Theresa W. Fossum, DVM, PhD, Diplomate ACVS
Pneumothorax is an accumulation of air or gas within the pleural space. Traumatic pneumothorax is the most frequent type of pneumothorax in dogs. It most often occurs due to blunt trauma (i.e., vehicular accidents, being kicked by a horse), which causes parenchymal pulmonary damage to the lung and a closed pneumothorax. When the thorax is forcefully compressed against a closed glottis, rupture of the lung or bronchial tree may occur. Alternately, pulmonary parenchyma may be torn due to shearing forces on the lung. Pulmonary trauma occasionally results in subpleural bleb formation, similar to those seen with spontaneous pneumothorax. Open pneumothorax occurs less commonly, but is also frequently due to trauma (i.e., gun shot, bite or stab wounds, lacerations secondary to rib fractures). Some penetrating injuries are called "sucking chest wounds" because large defects in the chest wall allow an influx of air into the pleural space when the animal inspires. These large, open chest wounds may allow enough air to enter the pleural space that lung collapse and marked reduction in ventilation occur. There is a rapid equilibration of atmospheric and intrapleural pressure through the defect, interfering with normal mechanical function of the thoracic bellows which normally provides the necessary pressure gradient for air exchange. Pneumomediastinum may be associated with pneumothorax, tracheal, bronchial, or esophageal defects, or may be due to subcutaneous air migration along fascial planes at the thoracic inlet.
Spontaneous pneumothorax occurs in previously healthy animals without antecedent trauma and may be primary (i.e., an absence of underlying pulmonary disease) or secondary (underlying disease such as pulmonary abscesses, neoplasia, chronic granulomatous infections, pulmonary parasites such as Paragonimus, or pneumonia are present). Based on the histologic appearance of the pulmonary lesion, both cysts and bullae have been reported in dogs. Primary spontaneous pneumothorax in dogs may be due to rupture of subpleural blebs; the remaining lung tissue may appear normal. These blebs are most commonly located in the apices of the lungs. Secondary spontaneous pneumothorax is more common in dogs than the primary form. In these animals, the subpleural blebs are associated with diffuse emphysema or other pulmonary lesions. It has been shown that volume strain from expansive pressure within the lung increases disproportionately at the apex as height increases. A majority of affected people are cigarette smokers, suggesting that the underlying pulmonary disease could be a result of interference of the normal function of alpha-1-antitrypsin in inhibiting elastase. It is believed that alpha-1-antitrypsin is inactivated in people who smoke, allowing increased elastase-induced destruction of pulmonary parenchyma.
Medical management of an animal with pneumothorax consists of initially relieving dyspnea by thoracentesis. If the pleural air accumulates quickly or cannot be effectively managed with needle thoracentesis, a chest tube should be placed. Tube thoracostomy is typically required in animals with spontaneous pneumothorax. Intermittent or continuous pleural drainage may be used, depending on the speed with which air accumulates. Continuous drainage may cause quicker resolution of pneumothorax in animals with large, traumatic defects. Providing an enriched oxygen environment may be beneficial, particularly in animals with concurrent pulmonary trauma (e.g., pulmonary contusion or hemorrhage). Providing analgesics to animals with fractured ribs or severe soft tissue damage may improve ventilation. Surgical intervention is seldom required in animals with traumatic pneumothorax. Thoracentesis should be performed as necessary to prevent dyspnea while the pulmonary lesion heals, usually within 3 to 5 days. Recurrence is uncommon. Conversely, animals with spontaneous pneumothorax commonly have recurrence of the pneumothorax if they are not operated. An open chest wound should be covered immediately with any readily available material. Once admitted to the hospital a sterile occlusive dressing should be applied as rapidly as possible and intrapleural air evacuated by thoracocentesis or tube thoracostomy.
Surgical therapy of animals with traumatic pneumothorax is seldom necessary. However, non-surgical management of spontaneous pneumothorax usually results in a less than satisfactory outcome. Mechanical pleurodesis of the lungs may decrease the recurrence of pneumothorax in animals operated for spontaneous pneumothorax. Mechanical pleurodesis damages the pleura such that healing results in adherence of the visceral and parietal pleural. Postoperative pneumothorax or pleural effusion must then be prevented as they will result in separation of the parietal and visceral pleura, precluding adhesion formation.
Care should be used when anesthetizing and ventilating animals with pneumothorax and/or pulmonary bullae. Intermittent positive pressure ventilation (IPPV) may rupture intact bullae or accelerate air leakage from the damaged lung or bronchial tree. Therefore, do not exceed inspiratory pressures of 10 to 12 cm H2O pressure in these animals until the chest cavity is opened; then, the adequacy of ventilatory pressures should be reevaluated. Because IPPV may induce a tension pneumothorax, immediate treatment of this condition (i.e., needle thoracentesis, chest tube placement) may be necessary and should be anticipated. The use of nitrous oxide is contraindicated in patients with pneumothorax.
If an underlying pulmonary lesion is readily identified (i.e., pulmonary abscess or neoplasia) and can be localized to one hemithorax, an intercostal thoracotomy allows lobectomy to be performed more readily than from a median sternotomy approach. However, diffuse, bilateral pulmonary disease with multiple bullae is usually present in dogs with spontaneous pneumothorax. A median sternotomy allows visualization of all lung lobes, plus partial resection of any diseased lobes. Mechanical pleurodesis should be performed in dogs with spontaneous pneumothorax to decrease recurrence.
Identify and remove diseased lung. If the source of the pleural air is not evident, fill the chest with warmed, sterile saline or water and look for air bubbles when the anesthetist ventilates the animal. If multiple, partial lobectomies are necessary, use an automatic stapling device to decrease operative time. Perform pleural abrasion using a dry gauze sponge. Gently abrade the entire surface of the lung and parietal pleura. Prior to closure, fill the chest cavity with warmed fluid and look for air bubbles when the animal is ventilated to ensure that there are no further air leaks. Place a chest tube and remove residual air before recovering the animal. Postoperatively, if continuous air leakage is present, or pleural effusion develops, place the animal on a continuous suction device. In animals with an open pneumothorax, definitive closure of large thoracic wall defects may require mobilization of adjacent muscle in order to provide an air-tight closure.
Mechanical pleurodesis has been advocated in numerous studies in the veterinary literature to prevent recurrence of spontaneous pneumothorax in dogs. This technique typically involves abrading the surface of the lung and body wall with a dry gauze sponge during open thoracotomy. Although, clinical studies suggest that this technique is effective, necropsy studies evaluating the mechanism of mechanical pleurodesis are not available. We have recently investigated mechanical pleurodesis and our studies suggest that any reduction in recurrence of pneumothorax is more likely related to fibrosis of the visceral pleura than it is to obliteration of the pleural space.
With appropriate monitoring and care, the prognosis is excellent for animals with traumatic pneumothorax in which therapy is initiated prior to extreme dyspnea or respiratory arrest. In a recent study of dogs with spontaneous pneumothorax, 100% of those treated with needle thoracentesis alone and 81% of those managed with chest tubes had recurrence of pneumothorax. The times until recurrence varied from 3 days to 30 months. Three of 12 dogs (25%) undergoing thoracotomy had recurrence; only 1 of these had intraoperative pleural abrasion performed.