The chest cavity does not normally contain air, except for air within the lungs. Any air in the chest cavity is abnormal. A pneumothorax develops when air is allowed into the chest cavity through a breach in the chest wall due to injury (external leak), and/or air leaks into the chest cavity due to a leak in the lung tissue or airways (internal leak).

The presence of air within the chest cavity exerts pressure on the lungs so they cannot expand or inflate when the pet tries to take a breath. Large volumes of air cause the lungs to collapse completely. If the lungs do not inflate normally or are collapsed, the pet cannot obtain enough oxygen and develops signs of difficulty breathing such as rapid, shallow breaths, and cyanotic (blue) gums and tongue. The volume of air and the rate at which it accumulates within the chest varies with the degree of traumatic injury or underlying problem.

Pets with pneumothorax require hospitalization for an average of two to five days.

Related Symptoms or Diseases

There are many other problems involving the lungs that could produce symptoms similar to those observed with pneumothorax, such as:

Pleural effusion. An accumulation of fluid in the chest cavity restricts the lungs from inflating normally when the animal takes a breath. Types of fluid that may accumulate, include blood (hemothorax), chyle (chylothorax), pus (pyothorax), and clear fluid (hydrothorax). The presence of fluid in the chest is abnormal regardless of its origin.

Neoplasia. Cancer can cause difficulty breathing by causing fluid to accumulate in the chest cavity, by replacing normal lung tissue with cancerous tissue, or simply by occupying space required by the lungs. Many different types of cancer can occur within the chest cavity.

Pulmonary contusions. Bruising and bleeding into the lungs are the result of trauma and cause difficulty breathing. Pulmonary contusions and pneumothorax are both common following trauma to the chest and must be distinguished from one another by auscultation and chest radiographs.

A diaphragmatic hernia. A tear in the diaphragm, the tissue that separates the chest from the abdominal cavity, allows abdominal organs to move into the chest cavity. A diaphragmatic hernia is often the result of trauma, although some cats may be born with this condition. This problem is diagnosed using X-rays or abdominal ultrasound.

Pneumonia. An infection in the lungs can result from aspiration of vomit or other debris into the airways or can be spread via the bloodstream. Pets with pneumonia often have a fever, which is not typical with a pneumothorax.