Fluid dynamic study of angioaccess venous anastomoses

Longer life expectancy of patients receiving chronic hemodialysis requires that patency is maintained in their arteriovenous angioaccess loop graft (AVLG) system. However, there still is a high incidence of complications leading to failure of the AVLG system. Earlier studies have indicated that hemodynamic behavior of blood may play an important role in the failure. Unfortunately, the characteristics of hemodynamic contributions to the failure have not been clearly identified. To characterize the role of hemodynamics in the etiology of the failure, an in vivo/in vitro AVLG system was created to study flow phenomena and blood reaction. The work reported here was the in vitro part of a comprehensive study undertaken to study hemodynamic activity in the AVLG system. Three elastic transparent flow models were fabricated from silicone rubber casts and represented the detailed luminal geometry of the in vivo AVLG loop system. The hemodynamic characteristics of the three models with different area reduction (AR), acute, minor and significant, were compared under identical in vitro flow conditions. Flow visualization indicated that a distinct boundary layer separation region was observed downstream from the venous anastomotic "toe". A stagnation region was created at the venous wall, opposite to the venous anastomosis, by the oscillatory impinging jet originating from the graft conduit Momentum calculations indicated that a strong hydrodynamic force, from the impinging jet acted at the stagnation region. Oscillating wall shear stresses, with opposite sign, were found at the region. Retrograde flow in the distal vein occurred in an "oscillating" manner following each cardiac cycle. Local velocity mapping, using Laser velocimetry, provided detailed characteristics of hemodynamic flow phenomena. A low wall shear stress region was observed in the boundary layer separation region near the venous anastomotic "toe", while relative high wall shear stress was observed at the proximal side opposite to the venous anastomosis. Low magnitudes of turbulence intensity and Reynolds shear stresses were presented in all the flow models studied. The analysis of power spectral density of velocity fluctuation indicated that pronounced spectral humps occurred at about 100 Hz in the flow model with the significant area reduction.