Three-dimensional macro-scale assessment of regional and temporal wall shear stress characteristics on aortic valve leaflets

Abstract

The aortic valve (AV) achieves unidirectional blood flow between the left ventricle and the aorta. Although hemodynamic stresses have been shown to regulate valvular biology, the native wall shear stress (WSS) experienced by AV leaflets remains largely unknown. The objective of this study was to quantify computationally the macro-scale leaflet WSS environment using fluid-structure interaction modeling. An arbitrary Lagrangian-Eulerian approach was implemented to predict valvular flow and leaflet dynamics in a three-dimensional AV geometry subjected to physiologic transvalvular pressure. Local WSS characteristics were quantified in terms of temporal shear magnitude (TSM), oscillatory shear index (OSI) and temporal shear gradient (TSG). The dominant radial WSS predicted on the leaflets exhibited high amplitude and unidirectionality on the ventricularis (TSM>7.50 dyn/cm(2), OSI < 0.17, TSG>325.54 dyn/cm(2) s) but low amplitude and bidirectionality on the fibrosa (TSM < 2.73 dyn/cm(2), OSI>0.38, TSG < 191.17 dyn/cm(2) s). The radial WSS component computed in the leaflet base, belly and tip demonstrated strong regional variability (ventricularis TSM: 7.50-22.32 dyn/cm(2), fibrosa TSM: 1.26-2.73 dyn/cm(2)). While the circumferential WSS exhibited similar spatially dependent magnitude (ventricularis TSM: 1.41-3.40 dyn/cm(2), fibrosa TSM: 0.42-0.76 dyn/cm(2)) and side-specific amplitude (ventricularis TSG: 101.73-184.43 dyn/cm(2) s, fibrosa TSG: 41.92-54.10 dyn/cm(2) s), its temporal variations were consistently bidirectional (OSI>0.25). This study provides new insights into the role played by leaflet-blood flow interactions in valvular function and critical hemodynamic stress data for the assessment of the hemodynamic theory of AV disease.

The Oscillatory Shear Index: Quantifications for Valve Tissue Engineering and a Novel Interpretation for Calcification

“…Moreover, the averaged OSI across the entire aortic valve surface was previously reported to be an OSI ~0.16 [41] which is close to the "sweet spot" OSI range (0.18-0.23) of the current study. However, we note that fluid-induced oscillations are considerably greater on the fibrosa surface as compared to the ventricularis surface and as a result, regional OSI can vary.…”

“…The bioreactor system which we utilized is able to mimic regionally varying shear stresses on the specimen surfaces and is analogous to native tri-leaflet heart valve shear stress distributions [25]. Indeed, the range of OSI (0.18 -0.23) identified here is closeto the typical OSI experienced by aortic valve tissues (OSI ~ 0.16) [41]. Previous studies on Flex-Flow conditions by [5] and [6] found that the 3-dimensional coupling of cyclic flexure, cyclic stretch and fluid-induced shear stresses manifest as oscillatory patterns surrounding the localized flow field of the heart valve leaflets.…”

“…Aortic valve calcification is a predominant cause for aortic insufficiency [43,18,40]. From a biomechanics standpoint, fluid-induced shear stresses across the leaflets have been closely linked to normal and pathological valve tissue remodeling activity [41,14,8,10,17,16,44], but specific shear stress patterns and magnitudes both spatial and temporal have not yet been strongly linked to the underlying cell autocrine and paracrine signaling events that regulate the valve extracellular matrix (ECM). Nonetheless, the incidence of low shear, disturbed laminar flow resulting in temporal blood flow oscillatory patterns on the fibrosa side of aortic valve leaflets has been shown to strongly correlate to regions of calcific nodule deposition and aggregation [10,17].…”

An Experimental Study of Pulsatile Flow in a Compliant Aortic Root Model under Varied Cardiac Outputs

“…Compared with mechanical valve replacements, compliant aortic valves, such as tissue-engineered bioprosthetic valves and polymeric valves, have soft leaflets that could result in better hemodynamics. The fluid dynamics and fluid-structure interaction (FSI) of bioprosthetics and native tissue aortic valves have been studied extensively in computational fluid dynamics and fluid-structure interaction simulations [22][23][24][25]. The dynamic responses of the soft valve leaflets under simulated physiological loadings have been studied by many finite element simulations [26,27].…”

Endothelial cells and cathepsins: Biochemical and biomechanical regulation

“…The geometry of the proximal ascending aorta produces native regions of oscillatory shear stress. Endothelial cells on the aortic side of the aortic valve are normally subjected to oscillatory shear stress after the valves close, and a loss of this oscillatory stress results in expression of adhesion proteins and cytokines consistent with endothelial activation [23, 24]. Cathepsin expression by endothelial cells are ultimately regulated by this complex inflammation cascade and altered biomechanical hemodynamic signaling [15, 16, 25, 26].…”