We use computational and experimental techniques to understand the flow dynamics of cannulation. We consider both metal needles and plastic cannulae.
For regular hemodialysis sessions, strong and large vascular access is needed. For this reason, vascular surgeons create an artificial connection of an artery to a vein in the forearm or upper arm. With the extra pressure and blood flow, the surgically created vascular connection matures as AVF to serve as the ideal vascular access for arterial and venous needles. However, the jet of fluid exiting through a venous needle in the blood vessel has been previously reported as a potential source of endothelial damage which can result in intimal hyperplasia. Intimal hyperplasia, a thickening of the blood vessel wall in response to injury, can compromise the vascular access. Therefore, better understanding of the hemodynamics is needed to determine the optimal placement and blood flow rate and minimize this complication. A computational fluid dynamics model provides a means to accurately predict the wall shear stress (WSS) to analyze the flow mediated effects on the blood vessel wall. The following shows an image of a plastic cannula (15-G Argyle Safety Fistula Cannula) and a cross-sectional cut of the structured mesh at the cannula tip.
The blood exiting the cannula at different flow rates (200, 300, 400 ml/min) and different locations is visualized using velocity isosurfaces as in the following. It shows that the jet dissipates immediately after exiting the needle with high levels of instability at high flow rates (middle and right columns). On the other hand, at the flow rates of 200 ml/min (left column), the jet maintains a laminar structure until closer to the impingement zone (near wall).
The effects of wall shear stress can be considered in a time averaged approach (TAWSS). In the following figure, TAWSS in normal range (<5 Pa) are in gray while higher values are coloured with the maximum value at 40 Pa. Higher flow rate (right column) and closer positioning of the cannula to the vessel wall resulted in significantly higher TAWSS highlighting the importance of these parameters.