Hydrodynamic Characteristics of Tapered Arterial Prostheses

Summary

The use of arterial prostheses with tapered lumen has important advantages over those with a uniform lumen. The flow is less likely to become turbulent, a better match in size can be achieved and the wall shear stresses are also expected to be higher. A stable flow with moderately high wall shear stresses is desirable since both turbulence and low shear stresses have been associated with the formation of thrombi. Moreover, the regions in the prostheses exposed to low shear stresses are more susceptible to neo—intimai hyperplasia, a common cause of failure of small diameter prostheses.

We have investigated the stability of flow and pressure losses through tapered prostheses with angle of taper (a) between 0.5°and 1.25? Transition from laminar to turbulent fow in the tapered tubes occured at much higher Reynolds numbers (Re) than in the cylindrical ones. It was found to increase with a and distance into the taper. Although Re would not normally reach these values in vivo, flow distrubances can be produced where there is a discontinuity in the cross—sectional area, e.g. at the anastomoses. It is shown that tapering has a damping effect on these distrubances, the effect being greater when a is high.

Wall shear rates were calculated from the measured velocity profiles. Because of the blunter profiles wall shear rates were much higher than in cylindrical grafts and increased with a and distance into the taper. Provided the flow is laminar the maximum shear stresses, even in the 1.0° prosthesis, are well below the values which cause endothelial cell injury.