Effect of the Alterations in Contractility and Morphology Produced by Atrial Fibrillation on the Thrombosis Potential of the Left Atrial Appendage

Abstract

Atrial fibrillation (AF) is a common arrhythmia mainly affecting the elderly population, which can lead to serious complications such as stroke, ischaemic attack and vascular dementia. These problems are caused by thrombi which mostly originate in the left atrial appendage (LAA), a small muscular sac protruding from left atrium. The abnormal heart rhythm associated with AF results in alterations in the heart muscle contractions and in some reshaping of the cardiac chambers. This study aims to verify if and how these physiological changes can establish hemodynamic conditions in the LAA promoting thrombus formation, by means of computational fluid dynamic (CFD) analyses. In particular, sinus and fibrillation contractility was replicated by applying wall velocity/motion to models based on healthy and dilated idealized shapes of the left atrium with a common LAA morphology. The models were analyzed and compared in terms of shear strain rate (SSR) and vorticity, which are hemodynamic parameters directly associated with thrombogenicity. The study clearly indicates that the alterations in contractility and morphology associated with AF pathologies play a primary role in establishing hemodynamic conditions which promote higher incidence of ischaemic events, consistently with the clinical evidence. In particular, in the analyzed models, the impairment in contractility determined a decrease in SSR of about 50%, whilst the chamber pathological dilatation contributed to a 30% reduction, indicating increased risk of clot formation. The equivalent rigid wall model was characterized by SSR values about one order of magnitude smaller than in the contractile models, and substantially different vortical behavior, suggesting that analyses based on rigid chambers, although common in the literature, are inadequate to provide realistic results on the LAA hemodynamics.

Non-Newtonian Blood Rheology Impacts Left Atrial Stasis in Patient-Specific Simulations

“…Patient-specific computational fluid dynamics (CFD) analyses are emerging as a tool to help assess LAA thrombosis risk [6,35,67]. However, previous analyses have considered constant fluid viscosity, neglecting the non-Newtonian rheology of blood at low shear rates [68]. Given that RBC rouleaux increase blood's echogenicity creating spontaneous echocardiographic contrast (a.k.a.…”

“…Multiple studies have established a relationship between LAA blood stasis, echocardiographic "smoke", and the risk of LAA thrombosis, especially in patients with AF [71,72,73,74,61,75,76]. Moreover, previous CFD analyses of LA hemodynamics have reported slow blood flow consistently report shear rates below 70 s −1 and residence times above 5 s inside the LAA, which, according to data from in vitro experiments, should cause RBC aggregation [33,35,67,68]. However, there is a lack of studies evaluating the importance of non-Newtonian blood rheology in the LA and LAA hemodynamics.…”

Impact of Atrial Fibrillation on Left Atrium Haemodynamics: A Computational Fluid Dynamics Study

“…Literature abounds with CFD studies of human atria under AF, both for idealized [20,21] and patient-specific geometries [22,23,24,25,26]. Concerning the numerical approach, in [23], CFD simulations were performed without the application of a turbulence model; in [24,25], the LA haemodynamics is modeled via the Navier-Stokes (NS) equations in Arbitrary Lagrangian-Eulerian (ALE) formulation; the Variational Multiscale Large Eddy Simulation (VMS-LES) method [27] is considered to model the transitional flow.…”

“…In many of the works regarding atrial haemodynamics simulations, the effects of the mitral valve (MV) is mimicked by means of switching boundary conditions [24,25,21,23]. Differently, in this paper we model the effect of the MV on the fluid flow by means of the Resistive Immersed Implicit Surface (RIIS) method [50].…”

“…Differently, in this paper we model the effect of the MV on the fluid flow by means of the Resistive Immersed Implicit Surface (RIIS) method [50]. In addition, we prescribe opening and closing times of the valves that are consistent with clinical findings, overcoming the classical over simplification of an instantaneous switch of the valvular status [24,25,21,23]. To the best of our knowledge, the only work in literature which simulates the left atrial haemodynamics considering also the presence of a MV is [51], where a fluid-structure interaction model is employed in order to perform an advanced analysis on the valvular movement.…”