Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: There has been a previous median sternotomy and coronary artery bypass surgery . Heart size , mediastinal and hilar contours are within normal limits and without change . Lungs are clear except for minimal linear atelectasis at the bases . No definite pleural effusion .
    OUTPUT:
    
Linear bibasilar atelectasis . Low lung volumes .