Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Median sternotomy wires , aortic valve replacement , mediastinal surgical clips are again noted , no change in alignment . There is persistent opacity at the left lung base , though aeration is improved from two days prior . Persistent linear opacities likely represent atelectasis . Left pleural effusion is small . Trace right pleural fluid is also present . Lungs are otherwise well aerated . There is no focal consolidation to suggest pneumonia . There is no vascular congestion or pulmonary edema . There is no pneumothorax .
    OUTPUT:
    
Improved aeration at the left lung base , with persistent linear atelectasis and small amount of pleural fluid .