Porcine Abdominal Aortas Research Articles

Ultraviolet-induced mechanical augmentation of the degraded porcine aortic adventitia: Its significance for preventing aneurysmal rupture

In the previous studies, we have shown that the tunica adventitia of porcine abdominal aorta could be mechanically reinforced by irradiating it with ultraviolet A (UV-A) rays to promote the photocrosslinking of constitutive collagen, and we have proposed the method as a procedure to prevent or delay abdominal aortic aneurysm (AAA) ruptures. We have also demonstrated that UV irradiation-induced mechanical augmentation despite the degradation of the adventitia through collagenolysis in vitro. Considering that elastolysis is equally a relevant event in the pathogenesis of AAA, the degradation of elastin was also investigated in the present study. A total of 50 porcine aortas were used in the study. All processed samples were evaluated in a mechanical tester before and after degradation and/or irradiation. The adventitial layer was isolated and subjected to elastase for either 1 – 48 h. We found that both elastin and collagen were digested by elastase, with the former being completely digested after 48 h. The samples degraded for 1 h were subsequently irradiated with UV-A (365 nm) in the presence of riboflavin as a photoinitiator, a process that induced an enhancement of the strength and stiffness of the tissue. This is an indication that a partly degenerated aortic wall, like that in an aneurysmal region, can be reinforced mechanically by UV irradiation, possibly to the extent of delaying, or preventing altogether, the wall’s rupture. It appears important to irradiate the wall as early as possible.

Abdominal Aortic Endograft Implantation Immediately Induces Vascular Stiffness Gradients That May Promote Adverse Aortic Neck Dilatation: Results of A Porcine Ex Vivo Study.

Endovascular aortic repair (EVAR) is the method of choice for most abdominal aortic aneurysm (AAA) patients requiring intervention. However, chronic aortic neck dilatation (AND) following EVAR progressively weakens the structural seal between vessel and endograft and compromises long-term results of the therapy. This experimental ex vivo study seeks to investigate mechanisms of AND. Porcine abdominal aortas (n=20) were harvested from slaughterhouse pigs and connected to a mock circulation. A commercially available endograft was implanted (n=10) or aortas were left untreated as controls (n=10). Vascular circumferential strain was assessed via ultrasound in defined aortic segments as a parameter of aortic stiffness. Histology and aortic gene expression analysis were performed to investigate potential changes of aortic wall structure and molecular differences due to endograft implantation. We found that endograft implantation acutely induces a significant stiffness gradient directly at the interface between stented and unstented aortic segments under pulsatile pressure. Comparing stented aortas with unstented controls, we detected increased aortic expression levels of inflammatory cytokines (Il6 and Ccl2) and matrix metalloproteinases (Mmp2 and Mmp9) after 6 hours of pulsatile pressurization. This effect, however, was abolished when repeating the same experiment under 6 hours of static pressure. We identified endograft-induced aortic stiffness gradients as an early trigger of inflammatory aortic remodeling processes that might promote AND. These results highlight the importance of adequate endograft designs to minimize vascular stiffness gradients and forestall late complications, such as AND. AND may compromise the long-term results following endovascular aortic repair. However, the mechanisms behind the underlying detrimental aortic remodeling are still unclear. In this study we find that endograft-induced aortic stiffness gradients induce an inflammatory aortic remodeling response consistent with AND. This novel pathomechanistic insight may guide the design of new aortic endografts that minimize vascular stiffness gradients and forestall late complications such as AND.

Effects of Ultraviolet-A Radiation on Enzymatically Degraded Tunica Adventitia of the Porcine Abdominal Aorta

In a previous study, it was demonstrated that mechanical stiffness and strength of tunica adventitia of the porcine abdominal aortic wall were significantly increased following irradiation of the tissue with ultraviolet-A (UV-A) rays, due to photochemically induced crosslinking of adventitial collagen. We have suggested that by applying the process to a dilated aortic region caused by abdominal aortic aneurysm (AAA), the mechanically enhanced adventitial layer might delay or prevent the aneurysmal rupture in human patients. Here, we extended our investigation to adventitial tissue that was subjected to in vitro collagenolysis, such mimicking a step in the aneurysmal pathomechanism. We used tissue isolated from the abdominal region of healthy porcine aortas. The adventitia was separated from the vessel wall and subjected to enzymatic degradation and exposed to UV-A radiation (365 nm) in the presence of a riboflavin derivative as the photo-initiator. Collagenolysis was carried out in buffered medium at two different durations (8 and 20 h). Three sets, each of 30 adventitial specimens (degraded only, irradiated prior to degradation, and irradiated after degradation), were evaluated in a mechanical tester. Mechanical testing of the irradiated specimens indicated an increase in both stiffness (Young’s modulus) and strength (ultimate stress) when compared to those degraded but not irradiated. The specimens first irradiated and then degraded displayed the highest gain in both stiffness and strength. In the tissue irradiated after degradation, the longer the duration of collagenolysis, the less significant was the enhancement of mechanical characteristics. Irradiation with UV-A rays of enzymatically degraded aortic adventitial tissue induced the enhancement of its mechanical characteristics (stiffness, strength). The earlier the irradiation, the more significant the enhancement.

High-Frequency Fluctuation Intensity as a Predictor of Post-Stenotic Dilatation in Swine.

To identify the turbulent components of blood flow facilitating aortic lumen dilatation in a post-stenotic dilatation (PSD) porcine model. The porcine abdominal aorta (AA) was moderately coarctated to induce overt flow turbulence in the downstream region and to lead to dilatation in time periods between four and twelve weeks. We propose a new metric, fluctuation intensity (FI), to quantify turbulent fluctuations of pulsatile aortic flow measured within twenty minutes post-coarctation. Lumen perimeter ratio (LPR) of the distal-to-suprarenal AA was used to assess the degree of PSD. Using k-means clustering analysis, we first divided FI frequency spectrum into low- and high-frequency fluctuation intensity (LFFI and HFFI), and subsequently grouped animals with coarctation. Receiver operating characteristic (ROC) analysis was performed to evaluate the ability of the proposed metric to predict PSD. The frequency band of the FI spectrum in facilitating aortic lumen dilatation was identified to be 40∼200 Hz. Using sham group as the reference, pigs receiving coarctation were clustered into two groups with (group A) and without (group B) increases in HFFI values. Coarctation significantly increased LPR values in group A, but not in group B. Moreover, group A exhibited a high probability density distribution on severe elastic fiber fragmentation. ROC analysis indicated HFFI to be capable of predicting PSD with excellent sensitivity and specificity. High-intensity, high-frequency components of blood flow fluctuations induced by moderate coarctation promote elastic lamella degradation and aortic lumen dilatation. HFFI application in flowmeter programs may provide a useful predictor of PSD.

Photocrosslinking of Adventitial Collagen in the Porcine Abdominal Aorta: A Preliminary Approach to a Strategy for Prevention of Aneurysmal Rupture

This study was aimed at generating data for designing a potential method to prevent the rupture of the abdominal aortic aneurysm (AAA). We found that the mechanical strength and stiffness of blood vessel walls was enhanced by the crosslinking of adventitial collagen through a photochemical process promoted by ultraviolet-A (UV-A) radiation. The experiments were carried out on samples isolated from 25 normal porcine aortas. The adventitial layer was separated from the other layers and exposed to UV radiation of 365-nm wavelength, in the presence of a riboflavin compound as the photosensitizer. Mechanical testing of 30 specimens, prior to and after exposure, indicated an increase in both strength (ultimate stress) and stiffness (Young’s modulus) of the adventitial specimens following irradiation. The crosslinking process also led to an enhanced resistance to experimental collagenolysis, as determined on six specimens. At this phase of conceptual design, we suggest that by applying this method to an aneurysmal dilated wall region, the stabilization of tunica adventitia may delay or prevent the rupture of the aneurysm and, with further investigation and refinement, can become a therapeutic strategy for arresting the progression of AAA.

Biomechanical Restoration Potential of Pentagalloyl Glucose after Arterial Extracellular Matrix Degeneration.

The objective of this study was to quantify pentagalloyl glucose (PGG) mediated biomechanical restoration of degenerated extracellular matrix (ECM). Planar biaxial tensile testing was performed for native (N), enzyme-treated (collagenase and elastase) (E), and PGG (P) treated porcine abdominal aorta specimens (n = 6 per group). An Ogden material model was fitted to the stress–strain data and finite element computational analyses of simulated native aorta and aneurysmal abdominal aorta were performed. The maximum tensile stress of the N group was higher than that in both E and P groups for both circumferential (43.78 ± 14.18 kPa vs. 10.03 ± 2.68 kPa vs. 13.85 ± 3.02 kPa; p = 0.0226) and longitudinal directions (33.89 ± 8.98 kPa vs. 9.04 ± 2.68 kPa vs. 14.69 ± 5.88 kPa; p = 0.0441). Tensile moduli in the circumferential direction was found to be in descending order as N > P > E (195.6 ± 58.72 kPa > 81.8 ± 22.76 kPa > 46.51 ± 15.04 kPa; p = 0.0314), whereas no significant differences were found in the longitudinal direction (p = 0.1607). PGG binds to the hydrophobic core of arterial tissues and the crosslinking of ECM fibers is one of the possible explanations for the recovery of biomechanical properties observed in this study. PGG is a beneficial polyphenol that can be potentially translated to clinical practice for preventing rupture of the aneurysmal arterial wall.

Histologic Analysis with the Newly Designed Exoskeleton Seal® Stent-Graft in the Porcine Abdominal Aorta.

To investigate the technical feasibility of a novel exoskeleton Seal® stent-graft and analyze early histologic changes in the porcine abdominal aorta. Six pigs received an abdominal stent-graft (Group I), and six received an iliac branch stent-graft (Group II). Groups were subdivided as follows: Group Ia, which received three bifurcated main-body stent-grafts; Group Ib, which received three bifurcated main-body stent-grafts with both iliac graft-stents; Group IIa, which received three simple uni-iliac tapered stent-grafts; and Group IIb, which received three uni-iliac tapered tapered stent-grafts with right straight limb and left branched limb. Statistical analyses were performed with the Wilcoxon signed-rank test and mixed-model regression analysis. The primary technical success rate (< 24h) was 83% because of two acute thromboses in the lumen of the stented abdominal aorta immediately after stent-graft placement. At 4weeks, late thrombosis occurred in two pigs. Higher mean neointimal hyperplasia areas (23.5% vs. 16.2%; P = .047), neointimal hyperplasia thicknesses (545.5μm vs. 422.2μm; P = .001), and degrees of collagen deposition (2.71 vs. 2.33; P = .002) were observed at the bare-metal stent-graft compared with the proximal exoskeleton portion of the stent-graft, with no significant differences between the patent and occluded groups or among the four types of stent-grafts. The exoskeleton stent-graft demonstrates 66% of patency rate during 1-month follow-up due to four cases of thromboses; however, the endothelialization on the junction of proximal graft showed no significant differences between the patent and occluded groups. Further studies should investigate long-term outcomes with prolonged neointimal hyperplasia.

Experimental and numerical studies of two arterial wall delamination modes.

Arterial wall dissection, which results from various pathophysiological processes, can lead to the occurrence of large area delamination in the aortic wall that can potentially block blood flow and lead to deleterious clinical conditions. Despite its critical clinical relevance, few studies have focused on investigating the failure mode of delamination in the arterial wall. In this study, we quantify the energy release rate of the medial layer of a porcine abdominal aorta via two delamination experiments: the mixed-mode delamination experiment and the "T"-shaped delamination experiment. A cohesive zone model (CZM) is applied to simulate the arterial wall delamination and Holzapfel-Gasser-Ogden (HGO) material model is used to capture the bulk arterial material behavior. A set of parameter values for the HGO and CZM models are identified through matching simulation predictions of the load vs. load-point displacement curve with experimental measurements. Then the parameter values and critical energy release rates obtained from experiments are used as input data for simulation predictions for two arterial wall delamination experiments. The simulation predictions show that the delamination front matches well with experimental measurements. Moreover, the mixed-mode delamination experiment reveals a shear mode-dominated failure event, whereas the "T"-shaped delamination experiment is an opening failure process. The integration of experimental data and numerical predictions of arterial delamination events provides a comprehensive description of distinct failure modes and aids in the prediction of aortic dissection.

A systematic comparison of biodegradation behavior of absorbable metal: in standardized immersion, arterial bioreactor and in vivo study

Event Abstract Back to Event A systematic comparison of biodegradation behavior of absorbable metal: in standardized immersion, arterial bioreactor and in vivo study Juan Wang1, 2*, Lumei Liu2*, Yeoheung Yun2* and Nan Huang1* 1 Southwest Jiaotong University, Dept. of Materials Science and Engineering, China 2 North Carolina Agricultural & Technical State University, NSF Engineering Research Center for Revolutionizing Metallic Biomaterials, United States Introduction: Absorbable metals have been widely tested in various in vitro environments to evaluate their biodegradation as vascular stent materials[1]. However, there exists a gap between in vivo and in vitro test results. A key step is to identify the relevant biochemical and biophysical microenvironments in test-systems. The purpose of this study was to establish an appropriate methodology for accurate and standardized determinations of degradation parameters in the pre- and post- endothelialization stage after stent implantation, which plays a vital part in predicting the fate of magnesium (Mg)-based stent. Materials and Methods: As-drawn Mg wires of 99.9% purity with a length of 10 mm and a diameter of 250 μm (Goodfellow, USA) were used. In the standardized immersion test, Mg wires were immersed in DMEM solution according to the standard protocol ASTM-D1141-98[2]. The LumeGen bioreactor (TGT DynaGen® Series, USA) as a vascular bioreactor was chosen to stimulate physiological aortal conditions (Fig. 1a). Each porcine abdominal aorta with a diameter of ~8 mm and a length of 5 cm was mounted into a chamber with a flow rate of 100 ml/min and a pulse pressure of 80-120 mmHg (Fig. 1b). In aortal in vivo test, rat abdominal aortas were used for implantation. Two Mg segments were symmetrically implanted into the lumen and wall of each aorta in the bioreactor or in vivo tests (Fig. 1c). Samples were analysed by X-ray computed tomography (CT). Fig. 1.(a) Photograph of bioreactor. Mg wires in the wall and lumen of a porcine aorta in the chamber (b) and rat aorta in vivo (c). Results and Discussion: One of two Mg wires exposed on the lumen and contacted the circulating medium to simulate a pre-endothelialization stage. The other Mg wire was embedded under the intima to simulate a post-endothelialization stage. The degradation product volumes, residual Mg volumes and average degradation rates were calculated utilizing the CT data. In Fig. 2, in vivo degradation was slower than in vitro degradation both in the standardized static and flow bioreactor conditions. In terms of the aortal bioreactor, flow convection on the lumen surface severely accelerated Mg degradation due to the increase of mass transfer, fluid shear stress and pulsatile stress, compared with low Mg degradation rate in the wall. In the aortal in vivo model, the degradation rate of Mg wire in the wall was faster than that in the lumen, and calcification was observed around Mg surrounding tissue. The degradation of Mg wire in the wall (diffusion limited region) was accelerated, which may result in the decrease of local pH from lysosome. Fig. 2. Volume ratios of residual Mg and corrosion products in the static no aorta, and the aortal lumen and wall of the bioreactor and in vivo conditions for 3-day. Conclusions: First-time investigation of degradation testing in the aortal bioreactor revealed that hydrodynamics plays a dominant role on the degradation. The established porcine aortal bioreactor and rat aortal in vivo model are expected to provide a better understanding of degradation mechanism of absorbable metallic stents. NIH grant (1SC3GM113728-01); NSF-ERC for revolutionizing metallic biomaterials (NSF-0812348); National Natural Science Foundation of China under Grant (81330031); Thanks Prof. Deling Kong, Dr. Zhihong Wang and Ms. Yifan Wu from Nankai University for in vivo test.

Regenerative potential of low-concentration SDS-decellularized porcine aortic valved conduits in vivo.

The aims of this study were to determine the functional biocompatibility of low-concentration SDS-decellularized porcine aortic roots in vivo. A previously developed process was modified for 9- and 15-mm-diameter aortic roots to facilitate implantation into the porcine abdominal aorta (n=3) and juvenile sheep right ventricular outflow tract (n=7), respectively. Native allogeneic aortic roots were used as controls. Acellular porcine roots explanted from pigs at weeks were largely repopulated with stromal cells of appropriate phenotype, and there was evidence that macrophages were involved in the regenerative process. Native allogeneic roots were subject to a classic allograft rejection response. Acellular porcine roots explanted from sheep at 6 months showed evidence of appropriate cellular repopulation, again with evidence of a role for macrophages in the regenerative process. There was some degree of calcification of two of the explanted acellular roots, likely due to incomplete removal of DNA before implantation. Native allogeneic ovine roots were subject to a classic allograft rejection response involving T cells, which resulted in overtly calcified and damaged tissues. The study highlighted (1) the importance of removal of DNA from acellular porcine valved roots to avoid calcification and (2) a role for macrophages in the regeneration of low-concentration SDS-decellularized aortic roots, as has been reported for other acellular biological extracellular matrix scaffolds.

A Novel Device for the Automatic Decellularization of Biological Tissues

Decellularized biological scaffolds represent a promising solution for tissue engineering. They offer a good substrate for cells in terms of biochemical composition, ultrastructure and mechanical properties without generating an immunogenic response. The aim of this study was to design and develop a device for the automatic decellularization of biological tissues to overcome manual operation limits, toward a good manufacturing practice-compliant process. A versatile, modular and easy-to-use device was designed, able to automatically exchange decellularization fluids and to provide mechanical shaking according to a user-defined protocol. Preliminary decellularization tests were made on porcine abdominal aortas comparing results between conventional process and device-operated process using water, sodium deoxycholate and DNase. Vessels were processed up to 4 cycles of the protocol and after each decellularization cycle histological analyses (hematoxylin-eosin, Movat pentachrome and DAPI stainings) were observed. Preliminary mechanical tests were also performed to compare the mechanical behavior of blood vessels processed with the 2 methods mentioned above. Briefly, the device consists of decellularization chambers, a shaking system and hydraulic modules for the exchange of fluids. The device was bench-tested for functionality and reliability with positive outcomes. The protocol used revealed to be effective, with a progressive tissue decellularization through repeated cycles. No difference between manual and automated operation was observed in histological or mechanical analyses. The developed device is able to automate the decellularization process lowering operator-related risks, and is a reliable and functional tool for clinical use.

Biodegradable polymer scaffold, semi-solid solder, and single-spot lasing for increasing solder-tissue bonding in suture-free laser-assisted vascular repair

We recently showed the fortifying effect of poly-caprolactone (PCL) scaffold in liquid solder-mediated laser-assisted vascular repair (ssLAVR) of porcine carotid arteries, yielding a mean ± SD leaking point pressure of 488 ± 111 mmHg. Despite supraphysiological pressures, the frequency of adhesive failures was indicative of weak bonding at the solder-tissue interface. As a result, this study aimed to improve adhesive bonding by using a semi-solid solder and single-spot vs. scanning irradiation. In the first experiment, in vitro ssLAVR (n=30) was performed on porcine abdominal aorta strips using a PCL scaffold with a liquid or semi-solid solder and a 670-nm diode laser for dual-pass scanning. In the second experiment, the scanning method was compared to single-spot lasing. The third experiment investigated the stability of the welds following hydration under quasi-physiological conditions. The welding strength was defined by acute breaking strength (BS). Solder-tissue bonding was examined by scanning electron microscopy and histological analysis was performed for thermal damage analysis. Altering solder viscosity from liquid to semi-solid solder increased the BS from 78 ± 22 N/cm(2) to 131 ± 38 N/cm(2) . Compared to scanning ssLAVR, single-spot lasing improved adhesive bonding to a BS of 257 ± 62 N/cm(2) and showed fewer structural defects at the solder-tissue interface but more pronounced thermal damage. The improvement in adhesive bonding was associated with constantly stronger welds during two weeks of hydration. Semi-solid solder and single-spot lasing increased welding strength by reducing solder leakage and improving adhesive bonding, respectively. The improvement in adhesive bonding was associated with enhanced weld stability during hydration.

A Magnetic Approach to Decrease Stent Graft Endoleak: Ex-Vivo Validation

Stent graft endoleak is a major problem in endovascular aneurysm repair (EVAR). Endoleak occurs after EVAR and may lead to aneurysm rupture, acute vessel thrombosis or occlusion. This study presents a novel design to potentially reduce endoleak with use of magnets. A ferromagnetic stent is deployed into the vessel lumen, and two external flexible magnetic rings are used to clamp on the proximal and distal necks of the stent. The rings impose epivascular magnetic pressure on the vessel wall to prevent the vessel wall from separating from the stent under elevated blood pressure. The geometry and magnetic properties of the stent and rings were designed to produce sufficient pressure, without overly compressing the vessel wall. Feasibility of this design was demonstrated with in vitro experiments using porcine abdominal aortas. The experiments showed that magnetic ring significantly improved the seal between the stent and vessel wall with use of moderate-sized stent. For aortas subjected to physiological axial stretch, rings that generate magnetic pressure of about 45 mmHg were found sufficient to prevent endoleak at pressure of 140 mmHg. Evaluation of this design in an in vivo animal model of aortic aneurysm is warranted.

Evidence forIn VivoGrowth Potential and Vascular Remodeling of Tissue-Engineered Artery

Nondegradable synthetic polymer vascular grafts currently used in cardiovascular surgery have no growth potential. Tissue-engineered vascular grafts (TEVGs) may solve this problem. In this study, we developed a TEVG using autologous bone marrow-derived cells (BMCs) and decellularized tissue matrices, and tested whether the TEVGs exhibit growth potential and vascular remodeling in vivo. Vascular smooth muscle-like cells and endothelial-like cells were differentiated from bone marrow mononuclear cells in vitro. TEVGs were fabricated by seeding these cells onto decellularized porcine abdominal aortas and implanted into the abdominal aortas of 4-month-old, bone marrow donor pigs (n = 4). Eighteen weeks after implantation, the dimensions of TEVGs were measured and compared with those of native abdominal aortas. Expression of molecules associated with vascular remodeling was examined with reverse transcription-polymerase chain reaction assay and immunohistochemistry. Eighteen weeks after implantation, all TEVGs were patent with no sign of thrombus formation, dilatation, or stenosis. Histological and immunohistochemical analyses of the retrieved TEVGs revealed regeneration of endothelium and smooth muscle and the presence of collagen and elastin. The outer diameter of three of the four TEVGs increased in proportion to increases in body weight and outer native aorta diameter. Considerable extents of expression of molecules associated with extracellular matrix (ECM) degradation (i.e., matrix metalloproteinase and tissue inhibitor of matrix metalloproteinase) and ECM precursors (i.e., procollagen I, procollagen III, and tropoelastin) occurred in the TEVGs, indicating vascular remodeling associated with degradation of exogenous ECMs (implanted decellularized matrices) and synthesis of autologous ECMs. This study demonstrates that the TEVGs with autologous BMCs and decellularized tissue matrices exhibit growth potential and vascular remodeling in vivo of tissue-engineered artery.

Splenic Artery Embolization and Endovascular Stapler Integrity in a Porcine Model

Splenic artery embolization is often used before laparoscopic splenectomy in cases of splenomegaly to reduce blood loss and facilitate the procedure. The aim of this study was to examine the general reliability of endovascular staplers when fired at the site of embolization coil deployment using a porcine model. Ex vivo and in vivo experiments were conducted on porcine abdominal aortas, which are similar in diameter to those of the splenic artery in the human. When the endovascular staplers were fired across the porcine vessels at the area of embolization coil deployment ex vivo, the staple lines all failed. In contrast, in vivo, the staple lines remained intact with no bleeding despite resistance imposed by the intravascular coils. Despite consistent failure in the ex vivo studies, in vivo all staple lines held and permitted safe transection of the vessel. We presume that the hemostatic properties of the coils caused sufficient thrombosis in this model, which mimics the clinical situation, to permit division of the previously embolized splenic vessel.

Vibrometry

Our objective was to test vibrometry as a means to measure changes in aneurysm sac pressure in an in vitro aneurysm model. Explanted porcine abdominal aortas and nitrile rubber tubes were used to model an aneurysm sac. An ultrasound beam was used to vibrate the surface of the aneurysm model. The motion generated on the surface was detected either by reflected laser light or by a second ultrasound probe. This was recorded at different aneurysm pressures. The phase of the propagating wave was measured to assess changes in velocity and to see if there was a correlation with aneurysm pressure. The cumulative phase shift detected by laser or Doppler correlated well with increasing hydrostatic pressure in both the rubber and the porcine aorta model. The square of the mean pressure correlated well with the cumulative phase shift when dynamic pressure was generated by a pump. However, the pulse pressure was poorly correlated with the cumulative phase shift. Noninvasive measurement of changes in aortic aneurysm sac tension is feasible in an in vitro setting using the concept of vibrometry. This could potentially be used to noninvasively detect wall stress in native aneurysms and endotension after endovascular aneurysm repair (EVAR) and to predict the risk of rupture.

A Statistical Method to Evaluate the Effect of Shear Stress on Endothelial Gene Expression in vivo

Objective To determine the effect of hemodynamic shear stress on endothelial gene expression in vivo. Methods Computational fluid dynamics (CFD) simulations were performed in three realistic models of the porcine abdominal aorta and its iliac artery branches. Using these calculations, a shear stress probability map in the iliac arteries was generated. This map showed the likelihood that a particular location would experience high, medium, or low time average shear stress. Based on this map, three anatomical regions likely to experience high, medium, and low shear stress levels were identified. The median time-average shear stress in these regions were 32.1, 15.2, and 9.7 dyn/cm2, respectively. Subsequently, endothelial cell RNA was collected from these regions in five additional porcine iliac arteries. Gene expression of select genes was measured in these samples using quantitative real time PCR. Results for the high and low shear regions are reported relative to the medium shear region. Results Expression of the genes tubulin, beta-catenin, c-jun, MCP-1, and VCAM was elevated in both the low and high shear stress regions relative to the medium shear region. ICAM expression was downregulated in both the low and high shear regions relative to the medium shear region. These effects were significant in nearly all cases. Conclusions This statistical method permits evaluation of phenotypic differences that arise due to hemodynamic forces in in vivo specimens. Arterial regions subjected to distinct levels of time average shear stress exhibit differences in endothelial gene expression. Supported by NIH grant H50442.

Noninvasive measurement of aortic aneurysm sac tension with vibrometry

Currently, the risk of aneurysm sac rupture after endovascular abdominal aortic aneurysm repair (EVAR) is estimated by using a group of anatomic variables. Available techniques for pressure monitoring include either direct measurement using catheter-based techniques or indirect measurement requiring implantation of a pressure sensor during aneurysm repair. None of these methods is without limitations. Radiation pressure, such as that generated by a modulated ultrasound (US) beam, can induce surface vibration at a distance. The velocity of the resulting surface waves depends on the tensile stress of the vibrated surface. By measuring the change in wave velocity, it is possible to detect the change in tensile stress and calculate the pressure through the vibrated surface. We tested this concept in an in vitro aneurysm model. Rubber tubes and explanted porcine abdominal aortas were used to model an aneurysm sac. The surface of the model was vibrated with an amplitude-modulated US beam. The resulting motion was detected either by reflected laser light or by Doppler US. The phase of the propagating wave was measured to assess changes in velocity with different pressures. Increasing hydrostatic pressure in the rubber model correlated well with the cumulative phase shift (R(2) = 0.96-0.99; P < .0001). By using a pump to generate dynamic pressure (between 110 and 200 mm Hg), the cumulative phase shift correlated well with the square of the mean pressure (R(2) = 0.92; P < .0001); however, the correlation with pulse pressure was poor (24-36 mm Hg; r = 0.38; P < .02). In the porcine in vitro aortic sac model, the cumulative phase shift detected with both laser (r = 0.94-0.99; P < .0001) and Doppler (r = 0.96-0.99; P < .0001) correlated well with the aneurysm pressure. Application of vibrometry for noninvasive measurement of aortic aneurysm sac tension is feasible in an in vitro setting. The concept of vibrometry may be used to detect endotension noninvasively after EVAR. Vibrometry may also be used to estimate wall stress in native aneurysms, and it may predict the risk of aneurysm rupture. Vibrometry may offer a technique for completely noninvasive monitoring of aneurysm sac pressure after EVAR. Vibrometry is based on the following principles: radiation pressure, such as that generated by modulated US, can induce surface vibration at a distance; by measuring the change in wave velocity of vibration, it is possible to detect changes in tensile stress and calculate the pressure through the vibrated surface. We tested this concept in an in vitro model and found that application of vibrometry for noninvasive measurement of aortic aneurysm sac tension is feasible. Vibrometry may also be used to estimate wall stress in native aneurysms.

Effects of advanced glycation end products on the proliferation and fibronectin production of smooth muscle cells.

The aim of this study was to evaluate the effects of advanced glycation end-products (AGEs) on the proliferative activity and fibronectin production of smooth muscle cells (SMCs). AGE-bovine serum albumin (AGE-BSA) was prepared by incubation with D-glucose at 37 degrees C for 60 days. Cultured SMCs were obtained from explants isolated from porcine abdominal aorta and used between passages 3 and 10. The proliferative activity of SMCs was examined by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) assay and by incorporation of 3H-thymidine into DNA. Fibronectin production was assessed by competitive ELISA assay for both fibronectin secreted into the culture medium (M-FN) and cell-associated fibronectin (C-FN), i.e., both intra- and peri-cellular fibronectin. Theassay revealed that AGE-BSA did not produce any change in optical density (A570) of SMCs at concentrations of up to 20 microg/ml, but decreased that of SMCs at a concentration of 40 microg/ml. The addition of PDGF (5 ng/ml) induced an increase in 3H-thymidine incorporation into DNA of quiescent SMCs, while the addition of AGE-BSA (20 microg/ml) had no effect. In contrast, AGE-BSA significantly increased C-FN of SMCs (30.8+/-8.58 ng/microg TP), compared to unmodified BSA (16.5+/-4.19 ng/microg TP). However, no difference in M-FN levels was observed between cells treated with AGE-BSA and unmodified BSA. The addition of anti-transforming growth factor (TGF)-beta antibody restored the levels of C-FN in SMCs cultured in 20 microg/ml of AGE-BSA, suggesting that TGF-beta might act as an intermediate factor in AGE-induced fibronectin production by SMCs. Our results suggest that interaction of AGE-modified proteins with SMCs may play a role in the development of atherosclerosis in diabetic and non-diabetic patients.

Development of Velocity Profiles and Retrograde Flow in the Porcine Abdominal Aorta under Different Haemodynamic Conditions

Low and/or oscillating wall shear stresses are related to the development of atherosclerosis and this oscillation is influenced by changes in basic haemodynamics (exercise). The objective of this study was to provide in vivo data on the development of velocity profiles and oscillating blood velocities in the abdominal aorta under varying haemodynamic conditions. Six anaesthetized, 90-kg pigs were used in the study. Abdominal aortic velocity profiles across the anterior-posterior diameter were acquired at different axial positions using 10 MHz pulsed Doppler ultrasound. Measurements were obtained under normal conditions and during cardiac pacing up to 170 beats/ min. Velocity profiles were obtained during heart rates ranging between 58 and 169 beats/min, and during flow rates ranging between 0.57 and 2.89 l/min. Main outcome measures included minimum velocities, frequency index, shape of velocity profiles (velocity distribution index), Reynolds' numbers, and Womersley's frequency parameter. Velocity profiles were blunted, with lowest velocities at the distal posterior vessel wall. Multiple regression analysis showed the development of velocity profiles to be inversely correlated with the pulsatility index, Womersley's frequency parameter and the mean Reynolds' number (r = 0.89, p < 0.0005). Minimum velocities were negatively correlated with the PI, Womersley's frequency parameter and positively with the mean Reynolds' number (r = 0.94, p < 10-8). Retrograde velocities (and hence oscillating wall shear stresses) were present at mean Reynolds' number < 1000. The oscillation of blood velocities at the wall in the porcine abdominal aorta was highly dependent on general haemodynamics (i.e. flow, heart rate and vessel diameter as expressed in the Reynolds' numbers and Womersley's frequency parameters). The velocity profiles in the abdominal aorta were found to be far from parabolic. These findings have important implications for the understanding and future modelling of the complex haemodynamics in the abdominal aorta and their relation to the development of atherosclerotic disease.