Julia R. Coleman

Introduction Traumatic pneumothorax (PTX) is a common occurrence in thoracic trauma patients, with a majority requiring tube thoracostomy (TT) for management. Recently, the “35-mm” rule has advocated for observation of patients with PTX less than 35 mm on chest computed tomography (CT) scan. This rule has not been examined in chest x-ray (CXR). We hypothesize that a similar size cutoff can be determined in CXR predictive of need for tube thoracostomy. Methods We performed a single-institution retrospective review of patients with traumatic PTX from 2018 – 2022, excluding those who underwent TT prior to CXR. Primary outcomes were size of pneumothorax on CXR and need for TT; secondary outcome was failed observation, defined as TT more than four hours after presentation. To determine the size cutoff on CXR to predict TT need, area under the receiver operating curve (AUROC) analyses were performed and Youden’s index calculated (significance at p < 0.05). Predictors of failure were calculated using logistic regression. Results There were 359 pneumothoraces in 322 patients (94.4% blunt trauma, median injury severity score 14). Of these, 82 (22.8%) had a TT placed within the first four hours. Fifty-five of observed patients (21.2%) failed, and these patients had a larger PTX on CXR (8.6 mm [5.0 – 18.0 mm] versus 0.0 mm [0.0 – 2.3 mm] ( p < 0.001)). CXR PTX size correlated moderately with CT size (r = 0.31, p < 0.001) and was highly predictive of need for TT insertion (AUC 0.75, p < 0.0001), with an optimal size cutoff predicting TT need of 38 mm. Conclusion CXR imaging size was predictive of need for TT, with an optimal size cutoff on CXR of 38 mm, approaching the “35-mm rule.” In addition to size, failed observation was predicted by presenting lactic acidosis and need for supplemental oxygen. This demonstrates this cutoff should be considered for prospective study in CXR. Level of Evidence: III; Therapeutic/Care Management