LAUSR

The anatomy of aortic root and its components has been shown to significantly affect the flow through the aorta. Aberrant flows in the ascending aorta are mainly due to aortic abnormalities of the aortic root or the aorta, which can be corrected by surgical repair. However, the effect of the aberrant flow on the aortic root and the aorta is less clear, although this has been previously explored. What is certain is that there is a dynamic interaction between the flow and the adjacent aortic root tissue that leads to continuous exchange of momentum between the two. From a mechanistic perspective, blood flow interacts with the adjacent tissue through the exchange of forces, as represented by either pressure or wall shear stress (WSS). The role of pressure in the cardiovascular system is already well known. However, effects of WSS on the neighbouring structures are more elusive. Vascular atherogenesis is an example where the role of WSS has been recognized since the early sixties. Low values of WSS were initially considered to simply facilitate lipid deposition on the vessel walls and buildup of atherosclerotic plaques. However, more recent studies suggest that the interplay between the flow and the endothelium is more complex than what was thought before. Plaque buildup is regulated at the cellular level such that the abnormal WSS can trigger proinflammatory responses through mechano-transduction that leads to atherosclerosis. Similarly, degradation of the endothelial layer can occur in the regions where WSS is high (e.g. when blood jet across a stenosis or bicuspid aortic valve (AV) deviates and impacts the adjacent wall in the aortic root). Thus, it is possible that abnormal WSS on the AV leaflets may eventually trigger endothelial dysfunction and inflammation, which ultimately leads to calcification. The sinuses of Valsalva play a crucial role in the fluid dynamics of the aortic root and in the interaction between the flow and AV leaflets. Preserving the protection that is facilitated by the fluid dynamic of sinuses of Valsalva is one of the basis for valve-sparing aortic root replacement (VSARR). Figure 1 compares computer simulation of aortic root’s blood flow pattern in presence or absence of sinuses of Valsalva. In early and peak systole, the vortex flow that develops along with the jet in the Valsalva sinuses creates a cushion between the leaflets and the wall preventing the leaflets from smashing against the wall of the aorta. This vortex flow may regulate WSS on the AV’s fibrosa layer. The central jet stream of aortic flow through the open AV regulates the WSS on the ventricularis surface of its leaflets. In late systole, the vortices in the sinuses of Valsalva mitigate the forces at which the leaflets appose during diastole, thus minimizing trauma to the leaflets. Therefore, the intricate fluid dynamics of the normal aortic root is in part intended to sustain the durability of the AV leaflets. The recent study by Hayashi et al. investigates the effect of aortic root geometry on WSS measured directly over the valve leaflets prior to and post-VSARR. Their results indicate that the abnormal WSS on the AV leaflets in an aneurysmal aortic root is reversed post-VSARR, when a Valsalva graft is used to replace the aneurysmal aortic root. The post-VSARR WSS on the AV leaflet was found similar to a control group of patients who underwent coronary artery bypass grafting surgery in whom the aortic root geometry was normal. These results suggest that preserving the sinuses of Valsalva during VSARR may improve durability of the native AV leaflets, although this remains a matter of debate. From a methodological standpoint, the authors have calculated WSS using blood velocities and estimations of velocity gradients obtained by colour flow Doppler-based vector flow mapping (VFM). This simplistic approach entails limitations because it is based