Left atrial enlargement of 45mm is considered mild. Patients are advised to seek immediate medical attention and follow the doctor’s instructions for active treatment to avoid accidents. Generally speaking, left atrial enlargement is called mild left atrial enlargement when it ranges from 35 to 50mm in adult women and 40 to 50mm in adult men. Left atrial enlargement is mainly caused by coronary heart disease, heart valve disease and other types of heart disease. Due to the increased pressure in the left atrium, patients are prone to palpitations, chest tightness, shortness of breath, and other circulatory discomforts, which in severe cases can lead to atrial fibrillation, thrombosis, and direct threat to the patient’s life. It is recommended that patients seek timely medical treatment and actively treat the condition after diagnosis by ECG and cardiac ultrasound. If atrial fibrillation exists, it is recommended to use aspirin, clopidogrel bisulfate and other drugs for anti-platelet aggregation treatment as prescribed by the doctor. Therefore, left atrial enlargement of 45mm is mild. It is recommended that the patient actively seek medical treatment and early treatment.