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Biomedical subjects

D A Vorp

Publications and source records attributed to D A Vorp.

26 records · Page 2Linked to original sources
PubMed1996

Potential influence of intraluminal thrombus on abdominal aortic aneurysm as assessed by a new non-invasive method.

Intraluminal thrombus may play a role in abdominal aortic aneurysm pathogenesis and rupture. The purpose of this work was to demonstrate the feasibility of a new non-invasive method for the determination of the biomechanical features of the aortic wall and luminal boundary in abdominal aortic aneurysm containing intraluminal thrombus. Automated ultrasonographic measures of infrarenal aortic cross-sectional area (A) were obtained on-line along with non-invasive arterial pressure (p) from eight patients of mean (s.e.m.) age 74(3) years, with abdominal aortic aneurysm (mean dimensions 5.9(0.4) x 5.3(0.5) cm) containing intraluminal thrombus. Luminal boundary and abdominal aortic aneurysm wall were scanned separately. Compliance (C) was computed as C = (Amax - Amin)/[Amax(Pmax - Pmin)], where 'max and 'min' represent maximum and minimum values, respectively. Mean compliance was lower for the abdominal aortic aneurysm wall alone than for the luminal surface enclosed by intraluminal thrombus: 4.0(0.9) x 10(-4)/mmHg versus 9.8(1.7) x 10(-4)/mmHg (P < 0.01). Intraluminal thrombus area was nearly constant over the cardiac cycle, indicating that the thrombus is virtually incompressible. This noninvasive method to assess biomechanical features of abdominal aortic aneurysm has potential to further the understanding of the influences of intraluminal thrombus on aneurysm disease.

Aged↗
PubMed1996

A device for the application of cyclic twist and extension on perfused vascular segments.

Cyclic strain is known to influence many aspects of vascular cell biology, including macromolecular biosynthesis, cell proliferation, and cell morphology. The simulation of the cyclic strain associated with the vasculature in situ has been carried out mostly using devices that manipulated flat membranes on which the cells of choice are cultured. The purpose of this work was to create an apparatus wherein cyclic strains consistent with those found in native blood vessels could be applied to intact three-dimensional (tubular) specimens perfused ex vivo. The cyclic strain protocols using our apparatus may be any combination of axial stretch and twist. Concurrently, the perfusion flow and shear stress are controlled as desired. Vessel diameter, intraluminal pressure, volume flow, and regional strain are measured and stored on line. To illustrate the application of the device, we present hemodynamic and kinematic data collected from a human saphenous vein segment perfused under steady-flow conditions while subjected to cyclic stretch and twist.

Blood Vessels↗
PubMed1995

Identification of elastic properties of homogeneous, orthotropic vascular segments in distension.

Characterization of the constitutive behavior of normal and pathological blood vessel segments could provide the clinician with a means to predict the onset and assess the severity of certain vascular maladies. Many of the constitutive models that have been proposed to date either fail to properly consider certain features of the anatomic structure and function of vascular tissue or are so mathematically complex that their utilization is intractable. We have developed a material identification technique that first required the adaptation and validation of a constitutive law describing the nonlinear, three-dimensional behavior of orthotropic, compressible, hyperelastic vascular segments. By coupling a nonlinear finite element program and experimental data with a robust nonlinear least-squares regression algorithm, a set of elastic parameters (moduli) is obtained. Regressions on data for a canine carotid artery and rabbit infrarenal aorta yielded coefficients of variation of 0.21 and 0.08, respectively. The estimated moduli demonstrated certain trends found by other investigators: both the canine carotid artery and rabbit aorta were found to be stiffer radially than circumferentially, and the former was found to be stiffer circumferentially than longitudinally. Using these material constants and measured arterial pressures, the stress distribution was computed for each specimen. The predicted radial stress was consistent with a transmural variation of approximately--p (applied luminal pressure) to approximately zero in both specimens, while the circumferential stresses ranged from 2.2p to 0.7p for the canine carotid, and from 6.4p to 3.7p for the rabbit aorta. The stress distributions qualitatively agreed with those reported in previous investigations, as well as with certain physiologic observations. Based on the results of our two sample cases, we believe that our technique could be beneficial to the assessment of the three-dimensional, anisotropic behavior of vascular tissue.

Animals↗
PubMedDate not supplied

Modeling the transmural stress distribution during healing of bioresorbable vascular prostheses.

Little attention has been given to the stresses within the wall of bioresorbable vascular prostheses and how they might affect the resorption process. We modeled the graft "complex" (inner tissue capsule, residual graft, and outer tissue capsule) as a three-layered compound tube under internal pressure. Using this biomechanical model, we studied the effects of alterations in the geometry (i.e., radius and thickness) and mechanical properties of each stratum on the overall transmural stress distribution. Hypothetical simulations were performed to investigate the possible sequence of and alterations in the radial and circumferential stresses during the resorption process. Our results suggest that early in the resorption phase, the inner tissue capsule is subjected to compressive hoop stresses and concentrated, large-magnitude compressive radial stresses. This distribution gives way to the more typical distribution for a thick-walled tube when equilibration (i.e., complete resorption) is approached. The prediction of the compressive stresses in the pseudo-intima during early resorption parallels findings of an elevated mitotic index in that region at that time. This leads to a new hypothesis, namely, that compressive stresses, both in-plane and out-of-plane with respect to the regenerated vascular cells, participate in the resorption process of bioresorbable vascular grafts by modulating elevated cellular proliferative activity and may play an important role in other aspects of vascular cell biology. Results of recent experimentation support this hypothesis.

Animals↗
PubMedDate not supplied

Ex vivo biomechanical behavior of abdominal aortic aneurysm: assessment using a new mathematical model.

Knowledge of the biomechanical behavior of abdominal aortic aneurysm (AAA) as compared to nonaneurysmal aorta may provide information on the natural history of this disease. We have performed uniaxial tensile testing of excised human aneurysmal and nonaneurysmal abdominal aortic specimens. A new mathematical model that conforms to the fibrous structure of the vascular tissue was used to quantify the measured elastic response. We determined for each specimen the yield (sigma y) and ultimate (sigma u) strengths, the separate contribution to total tissue stiffness by elastin (EE) and collagen (EC) fibers, and a collagen recruitment parameter (A), which is a measure of the tortuosity of the collagen fibers. There was no significant difference in any of these mechanical properties between longitudinal and circumferential AAA specimens, nor in EE and EC between longitudinally oriented aneurysmal and normal specimens. A, sigma y, and sigma u were all significantly higher for the normal than for the aneurysmal group: A = 0.223 +/- 0.046 versus A = 0.091 +/- 0.009 (mean +/- SEM; p < 0.0005), sigma y = 121.0 +/- 32.8 N/cm2 versus sigma y = 65.2 +/- 9.5 N/cm2 (p < 0.05), and sigma u = 201.4 +/- 39.4 N/cm2 versus sigma u = 86.4 +/- 10.2 N/cm2 (p < 0.0005), respectively. Our findings suggest that the AAA tissue is isotropic with respect to these mechanical properties. The observed difference in A between aneurysmal and normal aorta may be due to the complete recruitment and loading of collagen fibers at lower extensions in the former. Our data indicate that AAA rupture may be related to a reduction in tensile strength and that the biomechanical properties of AAA should be considered in assessing the severity of an individual aneurysm.

Aged↗
PubMedDate not supplied

Gene expression is altered in perfused arterial segments exposed to cyclic flexure ex vivo.

Certain regions of coronary and other arteries undergo cyclic flexure due to attachment to the heart or crossing of joints. Such motion gives rise to fluctuations in transmural stress and luminal shear stress. It is well known that cyclic variation of these biomechanical forces influences many aspects of vascular cell biology including gene expression. The purpose of this work was to investigate the hypothesis that cyclic flexure of arterial segments influences their gene expression. Bilateral porcine femoral arteries were obtained fresh from the abattoir. One vessel was mounted in an ex vivo perfusion system and subjected to an intraluminal pressure of 60 mmHg and flow of 50 ml/min to serve as a control. The other vessel was mounted in a second perfusion system with similar hemodynamic conditions, but also subjected to controlled cyclic bending consistent with that found in coronary arteries in vivo. Reverse transcriptase-polymerase chain reaction analysis demonstrated that E-selectin and matrix metalloproteinase-1 (MMP-1) were consistently and significantly downregulated in the specimens subjected to 4 h of cyclic bending as compared to the control (n = 8, p < 0.05). Our results show that cyclic flexure of arterial segments in vitro may influence their gene expression. Further investigation should follow this novel observation and focus on other known mediators to more carefully elucidate the consequence of cyclic flexure on arterial pathobiology.

Actins↗
PubMedDate not supplied

In vivo three-dimensional surface geometry of abdominal aortic aneurysms.

Abdominal aortic aneurysm (AAA) is a local, progressive dilation of the distal aorta that risks rupture until treated. Using the law of Laplace, in vivo assessment of AAA surface geometry could identify regions of high wall tensions as well as provide critical dimensional and shape data for customized endoluminal stent grafts. In this study, six patients with AAA underwent spiral computed tomography imaging and the inner wall of each AAA was identified, digitized, and reconstructed. A biquadric surface patch technique was used to compute the local principal curvatures, which required no assumptions regarding axisymmetry or other shape characteristics of the AAA surface. The spatial distribution of AAA principal curvatures demonstrated substantial axial asymmetry, and included adjacent elliptical and hyperbolic regions. To determine how much the curvature spatial distributions were dependent on tortuosity versus bulging, the effects of AAA tortuosity were removed from the three-dimensional (3D) reconstructions by aligning the centroids of each digitized contour to the z axis. The spatial distribution of principal curvatures of the modified 3D reconstructions were found to be largely axisymmetric, suggesting that much of the surface geometric asymmetry is due to AAA bending. On average, AAA surface area increased by 56% and abdominal aortic length increased by 27% over those for the normal aorta. Our results indicate that AAA surface geometry is highly complex and cannot be simulated by simple axisymmetric models, and suggests an equally complex wall stress distribution.

Aorta, Abdominal↗