INTRODUCTION

Injury to the respiratory system is common in any animal suffering from significant trauma. It has been estimated that 39% of dogs that incur skeletal injuries due to motor vehicular accidents also sustain thoracic injury. Of those injuries, pulmonary contusions predominate.^1,2 Over one half of the animals suffering from thoracic trauma have more than one thoracic injury.¹ Therefore it is imperative that the myriad of potential thoracic injuries be identified rapidly and treated appropriately. Thoracic injuries may vary in severity from those less likely to result in significant morbidity and mortality to those that are life threatening and warrant immediate intervention. This chapter will focus on the more common thoracic injuries, including the pathophysiology and treatment of these injuries.

PNEUMOTHORAX

The pleural space is a potential space that is created by the opposing surfaces of the parietal pleura and the visceral pleura (see Chapter 30, Pleural Space Disease). The normal pleural space is occupied by only a small amount of serous fluid that helps lubricate the surfaces of the pleurae. This space also maintains a resting negative intrathoracic pressure relative to the atmosphere. When this negative intrapleural pressure is not maintained, there is disruption of the normal expansive and relaxation properties of the lung. A pneumothorax is created when air accumulates within the pleural space. Air can be introduced into the pleural space via two mechanisms: alveolar rupture secondary to increased force applied to the chest with a closed glottis or secondary to laceration of the pulmonary parenchyma.³ Progression of the pneumothorax will depend on several factors: the respiratory pattern of the patient, the size of the defect, and whether the defect is unidirectional, prohibiting the escape of air from the pleural space.⁴

Air in the pleural space can lead to partial or complete lung atelectasis and disturbances in pulmonary and cardiac hemodynamics.³ Ventilation-to-perfusion mismatch is a result of instantaneous lung collapse and results in a decreased arterial partial pressure of oxygen. Minute ventilation is maintained by an increased respiratory rate that compensates for decreased tidal volume.⁴ However, if there is severe atelectasis from increased air in the pleural space, hypoxemia develops rapidly and overwhelms compensatory mechanisms. For example, a tension pneumothorax develops when the intrathoracic pressure exceeds the atmospheric pressure. This causes a decrease in venous return and can cause complete respiratory and hemodynamic collapse.³ If pleural pressure exceeds central venous and pulmonary artery pressures, there is decreased venous return to the heart. Tachycardia results in an effort to maintain cardiac output. Systemic hypotension results when the myocardial oxygen demand is higher than delivery, thus further decreasing cardiac output.⁴ These patients often present with clinical signs of shock, similar to that witnessed with cardiac tamponade, because atrial diastolic filling is compromised by decreased venous return to the heart from the increased intrapleural pressure.³

A pneumothorax may be open or closed. An open pneumothorax occurs when the pleural space communicates directly with the atmosphere. In this situation, the unaffected lung is not ventilated normally and there is a paradoxical decrease in pulmonary volume with inspiration and an increase on exhalation.³ A sucking chest wound is present if air is heard moving in and out of the pleural space with respirations.³ With a closed pneumothorax, air is contained within the pleural space. This may be accompanied by other signs of thoracic trauma (e.g., rib fractures) and may be a result of damage to the larger nonconducting airways (trachea and bronchi), other mediastinal structures (esophagus), or the pulmonary parenchyma itself (most common).^3,4

Diagnosis is made most commonly with thoracic radiographs, although a high clinical suspicion should exist based on physical examination alone. Dull dorsal breath sounds and hyperresonance on percussion of the affected lung dorsally are hallmarks of a pneumothorax. The best radiographic view is a ventrodorsal view. Radiography often reveals that a can cause retraction of the lungs from the body wall, elevation of the heart off the sternum, and atelectasis in patients with a pneumothorax.⁵ A diagnosis of tension pneumothorax is often suspected in patients that are air hungry, tachycardic, tachypneic, and cyanotic.³ In this case, immediate therapy is vital, and waiting for a thoracic radiograph may prove fatal.

The goal of treatment for pneumothorax is reexpansion of the collapsed lung. This may be accomplished by thoracocentesis or tube thoracostomy if the volume of air is such that negative pressure cannot be established within the pleural space or if repeated thoracocenteses are required (see Chapters 31 and 32, Thoracentesis and Thoracostomy Tube Placement and Drainage, respectively). Intermittent or continuous pleural drainage will be necessary following thoracostomy tube placement, depending on the rate of air accumulation. If an animal with a rapidly progressive pneumothorax presents, an immediate thoracotomy and intubation with positive-pressure ventilation may prove lifesaving. If an open pneumothorax is present, an occlusive dressing should be placed to create a closed pneumothorax. The dressing should be secured only on three sides to allow for air escape from the pleural space without risk of developing a tension pneumothorax.³ Alternatively, a full occlusive dressing can be placed and secured on all four sides if a thoracostomy tube is inserted.

Open chest wounds will require surgical exploration once the patient has been stabilized. Treatment decisions should be based on the respiratory and cardiovascular status of the patient. In an otherwise stable patient, repeated monitoring (physical parameters, pulse oximetry, and arterial blood gases) can take the place of immediate evacuation of the pleural space.

According to one study, the prognosis for patients with pneumothorax may depend on the need for thoracocentesis and the length of the intensive care unit stay. Animals that required repeated thoracocenteses or had shorter hospitalizations were more likely to be euthanized in one study. Animals that present dyspneic also tended to have a poorer prognosis.⁶ Overall, the prognosis for animals with a traumatic pneumothorax is good, with an 87% survival rate reported.⁶ However, animals may succumb to other serious concurrent injuries.

PULMONARY CONTUSIONS

Pulmonary contusions are the most common type of injury following blunt thoracic trauma (see Chapter 25, Pulmonary Contusions and Hemorrhage).⁷ The most frequent cause in human medicine is motor vehicle accidents; other possible causes include falls and penetrating chest trauma.³ In veterinary medicine, 17% of animals have evidence of pulmonary contusions after a motor vehicle accident.⁸ Pulmonary contusions rarely exist as an isolated injury and are often found in association with other thoracic injuries (e.g., rib fractures, pneumothorax, hemothorax, diaphragmatic hernia) (see Chapter 25, Pulmonary Contusions and Hemorrhage).⁷

RIB FRACTURES

Rib fractures may occur secondary to any thoracic trauma and are the most common type of thoracic injury in human patients. Rib fractures rarely occur in isolation and often occur in conjunction with pulmonary contusions or pleural space disease.³ They may be evident on initial physical examination if a flail segment or open fracture is present. However, a majority of rib fractures are most readily diagnosed with thoracic radiographs. It seems that the ribs can sustain forces greater than other long bones before a fracture occurs. Human cadaver studies indicate that the thorax can tolerate a 20% volume reduction before rib fractures occur.³ However, underlying lung tissue can sustain significant injury as a result of the concussive forces of the traumatic event.⁵

Rib fractures may cause hypoxemia indirectly by leading to lung injury; therefore oxygen supplementation is warranted. Rib fractures may also lead to hypoventilation as a result of pain. Pain management is of utmost importance and may consist of local or systemic use of analgesics. Local anesthetic agents (i.e., lidocaine and/or bupivacaine) are useful because they do not inhibit ventilation. Local blocks should be administered to the caudal surface of the fractured rib(s), both dorsally and ventrally to enhance efficacy.⁵ Systemic analgesia should be used cautiously to prevent respiratory depression.

FLAIL CHEST

Flail chest is a relatively uncommon condition in veterinary patients, but occurs most commonly secondary to dog-bite trauma and motor vehicle trauma.⁹ Flail chest occurs secondary to the fracture of two or more adjacent rib segments, both dorsally and ventrally. This segmental chest wall injury can lead to thoracic instability and paradoxical chest wall motion.^3,10 Intrathoracic injuries affect respiratory function more than the rib fractures themselves.

Pulmonary contusions have the greatest effect on oxygenation and ventilation, but hemothorax or pneumothorax and pain will also affect pulmonary mechanics.³ Patients usually present tachypneic or dyspneic, with paradoxical motion of the chest wall flail segment (see following paragraph). In most patients, the diagnosis is confirmed with thoracic radiography. Radiographs often reveal the flail segment and also highlight any concurrent injuries (pulmonary contusions or pleural space disease).

The flail component moves paradoxically in relation to the rest of the thorax. The flail segment moves inward with inspiration as a result of the negative intrathoracic pressure and outward with exhalation. The flail segment has been shown to cause a small reduction in the arterial partial pressure of oxygen, although this does not appear to be significant in experimental studies.¹⁰ In humans, the incidence of concomitant injuries, such as pulmonary contusions and a pneumothorax, are 50% and 77%, respectively.³ In animals, the likelihood of pulmonary contusions secondary to flail chest is estimated to be between 75% to 100%.¹¹ These injuries, in combination with pain, contribute the most to the hypoxemia and hypoventilation that occur in patients with flail segments.

Treatment of the underlying intrathoracic injuries depends on the severity of the injuries. Hypoxemia secondary to pulmonary contusions may require oxygen supplementation, and mechanical ventilation may be necessary for patients in respiratory failure (see Chapters 19 and 213, Oxygen Therapy and Basic Mechanical Ventilation, respectively). Pneumothorax may warrant thoracocentesis, placement of a chest tube, or an emergency thoracotomy. Hemothorax is usually self-limiting and related to the initial trauma,¹⁰ but may require surgical exploration if continued hemorrhage is suspected. Although rare, cases of cardiac rupture or aortic or great vessel laceration have been reported.¹⁰ Stabilization of the flail segment is often not necessary, but it should be considered if there is evidence of an open pneumothorax, continued hemothorax, or other evidence of continued trauma to the underlying tissues,⁵ and/or an exploratory thoracotomy is otherwise warranted. Additionally, patients that require mechanical ventilation as a result of severe intrathoracic injuries and respiratory failure may benefit from stabilization of the flail segment.¹⁰

Treatment is most dependent on assessment of pulmonary function. Pain management should be considered early, because pain impairs normal chest wall movement and ventilation (see Chapters 161 and 164, Pain and Sedation Assessment and Analgesia and Constant Rate Infusions, respectively). Pain contributes not only to hypoventilation, but also to atelectasis and a decreased cough reflex, allowing the accumulation of pulmonary secretions.⁹ The latter increases the likelihood for pneumonia. As with single rib fractures, local anesthetics provide analgesia without affecting ventilation centrally. The fractured ribs should be injected both dorsal and ventral to the fracture on the caudal surface of the rib. One rib caudal and cranial to the segment should also be included in the nerve block. Bupivacaine or lidocaine, or both, can be administered every 6 hours as needed. Epidural analgesia has shown improved benefit over patient-controlled analgesia in human medicine³ and may be underutilized in veterinary patients. Because of the potential for hypotension, cardiovascular stability is a prerequisite to epidural analgesia. Systemic analgesia with opioids is also effective, but they should be employed cautiously to minimize respiratory depression. Placing the animal in lateral recumbency with the flail segment down or a light external chest wrap may prevent excessive outward movement of the segment.