Left atrium 40mm, which is a mild enlargement, is generally abnormal. The general size of the left atrium is 30-35mm, and the left atrium, 40mm, is mildly enlarged, and if it exceeds 40mm it indicates that the left atrium is significantly enlarged. The human heart normally has four chambers, including the left atrium, right atrium, left ventricle, right ventricle. The left ventricle and left atrium are important structures of the heart. There are many causes of left atrial enlargement, including hypertension, atrial fibrillation, rheumatic heart disease, mitral stenosis, mitral valve insufficiency, mitral valve prolapse, dilated cardiomyopathy and so on. Patients are advised to go to the hospital for further improvement of electrocardiogram, regular review of cardiac ultrasound and other related examinations, and actively search for the causes of enlargement. At the same time, avoid strenuous exercise to avoid aggravating the burden on the heart.