Blood flow characteristics in the ascending aorta after aortic valve replacement--a pilot study using 4D-flow MRI

Abstract

Background: Aortic remodeling after aortic valve replacement (AVR) might be influenced by the postoperative blood flow pattern in the ascending aorta. This pilot study used flow-sensitive four-dimensional magnetic resonance imaging (4D-flow) to describe ascending aortic flow characteristics after various types of AVR.

Methods: 4D-flow was acquired in 38 AVR patients (n=9 mechanical, n=8 stentless bioprosthesis, n=14 stented bioprosthesis, n=7 autograft) and 9 healthy controls. Analysis included grading of vortex and helix flow (0-3 point scale), assessment of systolic flow eccentricity (1-3 point scale), and quantification of the segmental distribution of peak systolic wall shear stress (WSS(peak)) in the ascending aorta.

Results: Compared to controls, mechanical prostheses showed the most distinct vorticity (2.7±0.5 vs. 0.7±0.7; p<0.001), while stented bioprostheses exhibited most distinct helicity (2.6±0.7 vs. 1.6±0.5; p=0.002). Instead of a physiologic central flow, all stented, stentless and mechanical prostheses showed eccentric flow jets mainly directed towards the right-anterior aortic wall. Stented and stentless prostheses showed an asymmetric distribution of WSS(peak) along the aortic circumference, with significantly increased local WSS(peak) where the flow jet impinged on the aortic wall. Local WSS(peak) was higher in stented (1.4±0.7 N/m(2)) and stentless (1.3±0.7 N/m(2)) compared to autografts (0.6±0.2 N/m(2); p=0.005 and p=0.008) and controls (0.7±0.1 N/m(2); p=0.017 and p=0.027). Autografts exhibited lower absolute WSS(peak) than controls (0.4±0.1 N/m(2) vs. 0.7±0.2 N/m(2); p=0.003).

Conclusions: Flow characteristics in the ascending aorta after AVR are different from native aortic valves and differ between various types of AVR.

Keywords: 4D-flow; Aorta; Hemodynamics; Magnetic resonance imaging; Surgery; Valves.

Fig. 1

Methodological schematic: a) position of the analysis planes in the ascending aorta on the level of the sinotubular junction (S1), mid-ascending (S2) and distal ascending aorta (S3). b) Cross-section of the ascending aorta that describes the distribution of the segments along the aortic wall circumference. c) Peak systolic flow map at the level of the mid-ascending aorta demonstrating central systolic flow, mild and markedly eccentric systolic flow.

Fig. 2

Visualization of the blood flow in the ascending aorta using particle traces during peak systole for each type of AVR. Relevant anatomic and functional data are given for the individuals below.

Fig. 3

Visualization of the blood flow in the ascending aortic using streamlines during peak systole: A: Healthy volunteer with cohesive systolic streamlines with mild helical (a) and no vortical (b) flow. B: Two exemplary cases with AVR, who exhibited helical (a) and vortical (b) flow each graded as severe. a) A mechanical prosthesis (St. Jude Medical 21); b) a stented bioprostheses (Medtronic Freestyle 25). The helical flow is shown in a transverse cut plane, whereas the vortex is shown in a sagittal cut plane.

Fig. 4

Evaluation of vorticity, helicity and eccentricity of blood flow: The upper row shows the frequency of each score for the various AVR groups and controls. The lower row depicts the mean ± SD scoring results (*p < 0.05 vs. stentless, autografts and controls; ^†p < 0.05 vs. stentless, autografts and controls; ^‡p < 0.05 vs. controls; ^$p < 0.05 vs. stented, autografts, controls; ^║p < 0.05 vs. stentless, mechanical, autografts and controls).

Fig. 5

Segmental distribution of WSS_peak along the circumference of the aortic wall for aortic levels S1–S3 for all groups of AVR and the controls. The aortic segments 1–12 correspond to the anatomic regions as outlined in Fig. 1 (which is additionally displayed in miniature within the graphs here). Roughly, “3–5” = right, “9–11” = left, “12–2” = outer curvature, “6–8” = inner curvature. Asterisk indicates the presence of any significant inter-group difference at the given aortic location and refers to the statistical results of Table 2.