior leaflet of human mitral valves was investigated by subjecting the ventricular aspect of membranous samples of the tissue to sinusoidal fluid pressures. The frequency range of the stressing function used was from 0.5 Hz to 5 Hz. The storage modulus of the tissue was found to be independent of the stressing frequency and an average value of 2.55 (SE = 0-12) X 10(8) dyn cm-2 was observed. Losses were small and the phase shift between stressing function and resulting strain and hence the loss modulus increased with frequency. For the frequency range investigated the phase shift varied from 0.04 to 0.06 rad and the loss modulus was of the order of 10(7) dyn cm-2. From the observed data it was concluded that any tissue substitute used in mitral valve replacement should be rather inextensible and have a low loss modulus. Also under normal physiological conditions, the mitral valve cannot bulge into the left atrium during peak ventricular systole and hence events in the cardiac cycle, such as the presence of the atrial pressure 'c' wave, that involve the distensibility of the valve need to be re-examined.
Elasticity↗
The low frequency dynamic viscoelastic properties of human aortic valve tissue.
Membranous samples of human aortic valve cusps were subjected to sinusoidal fluid pressure variations (frequency range, 0.5-5 Hz) to assess their dynamic viscoelastic properties. The storage (elastic) and loss (viscous) moduli and phase lag between the stressing function and response were found to be independent of the frequencies applied. The respective average values were 1.35 (SE = 0.06) X 10(8) dynes cm-2, 4.14 (SE = 0.28) X 10(6) dynes cm-2, and 0.033 (SE = 0.002) rad. The small phase lag indicates that the tissue would recover almost completely to its original state on removal of any applied stress, and this and the relatively low extensibility should be considered in the design of leaflet-type valve prostheses. The storage modulus of the aortic valve cusps when compared to that of the mitral leaflet shows the mitral leaflet to be almost twice as stiff as the aortic valve cusps. This finding led us to conclude that the vibrations of these two cardiac valves alone cannot contribute in any significant way to the production of the observed lower frequency of the first and the higher frequency of the second heart sounds and that other factors must be considered to explain this finding.
Aged↗
Morphology and relationship to extensibility curves of human mitral valve chordae tendineae.
Human mitral valve chordae tendineae in which elastic response curves are nonlinear have also been found to exhibit extensibility that increases with chordal size and decreases with chordal age. We used selective enzymatic digestion and scanning transmission electron microscopy to explain these observations. Removal of the outer elastin sheath by enzymatic digestion did not significantly affect the elastic response of this tissue. Scanning electron microscopy revealed that the collagen fibers in the central core of young chordae exhibited a very wavy pattern but the pattern in adult specimens was relatively straight. The increased waviness accounted for the greater extensibility of the young specimens. The collagen fibers from young and old chordae consisted of a network of collagen fibrils that became more collapsed when the tissue was fixed under tension. This network arrangement of the fibrils explains the nonlinearity in the elastic response of the tissue. Transmission electron micrographs showed that the density of collagen fibrils decreased as chordal size increased. The number of fibrils per 10(-8) cm2 of the central core decreased from 182.4 (SE = 1.3) to 131.3 (SE = 1.6) as average chordal cross-sectional area increased from 0.0016 cm2 to 0.0268 cm2. This difference in fibril density provides an explanation for the greater extensibility shown by the thicker chordae. The collagen fibril diameters ranged from 516 A to 552 A.
Adolescent↗