Pulmonary Thromboembolism in Cats

  • About: Pulmonary thromboembolism is a blockage of one of the pulmonary arteries in the lungs by a substance (emboli) that have travelled there from another part of the body. Kittens and senior cats are most at risk.
  • Causes: The most common cause of pulmonary thromboembolism is a blood clot other causes include heartworm and
    a globule of fat. Blood can clot as a result of increased clotting disorders, heart disease, tumours, heartworm, polycythemia (increased red blood cells), and damage to the blood vessel walls.
  • Symptoms: Difficulty breathing, exercise intolerance, blue-tinged gums, lethargy, loss of appetite and sudden death.
  • Diagnosis: Baseline tests which will include a complete physical examination and medical history, blood count,
    biochemical profile and urinalysis, arterial blood gasses, coagulation profiles, x-ray, echocardiogram and pulmonary angiography.
  • Treatment: Stabilise the cat with oxygen therapy, fluid therapy and cage rest. Drugs to dissolve the clot and blood thinners.

What is pulmonary thromboembolism?

Pulmonary thromboembolism (PTE) is a blockage in the pulmonary artery or one of its branches in the lungs that is caused by an embolism, a substance that has travelled through the circulatory system from another part of the body.

It can occur in cats of all ages; however, it is seen most often in kittens and senior cats.

Pulmonary embolism in cats

Blood clot formation:

A thrombus is a blood clot that forms in a blood vessel; in some cases, a portion can break off. From there it travels (embolises) through the circulatory system to another location causing a partial or full blockage of the affected vessel.

Normally, a blood clot should only occur when there is a break in a blood vessel wall which allows blood to escape. Several functions stem the loss of blood. This process is known as hemostasis.

  • The damaged blood vessel constricts, which helps to slow down the flow of blood out of the blood vessel.
  • Platelets in the blood quickly form a plug over the area to further stop the blood from escaping.
  • The damaged vessel exposes clotting factors within the blood to the subendothelial tissue; this exposure triggers the coagulation cascade resulting in the formation of fibrin, which forms a mesh over the platelet plug.

Effects on the organs:

An embolus can affect many organs including the coronary artery supplying the heart (heart attack), the cerebral artery supplying the brain (stroke), renal arteries (kidneys), portal vein (liver), iliac supplying the hind legs (saddle thrombosis), eyes or intestines, this article relates to an embolus in the lungs. When this occurs, the flow of blood is reduced or completely blocked.

  • As blood builds up behind the blockage, the affected area is now no longer able to exchange oxygen and carbon dioxide.
  • The loss of oxygen to the area can cause lung tissue to die (infarct).
  • The blockage leads to an increase in pressure (pulmonary hypertension) to the right-hand side of the heart (which pumps the oxygenated blood to the lungs), which can lead to cardiac arrest.
  • If the blockage straddles the bifurcation of the pulmonary trunk (the junction supplying the left and right pulmonary arteries) sudden death can occur without any prior symptoms.

Other embolisms can also occur in cats, such as gas, dead heartworm which has broken apart and travelled to the narrower arteries in the lungs, a gas bubble, a broken-off piece of tumour or a fat globule. This article will focus predominantly on a blood clot in the pulmonary arteries.


Most cats with pulmonary thromboembolism have an underlying medical condition that can result in a thrombus forming. Three mechanisms can lead to the inappropriate formation of a blood clot. This is known as Virchow’s triad.

  1. Hypercoagulability (thrombophilia) – An increased tendency of the blood to clot. It may be genetic or acquired. Causes include platelet hypersensitivity, disseminated intravascular coagulation, elevated levels of clotting factors (which have several causes including hyperthyroidism, neoplasm and Cushing’s syndrome), immune-mediated hemolytic anemia, heart disease, thrombocytosis (increased number of platelets) which can be due to cancer, decreased production of antithrombin (an anticoagulant) due to protein-losing nephropathy or Cushing’s syndrome.
  2. Stasis – When blood flow becomes sluggish which can lead to the formation of blood clots. Heart disease, polycythemia (a higher than normal number of red blood cells or decreased plasma levels making the blood thicker), vascular tumours, partial venous occlusion due to neoplasia, heartworms, shock and hyperviscosity syndromes.
  3. Damage to the blood vessel walls – Damage to the inner lining of the blood vessels (endothelium) can occur due to physical, chemical, biological or immune factors. Any cause of vasculitis (inflammation of the blood vessels), which may be due to heartworm disease, immune-mediated disease, and pancreatitis, other conditions include neoplasia, endotoxemia (a systemic disorder which is caused by the cat’s response to a bacteria in the blood).


Pulmonary embolisms are usually quick to develop. Symptoms can vary depending on how large the clot is as well as the location within the lungs.


Your veterinarian will perform a complete physical examination of your cat and obtain a medical history from you including underlying medical conditions your cat may have as well as symptoms you have noticed. During the exam, he may observe an increased respiratory and heart rate.

Diagnostic workup:

  • Baseline tests: Complete blood count, biochemical profile and urinalysis won’t be able to diagnose pulmonary thromboembolism but may reveal a predisposing condition.
  • Arterial blood gases: May reveal hypoxemia (low blood oxygen) and hypocapnia (low carbon dioxide).
  • Coagulation profiles: To look for clotting disorders.
  • Thoracic X-rays: Imaging studies may reveal underlying abnormalities of the heart and lungs as well as evaluate the pulmonary arteries. Pulmonary infiltrates (abnormal substances, commonly fluid, blood or clots), pulmonary arterial enlargement and pleural effusion.
  • Echocardiogram: A non-invasive test that uses high-frequency sound waves to capture live images of the heart to examine the anatomy and function of the heart and valves.
  • Pulmonary angiography: A diagnostic method that involves the injection of contrast material into the pulmonary artery followed by an X-ray to observe the pulmonary arterial tree.


The goal of treatment is to manage symptoms as well as treat the underlying cause

Emergency care:

  • Supportive care: Cage rest, oxygen therapy, and intravenous fluids.
  • Thrombolytics or tissue plasminogen activator (t-PA) drugs: Streptokinase or Alteplase administered via IV to dissolve the clot. Bleeding may occur in cats given these medications.
  • Anticoagulants (blood thinners) and antiaggregants (antiplatelet): These drugs reduce the occurrence of blood clots forming. Anticoagulants work by delaying the clotting of blood; common medications include Warfarin and Heparin. Antiaggregants stop platelets from clumping together to form a plug. Aspirin and clopidogrel are two drugs in this category.

Sadly, pulmonary thromboembolism is very commonly fatal. Your veterinarian will do the best he can; however, the prognosis is not good.


  • Julia Wilson, 'Cat World' Founder

    Julia Wilson is the founder of Cat-World, and has researched and written over 1,000 articles about cats. She is a cat expert with over 20 years of experience writing about a wide range of cat topics, with a special interest in cat health, welfare and preventative care. Julia lives in Sydney with her family, four cats and two dogs. Full author bio

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