Aortic dilation in bicuspid aortic valve disease: flow pattern is a major contributor and differs with valve fusion type

Abstract

Background: Ascending aortic dilation is important in bicuspid aortic valve (BAV) disease, with increased risk of aortic dissection. We used cardiovascular MR to understand the pathophysiology better by examining the links between 3-dimensional flow abnormalities, aortic function, and aortic dilation.

Methods and results: A total of 142 subjects underwent cardiovascular MR (mean age, 40 years; 95 with BAV, 47 healthy volunteers). Patients with BAV had predominantly abnormal right-handed helical flow in the ascending aorta, larger ascending aortas (18.3±3.3 versus 15.2±2.2 mm/m²; P<0.001), and higher rotational (helical) flow (31.7±15.8 versus 2.9±3.9 mm²/s; P<0.001), systolic flow angle (23.1°±12.5° versus 7.0°±4.6°; P<0.001), and systolic wall shear stress (0.85±0.28 versus 0.59±0.17 N/m²; P<0.001) compared with healthy volunteers. BAV with right-handed flow and right-non coronary cusp fusion (n=31) showed more severe flow abnormalities (rotational flow, 38.5±16.5 versus 27.8±12.4 mm²/s; P<0.001; systolic flow angle, 29.4°±10.9° versus 19.4°±11.4°; P<0.001; in-plane wall shear stress, 0.64±0.23 versus 0.47±0.22 N/m²; P<0.001) and larger aortas (19.5±3.4 versus 17.5±3.1 mm/m²; P<0.05) than right-left cusp fusion (n=55). Patients with BAV with normal flow patterns had similar aortic dimensions and wall shear stress to healthy volunteers and younger patients with BAV showed abnormal flow patterns but no aortic dilation, both further supporting the importance of flow pattern in the pathogenesis of aortic dilation. Aortic function measures (distensibility, aortic strain, and pulse wave velocity) were similar across all groups.

Conclusions: Flow abnormalities may be a major contributor to aortic dilation in BAV. Fusion type affects the severity of flow abnormalities and may allow better risk prediction and selection of patients for earlier surgical intervention.

Keywords: aorta; bicuspid aortic valve; cardiac MRI; vascular function; wall shear stress.

Figure 1

Flow patterns in bicuspid aortic valve disease; (A) Normal flow pattern; (B) Right-handed helical flow; (C) Left-handed helical flow. The systolic flow angle (θ) is demonstrated on Figure B - the angle between the aortic mid-line (dashed) and the instantaneous mean flow vector at peak systole (arrow).

Figure 2

(A) Incidence of flow patterns in bicuspid aortic valve disease (BAV); (B) Mean rotational flow values in the different BAV flow patterns; (C) Mean ascending aortic diameter in the different flow patterns; Error bars indicate standard deviation; * p<0.05 compared to healthy volunteers (HV); † p<0.001 compared to HV

Figure 3

Wall shear stress (WSS) at all locations in the ascending aorta, in different bicuspid aortic valve flow patterns; (A) systolic WSS; (B) through-plane systolic WSS. A= anterior, LA= left anterior, L= left, LP= left posterior, P= posterior, RP= right posterior, R= right, RA= right anterior

Figure 4

Mean rotational flow (a) and peak systolic circumferentially-averaged wall shear stress (WSS) (b) in ascending aortic diameter tertiles; NS = not significant; AA/BSA = ascending aortic diameter/Body surface area

Figure 5

Proportion of patients with each ascending aortic flow pattern in the two main bicuspid aortic valve fusion sub-groups (RL-BAV and RN-BAV)

Figure 6

Wall shear stress (WSS) profiles in the leaflet fusion groups (RL-BAV and RN-BAV) with right-handed helical flow; Abbreviations as for Figure 3; *= p<0.05