Experimental study of pulsatile flows in models of abdominal aortic aneurysms

In order to investigate the mechanical factors that lead to rupture of abdominal aortic aneurysms (AAAs), velocity profiles and wall shear stresses are presented for pulsatile flow through a series of AAA models. Seven rigid models were constructed, with uniform lengths of 4d and diameters that ranged from 1.5 to 3.2 d, where d is the inner diameter of the undilated entrance tube. Pulsatile flow simulating resting in vivo aortic conditions, with Womersley number 11 and mean and peak Reynolds numbers of 360 and 2000 respectively, was delivered through the models. Flow fields were interrogated by laser-Doppler velocimetry, and wall shear stresses were derived from these measurements. During systole, the flow was unidirectional and nearly irrotational, with peak wall shear stresses at the distal bulge constriction. As the flow decelerated towards zero, a recirculating vortex developed in the proximal half of the bulge. During reversal, this vortex expanded both distally and radially, eventually developing into a retrogradely directed jet along the bulge wall that isolated a forward recirculation region in the core. During diastole, all velocities decayed towards a small forward value. However, in the larger models, a recirculation vortex remained in the distal half of the bulge, with a small outer secondary vortex. Regions of unstable flow were increasingly evident in the distal bulge of the larger models. Pressure was constant along the models, but maximum shear stresses reached levels extrapolating to greater than 80 dynes/cm(2) in vivo.