Energy loss and adults with congenital heart disease: a novel marker of cardiac workload beyond right ventricular size.

Right ventricular (RV) dysfunction after biventricular repair is critical in most adults with congenital heart disease (ACHD). Conventional 2D magnetic resonance imaging (MRI) measurement is considered as a 'gold standard' for RV evaluation; however, addition information on ACHD after biventricular repair is sometimes required. The reasons why adjunctive information is required is as follows: (I) to evaluate the severity of cardiac burden in symptomatic patients with normal RV size and ejection fraction (EF), (II) to determine the optimal timing of invasive treatments in asymptomatic ones, and (III) to detect proactively a potential cardiac burden leading to ventricular deterioration, from a fluid dynamics perspective. Energy loss (EL) using 4D flow MRI is a novel non-invasive flow visualisation method, and EL using 4D flow MRI can be a potential marker of cardiac burden. EL is the energy dissipated by blood viscosity, and evaluates the cardiac workload related to the prognosis of heart failure. The advantages are as follows: EL can detect cardiac overload which integrates both afterload and preload. EL is an independent parameter of current heart failure or cardiac remodeling state, such as chamber size or ventricular wall motion. This parameter is based on intuitive and clear physiological concepts, suitable for flow measurements using inner velocity profiles without a pressure-volume loop. The possible clinical applications of EL are as follows: (I) to follow the temporal changes in each patient and (II) to calculate the percentage of cardiac burden by combining pressure data from catheterisation. Although EL appears to be an ideal marker of haemodynamics from a fluid dynamics perspective, EL measurement using 4D flow MRI has some limitations. Flow dynamics software is still being developed, both technically and methodologically, and its clinical impact on long-term outcomes remains unknown. Therefore, further studies are warranted.