An occult pneumothorax is one that is visible on chest CT but not conventional chest xray. The pneumo can be a single bubble, or it can be a larger one that layers out over the lung but cannot be seen on plain xray. This air is generally watched for a period of time, typically 6 hours, then a repeat plain radiograph is obtained to see if it has become visible. 

The pneumothorax literature cautions us about watching visible pneumothoraces in patients who are placed on positive pressure ventilation. The rationale is that this may force more air out of an acutely injured lung, resulting in an enlarging pneumothorax. Many have recommended that a chest tube be placed in any patient with a visible pneumothorax on positive pressure ventilation to avoid the possibility of developing a tension pneumothorax.

But what about the occult pneumothorax? Since they are generally very small, do they pose the same risk? A paper from 2008 retrospectively reviewed 79 patients with occult pneumothorax , 20 of whom were placed on ventilators. 51 of 59 of the non-ventilated patients had no change in their occult pneumo (86%), while 16 of 20 of the ventilated patients had no progression (80%).

The study numbers are small, but suggest that occult pneumothoraces can be safely watched. The real question is, how long do you have to watch it? Typically, ventilated patients get regular chest xrays, so monitoring for progression of the pneumo should be easy.

Reference: American Surgeon 74(10):958, 2008.