Acute Respiratory Distress Syndrome (Shock Lung) in Cats

What is acute respiratory distress syndrome?

Acute respiratory distress syndrome (ARDS), also known as shock lung, is a life-threatening complication of critical illness. This condition was noted in humans in the 1960s and discovered in pets more recently. ARDS has primarily been studied in humans because it appears to be relatively rare in pets. As more is learned about the disease and veterinarians become more familiar with it, more cases will likely be diagnosed. 

What causes acute respiratory distress syndrome? 

Acute respiratory distress syndrome is a complication of severe, systemic disease. Diseases that affect the lungs directly can result in ARDS, such as pneumonia (due to infection or the inhalation of foreign materials), smoke inhalation, or near drowning.

Conditions that lead to massive body-wide inflammation can also lead to ARDs, such as sepsis (infection), pancreatitis, snake bite envenomation, severe trauma, burns, or shock. In most cases, ARDS occurs one to four days after the onset of the pet’s severe inflammatory disease. In many cases, this is a period when the cat is hospitalized for treatment.

In ARDS, massive inflammation and the release of various inflammatory chemicals cause capillaries (small blood vessels) to leak fluid into the lungs. This fluid interferes with the body’s ability to exchange oxygen effectively via the lungs. When the animal can no longer exchange oxygen, tissue damage occurs throughout the body and can lead to death.

What are the clinical signs of acute respiratory distress syndrome?

Signs of ARDS include increased respiratory rate, blue discoloration of the skin and mucous membranes (gums, insides of the eyelids) due to poor oxygen delivery, and, occasionally, coughing. Some affected cats may have pink, foamy fluid from the mouth or nose, which is coming from their lungs.

Abnormal or increased respiratory sounds may be heard when your veterinarian examines your cat and listens to her chest. If your cat is monitored for blood oxygen levels, these levels will be low because efficient oxygen exchange is not happening properly.

How is acute respiratory distress syndrome diagnosed?

The most important test in diagnosing ARDS is a blood test, called a blood gas analysis, which provides detailed information on the levels of oxygen and carbon dioxide in the blood. Your veterinarian will also likely perform a complete blood cell count, serum biochemistry profile, and urinalysis to assess your cat’s overall health. Chest X-rays and an echocardiogram (ultrasound of the heart) may also be performed.

In addition to these tests, your veterinarian will perform tests to determine the underlying cause of your cat’s illness (if it has not already been identified). These tests may include X-rays of the abdomen, ultrasound, exploratory surgery, bacterial cultures, and other diagnostic tests that vary based on the exam findings.

How is ARDS treated?

Cats with ARDS are critically ill. They require 24-hour care, ideally at a referral center with specialists in emergency and critical care. There are two components in ARDS treatment.

First, the underlying disease must be identified and treated to decrease inflammation and prevent further fluid leakage into the lungs. Second, as the underlying disease is being addressed, pets must receive support for their lung function. In the early stages, lung support often involves supplementing oxygen by using an oxygen cage, or a line that carries oxygen directly into the nasal passages. In more severe cases, cats often must be placed on a mechanical ventilator, which requires the cat to be fully anesthetized. A tube is passed into the trachea (windpipe) and the ventilator is used to breathe for the cat. The goal of this ventilation is to keep the body’s oxygen levels high enough for survival while treatment begins.

"...fluid therapy in patients with ARDS is a challenging balancing act."

Cats with ARDS are typically hospitalized on IV fluids to address their underlying disease, which presents challenges for the veterinary team: those fluids that keep blood flowing to your cat’s tissues are the same fluids that can pool in the lungs. Therefore, fluid therapy in patients with ARDS is a challenging balancing act.

Cats also often require supplemental nutrition. The increased effort required to breathe with ARDS involves significant energy exertion. Cats with ARDS typically receive nutrition by a feeding tube or by intravenous nutritional support.

No specific medication has proven beneficial in the treatment of ARDS. Several drugs are used in these cases, including antibiotics and medications that influence lung function and inflammation. The response to these medications is variable. The only known effective treatment for ARDS is time and supportive care, with treatment directed at the underlying cause of the severe inflammatory response.

What is the prognosis for acute respiratory distress syndrome?

Unfortunately, the prognosis for ARDS is poor. Even with aggressive treatment, including the use of a mechanical ventilator, many pets with ARDS do not survive. The mortality rate for veterinary patients is around 90%. Even in human medicine, with its extensive research and aggressive treatment approaches, mortality is approximately 40%-60% for this condition.