In the march 2013 issue of Western Journal of Emergency Medicine, a study done that has been described as having generalizability, as the ultrasound scans were by many different levels of physicians, prospectively, during a trauma assessment for pneumothorax, has caused quite a bit of discussion. Mostly due to some of the limitations of the study. It is great that a prospective study with some generalizability is seen, but I wonder about the details in the methodology. They begin by discussing the importance and relevance of ultrasound for pneumothorax:
“Rapid diagnosis and treatment of traumatic pneumothorax (PTX) is important to prevent tension physiology and circulatory collapse in patients with blunt and penetrating trauma. Supine chest radiograph (CXR) is traditionally employed; however, it misses up to 50% of PTXs.1 Thoracic ultrasound (TUS) was first described in 1995 for diagnosing PTX in humans when Lichtenstein noted that the absence of comet-tail artifacts and lung sliding were associated with PTX.2 Since then ultrasound has become a validated method of examining the pleura in multiple settings. In 2011 the Eastern Association for the Surgery of Trauma gave a level 2 recommendation for the use of ultrasound to identify PTX in its practice management guidelines.3 In most studies TUS has been found to have favorable results. In Lichtenstein’s study,2 TUS had a sensitivity and negative predictive value of 100% and 96.5%, respectively, for the detection of PTX in the intensive care unit setting.4 Dulchavsky5 subsequently demonstrated that this modality has a sensitivity of 95% in the detection of PTX in patients at a Level 1 trauma center. These reports used plain radiography as the gold standard: a diagnostic modality known to be inaccurate in the detection of PTX.6 In subsequent studies using dedicated chest computed tomography (CCT) as a reference standard, sensitivities of TUS have ranged widely from 49% – 98%, while finding that it is still consistently more accurate than supine CXR.7–13 Studies in which TUS is performed by emergency physicians (EP) for traumatic PTX have reported even higher sensitivities ranging from 86–97% with specificities of > 99%.14 While these latter numbers are desirable, they have the potential limitation of being less applicable due to a higher skill level of the sonologists involved. The actual performance of TUS for PTX would likely vary based on the sonologist’s skill and experience. The current investigation set out to determine the test characteristics of TUS for traumatic PTX in the hands of a large heterogenous group of potential sonologists representative of typical clinicians involved in trauma care.”
The full abstract is shown below:
“Introduction:
Prior studies have reported conflicting results regarding the utility of ultrasound in the diagnosis of traumatic pneumothorax (PTX) because they have used sonologists with extensive experience. This study evaluates the characteristics of ultrasound for PTX for a large cohort of trauma and emergency physicians.
Methods:
This was a prospective, observational study on a convenience sample of patients presenting to a trauma center who had a thoracic ultrasound (TUS) evaluation for PTX performed after the Focused Assessment with Sonography for Trauma exam. Sonologists recorded their findings prior to any other diagnostic studies. The results of TUS were compared to one or more of the following: chest computed tomography, escape of air on chest tube insertion, or supine chest radiography followed by clinical observation.
Results:
There were 549 patients enrolled. The median injury severity score of the patients was 5 (inter-quartile range [IQR] 1–14); 36 different sonologists performed TUS. Forty-seven of the 549 patients had traumatic PTX, for an incidence of 9%. TUS correctly identified 27/47 patients with PTX for a sensitivity of 57% (confidence interval [CI] 42–72%). There were 3 false positive cases of TUS for a specificity of 99% (CI 98%–100%). A “wet” chest radiograph reading done in the trauma bay showed a sensitivity of 40% (CI 23–59) and a specificity of 100% (99–100).
Conclusion:
In a large heterogenous group of clinicians who typically care for trauma patients, the sonographic evaluation for pneumothorax was as accurate as supine chest radiography. Thoracic ultrasound may be helpful in the initial evaluation of patients with truncal trauma.”
So what are the limitations? They describe a few of them:
The technique: “The TUS examination consisted of the consecutive sonographic interrogation of every intercostal space between the clavicle and the diaphragm on each hemithorax. Scans were performed in the mid-clavicular line. On the left side, if cardiac motion was encountered in the mid-clavicular line, the probe was moved laterally to the left anterior axillary line and the pleura seen in the remaining intercostal spaces was evaluated until the diaphragm/spleen was encountered. To use the ribs to assist in the identification of the rib spaces and the pleura, the probe was placed in a longitudinal plane for the entire exam.” So, would this have increased their sensitivity or specificity as they include all rib spaces? Not too sure. Is this truly generalizable if the technique is different than how most perform the quick E-FAST? no. The main reason for the technique, i imagine, is to find the lung point which is far more specific for pneumothorax.
The Probe and Machine – The low frequency curvilinear probe was used on an older ultrasound system – SonoSite Titan. Could this have affecte their results? Would the increased resolution of a linear probe have helped their evaluation on the newer machines? It is possible, but by how much? who knows.
The comparison group: “Not all subjects underwent CCT and instead just had CXR and clinical observation. It is possible that some patients in this latter group had radio-occult PTX that may have been visualized on CCT leading to misclassification bias. Such a bias could result in a lower sensitivity rate for both TUS and CXR, however would likely not affect the accuracy of these tests for determining clinically significant PTX.” It is tough to have a standard and if only the chest CT group were compared, it may have had different results.
Im hoping to see more studies like this one where more generalizability is seen, and not studies done only by the experts, so that we can have a true assessment. It is best done using the technique most commonly performed (using only the second intercostal space and mid clavicular line and trying to ind the lung point if absence of lung sliding is seen) at multiple-sites, with increased power to the study, all compared to a CT as the imaging gold standard. But, i can dream, as that is quite difficult to accomplish, and the authors did a pretty nice job with what they had, got pretty good numbers of subjects – something to ponder….
For a prior post on pneumothorax and a link to the CHEST meta-analysis, go here.
For a SonoTutorial post on pneumothorax ultrasound, go here.
Another study stating that ultrasound can be used to assess post=procedure pneumothorax published in June 2013 of JUM, go here.
SonoSpot: Topics in Bedside Ultrasound 