Blood Flow Helicity Pattern in Type III Arch Configuration as a Potential Risk Factor for Type B Aortic Dissection

Abstract

Introduction: Abnormal helical flow in the aortic arch has been proposed as a causative factor for aortic dilatation (1), for the formation of entry tears and consequent development of aortic dissections (AD) (2). Blood flow helicity derangements can be caused by the presence of a bicuspid aortic valve, but are also related to aortic arch morphology (3), and in particular to the presence of arch elongation and severe tortuosity (2). Previous studies showed that patients with Type B AD present a high prevalence of Type III arch configuration, which comprises recognized anatomic variables associated with an increased risk of AD, namely elongation, tortuosity, and angulation (4). Our aim was to assess whether the Type III arch configures a consistent helical flow pattern, which may account for its association with the occurrence of Type AD. Methods: Angiographic computed tomography (CT) scans of healthy aortas were selected to equally reflect Type I to III arches (each n=5). Computational fluid dynamics (CFD) was performed with LifeV open-source library to assess the intra-aortic helical flow field along the aortic Ishimaru's landing zones (5). CFD hemodynamic parameters, derived from the velocity field (V), included: vorticity (ω), the local spinning motion of the fluid; helicity (H), a measure of the V- ω alignment quantifying the corkscrew-like fluid motion about the principal flow direction; the absolute local normalized helicity (LNH) assuming values ranging between 0 (high flow rotation) and 1 (irrotational flow) (6). By novel application of the Serret-Frenet formulas to CFD results (7), curvature (κ) and torsion (τ) of blood velocity streamlines were assessed to quantify both in-plane and out-of-plane flow rotations within each zone. Results: Absolute ω distribution remained comparable within each arch Type while H significantly increased in Zone 3 of Type III (P=0.025). Overall absolute LNH was similar between the anatomies; however, the percentage of aortic region with absolute LNH above a 0.8 threshold markedly increased in Zone 3 of Type III (IQR: 29.5 ÷ 39.5 %) with respect to Type I (IQR: 21.5 ÷ 38.1 %) and Type II (IQR: 18.0 ÷ 32.0 %). Serret-Frenet elaboration of the velocity streamlines revealed the highest level of curvature in Type I for Zone 0 (P< 0.0001) and Type III for Zone 3. Within Zone 3, absolute κ significantly increased from Type I to III (P< 0.0001; median κ values equal to 0.19 mm-1, 0.26 mm-1 and 0.35 mm-1, respectively) while streamline torsion conversely decreased from Type I to III (P=0.0030; median τ values equal to 0.56 mm-1,0.44 mm-1 and 0.32 mm-1, respectively). In each arch Type, absolute κ significantly changed from Zone 0 to 3 (P=0.0069 in Type I, P=0.0179 in Type II and P< 0.0001 in Type III). Conclusion: Our proof of concept analysis detected arch-type regional differences in aortic helical bulk flow, highlighting in Type III arch a significant exacerbation of aortic helical flow localized in Zone 3, which identifies the most common site for proximal entry tear in Type B AD. Extension to in vivo scenarios is feasible exploiting velocity-encoded PC-MRI (4D Flow). Disclosure: Nothing to disclose