Aortic shear stress in patients with bicuspid aortic valve with stenosis and insufficiency

Abstract

Objectives: Bicuspid aortic valve, characterized by valve malformation and risk for aortopathy, displays profound alteration in systolic aortic outflow and wall shear stress distribution. The present study performed 4-dimensional flow magnetic resonance imaging in patients with bicuspid aortic valve with right-left cusp fusion, focusing on the impact of valve function on hemodynamic status within the ascending aorta.

Methods: Four-dimensional flow magnetic resonance imaging was performed in 50 subjects with right-left bicuspid aortic valve and 15 age- and aortic size-matched controls with tricuspid aortic valve. Patients with bicuspid aortic valve were categorized into 3 groups according to their aortic valve function as follows: bicuspid aortic valve with no more than mild aortic valve dysfunction (bicuspid aortic valve control, n = 20), bicuspid aortic valve with severe aortic insufficiency (n = 15), and bicuspid aortic valve with severe aortic stenosis (n = 15).

Results: All patients with right-left bicuspid aortic valve exhibited peak wall shear stress at the right-anterior position of the ascending aorta (bicuspid aortic valve vs trileaflet aortic valve at the right-anterior position: 0.91 ± 0.23 N/m² vs 0.43 ± 0.12 N/m², P < .001) with no distinct alteration between bicuspid aortic valve with severe aortic insufficiency and bicuspid aortic valve with severe aortic stenosis. The predominance of dilatation involving the tubular ascending aorta (82%, type 2 aortopathy) persisted, with or without valve dysfunction. Compared with bicuspid aortic valve control subjects, the bicuspid aortic valve with severe aortic insufficiency group displayed universally elevated wall shear stress (0.75 ± 0.12 N/m² vs 0.57 ± 0.09 N/m², P < .01) in the ascending aorta, which was associated with elevated cardiac stroke volume (P < .05). The bicuspid aortic valve with severe aortic stenosis group showed elevated flow eccentricity in the form of significantly increased standard deviation of circumferential wall shear stress, which correlated with markedly increased peak aortic valve velocity (P < .01).

Conclusions: The location of peak aortic wall shear stress and type of aortopathy remained homogeneous among patients with right-left bicuspid aortic valve irrespective of valve dysfunction. Severe aortic insufficiency or stenosis resulted in further elevated aortic wall shear stress and exaggerated flow eccentricity.

Keywords: bicuspid aortic valve; magnetic resonance imaging; wall shear stress.

Figure 1. Representative Images and Analytical Planes for 4D Flow MRI

AAo indicated ascending aorta; and DAo, descending aorta.

Figure 2. Flow Patterns Visualization

Three-dimensional (3D) streamline visualization of peak systolic flow in a BAV patient of R-L fusion type with no more than mild aortic dysfunction (BAV-CTL, Middle) in comparison with an aortic size-matched TAV volunteer (TAV-CTL, Top). Note the presence of distinct 3D outflow jet directing towards the right-anterior wall of ascending aorta via right-handed helix (black dashed arrows) in the R-L BAV subject. Bottom panels showed 3D velocity-coded vectors with uniform velocity limit of 1.2m/s in BAV patients with severe aortic insufficiency (BAV-AI), severe aortic stenosis (BAV-AS), BAV-CTL, and TAV-CTL at mid-ascending aorta. Different distributions of high velocity vectors can be appreciated between groups in correspondence to differential exertion of shear forces. BAV indicates bicuspid aortic valve; TAV, tricuspid aortic valve; R-L, right and left coronary cusp fusion type; A, anterior; LA, left anterior; L, left; LP, left posterior; P, posterior; RP, right posterior; R, right; and RA, right anterior.

Figure 3. Segmental Systolic Wall Shear Stress Measurements

Segmental systolic wall shear stress (WSS) measurements were displayed across 8 anatomic positions (A, anterior; LA, left anterior; L, left; LP, left posterior; P, posterior; RP, right posterior; R, right; and RA, right anterior) at proximal ascending aorta, mid-ascending aorta, distal ascending aorta, and proximal descending aorta. Error bars represent standard deviation of individual WSS variation. Asterisks indicate statistical significance (p<0.0125 after Bonferroni correction) for pairwise comparison among 4 groups. AAo indicated ascending aorta; and DAo, descending aorta.

Figure 4. Relevant Parameters for WSS Deviation

Correlation analyses were performed between aortic diameter and WSS-SD_8P at the level of mid-ascending aorta in BAV-CTL, BAV-AI, and BAV-AS patients. Positive correlation was found between peak aortic valve velocity and WSS-SD_8P at mid-ascending aorta in BAV-AS patients. WSS-SD_8P indicated standard deviation of circumferential wall shear stress across 8 anatomic positions.