Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Portable frontal chest radiograph demonstrates stable mild cardiomegaly with unchanged mediastinal and hilar contours . When compared to chest radiograph dated ___ , there appears to be a slight increased retrocardiac opacification , most likely left lower lobe atelectasis . There is no pneumothorax , pleural effusion , or pulmonary edema . A left subclavian line is seen terminating in the upper right atrium .
    OUTPUT:
    
Retrocardiac opacification most consistent with left lower lobe atelectasis .