Systematic Echocardiographic Assessment of the Univentricular Heart Across the Stages of Fontan Palliation: A Practical Guide with Focus on Emerging 3D and 4D Imaging Modalities and Thromboembolic Complications from AEPC Imaging Working Group

Although echocardiography remains the primary imaging modality for assessing Fontan palliation, a standardized systematic approach for evaluating the univentricular heart throughout the different stages of palliation has yet to be established. This document aims to provide a narrative review and practical guide for the echocardiographic assessment of the univentricular heart at various phases of Fontan palliation. Additional objectives include highlighting the potential of advanced three-dimensional (3D) and four-dimensional (4D) imaging modalities, as well as outlining a systematic strategy for detecting thromboembolic complications. We propose a sequential framework for echocardiographic evaluation, encompassing key anatomical and functional components of the univentricular heart. This includes the atrial septum and pulmonary veins, ventricular function, atrioventricular valve, aorta and neo-aorta, sub-aortic region, aortic arch, and pulmonary arteries. Furthermore, we detail the assessment of pulmonary blood supply at different stages of palliation, including the modified Blalock–Taussig–Thomas (mBTT) shunt, Sano conduit, Glenn procedure, and total cavo-pulmonary connection (TCPC). A comprehensive analysis of potential thrombus formation sites is provided, along with diagnostic pitfalls. Additionally, we outline methods for screening for extracardiac clots following Glenn and TCPC procedures. This document serves as a practical guide for the systematic echocardiographic evaluation of the univentricular heart across all stages of Fontan palliation, offering guidance for clinical practice. It also explores the capabilities of emerging 3D and 4D echocardiographic techniques in univentricular heart assessment and provides an in-depth review of thromboembolic complications, emphasizing key diagnostic challenges.