Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Portable upright chest film ___ at 04 :00 is submitted .
    OUTPUT:
    
Status post median sternotomy with valve replacement and stable postoperative cardiac and mediastinal contours . Persistent low lung volumes with patchy opacities at both bases likely reflecting atelectasis , although pneumonia can not be excluded . Probable small layering right effusion . No pneumothorax .