The aim of this study was to characterize the nonlinear anisotropic elastic behavior of healthy porcine coronary arteries under uniaxial and equibiaxial tension. Porcine coronary tissue was chosen for its availability and similarity to human arterial tissue. A biaxial test device previously used to test human femoral arterial tissue samples (Prendergast, P. J., C. Lally, S. Daly, A. J. Reid, T. C. Lee, D. Quinn, and F. Dolan. ASME J. Biomech. Eng., Vol. 125, pp. 692-699, 2003) was further developed to test porcine coronary tissue specimens. The device applies an equal force to the four sides of a square specimen and therefore creates a biaxial stretch that demonstrates the anisotropy of arterial tissue. The nonlinear elastic behavior was marked in both uniaxial and biaxial tests. The tissue demonstrated higher stiffness in the circumferential direction in four out of eight cases subjected to biaxial tension. Even though anisotropy is demonstrated it is proposed that an isotropic hyperelastic model may adequately represent the properties of an artery, provided that an axial stretch is applied to the vessel to simulate the in vivo longitudinal tethering on the vessel. Isotropic hyperelastic models based on the Mooney-Rivlin constitutive equation were derived from the test data by averaging the longitudinal and circumferential equibiaxial data. Three different hyperelastic models were established to represent the test specimens that exhibited a high stiffness, an average stiffness, and a low stiffness response; these three models allow the analyst to account for the variability in the arterial tissue mechanical properties. These models, which take account of the nonlinear elastic behavior of coronary tissue, may be implemented in finite element models and used to carry out preclinical tests of intravascular devices. The errors associated with the hyperelastic models when fitting to both the uniaxial and equibiaxial data for the low stiffness, average stiffness, and high stiffness models were found to be 0.836, 5.206, and 2.980, respectively.
The effectiveness of a cardiovascular stent depends on many factors, such as its ability to sustain the compression applied by the vessel wall, minimal longitudinal contraction when it is expanded, and its ability to flex when navigating tortuous blood vessels. The long-term reaction of the tissue to the stent is also device dependant; in particular some designs provoke in-stent restenosis (i.e., regrowth of the occlusion around the stent). The mechanism of restenosis is thought to involve injury or damage to the vessel wall due to the high stresses generated around the stent when it expands. Because of this, the deflection of the tissue between the struts of the stent (called prolapse or "draping") has been used as a measure of the potential of a stent to cause restenosis. In this paper, uniaxial and biaxial experiments on human femoral artery and porcine aortic vascular tissue are used to develop a hyperelastic constitive model of vascular tissue suitable for implementation in finite-element analysis. To analyze prolapse, four stent designs (BeStent 2, Medtronic AVE; NIROYAL, Boston Scientific; VELOCITY, Cordis; TETRA, Guidant) were expanded in vitro to determine their repeating-unit dimensions. This geometric data was used to generate a finite element model of the vascular tissue supported within a repeating-unit of the stent. Under a pressure of 450 mm Hg (representing the radial compression of the vessel wall), maximum radial deflection of 0.253 mm, 0.279 mm, 0.348 mm and 0.48 mm were calculated for each of the four stents. Stresses in the vascular wall were highest for the VELOCITY stent. The method is proposed as a way to compare stents relative to their potential for restenosis and as a basis for a biomechanical design of a stent repeating-unit that would minimize restenosis.
Endothelial cells at the interface between the bloodstream and the vessel wall are continuously subjected to mechanical stimulation in vivo, and it widely recognised that such stimulation plays an important role in cardiovascular physiology. Cell deformation is induced by mechanical forces such as cyclic stretch, fluid shear stress, and transmural pressure. Although much of the work in this field has dealt with the effect of fluid shear stress, very little is known about how cyclic forces modulate and alter the morphology of single endothelial cells, and thereafter, how they effect the confluent layer of endothelial cells lining the vessel wall. The aim of this study is to investigate the response of endothelial cells when subjected to substrate deformation of similar magnitude to those experienced in vivo. Human umbilical vein endothelial cells (HUVEC) were cultured on plasma-treated silicone strips and uni-axially cyclically stretched using a custom made mechanical device. Results showed that endothelial cells subject to 10% deformation for as little as 4 h reoriented perpendicular to the stretch direction. In addition, although no integrin coating was applied to the substrate, it was found that plasma-treated silicone provided a cell adhesion substrate comparable to the commonly used collagen type I. Thus the results show that the stretch stimulus alone affects the morphology of endothelial cells. Further studies are required to establish the relative importance of substrate strain vs. fluid flow stimuli.
ObjectivesTo determine the incidence of 'burnout' among UK and Irish urological consultants and non-consultant hospital doctors (NCHDs). The second objective was to identify possible causative factors and to investigate the impact of various vocational stressors that urologists face in their day-to-day work and to establish whether these correlate with burnout. The third objective was to develop a new questionnaire to complement the Maslach Burnout Inventory (MBI), more specific to urologists as distinct from other surgical/medical specialties, and to use this in addition to the MBI to determine if there is a requirement to develop effective preventative measures for stress in the work place, and develop targeted remedial measures when individuals are affected by burnout. Subjects and methodsA joint collaboration was carried out between the Irish Society of Urology (ISU) and the British Association of Urological Surgeons (BAUS). Anonymous voluntary questionnaires were sent to all current registered members of both governing bodies. The questionnaire comprised two parts: the first part encompassed sociodemographic data collection and identifying potential risk factors for burnout, and the second used the MBI to objectively assess for workplace burnout. To evaluate differences in burnout, 2 9 2 contingency tables and Fischer's exact probability tests were used. ResultsIn all, 575 urologists responded to the online survey out of a total of 1380 invites, yielding a 42% response rate. All respondents were aged <75 years (median age 45 years), with men representing 87.5% of respondents. In all, 75% of respondents worked in England, followed by the Republic of Ireland (9%), Scotland (8%), Northern Ireland (4%), and Wales (3%). In all, 79% of respondents were consultants, with 13% representing training posts, and 40% of respondents held a professorship/clinical lead position. Respondents' countries of origin included England, Scotland, Ireland, India, Wales, Malaysia, Pakistan and Sri Lanka. Overall, the mean emotion exhaustion (EE) score was 23.5, representing a moderate level of EE. The mean depersonalisation (DP) score was 8.2, representing a moderate level of DP. The mean personal achievement (PA) score was 17.1, representing high levels of PA. In all, 86 respondents (15%) reported self-medication with non-prescription drugs or alcohol to combat signs and symptoms of burnout, while 46 (8%) sought professional help for symptoms of burnout. In all, 460 respondents (80%) felt that burnout should be evaluated amongst members of the ISU/BAUS, and 345 (60%) would avail of counselling if provided. ConclusionsThis is the first study to address the issue of burnout across two separate health systems in the UK and Ireland. This study has shown previously undescribed high levels of burnout characterised by EE and DP, with associated significant levels of self-medication amongst a malepredominant cohort. Burnout was attributed to non-surgical administrative/institutional factors, with most respondents reporting support for staf...
Interventions for preventing the spread of infestation in close contacts of people with scabies.
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