Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: A single portable semi-erect chest radiograph was obtained . The lung volumes are low . The mediastinal structures are shifted to the left . The left costophrenic angle is obscured . Cardiomegaly is moderate . The aortic arch is calcified .
    OUTPUT:
    
Leftward mediastinal shift likely due to left lower lobe atelectasis .