The durability of a polymer trileaflet valve is dependent on leaflet stress concentrations, so valve designs that reduce stress can, hypothetically, increase durability. Design aspects that are believed to contribute to reduced leaflet stress include stent flexibility, parabolic coaptation curvature, and leaflet anisotropy. With this in mind, the purpose of this investigation was to elucidate what specific combinations of these parameters promote optimal acute and long-term valve function. A combination of four stent designs, seven leaflet reinforcement materials, and three coaptation geometries were evaluated through a combination of experimentation and modeling. Static tensile and Poisson’s ratio tests and dynamic tensile fatigue testing were used to evaluate the individual leaflet components; and hydrodynamic testing and accelerated valve fatigue was used to assess complete valve prototypes. The two most successful designs included a 0.40 mm thick knit-reinforced valve with a fatigue life of 10.35 years, and a 0.20 mm thick knit-reinforced valve with a 28.9 mmHg decrease in pressure drop over the former. A finite element model was incorporated to verify the impact of the above-mentioned parameters on leaflet stress concentrations. Leaflet anisotropy had a large impact on stress concentrations, and matching the circumferential modulus to that of the natural valve showed the greatest benefit. Varying the radial modulus had minimal impact. Varying coaptation geometry had no impact, but stent flexibility did have a marked effect on the stress at the top of the commissure, where a completely rigid stent resulted in a higher peak stress than a flexible stent (E = 385 MPa). In conclusion, stent flexibility and leaflet anisotropy do effect stress concentrations in the SIBS trileaflet valve, but coaptation geometry does not. Regions of high stress concentrations were linked to failure locations in vitro, so a fatigue prediction model was developed from the S/N curves generated during dynamic tensile testing of the 0.20 mm knit-reinforced leaflets. Failure was predicted at approximately 400 million cycles (10 years) at the top of the commissure. In vitro fatigue of this valve showed failure initiation after approximately 167 million cycles (4.18 years), but it was related to a design defect that is subsequently being changed.

One of the major achievements in cardiac surgery over the past 30 years has been the ability to replace severely diseased heart valves with prosthetic ones. The option of using prosthetic heart valves for the treatment of valvular diseases has improved and prolonged many lives. This is reflected in around 120,000 heart valve replacement operations carried out every year in North America alone to correct the cardiovascular problems of stenosis, insufficiency, regurgitation, etc. The development of artificial heart valves depends on reliable knowledge of the hemodynamic performance and physiology of the cardiovascular system in addition to a sound understanding, at the fundamental level, of the associated fluid mechanics. It is evident from the literature review that noninvasive measurements in a confined area of complex transient geometry, providing critical information relating to valve performance, are indeed scarce. This thesis presents results of an extensive test program aimed at measuring turbulence stresses, steady and transient velocity profiles and their decay downstream of the mitral valve. Three mechanical tilting disc-type heart valves (Björk-Shiley convexo- concave, Björk-Shiley monostrut, and Bicer-Val) and two biological tissue valves (Hancock II and Carpentier-Edwards supraannular) are studied. The investigation was carried out using a sophisticated and versatile cardiac simulator in conjunction with a highly sensitive, noninvasive, two-component three-beam laser doppler anemometer system. The study covers both the steady (valve fully open) and pulsatile (resting heart rate) flow conditions. The continuous monitoring of the parametric time histories revealed useful details of the complex flow as well as helped establish location and timing of the peak parameter values. In addition, orientation experiments are conducted on the mechanical valves in an attempt to reduce stresses by altering the position of the major orifice. The experiments suggest correlation between high stresses and orientation. Based on the the data, the following general conclusions can be made: (i) Hemodynamic test results should be presented in nondimensional form to render them independent of test facilities, flow velocities, size of models, etc. This would facilitate comparison of results by different investigators, using different facilities and test conditions. (ii) The valves tested showed very disturbed flow fields which generated high turbulent stresses presenting a possibility of thromboembolism and, perhaps, haemolysis. (iii) Implantation orientation of the valve significantly affect the mechanical prostheses flow field. The single vortex formation in the posterior orientation results in a reduction in stresses compared to the anterior configuration. (iv) The present results together with the earlier information on pressure drop and regurgitation provide a comprehensive and organized picture of the valve performance. (v) The information is fundamental to the improvement in valve design, and development of guidelines for test methodology and acceptable performance criteria for marketing of the valves. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

Left ventricular flows in the human heart are very complex and in the presence of a diseased condition, such as unhealthy or prosthetic heart valves, the complexity of the flow is further increased. The intricacy of the heart geometry combined with the pulsatile character of the flow, the interaction of high-speed jets with the flexible walls, and the unsteady motion of the heart valve leaflets generate inherently complicated flow fields. It is therefore essential that we study and understand the complex cardiac energetics and physics of blood flow in both healthy and diseased hearts. Although artificial heart valves, mechanical and biological, have evolved to a level of universal acceptance, they have never reached a level of performance comparable to that of the natural valves of the heart. Many of the problems are directly related to the fluid mechanics. Considering that mechanical heart valves (MHV) are more commonly implanted because of their durability, it is imperative to better understand their hemodynamic behavior. Yet to date, no study has documented in depth the complex hemodynamic characteristics of left ventricular flows and assessed the intricate structures that are generated in the left ventricle (LV) due to vortex formation (roll-up of shear layers shed past the valve leaflets), turbulence characteristics, and energetics. The flow through pivoted leaflets of MHVs induces a combination of flow characteristics that are dependent on the specific valve design and orientation. The aim of the present study is to provide new insight into the spatio-temporal dynamics of the flow distal to a mitral MHV by employing a state-of-the-art, high resolution, flow diagnostic method, Time Resolved Digital Particle Image Velocimetry (TRDPIV) in a flexible, transparent LV documenting the evolution of eddies and turbulence during a complete period of the heart cycle. The broad impact of the proposed research extends beyond the hemodynamics of heart valve prosthesis. The research herein will enable the development of a tool for application in all cardiac energetic studies (unhealthy valves, tissue engineered valves, cardiac remodeling stages, and even congestive heart failure) and aid in better diagnosis of the efficiency and performance of the heart. The last component of the dissertation involved the translation of my dissertation research into an engineering educational tool for undergraduate engineering students. / Ph. D.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2005. / This study involved the modeling and verification of the finger follower valve train arrangement with the focus on the non-linear behaviour of high speed valve springs. The project was divided into three phases namely; the measuring of valve train dynamics, modeling of the valve train and the verification of the dynamic models by comparing the results from the two aforementioned two phases. Acceleration and force were measured on a running engine. A force transducer was specially developed for this purpose. Digital signal processing was used in the analysis and implementation of all measured data. The spring model was developed systematically from a solid model, into a finite element model, and finally into a dynamic model. All development steps were continually checked with experiments and calculations. The primary concept used in the spring modeling lends itself to modal analysis theory in conjunction with the superimposing of non-linearities onto a linear model. The dynamic model was verified and good correlations were found, especially at high engine speeds where valve train dynamics play an important role. Parameter adjustments could be made in the dynamic model and the effect that some engine mechanisms, such as engine oil aeration, had on the valve train dynamics were identified. The project was concluded with a case study of a cam profile optimisation project. Dynamic problems were found that would normally not have been identified without the dynamic model.

Conventional echocardiographic parameters are currently used in determining the timing for surgery in patients with mitral regurgitation. Since brain natriuretic peptide (BNP) rises in response to ventricular muscle stretch, and is to detect early heart failure, we hypothesized that BNP would be activated in patients with regurgitant valvular heart disease and concomitant left ventricular dilatation. Aim/Objectives: We therefore studied the pattern of changes in NT-pro BNP in patients with chronic severe rheumatic mitral regurgitation who were undergoing mitral valve replacement and compared this with the newer modality of tissue Doppler imaging (TDI). Setting: Patients submitted to surgery were prospectively evaluated over 8 months at Inkosi Albert Luthuli Central Hospital, Department of Cardiology. Controls were obtained from the outpatients' follow-up clinic. Methods: Simultaneous quantification of the severity of mitral regurgitation (MR), left ventricular (LV) end systolic volume (ESV), left atrial (LA) volume and Doppler filling ratios (mitral (E)/annulus (Ea)) were performed at baseline in all patients and was repeated at 1-week and at the six-week follow-up visit in surgical patients. Results: Both groups were similar for age and gender and echo-Doppler parameters in all patients preoperatively except LA size (p< 0.01) and volume (p<0.004) which were more elevated in the surgical group. Mean NT-pro BNP levels were markedly elevated preoperatively (262 pmolll) in all surgical cases compared to controls (57 pmol/l; p=0.0001). NT-pro BNP levels increased further at one week post surgery (395 pmol/l) and subsided at the six week follow-up visit (94 pmol/I). These changes were accompanied by significant reduction in LA (p= 0.003) and LV chamber dimensions (EDD = 0.004) with an increase in the ejection fraction from 42% at one week to 52 % at six weeks. Four patients had abnormally elevated NT-pro BNP levels (>53pmol/l) at the 6-week follow-up visit. A ROC curve was constructed for all variables to separate surgical cases from controls. The area under the curve was highest for NT-pro BNP (sensitivity= 96%, specificity 45 %). Conclusion: 1. There was a significant difference in the left atrial chamber size and volume, as well as Em/Ea (TDI) and NT-proBNP levels preoperatively between the two groups. The lack of a significant difference in the LV parameters between surgical and control groups suggest an almost total reliance on symptoms in deciding the timing of surgery which was reflected by markedly elevated NT-pro BNP in all surgical patients. 2. Postoperatively, there was a significant reduction in LA and LV dimensions. 3. The high false positivity rate for NT-pro BNP suggests that the test is most likely reflecting early LV decompensation in the less symptomatic control patients who rightly need surgery. 4. Tissue Doppler indices had similar sensitivity but low specificity compared to NT-proBNP. 5. Serial estimations of NT-pro BNP may prove useful in selecting patients for surgery. / Thesis (M.Med.Sc.)-University of KwaZulu-Natal, 2007.

No description available.

Cardiac valves are dynamic, sophisticated structures which interact closely with the surrounding hemodynamic environment. Altered mechanical stresses, including pressure, shear and bending stresses, are believed to cause changes in valve biology, but the cellular and molecular events involved in these processes are not well characterized. Therefore, the overall goal of this project is to determine the effects of pressure and shear stress on porcine aortic valve leaflets biology. Results from the pressure study showed that elevated constant pressure (140 and 170 mmHg) causes significant increases in collagen synthesis. The increases were 37.5% and 90% for 140 and 170 mmHg, respectively. No significant differences in DNA and sGAG synthesis were observed under constant pressure. In the cyclic pressure study, the effects of both pressure magnitude and pulse frequency were studied. With the frequency fixed at 1.167 Hz, collagen and sGAG synthesis increased proportionally with mean pressure level. At a fixed pressure level (80-120 mmHg), collagen and sGAG synthesis were slightly increased by 25% and 14% at 0.5 Hz, respectively. DNA synthesis was significantly increased by 72% at 2 Hz. An experiment combining high magnitude (150-190 mmHg) and high frequency (2 Hz) demonstrated significant increases in collagen and sGAG synthesis (collagen: 74%, sGAG: 56%), but no significant changes in cell proliferation. Shear levels ranging from 1 to 80 dyne/cm2 were studied. Scanning electron microscopy results indicated that 48 hrs exposure to shear stress did not alter the circumferential alignment of endothelial cells. Collagen synthesis was significantly enhanced at 9 and 25 dyne/cm2, but not different from static controls under other shear conditions. Leaflets denuded of the endothelium were exposed to identical shear stress and showed very different responses. Collagen synthesis was not affected at any shear levels, but sGAG content was increased at shear of 9, 25 and 40 dyne/cm2. Further studies showed that the increases in collagen synthesis under pressure or shear stress was concurrent with a decline in the expression and activities of cathepsins L and S. This converse relationship between collagen synthesis and cathepsin activity indicated that cathepsins might be involved in valvular ECM remodeling.

Thesis (M.S.)--University of Melbourne, Dept. of Surgery (St.Vincent's Hospital),Faculty of Medicine, Dentistry and Health Sciences, 2006. / Typescript. Includes bibliographical references (leaves 118-141).

Valve wear has been a serious problem to engine designers and manufacturers for many years. Although new valve materials and production techniques are constantly being developed, these advances have been outpaced by demands for increased engine performance. The drive for reduced oil consumption and exhaust emissions, the phasing out of leaded petrol, reductions in the sulphur content of diesel fuel and the introduction of alternative fuels such as gas all have implications for valve and seat insert wear. The aim of the project has been, through the use of a representative bench test and engine testing, to diagnose the predominant wear mechanisms in diesel engine inlet valves and seats. This information was then to be used with other test data to develop a model for predicting valve recession and other tools to assist in solving valve failure problems. Test apparatus has been developed that is capable of providing a simulation of the wear of both inlet valves and seats used in automotive diesel engines. Investigations carried out using the apparatus have shown that the valve and seat wear problem involves two distinct mechanisms; impact of the valve on the seat insert on valve closure and sliding of the valve on the seat under the action of the combustion pressure. Wear has been shown to increase with valve closing velocity, combustion load and misalignment of the valve relative to the seat. Lubrication of the valve/seat interface leads to a significant reduction in valve recession. Valve rotation ensures even wear and promotes debris removal from the valve/seat interface. During testing it was established that resistance to impact was the key seat material property determining the amount of recession that occurred. A semi-empirical wear model for predicting valve recession has been developed based on the fundamental mechanisms of wear determined during test work. Model predictions were compared with engine tests and tests run on the bench test-rig. The model can be used to give a quantitative prediction of the valve recession to be expected with a particular material pair or a qualitative assessment of how parameters need to be altered in order to reduce recession. Flow charts have also been developed, based on the review of literature, failure analysis and modelling carried out, to assist in diagnosing and rectifying valve/seat failures and to help in reducing valve recession by design. The test apparatus, valve recession model and design tools can be integrated into an industrial environment in order to help reduce costs and timescales involved in solving valve/seat wear problems using the current trial and error methods.

No description available.