Located in the lungs are thousands of air sacs known as alveoli which are small balloon-like structures where the exchange of carbon dioxide and oxygen occurs. The alveoli are surrounded by tiny capillaries 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 is the buildup of fluid in the within these air sacs leading to a shortness of breath. It is not a disease in itself but a manifestation of another disease. There are many causes of pulmonary edema in cats, which are divided into cardiogenic (relating to the heart) or noncardiogenic. The main cause of pulmonary edema is problems with the heart, such as hypertrophic cardiomyopathy, which results in poor movement of blood through the capillaries in the lungs. As the blood flow slows, fluid leaks out of the vessels into the airways. 
There are several ways in which pulmonary edema occurs:
Increase in the pulmonary capillary pressure.
Increase of the permeability between the capillaries and the alveoli.
Obstruction of the lymphatic drainage of the lungs.
What are the symptoms of pulmonary edema?
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
Rapid breathing (tachypnea)
Crackling sound in the lungs
Cyanosis (blue-tinged gums and tongue)
How is pulmonary edema diagnosed?
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. He will need to determine what caused the pulmonary edema.
Diagnosis is based on patient history, symptoms, and test results.
Chest x-ray which 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 may also be taken to evaluate the overall health of your cat and look for diseases such as diabetes or kidney failure.
How is pulmonary edema treated?
Treating the underlying cause as well as supportive care while your cat recovers. This will include:
Diuretics which 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 fluid building up.
Sedatives and analgesics may be prescribed to relieve pain and anxiety.
Cage rest may be recommended while your 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 medications which may have caused pulmonary edema.
Anemia: Blood transfusion and supportive care.
Kidney disease: Low protein and phosphate diet, phosphorous binders and erythropoietin, a hormone which assists with red blood cell production.
Airway obstruction: Surgery to remove the obstruction.
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 may be recommended. Severe allergic reactions (anaphylaxis) may be treated with administration of adrenaline.
Head trauma: Surgery, where necessary.
Seizures: Anti-seizure medication and supportive care.
Prognosis is guarded and depends on the underlying cause and how successful initial treatment is.
 Cat Health Encyclopedia – Edited by Dr. Lowell Ackerman.
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