21,772 research outputs found
Sixteen year retrospective analysis of rheumatic and non-rheumatic heart disease patients undergoing valve procedures at Groote Schuur Hospital first incidence single aortic and mitral valve replacement
No full textIncludes abstract.Includes bibliographical references.Rheumatic heart disease is still the most common cause of valvular heart lesions requiring replacement or repair procedures worldwide. In South Africa, where there is an interesting mix of first and third world dynamics, factors sustaining the epidemic of rheumatic disease are still commonplace. The choice of appropriate valve procedure and prosthesis in our setting will depend on an adequate knowledge of short and long term outcomes of valve replacement and repair. The aim of this thesis was to evaluate the demographics and presentation of our rheumatic and non-rheumatic patients and to determine if our current implantation choices could be validated
Non-linear finite element analyses of the aortic heart valve
No full textIncludes bibliographical references.Finite element models of the aortic heart valve have been successfully used in the past to gain insight into the mechanics of the valve and to aid in understanding of valve failure. Moreover such models are indispensable tools for further developments in heart valve prosthetic design. In previous stress analyses linear elastic constitutive models have predominantly been used to model aortic valve leaflets, despite aortic valve tissue showing highly non-linear behaviour in tension tests. In view of recent developments towards tissue engineering of heart valves, these linear constitutive models of aortic valve leaflets are not likely to produce results sufficiently accurate to correlate cell behaviour with mechanical stimuli. To study how non-linear material behaviour affects the results of stress analyses of the aortic valve, static finite element analyses of the valve including the aortic root and leaflets have been carried out. An isotropic linear elastic material model was assigned to the aortic root with Young's modulus adjusted for the simulated compliance to match physiological values. Linear elastic models for the aortic valve leaflets with parameters used in previous studies were then compared with hyperelastic materials. The parameters used for the exponential strain energy function of the latter were obtained from fits of uniaxial tension test results of fresh porcine aortic valve leaflets. As natural leaflets show anisotropy with a pronounced stiff direction along the circumference of the valve, isotropic models of the leaflets were extended to account for this behaviour by incorporating transverse isotropy. The results display a stark impact of a transversely isotropic hyperelastic material on leaflet mechanics, Le. increased coaptation with peak values of stress and strain in the elastic limit. Interestingly, the alignment of maximum principal stress of all models seems to approximately follow the coarse collagen fibre distribution found ill aortic valve leaflets
Transcatheter Aortic Valve Implantation in Dialysis Patients
No full textBackground/Aims: Transcatheter aortic valve implantation (TAVI) has emerged as a new therapeutic option for high-risk patients. However, dialysis patients were excluded from all previous studies. The aim of this study is to compare the outcomes of TAVI for dialysis patients with those for patients with chronic kidney disease (CKD) stages 3 and 4 and to compare TAVI with open surgery in dialysis patients. Methods: Part I: comparison of 10 patients on chronic hemodialysis with 116 patients with non-dialysis-dependent CKD undergoing TAVI. Part II: comparison of transcatheter (n = 15) with open surgical (n = 24) aortic valve replacement in dialysis patients. Results: Part I: dialysis patients were significantly younger (72.3 vs. 82.0 years; p < 0.01). Hospital stay was significantly longer in dialysis patients (21.8 vs. 12.1 days; p = 0.01). Overall 30-day mortality was 3.17%, with no deaths among dialysis patients. Six-month survival rates were similar (log-rank p = 0.935). Part II: patient age was comparable (66.5 vs. 69.5 years; p = 0.42). Patients in the surgical group tended to stay longer in hospital than TAVI patients (29.5 vs. 22.5 days; p = 0.35). Conclusion: TAVI is a safe procedure in patients on chronic hemodialysis. Until new data become available, we find no compelling reason to refuse these patients TAVI. Copyright (C) 2012 S. Karger AG, Base
Idle Operation with Low Intake Valve Lift in a Port Fuel Injected Engine
No full textReducing fuel consumption is a prime objective in the automotive industry in order to meet regulatory and customer demands. Variable valve actuation offers many opportunities for improving the spark ignition engine’s performance in areas such as fuel economy and pollutant emissions. Our studies revealed that the ability to control maximum intake valve lift does indeed offer the ability to control intake air mass, but also has the added benefit that it improves the fuel-air mixing process thanks to an increased turbulence, caused by the increased intake flow velocity. This is particularly important at idle and low part loads when low maximum lifts are to be used for improving the fuel economy or for achieving the required power. The paper focuses on the experimental results obtained when approaching idle operation with different intake valve laws. Results indicating the potential of using low intake valve lift for fuel economy and cyclic dispersion improvement are presented in this paper
Transcatheter aortic-valve replacement for inoperable severe aortic stenosis.
No full textBACKGROUND: Transcatheter aortic-valve replacement (TAVR) is the recommended therapy for patients with severe aortic stenosis who are not suitable candidates for surgery. The outcomes beyond 1 year in such patients are not known. METHODS: We randomly assigned patients to transfemoral TAVR or to standard therapy (which often included balloon aortic valvuloplasty). Data on 2-year outcomes were analyzed. RESULTS: A total of 358 patients underwent randomization at 21 centers. The rates of death at 2 years were 43.3% in the TAVR group and 68.0% in the standard-therapy group (P<0.001), and the corresponding rates of cardiac death were 31.0% and 62.4% (P<0.001). The survival advantage associated with TAVR that was seen at 1 year remained significant among patients who survived beyond the first year (hazard ratio, 0.58; 95% confidence interval [CI], 0.36 to 0.92; P=0.02 with the use of the log-rank test). The rate of stroke was higher after TAVR than with standard therapy (13.8% vs. 5.5%, P=0.01), owing, in the first 30 days, to the occurrence of more ischemic events in the TAVR group (6.7% vs. 1.7%, P=0.02) and, beyond 30 days, to the occurrence of more hemorrhagic strokes in the TAVR group (2.2% vs. 0.6%, P=0.16). At 2 years, the rate of rehospitalization was 35.0% in the TAVR group and 72.5% in the standard-therapy group (P<0.001). TAVR, as compared with standard therapy, was also associated with improved functional status (P<0.001). The data suggest that the mortality benefit after TAVR may be limited to patients who do not have extensive coexisting conditions. Echocardiographic analysis showed a sustained increase in aortic-valve area and a decrease in aortic-valve gradient, with no worsening of paravalvular aortic regurgitation. CONCLUSIONS: Among appropriately selected patients with severe aortic stenosis who were not suitable candidates for surgery, TAVR reduced the rates of death and hospitalization, with a decrease in symptoms and an improvement in valve hemodynamics that were sustained at 2 years of follow-up. The presence of extensive coexisting conditions may attenuate the survival benefit of TAVR. (Funded by Edwards Lifesciences; ClinicalTrials.gov number, NCT00530894.)
Local versus general anesthesia for transcatheter aortic valve implantation (TAVR)- systematic review and meta-analysis
No full textThe hypothesis of this study was that local anesthesia with monitored anesthesia care (MAC) is not harmful in comparison to general anesthesia (GA) for patients undergoing Transcatheter Aortic Valve Implantation (TAVR).TAVR is a rapidly spreading treatment option for severe aortic valve stenosis. Traditionally, in most centers, this procedure is done under GA, but more recently procedures with MAC have been reported
A Valve Design Methodology For Improved Reciprocating Compressor Performance
No full textThe current work directly relates valve dynamics to the compressor energy efficiency. Majority of the previous studies have focused on reducing pressure losses due to valve geometry, towards improved compressor performance. On a complimentary note, analyzing the valve ‘flutter’ leads to a holistic valve development methodology. Traditionally, pressure actuated reed valves have been used in reciprocating hermetic compressors on the suction and discharge ports. A characteristic of these valve operation is the multiple opening and closing motions, during a single suction and discharge pulse, often referred to as the ‘valve flutter’. This is more prominent for the suction reed considering the longer (crank-angle) duration of the suction process. The valve dynamics is a highly coupled fluid structure interaction problem. In the present work, the reed valve dynamics has been simplified to a single degree of freedom spring mass system and is captured by a mathematical model within a 15% accuracy range for displacement prediction. Considering the current stage of development, this is within acceptable limits. To validate this model, in-compressor valve lift measurements (direct strain gauge measurements) in a closed loop refrigeration rig have been used. Considering the complexity and time involved with the in-compressor measurements, a simplified framework to characterize the dynamics of reed valves outside the compressor (indirect measurements) has also been proposed and developed. Since the basis of this study is analyzing the characteristic valve dynamics, physics-based transfer functions can translate these measurements to the actual compressor reed motion leading to a faster design cycle. Also, CFD has been used to provide a detailed insight to the flow physics. With all of the above inputs, the mathematical models help identifying key design parameters and help evaluate conceptual designs towards an ideal/ chosen valve response
Structure-function relationships in the aortic valve
No full textPhDGlobally, heart valve dysfunction constitutes a large portion of the cardiovascular
disease load, causing high rates of mortality in European and industrialized countries.
This is reflected in the database of the American Heart Association and the UK Valve
Registry, showing a progressive increase in the number and age of patients in need of
surgical interventions. Aortic valve (AV) dysfunction is significantly more prevalent
than pathologies associated with other heart valves, accounting for approximately
43% of all patients having valvular disease. These statistics highlight the essential
need for efficient and long term substitutes. However, the two types of replacement
valves currently available in practice, i.e. mechanical and bioprosthetic valves, have
only an estimated lifetime of around 10 years, after which the associated problems
necessitate re-operation in at least 50-60% of the patients. Moreover, for patients
under 35, the failure rate is nearly 100% within 5 years of the valve replacement
surgery. The significant numbers of patients suffering from AV dysfunction,
shortcomings to currently available valve substitutes, and the market demands for
replacement valves has prompted increasing interest in the study of AV biomechanics.A fundamental study of the AV structure-function biomechanics is presented in this
thesis. The mechanical behaviour of the AV is characterised at the tissue level, and
the associated microstructural mechanisms established. In addition to the experiments,
in depth mathematical models are developed and presented, to explain the observed
experimental data and elucidate the micromechanics of the AV constituents and their
contribution to the tissue behaviour. Tissue-level results indicate that the AV shows
‘shear-thinning’ behaviour, as well as anisotropic time-dependent characteristics. The
microstructural experimental data indicates that there is no direct translation of tissue
level mechanical stimuli to the ECM, implying that strain transfer is non-affine.
Modelling micro-structural mechanics has confirmed that collagen fibres do not need
to become fully straight before they contribute to load bearing, while the elastin
network has been shown to contribute to load bearing even at high strains, further
exacerbating the non-linear stress-strain relationship of the valve. The structural
mechanisms underlying time-dependent behaviour of the tissue can be explained at
the fibre level, stemming from fibre sliding and the dissipative effects arising due to
fibre-fibre and fibre-matrix frictional interactions, suggesting a unified structural
mechanism for both the stress-relaxation and creep phenomena. These outcomes
contribute to an improved understanding of the physiological biomechanics of the
native AV, and may therefore assist in optimising the design processes for substitute
valves and selecting appropriate materials to effectively mimic the native valve
function. Understanding AV micromechanics also helps quantify the mechanical
environment perceived by the residing cells, which can have significant implications
for cell-mediated tissue engineering strategies.EPSRC; Discipline Bridging Initiative; MR
Transcatheter versus surgical aortic-valve replacement in high-risk patients.
No full textBACKGROUND: The use of transcatheter aortic-valve replacement has been shown to reduce mortality among high-risk patients with aortic stenosis who are not candidates for surgical replacement. However, the two procedures have not been compared in a randomized trial involving high-risk patients who are still candidates for surgical replacement. METHODS: At 25 centers, we randomly assigned 699 high-risk patients with severe aortic stenosis to undergo either transcatheter aortic-valve replacement with a balloon-expandable bovine pericardial valve (either a transfemoral or a transapical approach) or surgical replacement. The primary end point was death from any cause at 1 year. The primary hypothesis was that transcatheter replacement is not inferior to surgical replacement. RESULTS: The rates of death from any cause were 3.4% in the transcatheter group and 6.5% in the surgical group at 30 days (P=0.07) and 24.2% and 26.8%, respectively, at 1 year (P=0.44), a reduction of 2.6 percentage points in the transcatheter group (upper limit of the 95% confidence interval, 3.0 percentage points; predefined margin, 7.5 percentage points; P=0.001 for noninferiority). The rates of major stroke were 3.8% in the transcatheter group and 2.1% in the surgical group at 30 days (P=0.20) and 5.1% and 2.4%, respectively, at 1 year (P=0.07). At 30 days, major vascular complications were significantly more frequent with transcatheter replacement (11.0% vs. 3.2%, P<0.001); adverse events that were more frequent after surgical replacement included major bleeding (9.3% vs. 19.5%, P<0.001) and new-onset atrial fibrillation (8.6% vs. 16.0%, P=0.006). More patients undergoing transcatheter replacement had an improvement in symptoms at 30 days, but by 1 year, there was not a significant between-group difference. CONCLUSIONS: In high-risk patients with severe aortic stenosis, transcatheter and surgical procedures for aortic-valve replacement were associated with similar rates of survival at 1 year, although there were important differences in periprocedural risks. (Funded by Edwards Lifesciences; Clinical Trials.gov number, NCT00530894.)
Double orifice mitral valve associated with Ventricular Septal Defect in an infant : case report
No full textDouble orifice mitral valve (DOMV) is an uncommon anomaly characterized by a
mitral valve with a single fibrous annulus with two orifices opening into the left
ventricle. Here we present a rare case of DOMV with ventricular septal defect (VSD)
which was detected by 2-D echocardiography.peer-reviewe
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