Management of respiratory emergencies

Pneumothorax

Pneumothorax is an accumulation of air or gas within the pleural space. Traumatic pneumothorax may be classified either as “open” or “closed”. 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.

Surgical treatment

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.

Prognosis

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.

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