Air within the mediastinum may be the result of spontaneous rupture, trauma, or the result of diagnostic or therapeutic interventions.
Iatrogenic causes include surgical trauma, mechanical ventilation and transtracheal aspiration biopsy. Any trauma to the cervical
trachea can result in pneumomediastinum. Pneumomediastinum is often asymptomatic and diagnosed only as the result of thoracic
radiographs. Dyspneic animals usually have accompanying pneumothorax and treatment is directed at the pleural space disease.
Mediastinitis is either acute or chronic and usually is the result of an infectious process. Acute causes are usually due
to perforation of the esophagus or trachea with leakage into the mediastinum. These cases are usually surgical emergencies
to fix the perforated organ and lavage the surrounding tissue. Granulomatous mediastinitis is occasionally the result of fungal
disease. Agents implicated in these chronic cases include Histoplasma, Crytococcus, Coccidioides, Actinomycoses and Nocardia
Pneumothorax is an accumulation of air in the pleural space and can develop secondary to pathologic or iatrogenic causes.
Trauma commonly causes pneumothorax. Spontaneous pneumothorax occurs secondary to underlying pulmonary disease or to rupture
of an air-containing space such as a pulmonary bulla or bleb. Spontaneous pneumothorax is classified as closed pneumothorax
(no communication with the atmosphere). Close pneumothorax develops due to diseases of trachea, bronchi, lungs, or esophagus.
Tension pneumothorax develops when there is a defect in the pleura that allows accumulation of air in the pleural space during
inspiration but the air does not escape during expiration. Iatrogenic pneumothorax can develop secondary to diagnostic procedures
that cross the pleural space (transthoracic aspiration, transthoracic biopsy, pericardiocentesis), intrathoracic surgery,
tracheal intubation, and excessive positive pressure ventilation.
Animals with pneumothorax present with some degree of increased respiratory rate, dyspnea, exercise intolerance, restrictive
(rapid, shallow) breathing pattern, cyanosis and poor capillary refill time. Abnormalities are due primarily to hypoventilation
resulting in hypoxia. Ventilation-perfusion mismatching and decreased cardiac output also contribute to clinical disease.
Diagnosis is based on history, signalment, physical examination findings, thoracocentesis, and thoracic radiographic findings.
Traumatic and iatrogenic pneumothorax can usually be managed with intermittent thoracocentesis or continuous suction via chest
tube. Spontaneous pneumothorax may need to be managed surgically depending on the primary etiology. Primary spontaneous pneumothorax
secondary to bulla or blebs often recurs and so some authors recommend surgical intervention.
Table 1. Differential diagnoses of pleural effusions based on cell number and protein content.
Pyothorax is diagnosed by cytology. Pleural lavage is the treatment of choice. Because the disease usually affects both hemi
thoraces bilateral chest tubes are usually necessary. The pleural space is lavaged 2-3 times daily with approximately 20 ml/kg
of warmed 0.9% saline or Ringer's solution. The lavage fluid should be instilled slowly; the injection should be discontinued
if respiratory distress occurs. The lavage fluid remains in the pleural space for 30-60 minutes unless the animal becomes
dyspneic. The patient will absorb approximately 25% of the initial lavage volume. Clinical findings, thoracic radiographs
and cytology of the pleural effusion are used monitor clinical progress. Most animals with successful pleural lavage will
have a decrease in fever and improvement in general attitude within the first 48 hours. Radiographic re-evaluation should
be performed 48 hours after tube placement and following the complete removal of all lavage fluid. Assess the radiographs
for pleural space fluid volume, atelectasis, and areas of encapsulated fluid. Cytology of pleural fluid is generally performed
prior to lavage. Numbers of neutrophils, macrophages and bacteria as well as the percentage of degenerate neutrophils are
estimated. Most cases with pyothorax will have a gradual decrease in inflammatory cell numbers over 3-5 days.
Thoracic radiographs are generally suggested at 7 and 28 days following tube removal. The combination of pleural lavage and
antibiotic therapy has been reported to successfully resolve pyothorax in 53% of dogs and 60% of cats. Surgical exploration
is indicated when pleural lavage does not result in a rapid resolution of clinical signs.