Hemangiosarcoma of the Heart

TL;DR. Cardiac hemangiosarcoma is a rapid, highly aggressive cancer of the heart's blood vessels that can cause sudden internal bleeding and life-threatening heart failure, requiring immediate emergency veterinary care.

Cardiac hemangiosarcoma typically develops on the right atrium, leading to fluid accumulation within the pericardial sac.

What is it?

Cardiac hemangiosarcoma is a highly malignant and aggressive cancer that arises from the endothelial cells, which are the cells that line the inside of blood vessels. Because blood vessels exist throughout the entire body, this cancer can theoretically develop anywhere. However, it has a strong preference for certain organs. In dogs, the heart is one of the most common primary sites for this tumor, specifically localizing in the right atrium or the right auricular appendage (the ear-like flap on the top-right chamber of the heart).

To understand why this disease is so dangerous, it helps to understand the anatomy of the heart. The heart is enclosed in a tough, double-walled sac called the pericardium. Under normal circumstances, a tiny amount of fluid lubricates the space between the heart and this sac, allowing the heart to beat smoothly.

Because hemangiosarcoma is a tumor made of malformed, fragile blood vessels, it is highly prone to rupturing and bleeding. When a tumor on the right atrium ruptures, it bleeds directly into the pericardial sac. This accumulation of blood is called hemorrhagic pericardial effusion. Because the pericardial sac is fibrous and cannot stretch quickly, the pooling blood rapidly builds up pressure around the heart. This pressure prevents the heart chambers from filling with blood between beats—a life-threatening emergency known as cardiac tamponade. Without immediate veterinary intervention to relieve this pressure, the heart cannot pump blood to the rest of the body, leading to acute collapse and death.

While this disease is most frequently diagnosed in dogs, it can also occur in cats. However, the presentation in cats has some distinct differences. According to a leading veterinary internal medicine reference:

"The age distribution of cats with cardiac tumors is different from that of dogs; about 28% are 7 years old or younger. It is unknown whether reproductive status affects relative risk for cardiac tumors in cats."

Because cardiac hemangiosarcoma is rare in cats, much of the medical guidance and treatment protocols are extrapolated from canine medicine, and long-term prognosis data for felines remains limited.

Causes & risk factors

The exact cause of cardiac hemangiosarcoma remains unknown. Like most cancers, it is likely driven by a complex combination of genetic mutations and environmental factors. There is no evidence that lifestyle, diet, or exercise plays a role in the development of this tumor.

We do know that certain breeds of dogs are statistically much more likely to develop hemangiosarcoma than others, pointing to a strong genetic predisposition. The disease is most commonly diagnosed in middle-aged to older dogs, though it can strike at any age.

In terms of where these tumors start, a leading veterinary internal medicine reference notes:

"Approximately 50% of the tumors originate in the spleen, 25% in the right atrium, 13% in subcutaneous tissue, 5% in the liver, 5% in the liver-spleen–right atrium, and 1% to 2% simultaneously in other organs (i. e. , kidney, urinary bladder, bone, tongue, prostate)."

This means that when a vet finds a tumor on the heart, they must also evaluate other organs like the spleen and liver, as the cancer may have already spread (metastasized) or may have originated elsewhere.

Signs to watch for

The signs of cardiac hemangiosarcoma can appear suddenly and without warning. A dog or cat may seem completely healthy one day and collapse the next. The symptoms are primarily caused by sudden internal bleeding, anemia, or the heart's inability to pump blood effectively.

Common Signs

• Collapse: Sudden, unexplained falling or inability to stand, which is a major medical emergency.
• Syncope (Fainting): Temporary loss of consciousness due to a sudden drop in blood flow to the brain.
• Weakness: Severe, sudden lethargy or reluctance to move.
• Anemia: Pale gums, caused by internal bleeding or the destruction of red blood cells.
• Spontaneous Bleeding: Slow or rapid bleeding from the tumor into the chest cavity or pericardial sac.
• Cardiac Arrhythmias: Irregular heartbeats, which occur because the tumor disrupts the heart's normal electrical pathways.
• Muffled Heart Sounds: Detected by your vet during an exam; the fluid surrounding the heart acts as an acoustic barrier, making the heartbeat sound distant.
• Right-Sided Congestive Heart Failure: Backpressure from the struggling heart can cause fluid to back up into the body.
• Bloodwork Abnormalities: Your vet may detect low platelets (thrombocytopenia), fragmented red blood cells (schistocytes), or immature red blood cells (nucleated red blood cells) on a complete blood count.

Occasional Signs

• Ascites: Fluid accumulation in the abdomen, causing a pot-bellied appearance.
• Acanthocytes: Abnormally shaped red blood cells with spike-like projections, visible under a microscope.
• Heart Murmur: An abnormal whooshing sound heard through a stethoscope, caused by turbulent blood flow.

Sudden weakness, lethargy, and pale gums are common warning signs of internal bleeding from a ruptured cardiac tumor.

How vets diagnose it

Diagnosing cardiac hemangiosarcoma requires a rapid, systematic approach, especially if the pet is in a state of collapse. Your vet will perform several key tests to identify the tumor, assess the heart's function, and check for metastasis.

• Physical Examination: Your vet will listen to your pet's heart and lungs. Muffled heart sounds, weak pulses, and pale gums are strong clinical indicators of fluid around the heart.

• Echocardiography (Heart Ultrasound): This is the primary tool for identifying cardiac masses. An ultrasound allows the vet to visualize the heart in real-time, locate the fluid in the pericardial sac, and identify the tumor on the right atrium. However, ultrasound is not always definitive. As noted in a standard textbook on clinical veterinary echocardiography:

  "Echography reveals an intrapericardial cystic mass compressing the cardiac cavities. However, this echographic aspect can be similar to that of a cardiac hemangiosarcoma..."

• Pericardial Fluid Cytology: If your vet performs a "tap" (pericardiocentesis) to drain the fluid around the heart, they will analyze the fluid under a microscope. However, diagnosing hemangiosarcoma this way is notoriously difficult. A leading veterinary internal medicine reference explains:

  "An additional problem with effusions is that they frequently contain reactive mesothelial cells that may resemble neoplastic cells, leading to a false-positive diagnosis of HSA. In general, a presumptive clinical or cytologic diagnosis of HSA should be confirmed histopathologically, if feasible."

• Complete Blood Count (CBC): This blood test looks for characteristic signs of hemangiosarcoma. The tumor's chaotic blood vessels physically damage passing red blood cells, cutting them into fragments called schistocytes. The body may also release immature nucleated red blood cells to compensate for blood loss, and platelets (clotting cells) are often depleted (thrombocytopenia).

• Plasma Cardiac Troponin I (cTnI) Assay: This blood test measures a protein released when heart muscle cells are damaged. Elevated levels can support a diagnosis of cardiac injury associated with a tumor.

• Thoracic Radiographs (X-rays) & Abdominal Ultrasound: These imaging tools are used to search for signs of cancer spread in the lungs, spleen, liver, or other abdominal organs.

• Histopathology (The Gold Standard): The only way to definitively diagnose hemangiosarcoma is to submit a tissue sample of the tumor to a pathologist. Because taking a biopsy of a beating heart is highly risky, this is typically done after surgical removal of the tumor or during a post-mortem examination.

Echocardiography is the primary diagnostic tool used to visualize the heart and identify tumors or fluid accumulation.

Treatment options

Treatment for cardiac hemangiosarcoma is divided into emergency stabilization and long-term management. Because this is a highly aggressive cancer, treatment is focused on extending quality of life rather than achieving a complete cure.

Emergency Stabilization

If your pet is in cardiac tamponade, the immediate priority is to remove the fluid pressing on the heart. Your vet will perform a pericardiocentesis, inserting a needle through the chest wall into the pericardial sac to drain the blood. This instantly relieves the pressure, allowing the heart to pump normally and bringing the pet back from the brink of collapse.

Surgical Options

If the tumor is localized to the right auricular appendage, a surgeon may attempt to remove that portion of the heart (auricular resection). They may also perform a pericardiectomy—removing a portion of the pericardial sac. This does not cure the cancer, but it prevents fluid from building up around the heart again, allowing any future bleeding to drain safely into the chest cavity where it can be reabsorbed.

First-Line Chemotherapy

Because hemangiosarcoma spreads so rapidly through the bloodstream, chemotherapy is highly recommended following surgery, or as a primary treatment if surgery is not possible. The standard protocol is the VAC protocol, which combines three drugs:

1. Vincristine (a vinca alkaloid)
2. Doxorubicin (an anthracycline antineoplastic)
3. Cyclophosphamide (an alkylating agent)

Regarding the efficacy of this protocol, a leading veterinary internal medicine reference notes:

"Sixty-seven dogs with HSA in different anatomic locations were evaluated retrospectively. All dogs received the VAC protocol, as adjuvant to surgery ( n = 50), neoadjuvant ( n = 3), or as the sole treatment modality ( n = 14). There was no signi ficant difference between the MST [median survival time] of dogs with stage III ( n = 25; 195 days) and stage I/II ( n = 42; 189 days) HSA..."

Second-Line and Supportive Care

If aggressive chemotherapy is not an option due to toxicity concerns or owner preference, glucocorticoids (corticosteroids) may be used as a palliative measure. These medications can help reduce inflammation, support appetite, and maintain energy levels, though they do not actively shrink the tumor.

Prognosis

The overall prognosis for cardiac hemangiosarcoma is poor. This is one of the most challenging diagnoses in veterinary medicine.

Without treatment, or with surgery alone, survival times are extremely short—often only a few days to a few weeks, as the tumor is highly likely to bleed again. However, combining surgery with the VAC chemotherapy protocol can extend survival times to a median of approximately 189 to 195 days. Some individual pets may exceed these averages, while others may succumb sooner if the tumor is exceptionally aggressive.

For cats, long-term prognosis data is extremely limited due to the rarity of the disease. Your veterinary oncologist will help guide you through what to expect based on your cat's specific clinical presentation.

Throughout treatment, the primary goal is maintaining a high quality of life. Your vet will work closely with you to monitor your pet's comfort, appetite, and energy levels.

Prevention

There is currently no known way to prevent cardiac hemangiosarcoma. Because the exact triggers of the genetic mutations that cause this cancer are unknown, preventative lifestyle changes or dietary adjustments do not exist.

For owners of high-risk breeds, some veterinarians suggest routine screening echocardiograms starting around middle age. While this may occasionally detect a tumor before it ruptures, these tumors can grow and change very rapidly, meaning a clean scan is not a guarantee of long-term health.

When to call your vet

Cardiac hemangiosarcoma can cause sudden, life-threatening crises. You must seek immediate, emergency veterinary care if you notice any of the following red flags:

• Sudden collapse or inability to stand
• Extreme, sudden weakness or lethargy
• Pale, white, or blue-tinged gums
• Rapid, shallow, or labored breathing
• Fainting spells (syncope)
• Sudden abdominal distension (a bloated appearance)

If your pet has already been diagnosed with cardiac hemangiosarcoma and is undergoing treatment, contact your vet immediately if they show signs of decreased appetite, vomiting, or a sudden drop in energy, as these can be early warning signs of a new bleeding episode or chemotherapy side effects.

For specific breeds

If you own one of the following breeds, it is highly recommended that you familiarize yourself with the signs of internal bleeding and sudden weakness, as these breeds are statistically overrepresented in cases of hemangiosarcoma:

• German Shepherd Dog
• Golden Retriever
• Labrador Retriever
• American Cocker Spaniel
• English Setter
• Miniature Poodle
• Afghan Hound

Being vigilant and knowing the signs of acute collapse can save precious time in getting your pet the emergency stabilization they need.

Sources

• Veterinary Internal Medicine, 5th Edition, pages 203, 1220, 1221, 1222.
• Clinical Echocardiography of the Dog and Cat, page 261.

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