Market Institutions and Quality Enforcement

A competitive market for an experience good is considered where high quality is enforced by repeated game trigger strategies. The goods are demanded by long run (LR) and short run (SR) customers, the former buying repeatedly, the latter only once. SR buyers can free ride on quality enforcement by LR buyers but, by doing so, they may prevent LR buyers from punishing firms for producing low quality. We characterize equilibria in different market institutions and show that non-exclusivity has a negative impact on quality enforcement when the market institution provides some public information. In decentralized markets with only match specific information, some firms sell high quality to LR, others low quality to SR buyers. By contrast, in auction markets where past trading prices and quantities are publicly observed, SR buyers can buy from firms with high price histories. When a firm starts to produce low quality, its LR customers migrate to other firms but the punishment probability is lowerd by sales to SR buyers. A partial market breakdown results. The outcome can be improved by coordination among LR buyers, which allows a better use of information. Auction markets with a larger information flow, produce high punishment probabilities even without coordination. Better outcomes can be obtained if firms are allowed to price discriminate between LR and SR buyers. Without discrimination or exclusivity, information spillovers from LR to SR can reduce welfare. Depending on the market institution, more public information can improve the outcome but can also make it worse.

Does low and oscillatory wall shear stress correlate spatially with early atherosclerosis? A systematic review

Low and oscillatory wall shear stress is widely assumed to play a key role in the initiation and development of atherosclerosis. Indeed, some studies have relied on the low shear theory when developing diagnostic and treatment strategies for cardiovascular disease. We wished to ascertain if this consensus is justified by published data. We performed a systematic review of papers that compare the localization of atherosclerotic lesions with the distribution of haemodynamic indicators calculated using computational fluid dynamics. The review showed that although many articles claim their results conform to the theory, it has been interpreted in different ways: a range of metrics has been used to characterize the distribution of disease, and they have been compared with a range of haemodynamic factors. Several studies, including all of those making systematic point-by-point comparisons of shear and disease, failed to find the expected relation. The various pre- and post-processing techniques used by different groups have reduced the range of shears over which correlations were sought, and in some cases are mutually incompatible. Finally, only a subset of the known patterns of disease has been investigated. The evidence for the low/oscillatory shear theory is less robust than commonly assumed. Longitudinal studies starting from the healthy state, or the collection of average flow metrics derived from large numbers of healthy vessels, both in conjunction with point-by-point comparisons using appropriate statistical techniques, will be necessary to improve our understanding of the relation between blood flow and atherogenesis

Computational fluid dynamic study on effect of Carreau-Yasuda and Newtonian blood viscosity models on hemodynamic parameters

Pulsatile blood flow through the human carotid artery is studied using Computational Fluid Dynamics (CFD) in order to investigate the effect of blood rheology on the hemodynamic parameters. The carotid artery model used is segmented and reconstructed from the Magnetic Resonance Images (MRI) of a specific patient. The results of a non-Newtonian (Carreau-Yasuda) model and a Newtonian model are studied and compared. The results are represented for each peak systole where it is observed that there is significant variation in the spatial parameters between the two models considered in the study. Comparison of local shear stress magnitude in different branches namely Common Carotid Artery (CCA), Internal Carotid Artery (ICA) and External Carotid Artery (ECA) show that the shear thinning property of blood influences the Wall Shear Stress (WSS) variation. This is observed in branches where there is reduction in diameter and where the diameter reduces due to plaque deposition and also in the region where there is flow recirculation like carotid sinus

Influences of Geometric Configurations of Bypass Grafts on Hemodynamics in End-to-Side Anastomosis

BACKGROUND: Although considerable efforts have been made to improve the graft patency in coronary artery bypass surgery, the role of biomechanical factors remains underrecognized. The aim of this study is to investigate the influences of geometric configurations of the bypass graft on hemodynamic characteristics in relation to anastomosis. MATERIALS AND METHODS: The Numerical analysis focuses on understanding the flow patterns for different values of inlet and distal diameters and graft angles. The Blood flow field is treated as a two-dimensional incompressible laminar flow. A finite volume method is adopted for discretization of the governing equations. The Carreau model is employed as a constitutive equation for blood. In an attempt to obtain the optimal aorto-coronary bypass conditions, the blood flow characteristics are analyzed using in vitro models of the end-to-side anastomotic angles of 45degrees, 60degrees and 90degrees. To find the optimal graft configurations, the mass flow rates at the outlets of the four models are compared quantitatively. RESULTS: This study finds that Model 3, whose bypass diameter is the same as the inlet diameter of the stenosed coronary artery, delivers the largest amount of blood and the least pressure drop along the arteries. CONCLUSION: Biomechanical factors are speculated to contribute to the graft patency in coronary artery bypass grafting.ope

The Hatfield-System versus the Weekly Undulating Periodised Resistance Training in trained males: Effects of a third mesocyle

We recently demonstrated that recreationally strength trained men, randomly assigned to either a Hatfield-System (HAT) group or a weekly undulating periodisation (WUP) group showed significant increases in strength and power during only 2 mesocycles (6 weeks) without differences between groups. The questions arise, whether an additional mesocycle would further enhance strength and power equally or differently between groups. The same 26 strength trained men, assigned to the HAT (n = 13; age = 26.8 ± 7.2 years) or to WUP (n = 13; age = 29.2 ± 9.0 years) performed an additional mesocycle (3 weeks). Anthropometric measures and strength testing were performed again after finishing the third mesocycle and were then compared with the results recorded after the second mesocycle. Both the HAT and WUP groups made significant (p ≤ 0.05) increases in strength and power – to approximately the same extent, again, without significant differences between groups. Thus, HAT and WUP are similarly effective over a nine-week training period, and the decision to use HAT or WUP depends on the preferences of the individual athlete

Optimisation of a novel spiral-inducing bypass graft using computational fluid dynamics

Graft failure is currently a major concern for medical practitioners in treatingPeripheral Vascular Disease (PVD) and Coronary Artery Disease (CAD). It is nowwidely accepted that unfavourable haemodynamic conditions play an essential role in the formation and development of intimal hyperplasia, which is the main cause of graft failure. This paper uses Computational Fluid Dynamics (CFD) to conduct a parametric study to enhance the design and performance of a novel prosthetic graft, which utilises internal ridge(s) to induce spiral flow. This design is primarily based on the identification of the blood flow as spiral in the whole arterial system and is believed to improve the graft longevity and patency rates at distal graft anastomoses. Four different design parameters were assessed in this work and the trailing edge orientation of the ridge was identified as the most important parameter to induce physiological swirling flow, while the height of the ridge also significantly contributed to the enhanced performance of this type of graft. Building on these conclusions, an enhanced configuration of spiral graft is proposed and compared against conventional and spiral grafts to reaffirm its potential benefits.<br/

Simulation of fluid-structure interaction with the interface artificial compressibility method

Partitioned fluid–structure interaction simulations of the arterial system are difficult due to the incompressibility of the fluid and the shape of the domain. The interface artificial compressibility (IAC) method mitigates the incompressibility constraint by adding a source term to the continuity equation in the fluid domain adjacent to the fluid–structure interface. This source term imitates the effect of the structure's displacement as a result of the fluid pressure and disappears when the coupling iterations have converged. The IAC method requires a small modification of the flow solver but not of the black-box structural solver and it outperforms a partitioned quasi-Newton coupling of the two black-box solvers in a simulation of a carotid bifurcation. Copyright © 2009 John Wiley & Sons, Ltd