Do morphological, haemodynamic and biomechanical parameters relate to aortic growth rate in chronic type B aortic dissection? A 4D flow CMR study
Authors: Ruiz Munoz A , Guala A , Dux-Santoy L , Teixido-Tura G , Casas G , Valente F , Servato M L , Fernandez-Galera R , Galian-Gay L , Gutierrez L , Gonzalez-Alujas T , Ferreira-Gonzalez I , Evangelista A , Rodriguez-Palomares J F 
Published: October 2021

Abstract

Background
Aortic dissection (AD) is the most devastating complication of thoracic aortic disease (1). In the chronic phase, yearly clinical and imaging follow-up of the maximum aortic diameter is recommended, since indication for thoracic endovascular aortic repair or surgery is suggested by guidelines in case of thoracic aortic enlargement or false lumen (FL) aneurysms (2). Most of the reported parameters related adverse events in chronic AD are focused on morphological variables (3) and not on the haemodynamics and biomechanics of the FL.

Purpose
To evaluate the relationship between aortic growth rate and anatomical variables, flow patterns and aortic stiffness in patients with chronic type B AD.

Methods
Forty-one patients with chronic type B aortic dissection, no connective tissue disorders and with an imaging follow-up including two computed tomography angiograms (CTA) acquired at least 3 years apart underwent contrast-enhanced 4D-flow CMR and MR angiography (MRA). The FL volume was segmented from MRA, and velocity data inside the 3D volume of the FL was extracted from 4D-flow CMR and used for parameter quantification. Retrograde systolic and diastolic flow, wall shear stress (WSS) and in-plane rotational flow (IRF) were calculated at 8 equidistant planes in the distal descending aorta (DAo), from the pulmonary bifurcation to the diaphragmatic level, and averaged values were used [4]. Aortic stiffness in the FL was assessed in terms of pulse wave velocity (PWV), which was calculated from the third supraortic trunk to the diaphragmatic level on 4D-flow CMR [5]. The percentage of thrombus in the FL was calculated as the ratio of thrombus and FL volumes on MRA. Dominant entry tear area was quantified on the baseline CTA (Figure 1). Aortic growth rate (GR) was defined as the difference between final and baseline aortic diameters as measured on CTA divided by follow-up duration.

Results
Anatomical, haemodynamic and biomechanical parameters are shown in Table. Twenty-five patients have repaired type A AD with residual entry tear and 16 have type B AD. Mean follow-up duration was of 4.9±2.7 years. In bivariate analysis, WSS, IRF and PWV were positively related to GR, whereas dominant entry tear area and percentage of thrombus in the FL showed a positive tendency with GR (Table) (Figure). In multivariate analysis IRF, PWV, dominant entry tear area and thrombus in the FL were positively and independently associated with GR (Table). Retrograde systolic and diastolic flow were not related to GR while WSS tended to statistical significance.

Conclusions
In-plane rotational flow, regional aortic stiffness, dominant entry tear area and percentage of thrombus in the false lumen are positively and independently related to aortic growth rate in patients with chronic type B aortic dissection. Further prospective studies are needed to confirm if the assessment of these parameters may help to identify patients at higher risk of adverse clinical events.

Funding Acknowledgement
Type of funding sources: None. Table 1Figure 1