Pulmonary Thromboembolism (Blood Clots in the Lungs) in Dogs

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

What is a pulmonary thromboembolism?

Pulmonary means lung, and the word thromboembolism describes an obstruction of a blood vessel by a blood clot that has become 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 has blocked blood flow into the portion of the lung served by that artery.

Pulmonary thromboembolism (PTE) seems to be more common in medium to large-breed dogs, and generally in middle-aged to older dogs.


What are the typical signs of pulmonary thromboembolism?

The typical signs of pulmonary thromboembolism include:

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

Your veterinarian may find several important clues pointing to a pulmonary thromboembolism during a physical examination. Your dog may have a very rapid heart rate with weak pulses, and he may have a heart murmur. His gums may be pale or even bluish from too low an oxygen level 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.

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

There are many important considerations and potential causes of PTE in dogs. Canine heartworm disease is one potential cause, emphasizing the need to provide appropriate monthly protection against heartworm disease. Other important potential causes of canine thromboembolism include cancer, excessive levels of steroids produced by the adrenal glands in Cushing’s disease, steroid medications, and kidney disease in which protein is lost into the urine.

Other potential causes of pulmonary thromboembolism include:

  • immune-mediated hemolytic anemia (IMHA), in which the dog’s immune system destroys the dog’s own red blood cells
  • pancreatitis
  • heart disease
  • bone surgery or trauma
  • bacterial infection in the bloodstream
  • liver disease
  • clotting of the blood within the blood vessels, called disseminated intravascular coagulopathy (DIC)


How is pulmonary thromboembolism in dogs treated?

Many dogs with thromboembolism are first treated as inpatients until the oxygen levels in their blood return to normal. Oxygen therapy may be a part of their treatment, but response to oxygen therapy may be variable. Once the dog returns home, it is important to restrict activity to prevent worsening of low levels of oxygen in the blood or fainting.

"Once the dog returns home, it is important to restrict activity to
prevent worsening of low levels of oxygen in the blood or fainting."

There are some medications available for treating this disorder. Heparin is a medication to prevent blood from clotting (anticoagulant) that may help to prevent future clots from forming, but will not cause existing clots to break up. Warfarin is another anticoagulant that may be considered for long-term treatment. With it, dosage adjustments are necessary to keep blood clotting at a specific level, and carefully balancing the risk of further clots with the risk of bleeding complications is needed to prevent development of further clots.

Anti-platelet medications such as clopidogrel (brand name Plavix®) or Aspirin are also sometimes used as a preventive medication in dogs with disorders that might put them at increased risk for developing PTE.

Medications to dissolve existing clots (thrombolytic drugs) have been tried but their effective use without complications is not clear.

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

Monitoring oxygen levels in the blood through pulse oximetry (a non-invasive sensor used on the outside of the body) allows for tracking of improvement in breathing function. Blood tests to check blood clotting status will be needed for adjusting 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 development of future blood clots.

What is the outlook for my dog?

The prognosis for dogs with pulmonary thromboembolism is generally guarded to poor, and it depends upon resolution of the underlying cause. Dogs for whom the cause of their pulmonary thromboembolism is trauma or generalized bacterial infection tend to have a better prognosis.

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.

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