Supine chest xrays have an extremely low sensitivity (12-24%) in detecting pneumothoraces. Because a pneumothorax layers anteriorly on an AP CXR film, the xray beam is perpendicular rather than tangential to the pneumothorax edge. This makes visualizing a small to moderate sized pneumothorax extremely difficult. So you are left to look for indirect signs such as a deep sulcus at the costophrenic angle or subcutaneous air. I’m often surprised at how large of a pneumothorax can be missed on CXR based on CT imaging. The image to the right shows a large left-sided pneumothorax on CT.
What if you have an equivocal bedside ultrasound result in looking for a pneumothorax, and the patient is too unstable to go to CT?
Matsumoto et al in Annals of EM just reported this great tip. Position a film plate oblique to the patient’s chest as shown in the above diagram from the article. “OPX” designates an Occult PneumothoraX. The x-ray beam is now more tangential to the edge of the anterior pneumothorax. This allow us to detect free air more easily.
Thanks to Dr. Fabio De Iaco and Dr. Gemma Morabito from MedEmIt for this CXR image.
Matsumoto S, Kishikawa M, Hayakawa K, Narumi A, Matsunami K, Kitano M. A method to detect occult pneumothorax with chest radiography. Ann Emerg Med. 2011, 57(4), 378-81. PMID: 20864214