Using CT to Diagnose Tracheal Rupture

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1 Jen-Dar Chen 1,2 Kathirkamanathan Shanmuganathan 2 Stuart E. Mirvis 2 Karen L. Killeen 2 Richard P. Dutton 3 Received July 3, 2000; accepted after revision October 25, Department of Radiology, Veterans General Hospital, Taipei and National Yang-Ming Medical School, Taipei, Taiwan. 2 Department of Radiology and Shock Trauma Center, University of Maryland School of Medicine, 22 S. Greene St., altimore, MD ddress correspondence to K. Shanmuganathan. 3 Department of nesthesiology and Shock Trauma Center, University of Maryland School of Medicine, altimore, MD JR 2001;176: X/01/ merican Roentgen Ray Society Using CT to Diagnose Tracheal Rupture OJECTIVE. retrospective study was performed to assess CT sensitivity for diagnosing tracheal rupture. Intubated cadaver tracheas were examined to assess endotracheal tube balloon overdistention and deformity and to evaluate the relationship of balloon pressures to tracheal injury. MTERILS ND METHODS. Neck or chest CT scans of 14 patients with tracheal rupture and 41 control trauma patients with pneumomediastinum but without tracheal injury were reviewed and compared to assess the presence and location of extrapulmonary air, whether direct visualization of tracheal wall disruption was possible, the size and shape of endotracheal tube balloon, signs of transtracheal balloon herniation in intubated patients, and the location of the extratracheal endotracheal tube. Intact and experimentally injured cadaver tracheas were used to evaluate tube balloon pressure and configuration. RESULTS. ll 14 patients with tracheal rupture had deep cervical air and pneumomediastinum. Overdistention of the tube balloon occurred in 71% (5/7) of the intubated patients, and balloon herniation occurred in 29% (2/7). Direct tracheal injury was seen in 71% (10/14) of the patients as a wall defect (n = 8) or deformity (n = 2). Overall, CT was 85% sensitive for detecting tracheal injury. Patients with tracheal injury had a significantly lower incidence of pneumothorax (p = 0.01) than did the control group. The CT appearance of balloon herniation through defects in the cadaver tracheas closely mimicked those of patients with tracheal injury. The amount of balloon pressure required to rupture the intubated trachea was extremely high and rupture was difficult to obtain. CONCLUSION. CT can reveal tracheal injury and can be used to select trauma patients with pneumomediastinum for bronchoscopy, leading to early confirmation and treatment. T horacic trauma causes 25% of the annual traffic deaths in the United States. mong patients sustaining chest injury, tracheobronchial injury is rare, occurring in only 0.8 2% [1 3]. Tracheal rupture constitutes approximately 15 27% of all tracheobronchial injury and is associated with an even higher morbidity and mortality [1, 4 7]. Most major airway injuries are not recognized initially. Diagnosis of tracheal rupture may be delayed as a result of its rare incidence, subtle and nonspecific clinical and radiologic manifestations, and the much more overt clinical signs of other more common associated injuries [5, 8]. Delayed or missed diagnosis can result in death or severe complications including ventilatory failure, mediastinitis, sepsis, airway stenosis, bronchiectasis, recurrent pulmonary infections, and permanent pulmonary function impairment [3, 7, 8]. CT is being used with increasing frequency to facilitate detection of chest injuries in patients with severe multisystem trauma and has proved useful in diagnosing unsuspected thoracic injuries [9, 10]. We undertook a retrospective study of patients admitted to our trauma facility who were diagnosed with tracheal injury. First, CT studies were reviewed to determine findings that indicate tracheal injury and to assess their sensitivity, specificity, and accuracy for diagnosis. Second, differences in CT findings that were related to penetrating versus blunt tracheal injury were assessed. Finally, using intubated cadaveric tracheas, the relationships among endotracheal balloon pressure, balloon configuration, and tracheal disruption were evaluated. JR:176, May

2 Chen et al. Materials and Methods Study Population The trauma radiology computer database (ccess [1997 version]; Microsoft, Redmond, W) and the trauma registry at our level 1 trauma center were searched for the 5-year period of January 1995 to December 1999 to identify patients with tracheal injury. There were 31 patients identified with major airway injuries including 14 patients with tracheal injury and 17 patients with bronchial injury. These 14 patients with isolated tracheal injury form the study population and include 10 males and four females, ranging in age from 14 to 62 years (mean age, 28 years), who underwent CT that included the neck (n = 8) or thorax (n = 14) at admission. The mechanism of injury included blunt force trauma (n = 8), penetrating injury (n = 5), and probable iatrogenic injury (n = 1). lunt force trauma (n = 8) resulted from motor vehicle collision (n = 4), assault (n = 2), horse-riding accident (n = 1), and bicycle collision (n = 1). Penetrating injury (n = 5) resulted from gunshot wounds (n = 3) and stab wounds (n = 2). Endotracheal intubation was performed before CT in 50% (7/14) of the patients in the study group. The original CT interpretations of the study patients were reviewed in the 13 cases for which they were available. Control Trauma Population To verify the significance of the anatomic location of extrapulmonary air and the configuration and size of the endotracheal tube balloon after tracheal injury, we reviewed admission CT studies of 41 consecutive chest trauma patients without tracheobronchial injury but with pneumomediastinum. ll 41 patients underwent bronchoscopy (n = 16) or clinical follow-up of at least 1 year (n = 25). mong this population, blunt force injury occurred in 33 patients and penetrating injury in eight. There were 32 males and nine females, ranging in age from 16 to 75 years (mean age, 32 years). ll 41 patients without tracheobronchial injury but with pneumomediastinum had CT scans of the neck (n = 33) or chest (n = 41). lunt force trauma (n = 33) resulted from motor vehicle collision (n = 25), fall (n = 7), and hanging (n = 1). Penetrating injury (n = 8) resulted from gunshot wounds (n = 5) and stab wounds (n = 3). Endotracheal intubation was performed before CT in 71% (29/41) of the patients in the control group. CT Technique and Evaluation of CT Findings Contrast-enhanced CT scans of the chest were obtained using helical CT (a collimation of 8 mm, table speed of 8 mm, and pitch of 1) or conventional CT (contiguous images with 8-mm collimation) from the thoracic inlet to the lung bases. Contrast-enhanced CT scans of the neck were obtained using helical CT (a collimation of 5 mm, table speed of 5 mm, and pitch of 1) or conventional CT (contiguous images with 5-mm collimation) from the skull base to the thoracic inlet. The CT images of the chest and neck were reviewed on soft-tissue (width, 320 H; level, 40 H) and lung (width, 1500 H; level, 500 H) settings for the following findings. First, CT images were reviewed for extrapulmonary air and its anatomic location. Extrapulmonary air was defined as air seen in the following anatomic locations including soft-tissue emphysema in the neck, pneumomediastinum, paratracheal air (Fig. 1) (extrapulmonary air in direct contact with the external tracheal wall), pneumothorax, retroperitoneal air, pneumoperitoneum, and pneumopericardium. Second, CT images were reviewed to assess whether direct visualization of the tracheal injury, including deformity or fracture of tracheal cartilage or cartilages or discontinuity (a defect) of the tracheal wall, was possible. Third, in patients with endotracheal intubation in both the study and control groups, the endotracheal tube balloon diameter was measured with electronic calipers on transverse CT images. The balloon was considered to be overdistended if the maximum diameter of the balloon cuff was greater than 2.8 cm (average tracheal diameter: males, 2.43 cm; females, 2.0 cm) [11]. Fourth, CT images were evaluated for evidence of herniation of the endotracheal tube balloon outside the trachea. Fifth, CT images were examined for an extratracheal location of the endotracheal tube. The CT images, including axial and multiplanar reformations, of the study and control groups were retrospectively reviewed by four of the authors, and an interpretation was rendered by consensus. The anatomic location of the tracheal injury was not known at the time of CT interpretation. Supine chest radiographs (n = 13) and lateral cervical spine radiographs (n = 10) of the patients in the study group were also retrospectively reviewed by consensus interpretation for the presence of extrapulmonary air and its anatomic location and for deformity in the shape of the balloon. The radiographic and CT findings were compared to determine whether CT provided additional information. Medical records were reviewed to determine bronchoscopic results and operative findings. The two-tailed Fisher s exact test (Statistical Software [1999 version of 6.0]; Stata, College Station, TX) was performed to verify the significance of extrapulmonary air in various anatomic locations as seen on CT in patients with tracheal injury and in patients in the control group. The significance level was assumed at a p value of 0.05 or less. The study was exempt by the University of Maryland Institutional Review oard. Cadaveric Tracheal Study To verify the normal configuration of the endotracheal tube balloon under various pressure loads and to reproduce endotracheal tube balloon deformities seen on CT studies of patients with tracheal injury, we intubated adult human cadavers with a high volume low pressure endotracheal tube (Hi-Lo [endotracheal tube no. 7.0]; Mallinckrodt, St. Louis, MO). In the first specimen, helical CT scans were obtained (collimation of 3 mm, pitch of 1.5, and reconstruction at 2.0-mm intervals) after inflation of the endotracheal tube balloon with 10, 20, 30, and 40 ml of air. These volumes of air were instilled both before and after creation of three tracheal injury patterns seen at surgery or bronchoscopy. These injury patterns included a through-and-through perforation of the anterolateral walls of the trachea from a gunshot wound, a short longitudinal perforation (approximately 1 cm) of the membranous trachea, and a long longitudinal laceration (approximately 7 cm) of the membranous trachea. nother study in adult cadavers was performed to determine if the trachea or the endotracheal tube balloon would rupture first from overinflation of the balloon. ttempts were also made to determine the highest endotracheal tube balloon pressure and the minimum volume of air needed to rupture the trachea or the endotracheal tube balloon after intubation with the endotracheal balloon outside the trachea using a cuffed endotracheal tube (Hi-Lo [no. 7.0]; Mallinckrodt). The incremental endotracheal tube balloon volumes and pressures were measured after each time 5 ml of air was introduced into the balloon until rupture of either the trachea or the endotracheal tube balloon. Results Study Population In all 14 patients with tracheal injuries, injuries were confirmed at bronchoscopy (n = 12) or surgery (n = 7). The tracheal in- Fig. 1. Icepick injury to trachea in 30-year-old man. CT scan at thoracic inlet shows extensive paratracheal air (arrows) JR:176, May 2001

3 Using CT to Diagnose Tracheal Rupture jury involved the membranous portion of the trachea in 57% (8/14) of the patients and most commonly resulted from blunt force trauma (blunt force trauma penetrating injury ratio = 6:1). The anterior trachea, including the cartilage, or the ligamentous portions between the tracheal rings were most commonly injured from penetrating injuries (blunt force trauma penetrating injury ratio = 2:5). One patient with a gunshot wound to the neck had injuries to both the membranous trachea and the anterior wall. The intrathoracic trachea was injured in nine patients and resulted most commonly from blunt force trauma (blunt force trauma penetrating injury ratio = 7:2), the cervical trachea was injured in four patients and resulted most commonly from penetrating injuries (blunt force trauma penetrating injury ratio = 1:3), and both portions of the trachea were injured in one patient. The intubation-related iatrogenic injury was a long tear involving the membranous portion of both the cervical and intrathoracic segments of the trachea. The most common CT finding of tracheal injury was deep cervical emphysema and pneumomediastinum; this finding was seen in 100% (14/14) of the patients (Fig. 1). Other CT findings included paratracheal air in 93% (13/14) of the patients (Fig. 1), pneumothorax in 36% (5/14), and pneumoretroperitoneum in 14% (2/14). No significant difference was observed in the anatomic location of extrapulmonary air between patients with blunt force trauma and those with penetrating injury. The tracheal wall injury was directly visualized on CT in 71% (10/14) of the patients, either as a defect or discontinuity of the wall of the trachea in 57% (8/14) (Fig. 2) or as a focal tracheal wall deformity or tracheal ring fracture in 14% (2/14) (Fig. 3). The anatomic location of the tracheal defect and focal wall deformity seen on CT correlated with bronchoscopic and operative findings in all 10 patients with direct CT findings of tracheal wall abnormality. Endotracheal intubation was performed in 50% (7/14) of the patients. n overdistended endotracheal tube balloon cuff was seen in 71% (5/7) of the patients with tracheal injury. Of these five patients, a tracheal wall defect was seen in two patients and a focal deformity of the tracheal wall was seen in one patient. The endotracheal tube balloon diameter was measured (range, cm; mean, 3.4 cm) on axial images at the point of maximal distention in these five patients. The overdistended endotracheal tube balloon was either spherical or ovoid (Figs. 4 and 5) in 43% (3/7) of the patients and was herniated outside the wall of the trachea in 29% (2/7) of the patients. In one of these two patients, the balloon herniated through the anterolateral walls of the trachea from a gunshot wound suggested the appearance of a mickey-mouse head (Figs. 6 and 7). In the Fig. 2. Direct visualization of tracheal injury is possible in two patients who sustained blunt force trauma., In 18-year-old man, CT scan obtained at level of upper mediastinum shows defect (arrowhead) in membranous trachea. Mediastinal emphysema (straight arrows) and left lung contusion (curved arrow) can be seen., In 31-year-old woman, CT scan obtained at level of bifurcation of trachea shows defect in membranous trachea with air tracking into mediastinum (arrowhead). Mediastinal emphysema (arrows) can also be seen. Fig. 3. Focal deformity of tracheal wall in 35-year-old woman who sustained blunt force trauma. CT scan shows deformity (straight arrow) of wall of trachea. Endotracheal tube (arrowhead) is seen outside wall of trachea. Note mediastinal emphysema (curved arrow) in superior mediastinum. JR:176, May

4 Chen et al. Fig. 5. Photograph of experimentally injured cadaveric trachea (length of laceration, approximately 7 cm) shows overdistended endotracheal tube balloon herniating through injury site. Fig. 4. Overdistended endotracheal tube balloon in 35-year-old woman who underwent emergent endotracheal intubation., Lateral radiograph of cervical spine obtained at admission shows extensive deep cervical emphysema, spherical or ovoid shape of distended endotracheal tube balloon (straight arrows), and dislodged tooth (curved arrow)., CT scan obtained at thoracic inlet shows overdistended endotracheal tube balloon (arrow) and soft-tissue air. ronchoscopy (not shown) revealed long longitudinal laceration at membranous wall of trachea. other patient with a small perforation of the membranous wall of the trachea, the balloon had a dumbbell shape (Figs. 8 and 9). n extratracheal location of the endotracheal tube tip was noted on CT in one patient (Fig. 3). tracheal wall injury or herniation of the endotracheal tube balloon could not be directly seen on CT in the remaining 14% (2/14) of the patients. In the original neck or chest CT interpretations of 13 study patients, the diagnosis of major airway injury was either made or suggested in 11 (85%). CT and Radiographic Findings in the Study Group Supine chest radiographs revealed pneumomediastinum in 85% (11/13) of the patients with tracheal injury. Soft-tissue emphysema in the neck was evident on either supine chest radiographs in 92% (12/13) of the patients or lateral cervical spine radiographs in 100% (14/ 14). Chest radiographs failed to show a pneumothorax that was detected on CT in 40% (2/5) of the patients. Direct signs of tracheal injury were seen on chest radiographs in only 50% (3/6) of the patients. Neither the dumbbell-shaped deformity of the endotracheal tube balloon evident on CT in one patient nor the overdistention of the balloon evident on CT in another patient was visible on chest or cervical spine radiographs. Control Trauma Population CT findings in the control group of 41 patients (Table 1) included paratracheal air, seen in 73% (30/41) of the patients, pneumothorax in 76% (31/41), pneumoretroperitoneum in 15% (6/41), pneumopericardium in 7% (3/41), and pneumoperitoneum without hollow viscus injury in 2% (1/41). There were 29 patients (71%) who had undergone endotracheal intubation. Of these 29 patients, the maximal endotracheal tube balloon diameter was 2.7 cm (range, cm). None of the endotracheal tube balloon cuffs was herniated or deformed. Statistical nalysis The comparison of the anatomic location of extrapulmonary air in patients with tracheal injury with that in the control group (Table 1) (two-tailed Fisher s exact test) showed that patients with tracheal injury had a statistically significant lower incidence of pneumothorax than the control group (p = 0.01). No other statistically significant difference was observed between the two groups of patients when extrapulmonary air was present in the paratracheal, retroperitoneal, or pericardial spaces. Within the control group, no statistically significant difference between the patients with penetrating and those with blunt force injury or between patients who underwent endotracheal intubation and those who did not was observed in the patients with paratracheal air (Table 2). The presence of paratracheal air was more likely in patients with tracheal injury (93% 1276 JR:176, May 2001

5 Using CT to Diagnose Tracheal Rupture [13/14]) than in control group patients without tracheal injury who had not been intubated (58% [7/12]), but this finding did not quite reach statistical significance (p = 0.056). When nonintubated study patients with blunt traumatic tracheal injury were compared with nonintubated control patients (100% [5/5]) with blunt trauma (56% [5/9]), the presence of paratracheal air was highly associated with tracheal rupture (p < 0.05; χ 2 = ). Cadaveric Tracheal Study In the adult cadavers, the diameter of the endotracheal tube balloon increased during incremental inflation, with 10, 20, 30, and 40 ml of air, of the trachea. The balloon always maintained a cylindric shape on scanograms and on axial or multiplanar reformation images. The balloon did not herniate beyond the tracheal wall. The endotracheal tube balloon cuff pressures during incremental inflation with air inside and outside the trachea are shown in Table 3. The intubated trachea ruptured before the endotra- Fig. 6. Herniation of endotracheal tube balloon through injury sites in anterolateral wall of trachea in 15-year-old boy who sustained gunshot wound to neck. and, Supine chest radiograph () and CT scan () of lower neck show herniation of endotracheal tube balloon (arrowheads) through site of injury in anterolateral wall of trachea mimicking the appearance of mickey-mouse head. ullet can be seen in lower neck (arrow, ) with soft-tissue emphysema. Fig. 8. Herniation of endotracheal tube through membranous tracheal injury site in 14-year-old boy with gunshot wound to neck., xial CT image of lower neck region shows dumbbell-shaped endotracheal tube balloon (arrow) as result of herniation through injury site., Sagittal reformation CT image shows posterior herniation of endotracheal tube balloon (arrow) into prevertebral soft tissues. Fig. 7. Injury sites in anterolateral wall of trachea in adult cadaver. and, Photograph () and CT scan () of experimentally injured trachea show nearly identical appearance as herniated endotracheal tube balloon (arrows, ) seen in patient with similar injury. JR:176, May

6 Chen et al. cheal tube balloon ruptured. Tracheal rupture occurred after 75 ml of air was introduced into the balloon, whereas 130 ml of air was required to rupture the endotracheal tube balloon outside the trachea. marked increase in resistance to further inflation of the balloon in the intubated trachea was observed after 50 ml of air was introduced into the balloon. Resistance to inflation of the endotracheal tube balloon was subjectively less outside the trachea than within, but resistance increased dramatically after ml was injected. The tracheal rupture occurred as a long longitudinal tear in the midline of the membranous portion of the trachea. TLE 1 CT Finding Note. NS = not statistically significant. Comparison of ir Leak Distribution etween Patients With Tracheal Injury and Patients Without Tracheal Injury ut With Pneumomediastinum Patients With Tracheal Injury (n = 14) Patients Without Tracheal Injury ut With Pneumomediastinum (n = 41) No. % No. % Paratracheal air Present NS bsent NS Pneumoretroperitoneum Present NS bsent NS Pneumopericardium Present NS bsent NS Pneumothorax Present bsent Relationship of Injury Pattern and Endotracheal Intubation to the Presence TLE 2 of Paratracheal ir in Patients Without Tracheal Injury ut With Pneumomediastinum Paratracheal ir Present Paratracheal ir bsent Patient Condition p No. % No. % Injury pattern Has penetrating injury (n = 8) NS Has blunt injury (n = 33) NS Intubated with endotracheal tube to assist ventilation Yes (n = 29) NS No (n = 12) NS Injury pattern and intubation Has penetrating injury or is intubated NS (n = 32) Has blunt injury and is intubated (n = 9) NS Note. NS = not statistically significant. p For the same volume of air introduced into the balloon before and after intubation of the trachea, the balloon cuff pressures after intubation were always higher and increased more abruptly and rapidly after 15 ml of air had been introduced into the balloon. The endotracheal tube balloon cuff pressures exceeded 120 cm H 2 O (maximal measurable pressure) when both the trachea and endotracheal balloon ruptured (Table 3). The endotracheal tube was placed into the cadaveric trachea after the three experimental injuries described earlier were created to represent injured tracheas. The endotracheal tube balloon became overdistended and progressively herniated through the experimental defects created in the tracheal wall with incremental inflation. The appearances of the balloon herniating through the tracheal injuries Fig. 9. Photograph of trachea of adult cadaver with short (approximately 1 cm) experimentally created laceration of membranous trachea. Note herniation of endotracheal tube balloon. Results of Overdistention of TLE 3 Endotracheal Cuff Inside and Outside Trachea in dult Cadavers Volume of ir Infused (ml) Cuff Pressure (cm H 2 O) Inside Trachea Outside Trachea >120 a >120 a >120 a >120 a 75 Tracheal >120 a rupture 130 alloon cuff rupture a Maximum pressure measurement on manometer is 120 cm H JR:176, May 2001

7 Using CT to Diagnose Tracheal Rupture corresponded almost exactly to its CT appearance in the clinical cases (Figs. 5, 7, and 9). Discussion Tracheal Injury Tracheal injuries are rare, but their incidence in the United States has risen dramatically in the past decades because of the increase in the number of blunt force trauma cases resulting from high-velocity motor vehicle collisions and ballistic injuries [3, 4, 6, 8, 12]. lunt trauma contributes to most tracheal injuries, predominantly involving the membranous portion of the intrathoracic trachea as a result of a sudden increase in the intraairway pressure with a closed glottis at the time of impact, as was seen in our study. Other mechanisms of tracheal injury include tearing across the lower tracheal carinal junction from anteroposterior chest compression forcing the lungs apart laterally, hyperextension of the neck resulting in stretching and compression of the trachea against a steering wheel or dashboard, direct crushing of the trachea between the sternum and thoracic spine, and sudden and rapid deceleration with shearing force applied to the relatively fixed cricoid cartilage and carina [3, 4, 6, 13]. Penetrating injury is a less frequent cause of tracheal injury and more commonly involves the anterior extrathoracic trachea including the cartilage or the ligamentous portions between the tracheal rings, with a higher incidence of associated esophageal and vascular injuries [3, 5, 7]. Early diagnosis and prompt surgical repair is crucial for survival and reduction of the morbidity resulting from tracheal injury. Radiologic signs of tracheobronchial injury are nonspecific and include pneumomediastinum, pneumothorax, and progressive extrapulmonary soft-tissue air [3, 8, 12, 14]. Case reports indicate that CT, performed with the appropriate window settings, can reveal the exact site of tracheal injury by directly showing focal defects or the circumferential absence of the tracheal wall, a contour deformity, or abnormal communication with other mediastinal structures [15 18]. In our study, using lung and soft-tissue window settings in the chest and bone and soft-tissue window settings in the neck allowed direct visualization of the tracheal injury on axial CT images in 71% (10/14) of the patients, without prior knowledge of the anatomic site of the tracheal tear. defect in the wall of the trachea was seen in 57% (8/ 14) of the patients and a focal wall deformity in 14% (2/14). The anatomic location of the tracheal wall abnormality seen on CT was confirmed at either bronchoscopy or surgery in all patients. n awareness of these specific CT findings that are suggestive of tracheal injury will help select patients for early confirmatory bronchoscopy. Herniation of the endotracheal tube balloon outside the tracheal wall has not, to our knowledge, been described as a specific sign of tracheal injury. alloon deformities in the shape of a mickey-mouse head and dumbbell were seen from herniation through a defect or defects in the tracheal wall. These CT findings not only were specific signs of tracheal injury but also were helpful to localize the site of the tracheal tear. In the remaining 14% (2/14) of the patients, neither a tracheal wall injury nor herniation of the endotracheal tube balloon could be directly visualized. Possible factors contributing to nondiagnostic CT include respiratory misregistration, small lesion size with obscuration from partial volume averaging, size of injury below the resolution of CT, and nondisplaced tears occurring in the transverse plane. The ability to scan with a thinner collimation using helical CT or multislice helical CT should improve the sensitivity of CT to directly show tracheal wall injuries. In our study, deep cervical emphysema and pneumomediastinum were the most common radiographic and CT findings seen in all 14 patients with tracheal injury. These findings were related to an air leak into the mediastinum and deep cervical fascial planes from the disrupted thoracic or cervical trachea. Some studies postulate that pneumomediastinum or deep cervical emphysema in patients who sustain closed trauma is strongly suggestive of tracheal or bronchial injury [6, 12, 19, 20]. However, pneumomediastinum is clearly not pathognomonic of tracheal injury. In our study, there was no significant difference in the number of patients with retroperitoneal or intrapericardial air between the study and control groups. The most common cause of pneumomediastinum is the Macklin effect, in which alveolar rupture, due to ventilator barotrauma or pulmonary laceration, produces an air leak into the pulmonary interstitium with retrograde dissection along the perivascular sheaths toward the hilum and into mediastinum [21]. Mediastinal air can also track into the retroperitoneum (14% of our patients) and peritoneal spaces from intrathoracic tracheal injuries. Other studies describe paratracheal air or air paralleling the airway as a significant finding for tracheobronchial injury [6, 8, 22, 23]. None of these studies used a control group to verify the significance of this finding. In the current study, paratracheal air was present in 93% of the patients with tracheal injury and in 73% of the patients without tracheal injury and pneumomediastinum. No statistically significant difference was observed in the number of patients with paratracheal air between these two groups, thus indicating that this CT finding is not specific for tracheal injury. However, patients with tracheal injury tended to have paratracheal air, whereas the control group of nonintubated patients with pneumomediastinum tended to not (p = 0.056). Thus, the presence of paratracheal air on CT or radiography in an unintubated trauma patient should arouse suspicion of tracheal injury and deserves further evaluation with bronchoscopy. Two studies have reported an absence of or delayed appearance of pneumomediastinum in patients with tracheobronchial injury on supine chest radiographs and attribute this finding to preservation of the integrity of paratracheal connective tissue sheath or temporary occlusion of a small tear by the endotracheal tube balloon [11, 22]. The failure to identify a small amount of mediastinal air on radiographs may be another contributory factor. These two observations were confirmed in our study. Two study patients with evidence of pneumomediastinum on CT did not have this finding detected on conventional radiographs. Subjectively, relatively small volumes of pneumomediastinum were seen in our patients with overdistention of the endotracheal tube balloon in this study, reflecting the tamponade effect of the endotracheal tube balloon in preventing air leak into the mediastinum. This study showed pneumothorax occurs infrequently in patients with isolated tracheal injury. significantly lower incidence of pneumothorax was seen in patients with tracheal injury (36%) than in control group patients (76%) (p = 0.01). Tears in the trachea produce a central air leak into the surrounding mediastinum without a pneumothorax unless there is also a tear in the mediastinal pleura [5, 6, 19, 22]. n animal study performed by Lloyd et al. [13] indicated that the presence of a pneumothorax may have a protective effect on the tracheobronchial tree by decreasing the JR:176, May

8 Chen et al. distracting stress at the carina during anteroposterior compression and abruptly increasing in the transverse thoracic diameter. major limitation of this study is the retrospective nature of the case review and the small sample size caused by the rarity of tracheal injury even in a major trauma center. mong 13 of the 14 study patients for whom the original neck or chest CT interpretation was available, the report mentioned possible or definite evidence of major airway injury in 11 patients (85%). Cadaveric Study n oval or spherically overdistended endotracheal tube balloon has been well described as a specific finding of tracheal injury on chest and cervical spine radiographs [11]. Our cadaveric studies show that only in the presence of a tracheal rupture can the endotracheal tube balloon be easily overinflated as a result of decreased tracheal wall resistance in patients with tracheal disruption. The absence of an intact tracheal wall is perceived as a lack of resistance frequently leading to overinflation or herniation of the endotracheal tube balloon through a defect or defects in the tracheal wall and reproducing deformities in balloon shape (mickeymouse head and dumbbell) seen in our patients with tracheal injury. larger than normal diameter of the endotracheal tube balloon might occasionally result from unintended excessive balloon inflation in the uninjured trachea. In four of our five patients, overdistention of the endotracheal tube balloon was obviously the result of, but not the cause of, tracheal rupture. However, in the remaining patient who had intubation-related tracheal injury, the cause of tracheal rupture was less apparent. Intubation-related tracheal rupture has been attributed to traumatic intubation in the field under suboptimal conditions, the inappropriate use of the stylet, the use of an oversized tube, the overinflation of the endotracheal tube balloon, or an underlying weakness of membranous trachea as may occur in the elderly [14, 24, 25]. Overdistention of the balloon may be seen in patients with tracheomalacia from chronic endotracheal intubation with high cuff pressure [26]. Despite reports favoring endotracheal tube cuff overinflation as the major cause of acute intubation-related tracheal rupture [24 27], only one of these reports convincingly supports this theory [27]. Our study of adult cadavers confirms that ml of air is needed in the endotracheal tube balloon to rupture the trachea. marked increase in resistance to hand injection of air was noted after introduction of 50 ml of air into the intubated trachea balloon. This finding suggests that it is unlikely and difficult to overinflate the balloon in an intact trachea with up to 7 8 times the normal volume ( 10 ml) of air, particularly given the marked resistance to injecting such a large volume. Intubationrelated tracheal injury, particularly to the membranous portion, can still result from the several other factors mentioned. In conclusion, this study shows the overall sensitivity of CT for the diagnosis of tracheal rupture was 85%. Specific CT findings include overdistention of the endotracheal tube balloon or herniation of the deformed endotracheal tube balloon beyond the trachea, the extraluminal position of the endotracheal tube, a focal tracheal wall defect or discontinuity, and a contour deformity or fracture of the trachea. With the increasing use of helical and multislice helical CT for examining patients with severe chest trauma, awareness of these CT signs can suggest the diagnosis of tracheal injury, thus leading to early confirmatory bronchoscopy and definitive treatment. In addition, CT findings of paratracheal air in an unintubated trauma patient can be used to improve selection of patients with relatively common posttraumatic pneumomediastinum for bronchoscopy. References 1. Conn JH, Hardy JD, Fain WR, Netterville RE. Thoracic trauma: analysis of 1022 cases. J Trauma 1963;3: Shorr RM, Crittenden M, Indeck M, Hartunian SL, Rodriguez. lunt thoracic trauma: analysis of 515 patients. nn Surg 1987;206: Lee R. Traumatic injury of the cervicothoracic trachea and major bronchi. Chest Surg Clin N m 1997;7: ertelsen S, Howitz P. Injuries of the trachea and bronchi. Thorax 1972;27: Wiot JF. Tracheobronchial trauma. Semin Roentgenol 1983;18: Kirsh MM, Orringer M, ehrendt DM, Sloan H. 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