Browsing by Author "Zilla, Peter"

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    Bioprosthetic heart valves : ultrastructure and calcification
    (1998) Zhang, Yinxing; Zilla, Peter
    Background: Due to the geographic distance between abattoirs and commercial valve plants delays between harvest and fixation usually range from 48 to 72 hours. In order to assess the pre-fixation tissue damage arising from the hypoxic period and the resulting calcific degeneration after implantation, we used an ultrastructural damage score and transmission electron microscopy. Materials and Methods: In a step by step manner, three major issues were clarified: 1) The degree of pre-fixation tissue damage was determined in the four most widely used commercially produced tissue heart valves. Since stentless bioprostheses represent the latest promising trend in the development of biological heart valves, stentless models of the following makes were compared: Baxter, Medtronic, St. Jude and Biocor. Due to the fact that the aortic wall component of these valves proved most resistant to all anticalcification treatments, aortic wall tissue stood in the centre of our analyses. 2) Subsequently, three main determinants of the fixation process namely: delay, temperature and fixative-concentration were varied with the goal of significantly improving the ultrastructural preservation of the bioprosthetic tissue. 3) Eventually, the influence of improved ultrastructural preservation on calcific degeneration was evaluated under in vivo conditions in the non-human primate and the rat model. Results: The comparison of the four most commonly used stentless bioprosthetic heart valves revealed a disturbing degree of tissue damage in all valves. Using a damage score from 1 to 21 (21 being the worst), aortic wall tissue of commercial valves ranged from 10 to 18 and that of leaflet tissue from 12 to 20. When fixation conditions were permutated, tissue damage could almost be abolished by immediate fixation (within 30 minutes of slaughter), low-temperature fixation(4°C) and high glutaraldehyde concentrations (> 1 %). Our in vivo experiments confirmed that commercially used fixation (delayed fixation, room-temperature and I ow concentrations of glutaraldehyde) with its concomitant high degree of tissue damage results in high levels of calcification. Apart from a distinctly improved calcification potential in ultrastructurally well preserved tissue, there was also an inverse correlation between tissue calcification and the concentration of glutaraldehyde used for fixation. Conclusion: We could demonstrate that commercially produced bioprosthetic heart valves uniformly show badly damaged tissue and that tissue damage contributes to the calcific degeneration of these valves. We were also able to determine ideal fixation conditions which in turn significantly reduced tissue calcification.
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    Cyclic stretch-mediated release of vascular endothelial growth factor by vascular smooth muscle cells : a role in improved vascular graft patency
    (1999) Smith, James Douglas; Zilla, Peter
    In the light of studies which show the upregulation of VEGF in contractile cells subjected to cyclic stretch and the profound effects which cyclic stretch has been shown to have on the release of other cytokines by SMC, this study investigates the role which cyclic stretch might play in VEGF expression by SMC in a compliant environment. Furthermore, following observations of receptor phosphorylation in response to cyclic stretch in vascular cells, the effect of cyclic strain on the KDR-mediated endothelial response to locally-released VEGF was also investigated. Low passage number bovine aortic SMC and EC were plated on collagen-coated elastomer plates and subjected to 10% repetitive strain at 1 Hz. The mRNA expression of VEGF in SMC and the phosphorylation of KDR on EC were determined by northern blotting and western blotting respectively. The biological activity on EC and levels of VEGF secreted into the medium by SMC under cyclic stretch were investigated using a migration assay and ELISA respectively. Cyclic stretch was found to cause a 3.3 (±1.5 p < 0.005) fold increase in VEGF mRNA levels over unstretched controls at 4 hours. This biomechanically-induced expression was found to drop slightly by 24 hours and to be approximately equivalent to expression induced by the cytokine bFGF over the same time course. These results correlated with an increase in VEGF levels in media from stretched SMC capable of inducing migration of EC by 1.6 fold although additional EC chemotactic factors appear to be released by stretch. Furthermore, although the levels of KDR remained constant under cyclic stretch, average KDR phosphorylation was found to increase weakly over time due to cyclic stretch. These results show that cyclic stretch affects the VEGF communication between SMC and EC at both the level of VEGF expression by SMC and at the level of VEGF recognition by the KDR receptor on EC. It is possible that through the nitric oxide (NO) pathway, VEGF release may alleviate abnormally high levels of cyclic strain. It is hoped that a better understanding of the role of VEGF communication between stretched SMC and EC will enable the design of a graft in which the level of compliance encourages SMC to maintain a functional endothelium. Following this it is hoped that the low levels of SMC and pericytes invading the graft, pacified by endothelial cell mediation, will not result in intimal hyperplasia but rather play a role in microvessel maintenance and more complete healing.
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    Design of an adventitial type reinforcement of prosthetic vascular grafts through mechanically affirmed material and structure modulation
    (2001) Millam, Ross David Alexander; Zilla, Peter; Bezuidenhout, Deon
    The high occurrence of vascular disease in the 20th century has been the driving force for researchers to produce a successful small diameter synthetic graft. Large diameter synthetic grafts remain patent for extended periods due to high flow rate, while smaller diameter grafts occlude more readily. Mechanical property mismatch between graft and host artery has been cited as one of the major factors that contribute to graft occlusion. It has thus been important to develop a readily available graft that is accepted by the body and does not cause flow abnormalities and stress-concentrations at graft-artery junctions. The object of this project was to ascertain the effect of an adventitial reinforcement on elastic compliance of synthetic porous polyurethane grafts.
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    Development of a numerical tool for the optimisation of vascular prosthesis towards physiological compliance
    (2007) Van der Merwe, Helena; Franz, Thomas; Reddy, B Daya; Zilla, Peter
    It has been proposed that if a vascular prosthesis is to more closely approximate the mechanical behaviour of a native vessel, it should similarly feature a multi-component structure. One of the components could be a metal support structure, similar to an endovascular stent. The objective of the project was to develop a numerical tool, using the Finite Element Method (FEM) to aid in the development and optimization of such a metallic support structure. This tool was used to simulate the behaviour of different designs under the simulated in vivo conditions. The numerical results of the predicted mechanical behaviour are then analysed.
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    Isolating and optimising transmural endothelialization as an independent mode of spontaneous vascular graft healing
    (2014) Pennel, Timothy Charles; Bezuidenhout, Deon; Zilla, Peter
    Background: Poorly designed animal models with short, non-isolated grafts have led researchers to exclusively investigate transanastomotic endothelialization(TAE), which is known to be limited to the perianastomotic region in humans. Alternative spontaneous forms of endothelialization (fallout and transmural) need to be independently investigated and optimised to redirect future conduit research. Methods: All implants were performed in infra-renal rat aorta. TAE was determined at 2, 4, and 6 weeks (n=6/time-point) (ePTFE;ID1.7mm;IND15-25µm). High-porosity polyurethane (internal diameter 1.7mm- 150mm pore size) grafts were interposed between ePTFE grafts for 2, 4, 6, and 8 weeks (n=6/time point). Grafts were looped to increase their length and were implanted for 6, 8, 12, and 24 weeks (n=8/time-point). Perigraft isolation included (PU skin, 50µm, or 350µm wall thickness wrap, (IND30µm;ePTFE) to prevent transmural ingrowth; 12-weeks. The mid-graft was further surface-treated with heparin or heparin-VEGF/PDGF and implanted for 3 weeks (n=10/group). Explanted samples were analysed by light, immunofluorescence and scanning electron microscopy for vascularization and endothelialization. Results: TAE occurred at 0.6±0.4mm/wk., which slowed from 0.8±0.4 to 0.5±0.3mm/wk., [p=0.039] from 2 to 6 weeks. Straight composite grafts separation zones were too short to isolate transmural ingrowth beyond four weeks. However, a broad endothelial-free zone was preserved in all looped composite grafts (23.6±10.1, 23.7±8.2, 13.4±11.0, 10.5±5.6mm for 6, 8, 12, and 24 weeks respectively), [p=0.0031]. Mid-graft pre-confluence was reached by 6 weeks (55±45%) and confluence between week 12 and 24 (95.0±10.0% and 84.0±30.13%). The subintimal thickness did not increase (91.8±93.9 mm vs. 71.4±59.4 mm at 6 and 24 weeks, respectively; NS). All sealed grafts occluded. Mid-graft endothelialization was not influenced by 50µmFilmwrap (93±8.3%), but the 350µm-Wrap significantly reduced coverage (16±30%, pre Conclusions: Transmural endothelialization can be clearly distinguished from TAE in a high throughput rat model. A looped interposition graft model provides sufficient isolation length to separate the two events for up to half a year without an increase in intimal hyperplasia. Growth factor surface modification further enhances this form of healing, while perigraft isolation confirms that fallout endothelialization is insufficient to heal the mid-graft with confluence. These findings may be useful for the development of clinical peripheral vascular (including endoluminal) grafts, as TM endothelialization remains a viable healing mode in humans.
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    Long term outcome and the validity of EuroSCORE II in native-valve surgery for active endocarditis in a South African cohort
    (2015) Koshy, Jithan Jacob; Zilla, Peter; Brink, Johan G; Engel, Mark E
    Infective endocarditis was initially described in the early 16th century and only methodically reviewed after the 19th century when Osler gave the drive to the Royal College of Physicians in 1885 through his contribution. The last 25 years has not shown much change in the mortality from infective endocarditis (IE) despite diagnostic and therapeutic advances. The current in-hospital mortality rate for patients with IE is 15% to 20%, with 1-year mortality approaching 40%. The morbidity associated with infective endocarditis includes valvular incompetence, embolization, cerebrovascular accidents and congestive heart failure and this has influenced the surgical options to a great extent. The EuroSCORE II is the current model available for predicting the early mortality after cardiac surgery. HYPOTHESIS: Infective endocarditis has a high risk for mortality due to certain risk factors and the currently available EuroSCORE II model may not predict early mortality accurately and may not be suitable for our patient population. OBJECTIVES: To evaluate the major risk factors for adverse short and long term outcomes in patients with active native valve infective endocarditis needing cardiac surgery, and to validate the EuroSCORE II in our cohort of patients. PATIENTS AND METHODS: A retrospective review will be undertaken on patients with infective endocarditis requiring cardiac surgery from 2000-2012 at the Christian Barnard Division of Cardiothoracic surgery (Groote Schuur Hospital, UCT Private Academic Hospital) and follow-up with respect to mortality, re-operation and major adverse cardiac events, as well as an evaluation of the validity of the EuroSCORE II. DATA COLLECTION AND ANALYSIS: The standardized data extraction form in the appendix will be used for extracting data from various databases and telephonic interviews. Data will be analyzed using STATA to determine the most significant predictors of adverse outcome and conducting Kaplan Meier actuarial analysis for early and late survival and freedom from adverse events. The EuroSCORE II will be evaluated and validated to our cohort of patients.
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    Mitral valve replacement for rheumatic heart disease in Southern Africa
    (BioMed Central Ltd, 2013) Zilla, Peter; Koshy, J; Brink, J; Human, P
    Background: Threshold countries like South Africa provide cardiac surgery to a largely indigent population with rheumatic heart disease. Although repairs are a preferred treatment modality many rheumatic mitral valves can only be replaced. In view of significantly improved primary health care and broad access of the indigent population to communication technology we revisited the efficacy of mitral valve replacement (MVR) at the interface of the developing and developed world. Methods: A cohort of 280 patients (mean age 40.7±13.7y/range 12-80y/median 41y; 76.4% female) with rheumatic heart disease (21% MR; 11% MS; 68% mixed) undergoing mitral valve replacement (MVR) (88.2% mechanical versus 11.8% tissue valves) was analyzed.
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    The performance of cross-linked acellular arterial scaffolds as vascular grafts; pre-clinical testing in direct and isolation loop circulatory models
    (2016) Pennel, Timothy; Bezuidenhout, Deon; Zilla, Peter
    There is a significant need for small diameter vascular grafts to be used in peripheral vascular surgery; however autologous grafts are not always available, synthetic grafts perform poorly and allografts and xenografts degenerate, dilate and calcify after implantation. We hypothesized that chemical stabilization of acellular xenogenic arteries would generate off-the-shelf grafts resistant to thrombosis, dilatation and calcification. To test this hypothesis, we decellularized porcine renal arteries, stabilized elastin with pentagalloyl glucose and collagen with carbodiimide/activated heparin and implanted them as trans- position grafts in the abdominal aorta of rats as direct implants and separately as indirect, isolation-loop implants. All implants resulted in high patency and animal survival rates, ubiquitous encapsulation within a vascularized collagenous capsule, and exhibited lack of lumen thrombogenicity and no graft wall calcification. Peri-anastomotic neo-intimal tissue overgrowth was a normal occurrence in direct implants; however this reaction was circumvented in indirect implants. Notably, implantation of non- treated control scaffolds exhibited marked graft dilatation and elastin degeneration; however PGG significantly reduced elastin degradation and prevented aneurismal dilatation of vascular grafts. Overall these results point to the outstanding potential of crosslinked arterial scaffolds as small diameter vascular grafts.
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    The regulation of matrix metalloproteinase (MMP) secretion in human vascular cells exposed to extracellular and wound matrices
    (2002) Bracher, Mona; Davies, Neil; Zilla, Peter
    Proteolytic degradation is a central feature of the cellular remodelling in wounds during healing. The regulation of matrix metalloproteinases (a key superfamily of proteases involved in the above process) by wound healing components is an area of ongoing investigation. However, a systematic study of the exposure of the human derived vascular cells - endothelial cells, fibroblasts, smooth muscle cells and macrophages - to well defined ECM (collagen type I, III and IV and laminin) and wound healing matrix proteins (fibrin and plasma fibronectin) has not yet beeen carried out.
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    Ten-year propensity matched cohort analysis of mitral valve repair and replacement for rheumatic heart disease at Groote Schuur Hospital
    (2011) Geldenhuys, Agneta; Zilla, Peter
    Includes abstract. Includes bibliographical references.
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    Ultrasonography Evaluation of Patency of Implanted Infra-Renal Vascular Grafts in the Rat Model.
    (2020) Da Silva, Natercia; Pennel, Timothy; Bezuidenhout, Deon; Hadebe, Nkanyiso; Zilla, Peter
    Introduction: Intensive research over the last six decades has resulted in minimal improvement in vascular graft development. Small animal models are the first line of species exposed to vascular graft implantation and invasive monitoring of experimental graft patency may contribute to pain, suffering, higher cost and earlier sacrifice. Non-invasive ultrasonographic evaluation of vascular implants during the conduction of animal studies allows for chronic follow-up with multiple assessments. This study aims to apply and endorse the utilization of ultrasound as a less invasive diagnostic method in determining patency of vascular grafts in units where imaging modalities like Computerized Tomography (CT) and Magnetic Resonance Imaging (MRI) are not readily available. Methods: Pre-operative control ultrasound evaluation of the ejection fraction, aortic diameter and aortic velocity were conducted on Wistar rats (250-350g). Infra-renal aortic vascular graft implantation was then performed, with 8 rats receiving straight (1.8mm ID, 18mm length) expanded polytetrafluoroethylene (ePTFE) grafts, while 12 rats received a long (1.8mm ID, 100mm length) looped ePTFE conduit with a sealed mid-graft (10mm length) section. Ultrasonography was conducted on days 1, 3, 7 and weeks 4, 8 and 12 post operatively. Grafts were explanted if there was any ultrasonographic evidence of occlusion or at twelveweek termination of the study. Explant was preceded by angiography and followed by histological assessment of the grafts for patency. Results: Three of the looped and all 8 of the straight grafts were patent at the 12 week explant time point, as correctly assessed by ultrasound and confirmed by angiography and histology. Three of the nine occluded looped grafts were explanted at eight weeks due to early ultrasonographic detection of occlusion; the remaining 6 were explanted at twelve weeks. There were two false positive results, which were incorrectly assessed as patent at twelve weeks of implantation on ultrasonographic evaluation, but confirmed to be occluded on angiography at explant. The results of ultrasonography evaluation of implanted infra-renal vascular grafts had a high specificity of 100% with a sensitivity of 78%. The outcome of the results between ultrasound and angiography corresponded in 18 out of 20 vascular grafts, with a calculated positive predictive value (PPV) of 100% and a negative predictive value (NPV) of 85%. 4 Conclusion: Ultrasound is easily available and a non-invasive diagnostic modality allowing for safe and reliable results, which may be repeated at different time frames following vascular implants in small animal models. Ultrasonographic limitations exist, emphasizing the need for an experienced operator with adequate knowledge and training. Its use may be complicated by tortuous geometries of vessels, which is technically more challenging to evaluate with ultrasound than with imaging techniques like CT and MRI. It does, however, add information without additional loss of life or increased use of animal numbers. Ultrasound is an essential additive diagnostic tool for chronic follow-up and evaluation of vascular graft implants.