CONDITION

Pulmonary Thromboembolism

Pulmonary thromboembolism describes a situation in which a clot forms in a blood vessel and travels to the lungs, where it blocks one or more of the arteries that carry blood through lung tissue. When this happens, parts of the lung may not receive the blood flow they need to function, and the heart may need to work harder to push blood through the remaining open vessels. Owners often arrive at this page because their dog or cat has developed sudden difficulty breathing, seems unable to settle, or is breathing rapidly even at rest. In some cases, the animal may have a known condition elsewhere in the body—such as a heart problem, a hormonal disorder, or an immune condition—that has been mentioned as carrying this risk. The signs can appear without warning and may vary depending on how much of the lung's blood supply has been affected. This page explores what patterns tend to raise concern, what may be happening in the circulation and lung tissue, how the picture is investigated when this process is suspected, and what approaches exist once the condition is identified.

Why this matters now

Pulmonary thromboembolism can occur at any age, though it tends to appear in animals who already carry another condition that affects how blood clots or how it flows through the body. Dogs with immune-mediated diseases, certain heart conditions, or hormonal disorders such as hyperadrenocorticism are more often affected. Cats may develop clots in the context of heart muscle disease or conditions that cause protein to be lost through the kidneys or gut. The event itself often arises without obvious warning, though the underlying risk may have been building quietly for weeks or months.

The course varies considerably depending on how much of the lung's circulation has been blocked and how quickly the body can begin to dissolve or work around the obstruction. In some animals the signs remain stable after the initial event, whilst in others breathing may become progressively more laboured over hours or days. Some clots begin to break down within the first week, allowing gradual improvement, though this process is unpredictable and in other cases the obstruction persists or new clots form. Recovery, when it occurs, may take weeks, and the risk of recurrence often depends on whether the underlying condition can be managed.

Signals & patterns

Early signals

Sudden rapid breathing at rest

The animal may begin breathing noticeably faster than usual even when lying quietly, often with a shallow, effortful quality. This reflects the fact that parts of the lung are not exchanging oxygen efficiently, so the body attempts to compensate by increasing the rate of breathing.

Restlessness or inability to settle

Many animals seem unable to find a comfortable position and may move from place to place, shift posture frequently, or appear anxious. This behaviour can reflect the discomfort of working harder to breathe or the body's response to lower oxygen levels.

Pale or grey gum colour

The mucous membranes inside the mouth may lose their usual pink tone and appear paler or take on a greyish tint. This change can indicate that less oxygen is reaching the tissues, though gum colour alone does not confirm the cause.

Coughing

Some animals develop a cough, which may be dry or occasionally produce small amounts of blood-tinged fluid. This can occur when inflammation develops in the affected parts of the lung or when fluid leaks into the airways.

Later signals

Bluish tinge to gums or tongue

In more advanced cases, the mucous membranes may take on a blue or purple hue, a sign that the blood is carrying significantly less oxygen than normal. This pattern tends to emerge when a large portion of the lung's circulation is obstructed.

Collapse or profound weakness

The animal may suddenly lie down and be unable to rise, or may show extreme lethargy and disinterest in surroundings. This can reflect severe oxygen deprivation or the strain placed on the heart as it attempts to push blood through narrowed or blocked vessels.

Open-mouth breathing in cats

Cats in particular may begin breathing with their mouth open, a behaviour that is uncommon in this species and typically signals significant respiratory distress. It suggests the animal is working very hard to move air and is no longer able to breathe comfortably through the nose alone.

Click to read about the biological mechanisms

How this is usually investigated

Investigation typically begins with a careful account of what has been observed at home and what other conditions are already known, followed by observation of breathing pattern, gum colour, and how comfortably the animal can settle. From there, the approach often moves toward imaging of the chest and tests that measure oxygen levels, clotting function, and markers of inflammation. Because no single test can confirm or rule out the condition with certainty, the picture is usually built from several pieces of information considered together.

Physical examination

Purpose: Listening to the chest may reveal crackles, harsh lung sounds, or an increased heart rate, and the colour of the gums can suggest whether oxygen is reaching the tissues adequately. The pattern of breathing—whether rapid, shallow, or accompanied by obvious effort—adds to the picture.
Considerations: Many of these signs can also appear with other lung or heart conditions, so the examination alone does not distinguish between causes. It provides a starting point and helps judge how much support the animal may need during further tests.

Thoracic radiography

Purpose: Radiographs of the chest can show areas where lung tissue appears denser than usual, fluid around the lungs, or enlargement of the blood vessels near the heart. Certain patterns, such as wedge-shaped areas of increased density, raise concern for clot-related changes.
Considerations: The changes may be subtle in the first hours after a clot lodges, and some animals with pulmonary thromboembolism have radiographs that appear normal. The images also cannot always distinguish between a clot and other causes of lung inflammation or fluid accumulation.

Blood gas analysis

Purpose: Measuring oxygen and carbon dioxide levels in the blood provides direct information about how efficiently the lungs are exchanging gases. A low oxygen level, particularly if it does not improve much when the animal breathes supplemental oxygen, can suggest that blood is bypassing areas of the lung.
Considerations: This test requires a small arterial blood sample, which can be more uncomfortable to collect than a routine blood draw, and the result reflects overall lung function rather than pinpointing the cause. Many other lung conditions can also reduce oxygen levels.

Coagulation testing

Purpose: Tests that assess how quickly blood clots and whether clotting proteins are being consumed faster than usual can help identify animals who are forming clots elsewhere in the body. Markers such as D-dimer, which rises when clots are breaking down, may support the possibility of active clot formation.
Considerations: Elevated D-dimer can occur with many inflammatory conditions and does not confirm that a clot has reached the lungs. Some animals with pulmonary thromboembolism have normal coagulation results, particularly if only a single clot has formed.

Computed tomography angiography

Purpose: CT imaging with contrast injected into a vein can show the pulmonary arteries in detail and may reveal filling defects where a clot is blocking flow. This is often considered the most direct way to visualise a clot in the lung circulation.
Considerations: Access to CT is limited to referral centres, and the procedure requires general anaesthesia or heavy sedation, which carries added risk in animals already struggling to breathe. Even with CT, very small clots or those in the smallest branches of the arteries may not be visible.

Options & trade-offs

Management tends to involve a combination of support for breathing, measures to reduce further clot formation, and treatment of any underlying condition that may have triggered the event. The balance between these elements varies depending on how severely the animal is affected, what other conditions are present, and what can be managed practically at home or in hospital. Different families find different approaches workable, and the picture often shifts as the animal's response becomes clearer over the first days and weeks.

Oxygen supplementation and rest

Providing supplemental oxygen through a mask, nasal prongs, or an oxygen cage can ease the work of breathing whilst the body begins to compensate for reduced lung function. Strict rest limits the body's demand for oxygen and reduces strain on the heart. In some cases this support is needed only for the first few days, whilst in others it may continue for a week or more.

Trade-offs: This approach does not address the clot itself but can make the animal more comfortable and stable enough for other treatments to take effect. Prolonged hospitalisation may be stressful for some animals, and not all families can manage oxygen therapy at home if early discharge is considered.

Anticoagulant medication

Drugs that interfere with the clotting process, such as heparin or oral anticoagulants, are often introduced to reduce the likelihood that new clots will form whilst the existing clot begins to break down naturally. Heparin is usually given by injection under the skin or into a vein, whilst oral medications can be continued long-term at home. The dose is sometimes adjusted based on blood tests that measure clotting time.

Trade-offs: Anticoagulants carry a risk of bleeding, particularly in animals who have other conditions affecting the gut, kidneys, or platelet function, and careful monitoring is needed during the first weeks. They do not dissolve clots that are already in place, and the response can be variable depending on the underlying cause and how much clot burden is present.

Treatment of the underlying condition

When pulmonary thromboembolism arises in the context of another illness—such as immune-mediated disease, hyperadrenocorticism, or protein-losing conditions—managing that primary condition may reduce ongoing clot formation and improve the overall outlook. This might involve immune-suppressing drugs, hormone regulation, or dietary changes, depending on what has been identified. The timeline for response often extends over weeks to months.

Trade-offs: Some underlying conditions cannot be fully controlled, and in others the treatment itself may carry risks or require long-term medication. The effect on clot risk may not be immediate, and managing two or more conditions simultaneously can become complex for both the animal and the family.

Thrombolytic therapy

In a small number of cases, drugs that actively dissolve clots—such as tissue plasminogen activator—may be considered, particularly when a large clot has caused severe breathing difficulty and other measures have not stabilised the animal. These medications are given by infusion and work over several hours. The decision to use them is usually made in a referral setting.

Trade-offs: Thrombolytics carry a substantial risk of serious bleeding, including into the brain, lungs, or abdomen, and their use is typically reserved for situations judged to be life-threatening. The evidence for benefit in dogs and cats is limited, and the cost and intensity of monitoring required make this approach impractical for many families.

Supportive care and monitoring at home

Once the animal is stable enough to leave hospital, care at home often involves continued rest, regular administration of medication, and observation for changes in breathing rate, gum colour, or energy level. Some families find it helpful to keep a simple log of breathing rate at rest, which can reveal trends over days. Follow-up visits may include repeat imaging or blood tests to assess how the lungs and clotting system are responding.

Trade-offs: Home care requires a degree of vigilance and confidence in recognising subtle changes, which can feel daunting for some owners. The animal's condition may fluctuate, and it can be difficult to distinguish between expected variation and signs that the situation is worsening.

Common misconceptions

Misconception:

"If the radiographs look normal, the animal cannot have a clot in the lungs."

Reality:

Radiographic changes may be subtle or absent in the early stages, and some animals with confirmed pulmonary thromboembolism have chest radiographs that appear unremarkable. The diagnosis often rests on a combination of clinical signs, blood test results, and in some cases more advanced imaging, rather than radiographs alone.

Misconception:

"Anticoagulant medication will dissolve the clot that is already present."

Reality:

Anticoagulants work by slowing or preventing new clot formation; they do not break down clots that have already formed. The body's own clot-dissolving systems may gradually reduce the size of an existing clot over time, but this process is slow and unpredictable, and anticoagulants support it indirectly by reducing the risk of further clots adding to the burden.

Misconception:

"Once the animal is breathing more comfortably, the risk has passed and medication can be stopped."

Reality:

Improvement in breathing often reflects the body compensating for the obstruction or the clot beginning to shrink, but the underlying tendency to form clots may persist for weeks or months, particularly if the primary condition has not been fully controlled. Stopping anticoagulant therapy too early can allow new clots to form, and the duration of treatment is usually guided by regular reassessment rather than symptom resolution alone.

Related conditions

Pulmonary Hypertension

Pulmonary hypertension can develop as a consequence of pulmonary thromboembolism when clots obstruct blood flow through the lungs, forcing the right side of the heart to work harder against increased resistance. In some cases, pre-existing pulmonary hypertension may itself predispose to clot formation in the pulmonary vessels.

Aortic Thromboembolism in Cats

Aortic thromboembolism in cats shares the fundamental mechanism of clot formation and vascular obstruction, though the site differs—the aorta rather than the pulmonary arteries. Both conditions often arise in the context of underlying heart disease or states that favour abnormal clotting.

Congestive Heart Failure in Dogs

Congestive heart failure in dogs can create conditions that favour clot formation, including sluggish blood flow and changes in vessel lining, which may raise the risk of pulmonary thromboembolism. The two conditions can also coexist, complicating the clinical picture when breathing difficulty appears.

Protein-Losing Enteropathy

Protein-losing enteropathy can lead to loss of proteins involved in regulating clotting and fluid balance, which may predispose to abnormal clot formation in the circulation, including the pulmonary vessels.

Nephrotic Syndrome

Nephrotic syndrome is associated with loss of proteins that help control clotting, and can result in a hypercoagulable state—a tendency for blood to clot more readily—which in turn may increase the risk of thromboembolism in various parts of the circulation, including the lungs.

Animals who have experienced pulmonary thromboembolism often carry other conditions that affect metabolism, immunity, or the heart, and exploring those connections may help clarify the broader picture and what might influence risk over time. The metabolic health pillar includes pages on some of these underlying processes, and understanding how they interact with clotting and circulation can be useful context for conversations that unfold over follow-up visits. In some cases, the event prompts a closer look at aspects of the animal's health that had not previously seemed directly related.