This is a moderate pneumothorax. You can see the cardinal sign which is the air ventral to the heart on a lateral radiograph.When a dog is sitting in sternal recumbency or standing the air will be located in the dorsocaudal part of the pleural space. The lungs will have moved ventrally so lung sounds will be quieter in the dorsocaudal chest. The lung sounds in the cranioventral chest will be normal or harsher than normal.