May 07, 2018

AVMA 2017: Respiratory Complications of Trauma

Thoracic injury can cause a variety of complications in the small animal patient, including pneumothorax, diaphragmatic hernia, and rib fracture.
By Natalie Stilwell, DVM, MS, PhD

Traumatic injury can result in multiple respiratory complications, according to Lori Waddell, DVM, DACVECC, clinical professor of critical care medicine at the University of Pennsylvania School of Veterinary Medicine (PennVet) in Philadelphia. At the 2017 American Veterinary Medical Association Convention in Indianapolis, Indiana, Dr. Waddell drew from her years of specialty training and clinical experience to educate the audience on this important topic in emergency medicine.

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The various causes of thoracic trauma, whether accidental or intentionally caused by abuse, are generally categorized as blunt or penetrating, she said. Blunt trauma most commonly occurs from a fall or vehicle strike, whereas penetrating trauma may involve a gunshot, knife, or bite wound, or impalement by a foreign object such as a stick or rod.
 

Initial Patient Assessment

Physical Examination
A thorough physical examination is valuable for identifying and localizing respiratory distress in patients that have experienced trauma, Dr. Waddell emphasized. During the initial examination, the veterinarian must determine whether the injured patient is hypoxemic or hypoventilating.

Hypoxemia results from ventilation-perfusion mismatch in the lungs as the lungs fill with blood or fluid. As blood flow to the affected area continues, fluid accumulation may prevent normal ventilation and oxygen exchange from occurring. This often results from pulmonary contusion or pleural space disease such as pneumothorax, which can result in atelectasis of the lung.

Hypoventilation arises from a variety of causes, including airway obstruction, pleural space disease, and diaphragmatic hernia, as well as severe neurologic injury to the brain, cervical spinal cord, or peripheral nerves innervating the diaphragm and intercostal muscles.

Diagnostic Imaging
Thoracic radiography is often recommended after suspected trauma, but Dr. Waddell noted that ultrasound can be a helpful alternative for detecting certain complications, such as pulmonary contusions, pneumothorax, hemothorax, and diaphragmatic hernia, in patients that are too unstable for restraint or sedation.

Thoracic focused assessment with sonography for trauma (TFAST) examination serves as a rapid, noninvasive screening tool for triage of the thorax and can be performed in standing patients or in lateral or sternal recumbency if the patient is particularly stressed or compromised. Standard TFAST approaches include diaphragmatic-hepatic and bilateral pericardial and chest tube sites. “The pericardial and diaphragmatic-hepatic views are ideal for detecting smaller volumes of free fluid,” she said. The chest tube sites are used to detect pneumothorax as well as evaluate the lungs. She described normal lung movement as a smooth, gliding motion along the thoracic wall, whereas air or fluid between the lung and thoracic wall may impair movement. Ultrasound can identify pulmonary parenchymal disease (eg, pulmonary contusion) by the presence of so-called B lines or rocket tails that look like search spotlights within the lung.

Although rarely performed on her trauma patients, Dr. Waddell said computed tomography may be a useful diagnostic tool for patients that are stable enough for sedation or anesthesia.
 
Figure 1. (Left) Severe right-sided pulmonary contusions and a small-volume left-sided pneumothorax with collapsed left lung in a dog that was hit by a car. (Right) Pneumothorax in a domestic shorthair cat that fell from a third-story window. Note the retracted lung lobes, especially in the caudodorsal lung elds, and the heart lifting slightly away from the sternum.
 

Complications of Thoracic Trauma

Thoracic trauma can lead to several common complications, Dr. Waddell said: pneumothorax, pulmonary contusion, hemothorax, diaphragmatic hernia, rib fracture, and tracheal injury. She offered details for each.

Pneumothorax
Pneumothorax—an abnormal collection of free air in the pleural space—is the most common complication Dr.Waddell encounters in patients suffering from blunt trauma to the thorax (Figure 1). She estimated that 45% to 50% of patients with fractured bones after being hit by a car experience pneumothorax,1 which she frequently observes in dogs and cats that fall from a height.

Pneumothorax is classified as open or closed, depending on the presence of a penetrating wound. An open pneumothorax is a life-threatening emergency; the patient may not be able to ventilate due to loss of the normal negative pressure within the pleural space. Blunt thoracic trauma can lead to closed pneumothorax by causing rupture of the pulmonary parenchyma or small airways. Open pneumothorax typically occurs due to a bite, gunshot, or knife wound, or from impalement. In the case of impalement, keeping the impaling object in place may actually aid ventilation by maintaining negative pressure within the chest, while removing it may lead to respiratory collapse. She recalled a case at PennVet involving a 2-year-old poodle that, despite presenting with a large rod impaling the cranial thorax, appeared in minimal respiratory distress on physical examination. The rod was removed carefully under anesthesia, the chest was lavaged and the wound closed, and the patient was discharged successfully from the hospital 3 days later.

Tension pneumothorax, a potentially life-threatening complication that occurs due to progressive air buildup within the thoracic space, gradually increases intrathoracic pressure and can compress the lungs, reduce venous return to the heart, and lower blood pressure, resulting in cardiovascular compromise. “Positive-pressure ventilation can exacerbate tension pneumothorax,” she said, “so it should be used with extreme caution in affected patients.”

Dr. Waddell suspects pneumothorax whenever a penetrating chest wound is present, but she says it is also common in patients with blunt thoracic trauma. The patient typically takes rapid, shallow breaths and has dull lung sounds on physical exam; if a tension pneumothorax is present, the animal may develop a barrel-chested appearance. Thoracic radiography may reveal an abnormally elevated heart position off the sternum, collapsed and retracted lungs, and decreased visibility of the peripheral vessels and airways.

“Thoracentesis can be a lifesaving treatment for pneumothorax,” she said, offering reassurance that the procedure is straightforward and requires minimal equipment. After the patient is placed in sternal or lateral recumbency and the site is prepped aseptically, the veterinarian inserts a needle attached to extension tubing, a 3-way stop-cock, and syringe cranial to a rib and advances it until positive pressure indicates correct placement into the pleural space. “When the tubing fogs up a little bit,” she said, “that’s how you can tell you’re in the right spot.”

Most patients with closed pneumothorax will respond to thoracentesis. Severe cases may also require placement of a chest tube with continuous suction. Open pneumothorax (eg, from bite wounds) necessitates surgical lavage and cleaning.

Pulmonary Contusion
Hemorrhage within the lung caused by damage to the pulmonary vasculature usually results from a direct crushing injury to the chest (see Figure 1). Dr. Waddell said this is the second most common complication of thoracic trauma she observes and is often associated with rib fracture and/or pneumothorax. Contusions range from mild to severe, and associated hemorrhage can impair normal lung function significantly.

Pulmonary contusion is typically diagnosed on physical examination, which reveals increased respiratory effort and harsh lung sounds (crackles). She emphasized that abnormal lung sounds may seem minor initially and worsen with time. Also, severe contusions may actually decrease lung sounds, as the presence of blood hinders normal air exchange. Patients with pulmonary contusion also may cough up blood or blood-tinged fluid. TFAST examination can aid in identifying pulmonary contusions.

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