Flow phenomena in compliant and noncompliant arteriovenous grafts

Oscillatory flows at venous anastomoses of two end-to-side angioaccess grafts were studied using: 1) a noncompliant Gore-Tex polytetrafluoroethylene (PTFE) graft and 2) a longitudinal compliant Ultraflex graft. The study was done in a transparent flow model fabricated from a femoral-to-femoral arteriovenous loop graft (AVLG) surgically implanted in dogs. The 6 mm x 25 cm loop graft reproduces that of the brachial artery-cephalic vein hemodialysis angioaccess loop graft commonly used in patients. At a pulse rate of 70 beats/min and blood flow of 0.7 L/min, flow visualization was done using a 15 mW He-Ne laser and a cylindrical lens system which converts the incident laser beam into a thin parallel light plane. Hydrogen bubble technique was used to illustrate the flow field at selected planes. Phasic pressure and flow oscillations were observed in the hypertensive host vein. The pressure wave arrived at the venous anastomosis with higher amplitude and shorter time of travel in the Gore-Tex graft than in the Ultraflex graft. The differences in wave attenuations and wave transmission time between grafts resulted in apparent changes of flow pattern in both the proximal (heart side) and distal (foot side) veins. The negative lags between the flow peaks in the distal vein and proximal artery are explained by the phasic pressure gradients measured at the anastomoses. Spectral analysis was done on the venous thrills, which are characteristic of almost all clinical AVLG implants. The power spectra taken at 60, 70, 90, and 120 beats/min clearly demonstrated a concentration of fluctuation energy at about 100 Hz in the Gore-Tex AVLG.