Fig. 1

Inferred atrial arrangement from the abdominal visceral situs. A Situs solitus: Normal atrial arrangement with right-sided right atrium (RA) and left-sided left atrium (LA). B Situs inversus: Mirror-image atrial arrangement with left-sided RA and right-sided LA. C Left atrial isomerism (LAI) with azygous continuation. D Left atrial isomerism with hemi-azygous continuation. In LAI, venous blood can return to the atria via the right superior vena cava (SVC), direct left SVC insertion or left SVC to coronary sinus (CS). There is direct hepatic drainage to the atriums, the CS is a left-sided structure and is often present, and whilst pulmonary veins often connect normally, they can drain abnormally (i.e. septal malposition or symmetrically to either side of the atrial septum). E Right atrial isomerism: cardiac anomalies are often more complex with inherent total anomalous pulmonary venous return, bilateral SVCs, absent CS and typically a common atrium. The concept of two morphologically identical atria/appendages is adopted for diagrammatic educational purposes only. Illustrations modified with permission from Geva L (2021) segmental approach to congenital heart disease. In Echocardiography in Pediatric and Congenital Heart Disease: From Fetus to Adult, 3rd edn, ch 3. Eds WW Lai, LL Mertons, MS Cohen, T Geva.