Wall shear stress oscillation and its gradient in the normal left coronary artery tree bifurcations

It is known that blood flow properties such as low/ oscillatory wall shear stress (WSS), high blood viscosity, low blood velocity and high concentration of low density lipoprotein (LDL) macromolecules, are some of the main flow parameters causing atherosclerosis. Limited research has been undertaken on the pulsatile WSS and WSS gradient (WSSG) analysis focusing in the differentiation between the bifurcation itself and the lateral to it walls in a normal left coronary artery (LCA). The results obtained show the flow characteristics and qualify the spatial and temporal distribution of WSS ant its gradient in regions close to the LCA tree flow dividers and in opposite to them areas.

A 3D computer generated model of the LCA tree based on averaged human data extracted from angiographies was developed for computational fluid dynamics analysis. Physiological phasic flow velocity is incorporated as entrance boundary condition.

The instantaneous min wall shear stress oscillates from 0.45 to 2.84 N/m(2) at the flow divider and from 0.25 to 1.28 N/m(2) at the lateral walls of the main bifurcation. However, for the D1-S1 bifurcation (first diagonal-first septal), the instantaneous min wall shear stress oscillates from 0.6 to 3.85 N/m(2) at the flow divider and from 0.6 to 2.65 N/m(2) at the lateral walls. Mean wall shear stress, from max systole to max diastole, experiences a 129.0 % increase at the main bifurcation flow divider. The difference between max and min wall shear stress for the flow divider of the main bifurcation, as it is compared with the max wall shear stress over the entire cardiac pulse, attains a maximum value of 81.1 % for the lateral walls and 60.0 % at the peak of diastole. At the D1-S1 bifurcation, the corresponding difference values are 69.0% and 57.0 % for the lateral walls and flow divider, respectively. The mean wall shear stress gradient experiences a 123.0 % increase from max systole to max diastole at the main bifurcation flow divider and 153.0 % at main bifurcation lateral walls.

Proximal LCA bifurcation exhibit lower spatial wall shear stress and lower wall shear stress gradient values compared to distal bifurcations. The lateral walls compared to the bifurcation itself are exposed to low WSS and WSSG. With regards to the temporal variation, wall shear stress and its gradient exhibited lower values throughout systole as compared to diastole, suggesting a possible atherogenic effect of both the systolic phase by itself as well as the phasic oscillation of wall shear stress and its gradient from systole to diastole.