The formation of mixed blood in the atria is known as unicentric atrium. Unicentric atrium is a rare congenital heart disease that results from the failure of both the first and second septum of the atrial septum to develop during embryonic development. Traces of the atrial septum are also absent, while the septum is intact, so it is also known as a two-chambered, three-chambered heart or a single-atrial, three-chambered heart. The single atrium may exist alone, but is often combined with a left superior vena cava and a right-sided heart, a left-sided heart or an abdominal visceral transposition malformation, and is particularly common with a cleft anterior mitral valve leaflet or even the presence of an atrioventricular canal malformation. What can cause mixed blood to form in the atria? The arterial and venous blood from the vena cava and pulmonary veins mix with each other in the single atrium. Due to the low filling resistance of the right ventricle, most of the blood enters the left ventricle, and only a portion of the blood flow from the pulmonary veins back to the atrium enters the left ventricle via the mitral valve and then enters the body circulation, so cyanosis may appear clinically. The oxygen saturation is almost the same in both sides of the atrium, ventricle, aorta and pulmonary artery. Univentricular combined with ectopic drainage of the vena cava is more common, such as the left superior vena cava draining into the coronary sinus or the left side of the common atrium, followed by the inferior vena cava draining through the odd or semi-oval vein and the hepatic vein directly into the right side of the common atrium, creating a mixture of blood in the atrium. Signs and symptoms are similar to those of giant atrial septal defect and atrioventricular canal malformation. Shortness of breath and cyanosis during crying and noise are common. Early onset of heart failure with progressive cyanosis and pestle-like fingers and toes, jet murmur in the pulmonary valve region, hyperactive fixed splitting of the 2nd tone, and systolic murmur of mitral valve closure insufficiency in the apical region. Single atrium needs to be differentiated from ventricular septal defect, complete anomalous pulmonary venous reflux, complete aortic misalignment, tricuspid atresia and complete atrioventricular canal malformation. The clinical signs and symptoms of monocentric atrium resemble those of a large atrial septal defect or atrioventricular canal malformation, but are characterized by early and severe onset of symptoms, cyanosis but increased pulmonary blood flow, and a large left-to-right shunt at the atrial level without evidence of significant pulmonary hypertension.