Insights into the co-localization of magnitude-based versus direction-based indicators of disturbed shear at the carotid bifurcation.

The observed co-localization of disturbed flow and lesion prevalence at predisposed districts such as the carotid bifurcation has led to the identification of the wall shear stress (WSS) as biomechanical localizing factor of vascular dysfunction. In particular, a proatherogenic role is attributed to low and oscillatory WSS. However, the endothelial cells (ECs) are exposed to a complex hemodynamic milieu that can be only partially described by low/oscillatory WSS. Recently, in the attempt to close this gap, descriptors of the complex multidirectional nature of WSS have been proposed, i.e., the axial component of WSS (aligned with the vessel׳s centerline, to quantify flow reversal), and the transverse WSS (transWSS, quantifying the WSS component orthogonal to the cycle averaged WSS direction). Here we explore the relationship between recently-proposed indicators quantifying WSS multidirectionality and "established" WSS-based hemodynamic descriptors of low/oscillatory WSS, in a representative sample (N=46) of subject-specific computational hemodynamics models of ostensibly normal carotid bifurcations. To do it, we quantitatively assess the co-localization of those descriptors at the luminal surface, aiming at providing connections among the peculiar hemodynamic features captured by the different descriptors. According to our findings: (1) regions of flow reversal are moderately co-localized with low WSS; (2) high WSS oscillations (quantified by the oscillatory shear index, OSI) at the carotid bulb are prevalently aligned with the main flow, where flow reversal is predominant; (3) regions where transWSS is high do not co-localize with the other descriptors. We suggest that the investigated WSS-based descriptors might represent different hemodynamic disturbances with different impact on ECs homeostasis, potentially being part of WSS phenotypes more effective in localizing the map of vascular atherosclerotic lesions.