An enlarged heart shadow may be life-threatening. The cause of an enlarged heart shadow may be pericardial effusion or an enlarged heart, both of which may be life-threatening. 1. Pericardial effusion: When pericardial effusion reaches a certain amount, imaging will show an enlarged cardiac shadow, and a larger amount of effusion may cause cardiac tamponade, which is life-threatening. For those with pericardial effusion, it is necessary to drain the effusion in time to relieve the pressure on the heart. 2. Heart enlargement: heart enlargement indicates that there is a certain change in the heart structure, there is ventricular remodeling, which is the inevitable process of the development of heart failure, heart failure, such as acute exacerbation of the existence of triggers is likely to endanger the life of the person. To summarize, enlarged heart shadow is life-threatening and requires prompt medical attention to determine the cause of the enlarged heart shadow and active treatment.