Biomedical subjects

I Vesely

Publications and source records attributed to I Vesely.

12 recordsLinked to original sources

PubMed1992

Micromechanics of the fibrosa and the ventricularis in aortic valve leaflets.

The elastic response of aortic valve cusps is a summation of its fibrous components. To investigate the micromechanical function of valve leaflet constituents, we separated the fibrosa and the ventricularis from fresh and glutaraldehyde-fixed leaflets and tested them individually. The ventricularis was stiffer circumferentially than radially (7.41 kPa vs 3.68 kPa, p less than 0.00001) and was more extensible radially (62.7% vs 21.8% strain to high modulus phase, p less than 0.00001). The fibrosa was also stiffer circumferentially than radially (13.02 kPa vs 4.65 kPa, p less than 0.0008), but had uniform extensibility. Glutaraldehyde fixation did not affect the circumferential elastic modulus of the fibrosa, but reduced its radial modulus from 4.65 kPa to 2.32 kPa (p less than 0.0078). The elastic modulus of the ventricularis remained unchanged. Fixation also reduced the extensibility of the ventricularis circumferentially (from 21.8% to 15.2% strain, p less than 0.018), but not radially, and increased the radial extensibility of the fibrosa from 27.7% to 46.1% (p less than 0.0048). These data show that while the ventricularis contains a large amount of elastin, the amount of radially oriented collagen is similar to that of the fibrosa. The fibrosa, by itself, has the same extensibility in both directions (about 23% strain), but can extend much more radially when connected to the rest of the leaflet because it is attached to the ventricularis in a highly folded configuration. The two layers therefore complement each other during aortic valve function, and become detrimentally altered by fixation in glutaraldehyde.

Animals↗

PubMed1992

Micromechanics and mathematical modeling: an inside look at bioprosthetic valve function.

A major contributing factor in the degeneration of glutaraldehyde-treated porcine xenograft bioprostheses is tearing of the valve cusps near their commissural attachment to the supporting stent. We have been examining aortic valves at the micromechanical level, and have developed several sensitive techniques to evaluate the biomechanical changes produced by the glutaraldehyde fixation process. Additionally, we have developed a mathematical modeling technique that stimulates valve function during the entire cardiac cycle. Our micromechanical tests have shown that compressive buckling is common to all fixed tissues, occurs at physiological bending curvatures, and is likely to be the primary mode of mechanical failure of bioprosthetic valves. We have also shown that existing glutaraldehyde fixation techniques inhibit the natural internal shearing of the valve cusps, and disable the interaction of the fibrosa and the ventricularis. With our modeling technique, we have shown that flexural stresses are indeed concentrated near the valve commissures, and that appropriate modifications of the supporting stent can reduce flexural deformations. With these new, more revealing techniques at hand, prospective valve designs can be better evaluated prior to large scale animals and clinical testing.

Animals↗

PubMed1991

An automated, video tape-based image archiving system.

We have developed an image storage and retrieval system that makes use of a Super-VHS video tape recorder, and a personal computer fitted with an interface board and a video frame grabber. Under PC control, video images are acquired into the frame grabber, a numeric bar code is graphically superimposed for identification purposes, and the composite images are recorded on video tape. During retrieval, the bar code is decoded in real-time and the desired images are automatically retrieved. This video tape-based system, enables the images to be previewed and retrieved much faster than if stored in digital format.

Electronic Data Processing↗

PubMed1991

Automated 3-D reconstruction of vascular structures from high definition casts.

Three-dimensional reconstruction and computer modeling is becoming recognized as a powerful tool for studying vascular structures. The computational approach, as well as the computer hardware selected for the task, however, depend upon the information desired. For the modeling of surface geometry, as in the case of the aortic valve, a surface formation technique is favorable over the more computationally demanding volume rendering approach. Automated surface formation, however, requires good quality, high contrast images. We therefore present a technique for producing high contrast images from high definition casts. We also describe the methodology used for automatic contour tracing, generating a mesh of variable density, and the schemes used to reconstruct bifurcating objects. With this approach, 98 mbytes of imaging data could be reduced to 180 kbytes of polygon vertices, and manipulated at near real-time speed on a medium performance graphics workstation. Such a system is therefore well suited for detailed, quantitative analyses of the reconstructed structures. Overall, this paper outlines the procedures used to create a high definition, three-dimensional computer model of any vascular structure.

Animals↗

PubMed1991

Aortic valve/root interactions in porcine hearts: implications for bioprosthetic valve sizing.

The implantation of aortic allografts as well as stentless, freehand porcine xenograft valves requires proper sizing of the graft for the recipient aortic root. To visualize the aortic valve in motion and measure the cyclic expansion of the aortic root, we developed an isolated porcine heart model and a computerized three-dimensional reconstruction technique. Dynamic and static expansions of the aortic root were obtained from beating and arrested porcine hearts, and additional static expansions at varying pressures were measured from reconstructed three-dimensional models of valves obtained with high-resolution magnetic resonance imaging. Measurements of aortic root expansion have shown that it is highly dependent upon the pressures imposed on the heart. Although the aortic root expanded by only 5% between systolic pressures of 60 and 100 mmHg, the total expansion was up to 40% between rest and cyclic pressurizing to 100 mmHg. This data suggest that unstented xenograft valves should be sized 30% to 40% larger than the collapsed size of the recipient aorta. Proper sizing of valves on stents should also be attempted to reduce the large amount of leaflet redundancy that current stenting techniques produce.

Animals↗

PubMed1991

Analysis of the Medtronic Intact bioprosthetic valve. Effects of "zero-pressure" fixation.

The long-term performance of current-design porcine xenograft valves has not been satisfactory. These valves are generally fixed at "low pressures" of about 3 to 5 mm Hg. The Medtronic Intact (Medtronic, Inc., Minneapolis, Minn.) valve is fixed at "zero pressure" and is proposed as a better alternative to existing xenograft valves. A mechanical analysis of this valve has been carried out to determine if the Intact valve differs significantly from the low-pressure fixed xenograft. Twelve circumferential strips of tissue 5 mm wide were cut from the leaflets of four clinical-grade Intact valves. Their stress/strain, stress relaxation, and flexural behavior were examined mechanically and histologically. The Intact valve was more extensible than the low-pressure fixed xenograft (22% versus 12% strain, p less than 0.001), relaxed faster (p less than 0.001), and was more pliable than the xenograft (p less than 0.05). It did not, however, buckle less than did the low-pressure fixed xenograft during enforced bending, and it buckled significantly more than did fresh porcine aortic valve tissue (p less than 0.001). The Intact valve also relaxed significantly more slowly than did the fresh tissue (p less than 0.05). Its bending stiffness had a stronger dependence on leaflet thickness than the bending thickness of fresh tissue had (p less than 0.001) but a weaker dependence than the bending thickness of the low-pressure fixed xenograft material had (p less than 0.001). The Intact valve demonstrated a very large variability in extensibility, bending stiffness, and buckling behavior, with little correlation between these parameters. Some valves appeared to have wrinkled leaflets; others were likely fixed at different pressures. The shrinkage of the leaflet material at these low fixation pressures is likely important, since it can modify the elastic behavior of the valve cusps. Overall, the Intact valve had a more "natural" elastic behavior than had low-pressure fixed xenograft, and it should therefore experience lower stresses during normal valve function. It can be concluded that zero-pressure fixation does preserve many of the desirable stress-reducing properties of aortic valve tissue.

Animals↗

PubMed1990

Effects of dynamic fixation on shear behaviour of porcine xenograft valves.

To evaluate an alternative valve fixation technique, we measured the ability of glutaraldehyde-fixed valve tissue to undergo internal shearing during bending. Porcine aortic valves were fixed statically using conventional means, and dynamically while opening and closing repeatedly in a pulse tank. Using a polarized light microscopy technique developed previously, we measured shear deformation angles in thin sections of bent leaflet tissue and calculated shear strains. Statically-fixed leaflet tissue sheared only 1.2% +/- 2.29% (Mean +/- SD) when bent to curvatures of 2.0 mm-1, while dynamically-fixed tissue sheared 5.1% +/- 2.63% (significant at P less than 0.05). It is likely that dynamic fixation increases the ability of prosthetic valve leaflets to shear during bending by reducing the number of interfibre cross-links that would otherwise impede such deformations. Because shear strains reduce internal fibre strains and protect the leaflets against fatigue, prosthetic valves constructed from dynamically fixed tissue should experience lower stresses and hence last longer.

Biomechanical Phenomena↗

PubMed1990

A computerized system for video analysis of the aortic valve.

A novel technique was developed to study the dynamic behavior of the porcine aortic valve in an isolated heart preparation. Under the control of a personal computer, a video frame grabber board continuously acquired and digitized images of the aortic valve, and an analog-to-digital (A/D) converter read four channels of physiological data (flow rate, aortic and ventricular pressure, and aortic root diameter). The valve was illuminated with a strobe light synchronized to fire at the field acquisition rate of the CCD video camera. Using the overlay bits in the video board, the measured parameters were super-imposed over the live video as graphical tracing, and the resultant composite images were recorded on-line to video tape. The overlaying of the valve images with the graphical tracings of acquired data enabled the data tracings to be precisely synchronized with the video images of the aortic valve. This technique enabled us to observe the relationship between aortic root expansion and valve function.

Animals↗

PubMed1990

Mechanical testing of cryopreserved aortic allografts. Comparison with xenografts and fresh tissue.

Reports indicate that cryopreserved aortic valve allografts have a better long-term survivability than other bioprostheses, such as the porcine xenograft. Unlike xenografts, allograft valves do not require treatment with glutaraldehyde and may therefore retain much of their original mechanical function. The effects of cryopreservation on the mechanical integrity of collagen fibers and mucopolysaccharides, however, are still largely unknown. We therefore compared the mechanical behavior of cryopreserved allograft leaflet material to that of fresh tissue and xenografts by measuring their bending stiffness (nine strips of tissue) and their uniaxial tensile stress/strain and stress/relaxation behavior (six strips of each tissue type). The bending tests showed no significant difference between the pliability of cryopreserved allografts and fresh pig aortic valve tissue, but the xenograft material was significantly stiffer than both (p less than 0.001). The mean circumferential tensile elastic moduli of the allografts, fresh tissue, and xenografts at a stress of 300 kPa were 9.1 +/- 5.4 MPa, 13.0 +/- 1.7 MPa, and 12.5 +/- 3.0 MPa, respectively, and were not significantly different from each other. We also found that the transition from a low to a high modulus on the stress/strain curves, a measure of extensibility, occurs at 23%, 22%, and 12% strain for the three materials. There was no significant difference between the allograft and the fresh tissues, but the xenograft material was less extensible than the other two (p less than 0.001). The xenograft tissue also had significantly lower rates of stress relaxation than the other two materials (p less than 0.005). Thus no detectable differences were found between the mechanical behavior of the cryopreserved allograft aortic leaflets and fresh tissue, whereas the xenograft material was less extensible and less capable of relaxing than both the allograft and fresh tissue. The ability of allografts valves to respond to tensile and flexural stresses in a manner similar to that of the natural aortic valve may therefore contribute to their good in vivo survivability.

Aorta↗

PubMed1989

Analysis of the bending behaviour of porcine xenograft leaflets and of natural aortic valve material: bending stiffness, neutral axis and shear measurements.

Flexibility of the materials used in the construction of bioprosthetic heart valves is essential for proper valve operation. We therefore examined the bending behaviour of glutaraldehyde treated porcine aortic valve cusps in comparison with fresh aortic valve tissue. We repeatedly bent a total of 35 strips of fresh and treated tissue to curvatures ranging from 0.2 to 2.2 mm-1. We compared the stiffness of the two materials between circumferential and radial bending, natural and reverse curvatures and constant or variable tensile stress (0.8-40 kPa). Our results showed a weak positive relationship between bending stiffness and applied tensile stress and a strong positive dependance of stiffness on tissue thickness (t). For the fresh tissue, the bending stiffness increased in proportion to t1.14 while for the glutaraldehyde treated tissue it increased with t2.18. Fourteen strips of fresh and treated tissue were also histologically processed, sectioned and examined with polarized light microscopy. Collagen fiber wavelengths and shear deformations were measured utilizing the tissue banding patterns produced by polarized light microscopy. The neutral axis of bending was found to lie very close to the outer surface of the tissue, suggesting that aortic leaflets have a very low compressive elastic modulus. The shear strains measured in fresh tissue were 10 +/- 2.7% vs 3 +/- 4.4% for the treated, indicating a stiffening of the tissue following glutaraldehyde fixation. We conclude that both natural and bioprosthetic valve cusps have a complex flexural behaviour that cannot be modeled using simple bending principles, although the bioprosthetic material more closely approximates the simple beam than does the fresh. The non-linear elastic modulus, high compressibility and shearing between fiber layers are likely responsible for the observed behaviour of the fresh tissue, while the cross-linking and dehydrating effects of glutaraldehyde are believed to be responsible for the alteration in bending properties observed in the treated tissue. Our study suggests that bioprosthetic valve material does not adequately mimic the mechanics of the natural valve tissue, and that the current glutaraldehyde fixation process eliminates many of the beneficial, stress-reducing properties of the aortic leaflet.

Animals↗

PubMed1988

Tissue buckling as a mechanism of bioprosthetic valve failure.

Current reports indicate that collagen fiber disruption resulting from cyclic leaflet bending is a factor determining long-term durability of bioprosthetic heart valves. Examination of the opening characteristics of porcine xenografts has shown two areas of high bending curvature that correlate well with sites of leaflet tearing. These are at the free edge and near the attachment of the leaflets to the aortic root. To determine the potential effects of sharp bends in leaflet material, we examined 15 strips each of fresh and glutaraldehyde-treated porcine aortic valve tissue. Leaflet strips were bent to curvatures of 0.18 mm-1 to 6.67 mm-1, histologically processed, sectioned, and examined under a light microscope. We observed severe compressive buckling in the samples taken from bioprosthetic valves but little in the fresh-tissue samples. At physiological curvatures (less than 0.28 mm-1), no buckling occurred in the fresh tissue; at high bending curvatures (2.0 mm-1), the depth of buckling observed in the treated tissue was 100% greater than that in the fresh. We believe that porcine xenograft failure is related to compressive buckling of the aldehyde-treated tissue and is mediated by the systematic breaking of collagen fibers at the site of buckling. We suggest that alternative valve designs and preservation techniques be employed to prevent such abnormal leaflet deformations.

Animals↗

PubMed1985

A multipurpose tissue bending machine.

A unique tissue bending machine was developed to test the bending properties of normal and bioprosthetic heart valve material. It can be operated in air or in a tissue bath and can measure bending torques with an accuracy in excess of 1.0 microN m. Three contrasting substances were tested to compare their stiffness and to demonstrate the machine.

Biophysics↗