Occult pneumothorax is the most common incidental finding on CT imaging, occurring in 2% to 10% of trauma patients. By definition, an occult pneumothorax is a pneumothorax that is seen only on CT and not a conventional chest x-ray. When detected, the question that comes to mind is, will this patient need a chest tube?
The AAST conducted a trial encompassing the experience at 16 Level I and II trauma centers around the US. They looked at injury severity, specific chest injuries, ventilator settings if on positive pressure ventilation (PPV) and size of pneumothorax. The size was calculated by measuring the largest air collection along a line perpendicular to the chest wall (see image above). Failure of observation meant that a thoracostomy tube was placed.
The 2 year study looked at a total of 448 occult pneumothoraces that were initially observed. Key findings of the study were:
Injury severity was no different between failure and non-failure groups
There was a 6% failure rate overall
PPV alone was associated with an increased failure rate of 14%
Surgical intervention requiring PPV was not associated with an increased failure rate
Pneumothorax size > 7mm, positive pressure ventilation, progression of the pneumothorax, respiratory distress and presence of hemothorax were associated with failure.
Pneumothorax size was not entirely reliable for predicting failure, since patients with sizes as small as 5mm on PPV and 3mm not on PPV failed in this series
Bottom line: Most blunt trauma patients with an occult pneumothorax can be safely observed. A followup chest x-ray should be obtained to look for progression. If the patient progresses, is placed on PPV, has a hemothorax or develops respiratory distress, have a low threshold for inserting a drainage tube. Maximum pneumothorax size may predict failure when large, but it can still happen with very small air collections.
An occult pneumothorax is one that is seen only on CT scan, but not on conventional chest x-ray. They are noted in somewhere between 2% and 10% of major blunt trauma patients. Although management is usually conservative, this has not been well studied.
A paper was presented at the AAST earlier this year based on a prospective, multicenter trial. The authors attempted to determine what factors were predictive of failure of observation. They were able to quantify the size of the pneumothorax by measuring a line from the largest collection to the chest wall.
Sixteen centers participated and enrolled 569 patients, who had 588 occult pneumothoraces. Of those, 21% had immediate chest tube drainage (no reasons were given). The remaining 448 patients were observed, and 27 of those patients failed. Failure was determined if they had progression of the pneumothorax, developed respiratory distress, or developed a hemothorax.
Risk factors were found to be: positive pressure ventilation (14% of observed group failed), size > 7mm, respiratory distress.
The authors recommend that patients with respiratory distress and those placed on positive pressure ventilation have a drainage system inserted. Those with pneumothoraces greater than 7mm bear close watching.
Our practice is to monitor any patient with an occult pneumothorax with a followup chest x-ray (one view only) performed after six hours. If the pneumo is still not visible, no further observation is done. If it becomes visible, serial 6 hour x-rays are obtained until it is stable or requires a chest tube.
Reference: Management of blunt traumatic occult pneumothorax: is observation harmful? Results of a prospective multicenter study. Forrest O Moore, et al. Paper #5 presented at 69th Annual AAST Meeting, September 22, 2010.
Occult pneumothorax occurs somewhere between 2% and 12% in all blunt trauma patients. Many of these pneumothoraces never progress and thus never need treatment. Is there a way that we can identify ones that are likely to get worse?
A retrospective study of 283 blunt trauma patients with occult pneumothorax was presented at the EAST Annual Scientific Assembly last January. A total of 98 of these patients underwent chest tube insertion within 7 days, and 185 patients were successfully observed.
The authors noted an inverse relationship between age and successful conservative management. Patients with more serious injuries failed expectant management more frequently. Finally, patients with more rib fractures also tended to fail.
The authors estimated the risk of failure of expectant management based on these critieria and found:
Age > 35 – 36%
ISS > 24 – 20%
Rib fractures >= 4 – 53%
The risk with having none of these was 10%, and the risk with all was 75%!
The time interval for placement was also interesting. 80% of the failures requiring a chest tube occurred within 24 hours, with most occurring in the first 2 hours. The authors also found that 40% of patients who were placed on a ventilator failed.
Obviously, this is a small retrospective study and the exact criteria for placing a chest tube were not specified. Nevertheless, it provides a simple tool that allows us to keep an eye on a subset of patients who are likely to fail observation of occult pneumothorax.
Reference: Factors Predicting Failed Observation of Occult Pneumothoraces in Blunt Trauma. Selander, Med Univ of South Carolina. EAST 2010 Annual Scientific Assembly.
Occult pneumothorax is a pleural air collection that is seen only on CT. It is not detected by standard chest xray either because of small size, location of the air, or position of the patient during xray (usually supine).
Approximately 15% of major trauma patients undergoing CT are diagnosed with an occult pneumothorax. The tough question is, what to do about it. Larger pneumothoraces are frequently treated with thoracostomy, but this procedure has its own list of associated complications. Patients undergoing positive pressure ventilation with a visible pneumothorax have an increased risk for progression to tension pneumothorax.
At our trauma center, we manage occult pneumothorax expectantly. If a pneumothorax is seen on the chest portion of a CT scan but not on the initial supine chest xray, a repeat conventional chest xray is scheduled for 6 hours later. Ideally, this xray is taken using the best technique (upright, PA, xray source 6ft from patient). However, this is not always practical for severely injured patients.
If the pneumothorax remains occult on the followup xray, no further monitoring is performed. If the pneumothorax becomes visible, repeat chest xrays are obtained every 6 hours until it is stable or it becomes large enough to warrant insertion of a chest tube.
How large is large enough for a chest tube? That’s the subject for another day.
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