Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: A portable frontal chest radiograph demonstrates a left chest wall pacer with the leads overlying the right atrium and ventricle . Low lung volumes result in increased prominence of the cardiac silhouette and bronchovascular crowding . A retrocardiac opacity could represent either atelectasis or pneumonia . Calcifications of the aortic knob are noted . There is no large pleural effusion or pneumothorax . The visualized upper abdomen is unremarkable .
    OUTPUT:
    
Low lung volumes . Retrocardiac opacity could represent either atelectasis or pneumonia .