Validating aortic stiffness parameters by cardiac magnetic resonance against mechanical testing of ex-vivo aortic aneurysms specimens

Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Instituto de Salud Carlos III, Sociedad Española de Cardiología Background Aortic stiffness is a predictor of incident cardiovascular events and mortality in the general population (1). Cardiovascular magnetic resonance (CMR) imaging allow to compute a number of parameters, such as pulse wave velocity (PWV), distensibility (AD) and aortic longitudinal strain, theoretically related to aortic wall stiffness, but no formal validation has ever been reported. Purpose To test the correlation between aortic PWV, AD and longitudinal strain and aortic wall stiffness as assessed via ex-vivo mechanical testing of aortic specimens. Methods Patients undergoing ascending aorta (AAo) aneurysms resection underwent a CMR study which included bSSFP images and a 4D flow CMR of the thoracic aorta. At surgery 2 AAo specimens (each 15x5 mm, one oriented in the longitudinal and the other in circumferential aortic direction) were extracted, maintaining a record of their in-vivo position and orientation, and frozen. Specimens were tested under control for extension force (I5558, INSTRON) and elongation was quantifies by laser video extensometer (2663–281, INSTRON). The Young's modulus (E), i.e. tangent of the stress-strain curve, was quantified at the patient’s diastolic pressure during CMR (2). AAo PWV was extracted from 4D flow CMR study and the product of Eh (h being wall thickness) was computed via the Moens-Korteweg equation using the mean AAo diameter(3,4). AD was quantified from cine CMR at the pulmonary artery level using ARTFUN software (INSERM U678, France) while AAo longitudinal strain was assessed as mean deformation of the AAo by tracking the aortic root during the cardiac cycle (5). Results Twenty patients were included: age 52.5 [37.3 – 71.0] years, 13 (65%) male, BSA 1.94 [1.74 – 1.94] m2. Most of patients had AAo aneurysms of degenerative origin and a tricuspid aortic valve, while 2 patients had a bicuspid aortic valve, and 9 patients had connective tissue disorders. Due to missing data, one AD a two PWV could not have been computed while the specimen in circumferential direction of a patient could not be tested. AAo PWV, Eh and AD were strongly associated with circumferential elastic modulus at diastolic blood pressure (R = 0.652, R = 0.602 and R = -0.502, respectively) (Figure 1). Strong association was also obtained between AAo longitudinal strain and longitudinal elastic modulus at diastolic blood pressure (R = -0.513). Conclusion Pulse wave velocity and the derived Eh product, aortic distensibility and aortic longitudinal strain as non-invasively assessed by CMR are reliable representation of aortic wall stiffness.