Red cell injury assessed in a numeric model of a peripheral dialysis needle

The highest shear stresses in a dialysis system are expected to be found in the needle, where the largest velocity-diameter ratio appears. Shear is a known source of hemolysis and related patients' discomfort. To assess the magnitude of blood cell injury and the location of its sources, a finite element model is used to calculate three-dimensional velocities and shear stresses in peripheral dialysis needles, concentrically placed in a rigid wall fistula. The boundary conditions consist of time dependent in vivo measured pressures. Cell damage is computed for different cell tracks into the needle by means of Wurzinger's empirical formula, which expresses the hemoglobin (Hb) release as a function of shear stress and shearing time. Near the needle wall, velocities are low and shear stresses high, resulting in a significantly higher level of cell damage: 0.1% vs 0.001% in bulk flow for a mean flow of 91 ml/min into a 14G needle with a peak velocity of 220 cm/sec. The deviation from the classic Poiseuille velocity profile is shown. Less than 5% of the flow passes through this high damage path. A vortex at the inner side of the needle has a cumulative damage of 0.007% per 0.23 sec trip around the vortex.