Pulmonary Thromboembolism (Blood Clots in the Lungs) in Cats

By Courtney Barnes, BSc, DVM; Malcolm Weir, DVM, MSc, MPH; Robin Downing, DVM, DAAPM, DACVSMR, CVPP, CRPP

What is a pulmonary thromboembolism?

Pulmonary means lung, and thromboembolism describes an obstruction of a blood vessel by a blood clot that has dislodged from another site. A pulmonary thromboembolism (PTE) describes a blood clot that has moved through the blood vessels, lodged in one of the pulmonary arteries, and blocked blood flow into the portion of the lung served by that artery.

PTE seems to be more common in cats less than 4 years of age or older than 10 years of age. No sex or breed tendency has been found.

What are the typical signs of pulmonary thromboembolism?

The signs of pulmonary thromboembolism include:

  • sudden difficulty breathing
  • rapid breathing
  • decreased appetite (anorexia)
  • fainting/collapse
  • coughing
  • spitting up blood
  • weakness
  • exercise intolerance
  • inability to get comfortable

How is pulmonary thromboembolism diagnosed?

Your veterinarian may find several important clues during a physical examination that point to a pulmonary thromboembolism. Your cat may have a rapid heart rate with weak pulses, and she may have a heart murmur. Her gums may be pale or even bluish from low oxygen levels in the blood. When your veterinarian presses on the gums to push away the blood, it takes longer than normal for the small blood vessels to refill with blood. An oxygen sensor placed on the gums or skin will often read too low.

"Chest X-rays are often normal, but may show abnormalities such as fluid in or around the lungs or enlargement of pulmonary blood vessels."

Blood testing can reveal clues about why your cat is forming blood clots. Chest X-rays are often normal, but may show abnormalities such as fluid in or around the lungs or enlargement of pulmonary blood vessels.

Other testing may be recommended, including echocardiography (ultrasound of the heart) and blood clotting profiles. Definitive diagnosis can be made with a CT scan using contrast dye (angiography) to show the blood vessels.

What could have caused my cat to have a pulmonary thromboembolism?

There are many important considerations and potential causes of PTE in cats:

  • feline heartworm disease
  • cancer
  • Cushing’s disease, in which excessive steroids are produced by the adrenal glands
  • administration of steroid medications
  • protein-losing nephropathy (kidney disease in which excessive protein is lost through the urine)
  • protein-losing enteropathy (intestinal disease in which protein leaks into the intestines and is lost into the stool)
  • immune-mediated hemolytic anemia (IMHA), in which the immune system destroys its own red blood cells
  • pancreatitis
  • heart disease
  • bone surgery or trauma
  • bacterial infection in the bloodstream (sepsis)
  • liver disease
  • clotting of the blood within the blood vessels, called disseminated intravascular coagulopathy (DIC)

How is pulmonary thromboembolism in cats treated?

Many cats with thromboembolism are first treated as inpatients until the oxygen level in their blood returns to normal. Oxygen therapy may be a part of their treatment, but response to oxygen therapy is variable. Once your cat returns home, it is important to restrict activity to prevent fainting and further decline in blood oxygen levels.

Some medications are available for treating this disorder. Heparin is a medication to prevent blood from clotting (anticoagulant). It may help to prevent future clots but will not break up existing clots. Warfarin is an anticoagulant that may be considered for long-term treatment. With warfarin, dosage adjustments are necessary to keep blood clotting at a specific level, and to balance the risk of further clots with the risk of bleeding complications.

Anti-platelet medications such as clopidogrel (e.g., brand name Plavix®) are also sometimes used as preventive medications in cats with disorders that might put them at increased risk of developing PTE.

Medications to dissolve existing clots (thrombolytic drugs) have been tried in cats but have been associated with increased complications and death.

What kinds of monitoring and follow-up will my cat need?

Your veterinary team will monitor the oxygen in your cat's blood using pulse oximetry (a sensor used on the outside of the body), which allows them to track improvement in breathing function. Your cat will need blood tests to check blood clotting status, which is required to adjust the dose of warfarin. Bleeding complications may arise in pets treated with anticoagulant medications.

Controlled activity or physical therapy may improve general blood flow and prevent the development of future blood clots.

What is the outlook for my cat?

The prognosis for cats with pulmonary thromboembolism is generally guarded to poor, and it depends on resolution of the underlying cause. Cats tend to have a better prognosis when the cause of their pulmonary thromboembolism is trauma or generalized bacterial infection.

Future episodes of pulmonary thromboembolism are likely unless an underlying cause is identified and corrected. These blood clots are often fatal, and sudden death is not unusual. Treatment with anticoagulant medications can lead to bleeding complications, necessitating frequent re-evaluation of clotting times, and these medications may be required long-term.

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