Chest Trauma

Published on 12/06/2015 by admin

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CHAPTER 2 Chest Trauma

Laura Avery, Robert A. Novelline

Thoracic injuries account for about 25% of trauma deaths, second in number only to head trauma. In the United States each year, more than 300,000 patients are hospitalized and more than 25,000 die as a result of sustaining chest trauma. Blunt trauma accounts for 90% of chest trauma, and the most common cases of blunt trauma are motor vehicle collisions and falls.

The imaging protocol of patients with suspected thoracic injuries usually begins with a supine portable anteroposterior chest radiograph. Many obvious thoracic injuries such as displaced rib fractures, large pneumothoraces, and hemothoraces may be quickly detected with this exam. The chest radiograph is also useful in confirming the correct positioning of an endotracheal tube or nasogastric tube. Other traumatic conditions, such as small pneumothorax, small hemothorax, parenchymal lung laceration, aortic trauma, tracheobronchial injury, cardiac injury, diaphragm rupture, and thoracic spine injury, usually require further imaging with computed tomography (CT). Multidetector CT (MDCT) scanners can quickly and accurately diagnose and display a wide variety of thoracic injuries. The use of multiplanar and volumetric images derived from the acquired axial images may not only aid in diagnostic evaluation but also better display the extent of injuries, and this may assist optimal treatment planning.

INJURIES OF THE PLEURAL SPACE

A hemothorax, blood in the pleural space, may result from injury to the chest wall, diaphragm, lung, or mediastinal structures. CT may confirm a hemothorax when a pleural fluid collection in a trauma patient seen on CT measures blood density over 35 to 40 Hounsfield units (HU) (Fig. 2-1). A pneumothorax, air in the pleural space, may result from a lung injury, tracheobronchial injury, or esophageal rupture. The most common cause is lung injury associated with a rib fracture. A pneumothorax occurs in about 15% to 40% of patients with acute chest trauma. Many small and even moderate-sized pneumothoraces that are not visible on the supine chest film may be identified on CT. A pneumothorax seen on CT that cannot be identified on a supine chest film is referred to as an “occult” pneumothorax. Studies estimate that 10% to 50% of pneumothoraces seen on CT are not evident on the supine anteroposterior film. Radiographic signs of pneumothorax may be subtle. In the supine patient air collects in nondependent locations such as the anterior costophrenic sulcus. This region extends from the seventh costal cartilage to the eleventh rib in the midaxillary line. The air collection appears as an abnormal lucency in the lower chest or upper abdomen, frequently referred to as the “deep sulcus” sign. Additional signs of pneumothorax in a supine patient include a sharply outlined cardiac or diaphragmatic border and depression of the hemidiaphragm (Fig. 2-2). Detection of even a small pneumothorax is important as it may enlarge during positive-pressure ventilation or general anesthesia. A tension pneumothorax is an emergency condition resulting from a lung or airway injury associated with a one-way accumulation of air within the pleural space. As intrapleural pressure rises the mediastinal structures are compressed, decreasing venous return to the heart, leading to hemodynamic instability. Radiography and CT will show mediastinal shift to the contralateral hemithorax, hyperexpansion of the ipsilateral thorax, and depression of the ipsilateral hemidiaphragm.

ESOPHAGEAL INJURIES

Blunt trauma to the esophagus is extremely rare since this structure is well protected in the posterior mediastinum. Most esophageal injuries occur from penetrating or iatrogenic trauma. Blunt trauma may result in rupture or intramural hematoma. These injuries normally involve the upper thoracic esophagus or the lower esophagus just above the gastroesophageal junction. The most commonly accepted theory regarding the pathophysiology of rupture is similar to the mechanism in Boerhaave syndrome in that an increase in intraluminal pressure against a closed glottis results in a tear at the weakest point of the esophageal wall, usually the distal third of the esophagus on the left where there is less protection from the pleural lining and the heart. Other etiologies for injury include disruption of the esophageal blood supply, resulting in ischemia and late perforation, and a blast effect caused by a concomitant tracheal injury. Direct injury may also result from adjacent thoracic spine fractures or compression between the sternum and thoracic spine, as observed in high-speed road traffic accidents. Esophageal injuries are often associated with clinical symptoms, including blood in the esophagus or pain on swallowing. CT may suggest the diagnosis of traumatic esophageal perforation with the presence of pneumomediastinum, mediastinitis, hydropneumothorax, or leakage of oral contrast medium into the mediastinum or the pleural space. Water-soluble contrast esophagography, followed by flexible esophagoscopy, may be required to fully evaluate the site of injury.

AORTIC AND GREAT VESSEL INJURIES

Injuries of the aorta account for a significant number of fatalities following blunt trauma. Seventy percent of all thoracic aortic injuries are fatal at the scene of trauma. Of patients who are transported to the hospital, 90% of aortic rupture occurs at the aortic isthmus, located at the junction of the posterior aortic arch and descending aorta, just distal to the origin of the left subclavian artery. The proposed mechanism of injury is rapid deceleration producing shear injury at the site where the rate of deceleration of the mobile aortic arch differs from that of the relatively fixed descending aorta. In addition, bending stress occurs because the aorta is flexed over the left pulmonary artery and left mainstem bronchus. Only 5% of aortic injuries in clinical series involve the ascending aorta, and these injuries may be associated with life-threatening cardiac and pericardial injuries. Rarely, aortic injuries may involve the descending aorta at the level of the diaphragmatic hiatus.

A normal chest radiograph has a high negative predictive value (98%) but a low positive predictive value for aortic injury. The chest film findings suggestive of aortic injury include mediastinal widening greater than 8 cm, loss of the normal aortic arch contour, a left apical pleural cap, displacement of the nasogastric tube to the right, widened paraspinal lines, and loss of the descending aortic line. Most of the plain film findings of aortic injury are nonspecific. The gold standard for the diagnosis of aortic injury has traditionally been aortography; however, at most trauma centers today, aortography has been replaced with MDCT.

The sensitivity of CT has been reported to be 92% to 100% and specificity 62% to 100% for the detection of aortic injury. The accuracy of aortic trauma detection with CT has been improving in parallel with technological improvements in CT scanning. Current fast MDCT scanners decrease motion artifact and provide higher-quality two- and three-dimensional reformations for diagnosis and treatment planning.

The CT findings of aortic trauma include indirect signs, such as mediastinal hematoma surrounding the posterior aortic arch and proximal descending aorta, as well as the direct signs of intimal tear/flap, aortic contour abnormality, thrombus protruding into the aortic lumen, false aneurysm formation, pseudocoarctation, and extravasation of intravenous contrast material. If only direct signs are utilized, the sensitive and negative predictive value remains at 100% but the specificity increases to 96% (Fig. 2-4).

A common aortic injury is a traumatic false aneurysm resulting from disruption of the vessel intima and media while the adventitia remains intact. The intravascular blood confined by only the adventitia bulges outward forming a pseudoaneurysm. In many cases, the aortic injury may be limited to a partial circumferential tear. CT findings typically consist of a saccular out-pouching demarcated from the aortic lumen by torn intima. It frequently results in hemomediastinum. Treatment of a pseudoaneurysm may today be performed with intravascular stent grafting. False positive examinations may be related to a prominent ductus diverticulum or an ulcerated atheromatous plaque. A traumatic pseudoaneurysm is usually surrounded by mediastinal blood whereas a ductus diverticulum and an ulcerated atheromatous plaque are not (Figs. 2-5 to 2-7).