Don’t Believe the Headline: Ultrasound vs CXR in Traumatic Pneumothorax Diagnosis

Background: 

         The use of ultrasound is well established for trauma patients in the emergency department, with almost every patient receiving a FAST (Focused Assessment with Sonography in Trauma) examination as part of the “ABC’s” of trauma. Though the initial focus of the FAST exam was for detection of abdominal free fluid, the eFAST (Extended FAST) is more commonly used and adds thoracic windows helping to identify pneumothorax (Musthafa 2014).

Historically, the chest x-ray (CXR) was the initial imaging modality used during the primary survey to look for pneumothorax (PTX) but it is notoriously insensitive. Ultrasound has been found to have higher accuracy and confers the advantage of being performed rapidly at the bedside. Prior studies have noted sensitivities of ultrasound for pneumothorax ranging from 91-98%, in comparison to CXR, which ranges from 47-75% (Blaivas 2005;Chan 2020). This has led an array of clinicians to rely on ultrasound over CXR to make this diagnosis when the stakes are high, on our sickest trauma patients. As always, we should consider new data as it emerges.

Paper: Santorelli JE, Chau H, Godat L, Casola G, Doucet JJ, Costantini TW. Not so FAST- Chest ultrasound underdiagnoses traumatic pneumothorax. J Trauma Acute Care Surg. 2022 Jan 1;92(1):44-48. PMID: 34932040.

Clinical Question: Is ultrasound or supine CXR more sensitive in the detection of pneumothorax?

What they did:

• Retrospective study performed at a level 1 trauma center of patients diagnosed with pneumothorax from 2018-2020.
• Patients had to have a CXR and complete ultrasonography of trauma (CUST) performed prior to any interventions and prior to confirmatory CT chest imaging.
• CUST was done by a registered sonographer while the trauma team observed for any concerning findings.
• All images were then uploaded for attending radiologists to read in real time.
• CUST is a more comprehensive examination than the traditional eFAST. CUST imaging includes:
  • Seven abdominal regions
  • Evaluation of visceral organs for abnormalities
  • Pericardial sac for effusion
  • Four thoracic views for pneumothorax/hemothorax. The sonographer would evaluate for absence of lung sliding, and if identified, m mode utilized to assess for a “barcode” or “stratospere” sign
• The CUST scanning time was around 3 to 5 minutes and included all views that would be seen in a traditional eFAST.

Inclusion:

• Patient’s diagnosed with pneumothorax who also had a CXR and CUST prior to confirmatory CT chest imaging or thoracostomy placement.

Exclusion:

• Patients diagnosed with pneumothorax who did not receive routine CUST exam
• Patients who had thoracostomy tube placed prior to CUST
• Patients with occult pneumothorax; occult pneumothorax was defined as one visible on CT but not on CXR or US

Results:

• Primary Results
  • 568 patients were identified with diagnosis of pneumothorax
  • 362 patients were included in the study
  • 191 were found to have an occult pneumothorax
• Critical Results
  • For all patients diagnosed with Pneumothorax:
    • CUST sensitivity – 35%
    • CXR sensitivity – 43%
  • When removing occult pneumothoraces (171 patients)
    • CUST sensitivity – 65%
    • CXR sensitivity – 78%
    • Of patients with a false negative CUST, 50% went on to chest tube placement, with the majority of them done within 8 hours.

Strengths:

• This study asks a clinically relevant question.
• Ultrasounds performed included multiple views by registered sonographers.
• Trauma teams were able to assess the images in real time at bedside while the images were simultaneously uploaded for final radiologist read.
• Retrospective nature of study may eliminate Hawthorne effect, as ultrasounds obtained in a prospective study may be more rigorously performed than in this one.

Limitations:

• Single center retrospective study
• Patients included had confirmed diagnosis of pneumothorax.
  • Patients without confirmatory CT chest imaging were excluded which creates a massive selection bias.
  • There is no data on patients with false positive CUST or CXR. As a result, we are unable to assess specificity and calculate likelihood ratios for these imaging modalities.
• Many imaging reports did not specify whether pneumothorax was present or not; thus, further limiting sample size.
• The authors do not mention how they accounted for this missing data, aside from not including them in the study.
• In the trauma bay it is usually the emergency and/or trauma physician who is performing the eFAST. Lung ultrasound is not a common modality performed by registered sonographers nor do radiologists interpret these regularly. This feature of the study limits generalizability as well as applicability.
• The sonographer’s images were reviewed by radiologists/trauma attendings in real time; however, the images/clips were not reviewed after the fact to assess the interrater reliability of the CUST interpretations.
• There is no mention of any training for the data abstractors and if their performance was monitored during the study. For example: If there was a discrepancy in the charts between the sonographer’s, trauma attending’s, and/or radiologists’ CUST interpretation, which one was included in their data analysis?
• There is no mention of whether the abstractors were blinded to the study hypothesis, which could further introduce bias.

Discussion:

• Lung ultrasound in the trauma bay is important for its ability to rule in life threatening PTX. The EFAST is often used for its specificity, which cannot be assessed based on this studies data set limiting clinical utility of this study.
• Although M-mode was utilized, lung ultrasound is better performed as a dynamic scan in the hands of a sonographer who performs these regularly. Ultrasound is user dependent and oftentimes interpretations will vary widely based on experience.
• 50% of patients with non-occult PTX required chest tube placement within 8 hours.
  • This is an alarmingly high number.
  • This may reflect changing status of the patient over time but it is not clear from the manuscript why the chest tubes were placed.
  • Regardless, progression is irrelevant in terms of accuracy of US for detecting PTX.
• An initial CUST/eFAST performed in the trauma bay is done to triage life threatening pathology that may require immediate intervention, so perhaps a shorter time interval than 8 hours has more clinical relevance when it comes to eventual chest tube placement.
• Although a retrospective study can eliminate some bias of image acquisition as discussed above, prospective studies tend to give us better information on a topic and there have been several which refute this article’s conclusions.
• One of the largest, Chan et. all included 13 studies, 1271 patients, and 410 patients with a diagnosis of traumatic pneumothorax. This systematic review included only prospective studies and found a sensitivity of 91% for US and 47% for CXR (Chan 2020).
• Despite this article being thought provoking, it is hard to discount the several prospective studies that argue the opposite on this topic.

Author conclusion:

“Complete ultrasonography of trauma performed on initial trauma evaluation had lower sensitivity than CXR for identification of pneumothorax including clinically significant pneumothorax requiring tube thoracostomy. Using CUST as the primary imaging modality in the initial evaluation of chest trauma should be considered with caution”

Our conclusions:

 Although this paper suggests that US is inferior to CXR for pneumothorax, we would interpret this with caution given the retrospective nature of the data the massive selection bias and the way in which the ultrasound was performed and interpreted. Prior high-quality literature clearly supports the superiority of US and this should continue to be our primary modality.

References

1. Abdulrahman Y, Musthafa S, Hakim SY, Nabir S, Qanbar A, Mahmood I, Siddiqui T, Hussein WA, Ali HH, Afifi I, El-Menyar A, Al-Thani H. Utility of extended FAST in blunt chest trauma: is it the time to be used in the ATLS algorithm? World J Surg. 2015 Jan;39(1):172-8. doi: 10.1007/s00268-014-2781-y. PMID: 25205343.
2. Blaivas M, Lyon M, Duggal S. A prospective comparison of supine chest radiography and bedside ultrasound for the diagnosis of traumatic pneumothorax. Acad Emerg Med. 2005 Sep;12(9):844-9. doi: 10.1197/j.aem.2005.05.005. PMID: 16141018.
3. Chan KK, Joo DA, McRae AD, Takwoingi Y, Premji ZA, Lang E, Wakai A. Chest ultrasonography versus supine chest radiography for diagnosis of pneumothorax in trauma patients in the emergency department. Cochrane Database Syst Rev. 2020 Jul 23;7(7):CD013031. doi: 10.1002/14651858.CD013031.pub2. PMID: 32702777
4. Santorelli JE, Chau H, Godat L, Casola G, Doucet JJ, Costantini TW. Not so FAST-Chest ultrasound underdiagnoses traumatic pneumothorax. J Trauma Acute Care Surg. 2022 Jan 1;92(1):44-48. doi: 10.1097/TA.0000000000003429. PMID: 34932040.

Authors: