Left atrium 42mm is considered left atrial enlargement, whether it is serious can not rely on a single indicator to judge, left atrial enlargement can be seen in a variety of diseases, should be combined with other tests to determine the patient’s condition. The left atrium is generally measured by the anterior and posterior diameters of the left atrium to determine whether the left atrium is enlarged, generally the anterior and posterior diameters of the left atrium of a normal adult female are not more than 38mm, and the anterior and posterior diameters of the left atrium of a normal adult male are not more than 40mm, so the left atrium 42mm is the left atrium is enlarged, and it can be seen in hypertension, mitral stenosis, mitral valve closure insufficiency, dilated cardiomyopathy, and other diseases. Hypertension can lead to enlargement of the atria and ventricles due to increased afterload on the heart; mitral stenosis can lead to enlargement of the atria; mitral valve insufficiency can lead to enlargement of the atria and ventricles; and dilated cardiomyopathy can lead to enlargement of the whole heart; it is necessary to determine which disease is causing the left atrium to enlarge, and then combine with the results of other tests in order to determine the severity of the condition. If the examination finds that the left atrium is 42mm, please follow the doctor’s instructions to improve the relevant examination and treatment.