Trick of the Trade: Oblique CXR to look for pneumothorax
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?