This review article describes the current state of affairs concerning in vivo, in vitro and in numero studies on the hemodynamics in vascular access for hemodialysis. The use and complications of autogenous and non-autogenous fistulas and catheters and access port devices are explained in the first part. The major hemodynamic complications are stenosis, initiated by intimal hyperplasia development, and thrombosis. The different in literature proposed conceivable causes of intimal hyperplasia development like surgical interventions, compliance mismatch, wall shear stress (WSS) and shear rate, vessel wall thrill and blood pressure are discussed on the basis of in vivo, in vitro and in numero studies.

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A three-dimensional finite volume model of the blood-dialysate interface over the complete length of the dialyzer was developed. Different equations govern dialyzer flow and pressure distribution (Navier-Stokes) and radial transport (Darcy). Blood was modeled as a non-Newtonian fluid with a viscosity varying in radial and axial direction determined by the local hematocrit, the diameter of the capillaries, and the local shear rate. The dialysate flow was assumed to be an incompressible, isothermal laminar Newtonian flow with a constant viscosity. The permeability characteristics of the membrane were calculated from laboratory tests for forward and backfiltration. The oncotic pressure induced by the plasma proteins was implemented as well as the reduction of the overall permeability caused by the adhesion of proteins to the membrane. From the calculated pressure distribution, the impact of flow, hematocrit, and capillary dimensions on the presence and localization of backfiltration can be investigated.

The hemodynamics in the vascular access graft are influenced by the flow aspirated and injected through the two needles during hemodialysis. For the first time, the impact of needle flow on vascular access performance, measured in an in vitro set up, is reported. A vascular access model, consisting of a loop polytetrafluoroethylene graft sewn to a compliant artery and vein, simulated the patient. The extracorporeal circuit was connected to the model. Three mean access flow rates (QG; 500, 1,000, and 1,500 ml/min) and five roller pump flow rates (Q(R); 0, 200, 300, 400, and 500 ml/min) were studied. Mean, systolic, and diastolic pressure and according pressure drops were derived at 14 loci. Systolic, diastolic, and mean pressures drop along the graft decreased with increasing Q(R) and decreasing Q(G). At Q(R) = 500 ml/min and Q(G) = 500 ml/min, the mean pressure drop over the graft was negative (-10 mm Hg), indicating a reversed pressure profile, originating at the puncture site of the venous needle. Mean pressure in the venous outlet segment was about 100 mm Hg compared with only 75 mm Hg without needle flow. The combination of a low Q(G) (500 ml/min) and high Q(R) (> 300 ml/min) must be avoided because venous pressures can rise to 100 mm Hg and load the venous system. The results of this in vitro setup indicate that high Q(R) (> 400 ml/min) should be avoided at Q(G) up to 1,000 ml/min; however, in vivo tests have to be performed to prove this thesis. This study demonstrates the need for a well-functioning vascular access (Q(G) > 600 ml/ min) to perform adequate dialysis and to avoid venous system loading.