Patient-Specific Hemodynamic Evaluation of an Aortic Coarctation under Rest and Stress Conditions
Computational fluid dynamics (CFD) simulation of internal hemodynamics in complex vascular models can provide accurate estimates of pressure gradients to assist time-critical diagnostics or surgical decisions. Compared to high-fidelity pressure transducers, CFD offers flexibility to analyze baseline hemodynamic characteristics at rest but also under stress conditions without application of pharmacological stress agents which present undesirable side effects. In this study, the variations of pressure gradient and velocity field across a mild thoracic coarctation of aorta (CoA) is studied under pulsatile ascending aortic flow, simulative of both rest and stress cardiac output. Simulations were conducted in FLUENT 14.5 (ANSYS Inc., Canonsburg, PA, USA) - a finite volume solver, COMSOL 4.2a (COMSOL Multiphysics Inc., Burlington, MA) - a finite element solver, and an in-house finite difference cardiovascular flow solver implementing an unsteady artificial compressibility numerical method, each employing second-order spatio-temporal discretization schemes, under assumptions of incompressible, Newtonian fluid domain with rigid, impermeable walls. The cardiac cycle-average pressure drop across the CoA modeled relative to the given pressure data proximal to the CoA is reported and was found to vary significantly between rest and stress conditions. A mean pressure gradient of 2.79 mmHg was observed for the rest case as compared to 17.73 mmHg for the stress case. There was an inter-solver variability of 16.9% in reported mean pressure gradient under rest conditions and 23.71% in reported mean pressure gradient under stress conditions. In order to investigate the effects of the rigid wall assumption, additional simulations were conducted using a 3-element windkessel model implemented at the descending aorta, using FLUENT. Further, to investigate the appropriateness of the inviscid flow assumption in a mild CoA, CFD pressure gradients were also compared results of a simple Bernoulli-based formula, used clinically, using just the peak blood flow velocity measurements (in m/s) obtained distal to the aortic coarctation from CFD. Helicity isocontours were used as a visual metric to characterize pathological hemodynamics in the CoA.
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