Impact of ascending aortic prosthetic grafts on early postoperative descending aortic biomechanics on cardiac magnetic resonance imaging

Abstract

Objectives: Among patients with ascending thoracic aortic aneurysms, prosthetic graft replacement yields major benefits but risk for recurrent aortic events persists for which mechanism is poorly understood. This pilot study employed cardiac magnetic resonance to test the impact of proximal prosthetic grafts on downstream aortic flow and vascular biomechanics.

Methods: Cardiac magnetic resonance imaging was prospectively performed in patients with thoracic aortic aneurysms undergoing surgical (Dacron) prosthetic graft implantation. Imaging included time resolved (4-dimensional) phase velocity encoded cardiac magnetic resonance for flow quantification and cine-cardiac magnetic resonance for aortic wall distensibility/strain.

Results: Twenty-nine patients with thoracic aortic aneurysms undergoing proximal aortic graft replacement were studied; cardiac magnetic resonance was performed pre- [12 (4, 21) days] and postoperatively [6.4 (6.2, 7.2) months]. Postoperatively, flow velocity and wall shear stress increased in the arch and descending aorta (P < 0.05); increases were greatest in hereditary aneurysm patients. Global circumferential strain correlated with wall shear stress (r = 0.60-0.72, P < 0.001); strain increased postoperatively in the native descending and thoraco-abdominal aorta (P < 0.001). Graft-induced changes in biomechanical properties of the distal native ascending aorta were associated with post-surgical changes in descending aortic wall shear stress, as evidenced by correlations (r = -0.39-0.52; P ≤ 0.05) between graft-induced reduction of ascending aortic distensibility and increased distal native aortic wall shear stress following grafting.

Conclusions: Prosthetic graft replacement of the ascending aorta increases downstream aortic wall shear stress and strain. Postoperative increments in descending aortic wall shear stress correlate with reduced ascending aortic distensibility, suggesting that grafts provide a nidus for high energy flow and adverse distal aortic remodelling.

Keywords: Aortic remodelling; Prosthetic graft; Thoracic aortic aneurysm.

Figure 1:

Pre- and postoperative descending aortic wall shear stress. Wall shear stress quantified by 4D flow CMR at landmarks in the aortic arch (top), mid-descending aorta (middle) and thoraco-abdominal aorta (bottom) partitioned by TAA aetiology (data shown as median [interquartile range]). Note that native wall shear stress differed in relation to TAA aetiology, with heritable patients manifesting higher wall stress compared to patients with degenerative aneurysms [asterisks refer to significant comparisons (P < 0.05) between heritable or BAV groups with degenerative TAA]. BAV: bicuspid aortic valve; TAA: thoracic aortic aneurysm.

Figure 2:

Downstream aortic flow and geometry in postoperative heritable TAA patients and normative controls. Aortic flow physiology [wall shear stress, velocity (left)] and geometric indices [area, maximum diameter (right)] as quantified in co-registered landmarks among normative controls and postoperative heritable TAA patients (blue = flow indices, grey = geometry). Note that wall shear stress and velocity were higher among heritable TAA patients who had undergone proximal grafting despite non-significant differences in aortic size (data shown as median [interquartile range]). TAA: thoracic aortic aneurysm.

Figure 3:

Ascending aortic distensibility in relation to downstream wall shear stress. Scatterplots demonstrating correlations between change in ascending aortic distensibility (pre- vs post-thoracic aortic aneurysm graft implantation) in relation to change in native aortic wall shear stress as measured in co-registered landmarks in the aortic arch (top) and mid-descending aorta (bottom).