Numerical study on the effect of secondary flow in the human aorta on local shear stresses in abdominal aortic branches.

Flow in the aortic arch is characterized primarily by the presence of a strong secondary flow superimposed over the axial flow, skewed axial velocity profiles and diastolic flow reversals. A significant amount of helical flow has also been observed in the descending aorta of humans and in models. In this study a computational model of the abdominal aorta complete with two sets of outflow arteries was adapted for three-dimensional steady flow simulations. The flow through the model was predicted using the Navier-Stokes equations to study the effect that a rotational component of flow has on the general flow dynamics in this vascular segment. The helical velocity profile introduced at the inlet was developed from magnetic resonance velocity mappings taken from a plane transaxial to the aortic arch. Results showed that flow division ratios increased in the first set of branches and decreased in the second set with the addition of rotational flow. Shear stress varied in magnitude with the addition of rotational flow, but the shear stress distribution did not change. No regions of flow separation were observed in the iliac arteries for either case. Helical flow may have a stabilizing effect on the flow patterns in branches in general, as evidenced by the decreased difference in shear stress between the inner and outer walls in the iliac arteries.

Numerical study on the effect of steady axial flow development in the human aorta on local shear stresses in abdominal aortic branches.

The three-dimensional flow through a rigid model of the human abdominal aorta complete with iliac and renal arteries was predicted numerically using the steady-state Navier Stokes equations for an incompressible. Newtonian fluid. The model adapted for our purposes was determined from data obtained from cine-CT images taken of a glass chamber that was constructed based on anatomical averages. The iliac arteries had a bifurcation angle of approximately 35 and a branch-to-trunk area ratio of 1.27. whereas the renal arteries had left and right branch angles of 40 and an area ratio of 0.73. The numerical tool FLOW3D (AEA Industrial Technology, Oxfordshire, UK) utilized body-fitted coordinates and a finite volume discretization procedure. Purely axial velocity profiles were introduced at the entrance of the model for a range of cardiac outputs. The four-branch numerical model developed for this investigation produced flow and shear conditions comparable to those found in other reported works. The total wall shear stress distribution in the iliac and renal arteries followed standard trends. with maximum shear stresses occurring in the apex region and lower shear stresses occurring along the lateral walls. Shear stresses and flow rate ratios in the downstream arteries were more effected by inlet Re than the upstream arteries. These results will be used to compare further simulations which take into effect the rotational component of flow which is present in the aortic arch.

Confirmation and initial documentation of thoracic and abdominal aortic helical flow. An ultrasound study.

Aortic helical flow may play an important role in plaque deposition, dissection formation, and organ perfusion. The authors have previously demonstrated, using in vitro flow models and transesophageal echocardiography, that helical flow begins in the mammalian aortic arch and continues into the descending thoracic aorta. The purpose of this study was to confirm thoracic aortic helical flow and document its extent into the abdominal aorta using direct measurements. Twelve mongrel dogs underwent surgery with exposure of the abdominal aorta up to the diaphragm. Six of the 12 underwent further thoracotomy with thoracic aorta exposure. Color Doppler ultrasound images were obtained using a 5 megaHz esophageal transducer, hand held, directly applied, and visually aligned for transverse aortic imaging. Helical flow was considered present with the appearance of red/blue hemicircles during a systolic wave when the aorta was imaged transversely. All six dogs that had thoracotomy showed clockwise thoracic aortic helical flow (along the direction of blood flow) at the retro left ventricular region. In all dogs, clockwise helical flow was demonstrated to and immediately beyond the renal arteries. In 11 of 12 dogs, clockwise helical flow was demonstrated 7 cm below the renal arteries. The study confirms the presence of helical flow in the thoracic aorta and documents its extent into the abdominal aorta below the level of the renal arteries. The teleologic flow pattern of mammals may extend to other classes of vertebrates and must be accounted for in studies of endothelial shear and flow separation. In addition, tangential velocities imparted by helical flow may affect organ perfusion.

In vitro hemodynamic analysis of flexible artificial ventricles.

An in vitro fluid dynamic study was performed to compare the hemodynamic characteristics of a rigid and a flexible total artificial heart. The artificial ventricles were incorporated into a mock circulatory system, and pressure signals within the ventricular chamber, proximal to the inflow valve and distal to the outflow valve, were obtained. The instantaneous flow rate through the inflow and outflow valves was measured with electromagnetic flow probes. Flow visualization studies performed on the flexible ventricle suggested a vortical motion within the chamber with a smooth washout of fluid in the next pumping phase, but flow disturbances were observed near the wall of the ventricle as well as near the outflow valve. The rate of pressure increase (dP/dt) was smaller in the flexible ventricle as compared with the rigid ventricle for comparable flows and heart rates. The results of the present study indicated that the flexible ventricle with polyurethane valves, having the advantage of ease of implantation and cost savings, can be a viable alternative as a bridge to transplant.