Attenuation of Hydrodynamic Disturbances in Tapered Arterial Prostheses

Abstract

Synthetic arterial grafts are widely used for the replacement and bypass of diseased arteries. They perform satisfactorily in the high blood flow environment of the large central blood vessels. The small diameter prostheses used in peripheral arterial reconstructions, however, have a high occlusion rate owing to thrombosis, often related to the development of pseudo-intimal hyperplasia. Hemodynamic disturbances such as turbulence and flow separation, low flow velocity and low wall shear stress are thought to be partly responsible for their poor performance Although most arterial prostheses used at present are uniform cylindrical conduits, we suggest that prostheses having a tapered lumen should be considered as these posssess hydrodynamic advantages, in terms of greater flow stability and higher wall shear stresses, which may improve their performance in vivo.

The purpose of this study was to investigate the flow stability in tapered prostheses in comparison with that in a cylindrical prosthesis. Flow disturbances were measured in tapered prostheses having half angles of taper between 0.5° and 1.0° which were placed in an in vitro flow circuit. The measurements were made using a single channel 20 MHz range-gated ultrasound Doppler velocimeter. The increase in transition Reynolds number with angle of taper and axial distance was determined under steady flow conditions. The instantaneous centerline velocity was measured in steady and pulsatile flow downstream of a 50% area stenosis positioned at the start of each tapered tube and from which the disturbance intensities were calculated. Significant reductions in post-stenotic disturbance intensity were recorded in the tapered as opposed to the cylindrical prostheses. In pulsatile flows with a large reverse flow component, there was an increase in disturbance intensity owing to diverging flow conditions on flow reversal. This effect was observed only in the 1.0° tapered prothesis. It is concluded that in designing tapered arterial protheses, the angle of taper should be made less than 1.0° in order that the advantages of taper can be fully realized.