WALL SHEAR RATE DISTRIBUTION IN AN ABDOMINAL AORTIC BIFURCATION MODEL- EFFECTS OF VESSEL COMPLIANCE AND PHASE-ANGLE BETWEEN PRESSURE AND FLOW WAVE-FORMS

The goal of this study was to determine how vessel compliance (wall mo tion) and the phase angle between pressure and flow waves (impedance p hase angle) affect the wall shear rate distribution in an atherogenic bifurcation geometry under sinusoidal flow conditions. Both rigid and elastic models replicating the human abdominal aortic bifurcation were fabricated and the wall shear rate distribution in the median plane o f the bifurcation was determined using the photochromic pow visualizat ion method. In the elastic model, three phase angle conditions were si mulated (+12, -17, -61 deg), and the results compared with those obtai ned in a similar rigid model. The study indicates a very low (magnitud e close to zero) and oscillatory wall shear rate zone within 1.5 cm di stal to the curvature site on the outer (lateral) wall. In this low sh ear rate zone, unsteadiness (pulsatility) of the flow greatly reduces the mean (time-averaged) wall shear rate level. Vessel wall motion red uces the wall shear rare amplitude (time-varying component) up to 46 p ercent depending on the location and phase angle in the model. The mea n wall shear rate is less influenced by the wall motion, but is reduce d significantly in the low shear region (within 1.5 cm distal to the c urvature site on the outer wall), thus rendering the wall shear rate w aveform more oscillatory and making the site appear more atherogenic. The effect of the phase angle is most noteworthy on the inner wall clo se to the flow divider tip where the mean and amplitude of wall shear rate are 31 and 23 percent lower, respectively, at the phase angle of -17 neg than at -61 deg. However, the characteristics of the wall shea r rate distribution in the low shear rate zone on the outer wall that are believed to influence localization of atherosclerotic disease, suc h as the mean wall shear-rate level, oscillation in the wall shear rat e waveform, and the length of the low and oscillatory wall shear rate zone, are similar for the three phase angles considered. The study als o showed a large spatial variation of the phase angle between the wall shear stress waveform and the circumferential stress waveform (hoop s tress due to radial artery expansion in response to pressure variation s) near the bifurcation (up to 70 deg). The two stresses became more o ut of phase in the low mean shear rare zone on the outer wall (wall sh ear stress wave leading hoop stress wave as much as 125 deg at the pre ssure-flow phase angle of -61 deg) and were significantly influenced b y the impedance phase angle.