Experimental analysis of the hemodynamics in punctured vascular access grafts

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.