Occult pneumothorax in patients with blunt chest trauma: key findings on supine chest radiography

Introduction

Blunt trauma to the chest is the second leading cause of mortality among all blunt trauma-related deaths, accounting for up to 20% of all deaths (1). Traumatic pneumothorax and rib fractures are common injuries found in chest trauma patients, and they can lead to severe complications like tension pneumothorax, which requires immediate treatment (2). In multiple trauma patients, a supine chest radiography [chest X-ray (CXR)] is preferred over an erect CXR due to inconsistent vital signs or devices such as pelvic fixations. However, this method has limitations in diagnosis because blood and air may overlap with normal lung markings, making pneumothorax or hemothorax difficult to detect (3). The use of multidetector chest computed tomography (CT) has been shown to increase the likelihood of detecting traumatic pneumothorax or hemothorax that may have been missed when conducting a supine CXR by 20–30%. However, the precise data on the frequency and subsequent treatment of traumatic occult pneumothorax (OP) is currently limited and under investigation (4,5).

Generally, traumatic pneumothorax can be quickly treated with a closed drainage technique (3). Radiologically, this is described as intrapleural gas or air between the parietal and visceral pleural layers on a CXR or chest CT (4). However, a subsequent chest CT may be delayed or not conducted in patients with major trauma due to unstable vital signs, ongoing resuscitation, or emergent procedures. A pneumothorax that is not detected before a chest CT scan is performed is known as an OP. Therefore, detecting an OP early on is critical because it can progress into a tension pneumothorax over time, especially in situations where positive ventilation is required.

The aim of this study was to investigate the frequency and risk factors of OP in blunt chest trauma patients when a chest CT scan is difficult to achieve. We also investigated the supine CXR-based diagnosis of injuries seen in trauma patients. The findings may help clinicians improve the primary survey and management of patients with blunt chest trauma. We present this article in accordance with the STARD reporting checklist (available at https://jtd.amegroups.com/article/view/10.21037/jtd-23-541/rc).

Methods

Study cohort and data collection

The patients included in this study were who suffered thoracic trauma at the Level 1 Regional Trauma Center of Wonju Severance Christian Hospital between 2015 and 2022. All patients were at least 18 years old. The study acquired clinical information from electronic medical records, including age, gender, mechanism of injury, Injury Severity Score (ISS), chest Abbreviated Injury Scale (AIS), image study, time of imaging, and procedural interventions (6,7). An AIS score of 3 or higher was characterized as severe trauma. The study excluded certain patients, including those with cut or penetrating injuries, as well as patients who did not undergo a supine CXR or chest CT during the study period. Additionally, subjects for whom data acquisition and analysis could not be obtained due to insufficient medical records were also excluded from the study. The researchers examined all the patients’ supine CXR and chest CT images and classified them into five radiographic diagnoses: pneumothorax, rib fracture, subcutaneous emphysema, lung contusion, and pneumomediastinum. One out of three thoracic surgeons with Trauma specialty immediately confirmed CXR at the patient’s Trauma Center visit, and at Trauma rounding the next day, two of thoracic surgeons and one radiologist checked the images together and made a diagnosis. Figure 1 shows the numerous radiographic findings of OP in supine CXR and chest CT. The researchers calculated the sensitivity, specificity, positive predictive value, and negative predictive value for pneumothorax diagnosis on supine CXR versus chest CT. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the institutional review board of Wonju Severance Christian Hospital (No. CR322100) and individual consent for this retrospective analysis was waived.

Figure 1 Various supine chest radiography and chest computed tomography findings of occult pneumothorax in Blunt chest trauma. (A,C,E) Arrows show the subcutaneous emphysema, rib fracture and lung contusion in supine chest radiography. (B,D,F) Arrows show the subcutaneous emphysema, rib fracture, lung contusion and traumatic pneumothorax in same patient. AP, anterior-posterior; Lt, left; R, right.

Statistical analysis

The t-test was employed to evaluate continuous variables, while categorical variables were evaluated using the chi-squared test. Means, proportions, and associated standard deviations were computed where applicable. For descriptive statistics, Pearson χ² tests were used to evaluate related factors between the two groups, and binary logistic regression analysis was employed to assess risk factors. In univariate analysis, factors that would have affected pneumothorax were analyzed, and in multivariate analysis, factors that affected pneumothorax were analyzed and significant results were presented in a table. All significance tests were two-sided, with a P value <0.05 deemed significant. All statistical analyses were carried out using SPSS ver. 26 (SPSS, Chicago, IL, USA), SAS statistical software version 9.4 (SAS Institute, Cary, North Carolina, USA), and R version 4.2.1 (R core Team, Vienna, Austria).

Results

Of a total of 1,836 patients originally included in the analysis, 552 cases were then excluded due to penetrating injuries, a lack of supine CXR or chest CT, drowning, or burns. The study included 1,284 patients, with confirmed diagnoses of lung contusion, rib fracture, pneumothorax, subcutaneous emphysema, and pneumomediastinum on supine CXR and chest CT. Table 1 shows the general characteristics of the included patients. The average age was 58.3±15.2 years, and the average ISS was 18.9±9.5. Traffic accidents were the most common cause of injury (690 cases), followed by falls (344 cases), collisions with objects (126 cases), slips (77 cases), mechanical press accidents (26 cases), and assaults (21 cases). Most Chest AIS score were 3, accounting for 79.5% of the total cases.

On supine CXR, rib fractures were diagnosed in 863 cases, lung contusions in 497 cases, subcutaneous emphysema in 270 cases, traumatic pneumothorax in 100 cases, and pneumomediastinum in 13 cases, according to Table 2. On chest CT scans, 1,184 cases had rib fractures, 1,130 had lung contusions, 478 had subcutaneous emphysema, 783 had traumatic pneumothorax, and 66 had pneumomediastinum. The total number of OP cases was 683. Compared to chest CT scan, the accuracy and sensitivity of pneumothorax diagnosis on supine CXR were lower with 46.7% and 12.7%, respectively. Accuracy, sensitivity, specificity, positive predictive value, and negative predictive value of CXR diagnoses are presented in Table S1.

Table 3 shows the results of univariate analysis. Rib fracture, lung contusion, and subcutaneous emphysema on supine CXR were all statistically significant concerning traumatic OP with a P<0.05. Although, when the AIS was clustered into two groups by score, there was no significant finding in supine CXR for OP for in the group with an AIS score of 2 or less. In contrast, in the group with AIS of 3 or higher, rib fracture, lung contusion, and subcutaneous emphysema on supine CXR were all determined to be statistically significant (P<0.05) in OP. In multivariate analysis, the risk factors for OP were lung contusion [odds ratio (OR), 1.440; 95% confidential interval (CI): 1.115–1.860; P=0.005] and subcutaneous emphysema [OR, 25.883; 95% CI: 13.155–50.928; P<0.001] on CXR which were expressed in Table 4. Furthermore, there was no risk factor in the low AIS group, but as expressed in Table 4, in the high AIS group, rib fracture was P=0.002 (OR, 1.544; 95% CI: 1.173–2.033) and lung contusion was P=0.001 (OR, 1.580; 95% CI: 1.208–2.066), with subcutaneous emphysema P<0.001 (OR, 24.930; 95% CI: 12.612–49.282).

Discussion

Trauma patients who visit the emergency department are usually studied with CXR before having a torso CT scan. However, CT scans are frequently avoided when the patient’s vital signs are unstable. The most common injuries identified on a supine CXR include pneumothorax, rib fracture, subcutaneous emphysema, lung contusion, and pneumomediastinum. However, OP’s situation differs slightly. If a pneumothorax is not detected on a supine CXR, decompression may not be essential, especially if the pneumothorax is small or positive pressure ventilation is not required. Nevertheless, in critical situations, an undetected pneumothorax on CXR may progress to a delayed pneumothorax while the patient is being transported to another procedure or admission. OP is especially dangerous in trauma patients with compromised cardiorespiratory reserve or those requiring positive-pressure mechanical ventilation. It frequently delays life-threatening signs and increases the risk of tension pneumothorax (8-10). Therefore, identifying risk factors for OP based on supine CXR results is essential.

Based on these identified risk factors, treatment decisions can be made for patients at significant risk of developing OP. These risk factors can also help achieving early treatment decisions during resuscitation in critically injured patients because supine CXR has low diagnostic sensitivity (2). In the phase 3 retrospective study of 338 patients with severe injuries, subcutaneous emphysema (OR, 5.47), lung contusion (OR, 3.25), and rib fracture (OR, 2.65) were recognized as clinical risk factors for OP (11). However, in a following prospective level-II study by the same group, only subcutaneous emphysema stayed independently predictive of OP (12). It should be known that while 15% of patients with OP have concurrent subcutaneous emphysema, nearly all subcutaneous emphysema patients have an underlying overt or OP.

Risk factor of OP in blunt trauma

In our study, univariate analysis revealed that all injuries found on supine CXR were associated with pneumothorax, but this is not considered statistically significant because chest injuries are typically complexly associated. However, in multivariate analysis, lung contusion, and subcutaneous emphysema were determined as statistically significant risk factors for OP in all patients. Furthermore, rib fracture was discovered to be a risk factor for OP in the group with an AIS of 3 or severe chest injury. According to the findings of this study, if the parietal pleura ruptures due to rib fracture, it can lacerate the visceral pleura of the lung, leading to the accumulation of air in the thoracic cavity. The air in the thoracic cavity then passes through the injured parietal pleura, resulting in subcutaneous emphysema. Additionally, severe lung injury caused by inertial deceleration appears as lung contusion on a supine CXR and causes air leakage from the lung parenchyma. All these findings observed on a supine CXR strongly indicate the presence of OP. Further research is needed to develop an improved diagnostic method for OP, potentially utilizing a scoring system or artificial intelligence analysis of supine CXRs. This research should consider factors such as the quantity of rib fractures, lung contusion, and subcutaneous emphysema.

Clinical values of supine CXR in blunt chest trauma

The increased use of CT scans and chest ultrasounds in the investigation of thoracoabdominal blunt trauma results in the detection of pneumothorax, which cannot be diagnosed using traditional supine CXR (2,13,14). In polytrauma patients, initial evaluation is often performed with focused assessment with sonography for trauma (FAST). Recently, ultrasound has been widely used in trauma patients, with reported sensitivity of 92–100% for pneumothorax diagnosis. However, there may be rare cases where ultrasound cannot be used for any situation of emergency room. And although using ultrasound, the presence of subcutaneous emphysema can limit the field of view, making it challenging to diagnose pneumothorax using ultrasound. Paradoxically, subcutaneous emphysema can be considered an overt predictor of pneumothorax. This evidence is unsurprising given that supine CXR is the least sensitive of all commonly used radiographic techniques for detecting pneumothorax (8). Furthermore, the percentage of OP in the supine CXR ranged from 29% to 72% (2,8,13,15-17).

In our study, 261 out of 270 patients (96.7%) with subcutaneous emphysema on CXR were diagnosed with pneumothorax on CT, whereas 522 out of 1,014 patients (51.5%) without subcutaneous emphysema on CXR were diagnosed with pneumothorax on CT in all included patients. Also, among 1,184 patients without pneumothorax on CXR, 218 out of 227 patients (96.0%) with subcutaneous emphysema on CXR were diagnosed with pneumothorax on CT, while 465 out of 957 patients (48.6%) without subcutaneous emphysema on CXR were diagnosed with pneumothorax on CT. According to the findings, the absence of subcutaneous emphysema cannot rule out an OP. Inspecting the 9 patients with subcutaneous emphysema in CXR but no pneumothorax in chest CT from the above paragraph, all patients had lung diseases such as tuberculosis, bronchiectasis, and emphysema. These lung diseases cause severe pleural adhesions on the entire pleura and can be attributed to the inhibition of pneumothorax in chest CT. The specificity of CXR in detecting thoracic trauma was confirmed to be almost 100% in all chest trauma diagnoses. In other words, most findings that were not visible on CT did not appear on CXR. Pneumomediastinum was found in 66 patients on chest CT, but in 13 patients in CXR, the detection rate was very low. CXR had the highest sensitivity in detecting rib fractures, at 72.9%.

Consequently, an OP cannot be detected using CXR but can be identified using a CT scan. Although CT scans are widely used nowadays, identifying OP using plain CXR is critical when multiple trauma patients require mechanical ventilation or when CT scans are difficult to perform due to poor vital signs, time constraints, or the need for immediate emergency surgery. Failure to promptly detect and treat an OP can result in the development of tension pneumothorax, which can be fatal.

This study has a significant limitation in that it only represents the experience of a single institution. Furthermore, the study was conducted retrospectively and relied on accurate and complete electronic medical records. And this study only involved patients with blunt trauma. Cases where pneumothorax was fully evident on initial CXR, were immediately treated with chest tube insertion before a CT scan. Therefore, the included pneumothorax cases in this study may not presentative. Lastly, in cases where vital signs remained unstable despite vigorous resuscitation, these patients were unable to undergo a chest CT scan due to the immediate need for a procedure or operation. Therefore, the included pneumothorax cases in this study may not be representative of critically injured patients. The decision to insert a chest tube should not depend solely on the size of the pneumothorax.

Conclusions

The presence of lung contusion and subcutaneous emphysema in trauma patients underwent CXR, indicates the existence of OP. Therefore, if chest CT scan cannot be performed, it may be necessary to recognize the above clinical signs of OP and consider performing preventive procedures.

Acknowledgments

Funding: None.

Footnote

Reporting Checklist: The authors have completed the STARD reporting checklist. Available at https://jtd.amegroups.com/article/view/10.21037/jtd-23-541/rc

Data Sharing Statement: Available at https://jtd.amegroups.com/article/view/10.21037/jtd-23-541/dss

Peer Review File: Available at https://jtd.amegroups.com/article/view/10.21037/jtd-23-541/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jtd.amegroups.com/article/view/10.21037/jtd-23-541/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the institutional board of Wonju Severance Christian Hospital (No. CR322100) and individual consent for this retrospective analysis was waived.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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