PubMed · 15176331
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Dorothea Rohrmann. 2003. The artificial urinary bladder.. https://doi.org/10.1007/978-1-4419-8889-8_56
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The proliferation of disease at the bed of the distal junction of an end-to-side anastomosis is attributed to abnormal wall shear stress (WSS) distribution. WSS is proportional to the viscosity and shear rate of the flowing fluid. Blood is characterised by a shear rate dependent viscosity. Various constitutive equations have been developed to represent the shear rate dependence of blood viscosity: Newtonian, Carreau, Power law, Carreau-Yasuda, Bi-exponential, Cross, Modified Cross, Herschel-Bulkley, etc. In the femoral artery, the instantaneous shear rate varies from 1-1200 s(-1) over a cardiac cycle. An idealised, 45 degrees rigid, 6mm diameter, end-to-side femoral anastomosis was modelled on a Computational Fluid Dynamic software package Fluent 6.0. A steady flow of 0.15 and 0.01 m/s was applied to the inlet to model high and low wall shear rate environments respectively. Blood was modelled using the various constitutive equations. The resulting WSS distribution on the bed of the artery was then obtained. At high shear rates there was no significant difference between WSS distribution. At low shear rates there were qualitative differences of up to 300%. In conclusion, the choice of blood constitutive equation has to be based on the particular situation under study e.g. flow rate, steady/unsteady flow, and geometry.
One of the main causes of long-term failure of ePTFE grafts is the development of anastomotic intimal hyperplasia which leads to graft thrombosis. Experimental studies with bypass grafts have shown an inverse relationship between mean wall shear stress and intimal hyperplasia. The geometry of the anastomosis has a strong influence on the flow pattern and wall shear stress distribution. The aim of this in vitro study was to investigate the influence of non-planarity in a model of a distal anastomosis with interposition vein cuff, an anastomosis configuration that is increasingly being used because of improved clinical results. Laser Doppler anemometer measurements were carried out in silicone rubber models of interposition vein cuff anastomoses with planar and non-planar inflow. The pulsatile flow waveforms were typical of those found in femoro-infrapopliteal bypass. Axial and radial velocities were measured in the proximal and distal outflow segments. As expected a symmetrical helical flow pattern (Dean flow) was evident in the planar model. The model with non-planar inflow, however, gave rise to swirling flow in both the distal and proximal artery outflow segments for during the systolic phase. In patients, the anastomosis is usually non-planar. Since the configuration depends in part upon the tunnelling of the graft, this may be altered to some extent. Non-planar anastomotic configurations induce a swirling flow pattern, which may normalise wall shear stress, thereby potentially reducing intimal hyperplasia.
The simultaneous occurrence of intussusception and volvulus in the paediatric age group is rare. We report the case of a volvulus of an ileoileal intussusception in an 8-year-old boy. This is the first time that computerised tomography (CT) images of a volvulus of an ileoileal intussusception have been published, and they clearly demonstrate both pathologies. This case highlights the use of CT in determining the nature of an abdominal mass and demonstrates how helpful it can be in diagnosing the cause of small bowel obstruction in children.