Structural Disorders

Pneumothorax

Pathophysiology

Entrance of air into the pleural cavity results in positive intrapleural pressure that causes collapse of the lungs with a subsequent drop in venous return and cardiac output. The most common cause of pneumothorax is blunt or penetrating chest trauma. Esophageal perforationor foreign body migration can also lead to pneumothorax. Spontaneous pneumothorax results from rupture of an emphysematous bulla, pleural bleb, neoplasm, Paragonimus kellicotti cyst, or of a necrotic or abscessed lung lobe. Multiple lung lobes can be involved depending on the disease process, and cranial lobes are affected most commonly in spontaneous pneumothorax.

A specific form of pneumothorax is a tension pneumothorax, where a piece of pulmonary tissue or pleura acts as a ball-valve, allowing passage of air into the pleural space during inspiration with inhibition of escape of air via the same pathway. In this situation, air continues to accumulate in the pleural space, leading to a continual rise in intrapleural pressure and circulatory collapse.

History and signalment

Any age or breed of dog or cat can be affected. Spontaneous pneumothorax associated with underlying lung disease appears to be more common in middle-aged large-breed dogs, and in one study, Siberian Huskies were overrepresented (Puerto et al. 2002). It is less common in cats but can occur secondary to chronic bronchial disease. The most common cause of pneumothorax in dogs and cats is trauma, from a car collision, falling from a height, or a thoracic bite wound. In suburban and rural areas, migrating grass awns are also implicated as a cause for pneumothorax. Iatrogenic pneumothorax occurs secondary to barotrauma during anesthesia, endotracheal tube injury, fine-needle lung aspiration, or bronchoscopy.

The primary clinical complaints associated with pneumothorax are difficulty breathing or tachypnea, and these are usually seen immediately after the injury or rupture of a pulmonary lesion. Some cases with spontaneous pneumothorax may lack a history of signs referable to the respiratory tract, and will display chronic, nonspecific signs of lethargy and anorexia prior to development of worsening respiratory difficulty.

Physical examination

Accumulation of air in the pleural space leads to rapid and shallow breathing, and lung sounds are usually difficult to discern. Air in the pleural space will rise, leading to an absence of breath sounds dorsally and hyperresonance on percussion. Animals involved in traumatic episodes should be closely evaluated for additional injuries including broken ribs, diaphragmatic hernia, and pulmonary contusions.

Diagnostic findings

Pneumothorax is usually bilateral and is recognized by an absence of lung parenchyma and pulmonary vasculature in the periphery of the lung on thoracic radiographs. The heart is usually lifted off the sternum on the lateral view (Figure 7.1). While pneumothorax is readily apparent on chest radiographs, the underlying lesion is often not visible, particularly when bullous lung disease or pleural blebs are the cause of pneumothorax. Cross-sectional imaging with computed tomography (CT) improves definition of lung regions (Figure 7.2), and when sufficient lung expansion can be obtained to alleviate atelectasis, it can be helpful in identifying diseased regions (Au et al. 2006).

Figure 7.1. Right lateral (a) and dorsoventral (b) radiographs from a 6-year-old MI Labrador Retriever with spontaneous pneumothorax.

Figure 7.2. Computed tomographic slice through the caudal thorax in the dog from Figure 7.1 illustrates a soft tissue structure in the distal portion of the right caudal lung lobe. Histopathology revealed a pulmonary adenocarcinoma.

For animals from the Midwest or Gulf States or those that may have been exposed to Paragonimus, a trematode that employs snails and crustaceans as intermediate hosts, a fecal sedimentation or zinc sulfate centrifugation–flotation should be performed. In dogs, thoracic radiographs may show multiple thin-walled cavitated cysts, while in the cat, thick-walled granulomatous lesions and pleural involvement are more typical.

Treatment

Thoracocentesis should be performed when pneumothorax is suspected and may be required as a life-saving procedure prior to radiographic confirmation of pneumothorax in order to resolve respiratory distress. With traumatic pneumothorax, a single chest tap and alleviation of respiratory distress through sedation and administration of oxygen will often restore pleural integrity and result in cure. If Paragonimus is diagnosed, praziquantel (25 mg/kg PO TID for 3 days) or fenbendazole (50 mg/kg/day for 2 weeks) have been recommended as effective (Bowman et al. 1991), although surgical resection of the diseased lung region is also required.

If air continues to accumulate within the pleural space after several chest taps, placement of a chest tube is required (see Chapter 3). Generally, spontaneous pneumothorax and tension pneumothorax will require placement of a chest tube and constant thoracic drainage to resolve air leakage. Because underlying lung disease is often present with spontaneous pneumothorax, surgical intervention is usually required to avoid recurrence. When pneu-mothorax does not resolve within 3–5 days or if parenchymal lesions causing pneumothorax are visualized on radiographs, an exploratory thoracotomy should be performed. Median sternotomy is usually preferred to allow full exploration of the lungs.

Prognosis

Traumatic pneumothorax generally has a good prognosis if traumatic myocarditis and pulmonary contusions do not complicate the presentation. The prognosis for spontaneous pneumothorax depends on the underlying disease responsible for air leakage. Surgical resection of the affected area is associated with an excellent outcome in most dogs with emphysematous lung disease, foreign body pleuropneumonia, or Paragonimus infection.

Pneumomediastinum

Pathophysiology

Pneumomediastinum is an accumulation of air around the structures in the cranial mediastinum (carotid arteries, vagosympathetic trunk, trachea, esophagus, and cranial vena cava). It occurs most commonly from endotracheal tube injury or overinflation of the cuff, traumatic jugular venipuncture, or a transtracheal wash. It is also associated with trauma from a dog fight or from overventilation during anesthesia. It can occur with bronchial, tracheal, or alveolar rupture when air tracks along fascial planes to reach the mediastinum.

History and signalment

Any age dog or cat can be affected. Usually a history of trauma or iatrogenic injury is present immediately prior to the onset of signs or within a week of presentation. In cats, a recent dental procedure is often found in the history. Often no clinical signs are noted, and pneumomediastinum is an incidental finding on radiographs, following traumatic venipuncture or tracheal wash. In more severely affected animals, tachypnea, respiratory distress, or collapse associated with pneumothorax may result. Owners may detect subcutaneous emphysema concentrated over the neck, thorax, and head.

Physical examination

Mild, moderate, or severe subcutaneous emphysema may be detected. When pneumothorax is present, tachypnea is often evident, and lung sounds are absent dorsally.

Diagnostic testing

Diagnosis is based on radiography. Pneumomediastinum is evident when outer borders of the trachea, esophagus, carotid arteries, aorta, and azygous veins are visible because of contrast with mediastinal air. Subcutaneous emphysema may be detected over the trunk (Figure 7.3). If tracheal rupture is suspected, tracheoscopy can occasionally be helpful in locating the lesion; however, surgical exploration is usually required.

Figure 7.3. Right lateral radiograph from an 18-month-old FS DSH that had been anesthetized 2 weeks previously and developed subcutaneous emphysema and pneumomediastinum.

Treatment

Generally no specific treatment is required and air will resorb within 10–20 days. Concurrent pneumothorax requires thoracocentesis. If respiratory distress is present, oxygen therapy is helpful for reducing the work of breathing, and judicious use of sedatives to reduce stress is important. If tachypnea or respiratory effort increases, surgical repair of the lesion is necessary.

Prognosis

Prognosis is usually excellent for resolution of disease. When pneumomediastinum is detected, restraint methods and diagnostic techniques should be reviewed for predisposing features that might have led to airway injury. In cats, <3 mL of air is generally required to inflate the cuff of an endotracheal tube sufficiently.

Diaphragmatic Hernia

Pathophysiology

Blunt trauma to the abdomen increases intracavitary pressure leading to rupture of the muscular portion of the diaphragm and herniation of abdominal organs into the thoracic cavity. The liver is involved most frequently, followed by the small intestine and stomach. The lungs collapse because of loss of contact between visceral and parietal pleura and from compression by abdominal organs. Fluid accumulation in the thorax due to hemorrhage or transudation from the organ surface further restricts lung expansion and worsens oxygenation and ventilation. Animals with herniation of the stomach into the chest cavity can suffer rapid decompensation if aerophagia results in continual expansion of the stomach. The space occupying effect of gastric dilation augments lung compression, with a decrease in venous return and subsequently cardiac output.

History and signalment

Affected animals may have a history of recent trauma within the past several hours or may have experienced injury years in the past. Most dogs and cats are young (3–4 years) in age. Clinical signs are usually respiratory in origin and include tachypnea and difficulty breathing, but some animals are presented for acute or chronic vomiting or regurgitation associated with gastrointestinal obstruction or strangulation. Still others may develop progressive exercise intolerance or respiratory difficulty associated with gradual accumulation of pleural fluid.

Physical examination

Animals with organ herniation or pleural effusion often have muffling of heart and lung sounds ventrally or on one side of the chest. When herniation has been present for >24–48 hours, intestinal borborygmi may be auscultated over the thorax (Figure 7.4). Occasionally, abdominal palpation may reveal an absence of contents. Traumatic injury to other structures in the thorax, abdomen, or skeleton can be found in 25–40% of cases, and a complete physical examination should be performed to detect findings consistent with traumatic myocarditis, urinary system rupture, rib fractures, or orthopedic injuries (Gibson et al. 2005).

Figure 7.4. Dorsoventral radiograph from a 12-year-old MC Labrador retriever with exercise intolerance reveals gastrointestinal loops in the right hemithorax. Physical examination findings included absent lung sounds in the right hemithorax and the presence of gastrointestinal sounds.

Diagnostic findings

Thoracic and abdominal radiographs can usually confirm the diagnosis of a diaphragmatic hernia; however, if pleural effusion is present, thoracocentesis and repeat radiographs may be required to allow definition of the diaphragmatic silhouette and thoracic structures. Chronic cases are often more difficult to diagnose radiographically. A barium series can confirm the presence of intestines in the thoracic cavity or provide better definition of the position of the liver or small intestines, and ultrasound can occasionally be helpful. CT and/or exploratory surgery may be required for confirmation of the diagnosis.

Treatment

Surgical repair of the diaphragm is indicated, and artificial ventilation must be provided once the abdominal cavity has been opened. Cases with stomach herniation generally represent a surgical emergency because respiratory difficulty causes aerophagia, progressive stomach distention, and further respiratory compromise. Standard stabilization methods with intravenous fluid support and withdrawal of pleural fluid prior to anesthesia are important. Oxygenation, ventilation, and cardiac rhythm must be monitored carefully throughout the procedure.

Prognosis

Improvements in anesthetic drugs, monitoring, and assisted ventilation have vastly improved response to surgery. Diaphragmatic hernia repair is associated with an 80–90% survival rate (Gibson et al. 2005), regardless of whether the hernia is acute or chronic in nature and regardless of the time between admission and surgery. Owners should be aware that additional surgery might be required to repair soft tissue or orthopedic injuries that are not as life-threatening as the diaphragmatic hernia.

Infectious Disorders

Hematology usually shows mild anemia and leukocytosis characterized by neutrophilia and monocytosis. Increased serum globulin is a nonspecific indicator of chronicity. Thoracic radiographs reveal unilateral or bilateral pleural effusion with obscuring of the cardiac silhouette and diaphragm, scalloping of ventral lung margins, and blunting of costophrenic angles (Figure 7.5). Evidence of a foreign body or a consolidated lung lobe is occasionally present. Thoracic ultrasound can be helpful in revealing flocculent or viscous pleural fluid and can sometimes detect lobar consolidation or a foreign body.

The initial step in confirming a diagnosis of pyothorax is to perform a chest tap (see Chapter 2). Cytologic analysis reveals an exudative fluid (high protein and high cell count) comprised primarily of degenerate neutrophils. Intracellular bacteria can be observed in up to 91% of cats and 68% of dogs (Walker et al. 2000) (Figure 7.6). A Gram stain can provide information on the type of bacteria present and improves choice of antibiotics while cultures are pending. Both aerobic and anaerobic cultures should be requested on a pleural exudate because mixed bacteria are usually found (Table 7.1). Note that a special transport medium may be required to obtain an accurate result for anaerobic bacteria.

Treatment

Successful treatment usually requires placement of uni- or bilateral chest tubes (see Chapter 3) and thoracic lavage with warm saline (10–20 mL/kg BID–QID). Addition of heparin to the lavage fluid (1500 units/100 mL) can reduce adhesions and promote drainage. The fluid is left in the chest cavity for 1 hour and then aspirated. The patient resorbs 10–25% of the fluid. Broad-spectrum antibiotics that are active against both aerobes and anaerobes are administered systemically. Anaerobic coverage should be provided even if cultures are negative. Penicillins and cephalosporins are commonly used alone or in combination with a fluoroquinolone. Trimethoprim sulfonamide is often employed if infection with No-cardia spp. is suspected. There is no advantage to adding antibiotics to the lavage fluid because the primary route of exposure of the pleura to antibiotics is through the systemic circulation.

Figure 7.5. Right lateral (a) and dorsoventral (b) radiographs from a 10-year-old MC DMH presented for anorexia and weight loss. Physical examination revealed dampened heart and lung sounds ventrally, and radiographs confirmed the presence of pleural fluid.

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