High-degree Norwood neoaortic tapering is associated with abnormal flow conduction and elevated flow-mediated energy loss.

Abstract Objective The Norwood neo-aortic arch biomechanical properties are abnormal due to reduced vessel wall compliance and abnormal geometry. Others have previously described neo-aortic geometric distortion by the degree of diameter reduction (tapering) and associated this with mismatched ventricular-neo-aortic coupling, abnormal flow hemodynamics, and worse patient outcome. Our purposes were to: 1) investigate the influence of neo-aortic tapering (i.e., diameter reduction) on flow mediated viscous energy loss (EL’) in post-Norwood palliated hypoplastic left heart syndrome (HLHS) patients, and 2) correlate flow-geometry with single ventricle power generation. Methods Twenty-six palliated HLHS patients underwent comprehensive cardiac evaluation with 4D-Flow MRI. Patients were grouped into high (group H, n=13) and low (group L, n=13) degree neo-aortic tapering using the median cut-off value of neo-aortic diameter variance. EL’ was calculated along standardized segments using 4D-Flow MRI. Flow mediated power loss as a percentage of total power generated by the single ventricle was determined. Results Group H had a higher prevalence of abnormal recirculating flow in the neo-aorta and elevated neo-aortic EL’ in the ascending aorta (1.0 vs. 0.6 mW, P = 0.004). Group H EL’ was increased across the entire thoracic aorta (2.6 vs. 1.3 mW, P = 0.002) and accounted for 0.7% of generated ventricular power vs. 0.3% in group L (P = 0.024). EL’ directly correlated with the degree of ascending aortic dilation (R = 0.49, P=0.012). Conclusion Patients with high degree neo-aortic tapering have significantly more perturbed flow through the neo-aorta and increased EL’. Flow-mediated energy loss due to abnormal flow represents irreversibly wasted power generated by the single right ventricle. In patients with high degree neo aortic tapering, EL’ was more than two-fold greater than low degree tapering patients. These data suggest that oversizing the Norwood neo-aortic reconstruction should be avoided and that patients with distorted neo-aortic geometry may warrant increased surveillance for single ventricle deterioration.