Hemodynamic differences in the outflow of access vascular grafts
Access vascular (AV) prostheses are commonly used for haemodialysis. Their low patency rates remain a challenge with restenosis in the distal anastomosis being the main reason of failure. The blood flow profile affects the wall shear stress which is a crucial factor for the endothelial function. Single spiral flow has been found to be a normal physiologic characteristic of vascular blood flow. This study compared the Spiral Laminar FlowTM AV graft (Vascular Flow Technologies, UK) which is designed to induce spiral flow against a control AV device.
Materials and Methods
The prostheses were integrated in an in-house ultrasound flow phantom which was consisted of blood, vessel, tissue mimicking materials and a piston pump. The devices were tested using a curve and straight configuration to mimic vascular surgical implantation geometries. Steady flow rates up to 720 ml/min were applied. Colour Doppler ultrasound imaging was utilized in a number of cross-sectional planes distally from the grafts outflow. To visualize and quantify rotational flow patterns, a vector Doppler technique was developed using Matlab (MathWorks, USA). Two-dimensional velocity magnitude and directional maps were created and an analysis based on the magnitude of the peak radial velocity was performed.
A single spiral flow was detected in the outflow of the spiral graft and two or three spirals in the outflow of the control device, for both types of anastomosis. Flow separation and areas of stagnation were detected when more than one helix existed. The radial velocity was consistently higher for the single spiral in comparison to that of the multi-spiral patterns under all the applied conditions. The flow pattern under high flow rates was stable for the spiral graft and disturbed for the control device.
The single helical pattern created by the spiral graft was associated with increased magnitude of radial velocity. This is an index of increased wall shear stress which is considered atheroprotective. These results support the hypothesis that spiral graft improves flow stability and coherence which may relate to increased graft patency rates.