Pulmonary edema is the buildup of fluid in the air sacs of the lungs leading to shortness of breath. The air sacs (alveoli) are small balloon-like structures where the exchange of carbon dioxide and oxygen occurs. Tiny capillaries surround the alveoli on the outside. When the cat breathes in, air enters the lungs, causing the alveoli to expand, oxygen passes from the alveoli and into the capillaries, carbon dioxide from the capillaries pass into the alveoli and is exhaled out.
Pulmonary edema not a disease in itself but a manifestation of an underlying disorder.
There are many causes of pulmonary edema in cats, which are divided into cardiogenic (relating to the heart) or noncardiogenic.
The most common causes of pulmonary edema relate to problems with the heart, such as hypertrophic cardiomyopathy. Heart diseases can lead to reduced movement of blood through the capillaries in the lungs. As the blood flow slows, fluid leaks out of the vessels into the airways. 
Other causes include electric shock, certain medications, anemia, kidney disease, airway obstruction, cancer, pneumonia, lungworm, allergic reactions, seizures, smoke inhalation, hypervolemia (fluid overload due to the administration of fluids), and head trauma.
There are several ways in which pulmonary edema occurs:
- Increase in pulmonary capillary pressure.
- Increase of the permeability between the capillaries and the alveoli.
- Obstruction of the lymphatic drainage of the lungs.
Respiratory distress is the primary symptom of pulmonary edema. The cat will try to remove the fluid by coughing and gagging. Pulmonary edema can occur very quickly (acute) or be slow and progressive (chronic).
- Difficulty breathing (dyspnea)
- Coughing and gagging
- Open-mouthed breathing
- Rapid breathing (tachypnea)
- Crackling sound in the lungs
- Cyanosis (blue-tinged gums and tongue)
Your veterinarian will perform a physical examination and obtain a medical history from you including any medications your cat may have recently taken and other symptoms you may have noticed.
- A chest x-ray will show a collection of fluid in the lungs.
- Echocardiography – Ultrasound of the heart to evaluate size and function.
- Biochemical profile, complete blood count, and urinalysis to evaluate the overall health of your cat and look for diseases such as diabetes or kidney failure.
The goal of treatment is to manage symptoms as well as address the underlying cause.
- Diuretics help in the removal of excess fluids by increasing urine output.
- Oxygen therapy to help your cat to breathe.
- Vasodilators to open up the vessels, preventing the fluid from building up.
- Sedatives and analgesics to relieve pain and anxiety.
- Cage rest while the cat recuperates.
Treatment for specific causes may include:
- Electric shock: Antibiotics and painkillers.
- Hypertrophic cardiomyopathy: Medications such as beta-blockers or ACE inhibitors to relax the heart, low salt diet and blood-thinning drugs.
- Medications: Cessation of medicines that may have caused pulmonary edema.
- Anemia: Blood transfusion and supportive care.
- Kidney disease: Low protein and phosphate diet, phosphorous binders and erythropoietin, a hormone that assists with red blood cell production.
- Airway obstruction: Surgery to remove the blockage.
- Cancer: Surgery and/or radiotherapy or chemotherapy.
- Pneumonia: Antibiotics and supportive care.
- Lungworm: Anti-worming medications to kill the worms.
- Allergic reactions: Avoidance of the allergen if possible. Antihistamines can relieve symptoms. Severe allergic reactions (anaphylaxis) may be treated with the administration of adrenaline.
- Head trauma: Surgery, where necessary.
- Seizures: Anti-seizure medication and supportive care.
Prognosis is poor and depends on the underlying cause and how successful initial treatment is.
 Cat Health Encyclopedia – Edited by Dr Lowell Ackerman.