The formation of mixed blood in the atria is known as monoventricular. Monoventricular is a rare congenital heart disease, which is caused by the lack of development of both septum 1 and septum 2 of the atrial septum during embryonic development. Traces of the interatrial septum are also absent, and the interventricular septum is intact, so it is also called a two-chambered, three-chambered heart or a single-ventricle, three-chambered heart. A single atrium may exist alone, but it is often combined with a left superior vena cava and a right heart, a left heart or an abdominal visceral transposition deformity, and it is particularly common to have a cleft anterior mitral leaflet, or even to have an atrioventricular canal deformity. How to check for mixed blood formation in the atria? Symptoms and signs of mixed blood formation in the atria are similar to those of giant atrial septal defects and atrioventricular canal malformations. Shortness of breath and cyanosis on crying are common. Early onset of heart failure with progressive cyanosis and pestle fingers and toes, jet murmur in the area of the pulmonary valve, hyperactive fixed splitting of the 2nd tone, and systolic murmur of mitral insufficiency of closure in the apical area. Do an ambulatory electrocardiogram. Monoventricle needs to be differentiated from ventricular septal defect, complete anomalous pulmonary venous return, complete transposition of the great arteries, tricuspid atresia, and complete atrioventricular canal malformation. Clinical signs and symptoms of monoventricular are similar to those of large ventricular septal defects or atrioventricular canal malformations, but are characterized by early and severe onset of symptoms, cyanosis but increased pulmonary blood flow, and large left-to-right shunts at the level of the atria but no evidence of significant pulmonary hypertension. The disease is a congenital disorder, so there is no effective prophylaxis. However, early surgery should be pursued in children with a clear diagnosis, as long as severe obstructive pulmonary vascular lesions have not yet occurred. Good results are expected.