Fluid around the heart in dogs, known as pericardial effusion, is most often caused by cancer, but it can also result from infections, heart failure, or unknown causes. The heart sits inside a thin, flexible sac called the pericardium, and when fluid builds up in the space between this sac and the heart itself, it puts dangerous pressure on the heart and can become life-threatening quickly.
How Fluid Buildup Affects the Heart
The pericardial sac normally holds a small amount of lubricating fluid. When that fluid increases, it changes the way the heart fills with blood. The right side of the heart, which operates at lower pressures, gets squeezed first. The right atrium and ventricle can’t fully expand to receive returning blood, so less blood flows through the heart with each beat.
When enough fluid accumulates to compress the heart significantly, the condition is called cardiac tamponade. During tamponade, the right chambers of the heart actually collapse under the external pressure, causing a dramatic drop in the volume of blood pumped out. The body tries to compensate by speeding up the heart rate and tightening blood vessels, but if the fluid keeps building, even the left side of the heart gets compromised and blood pressure falls. Acute tamponade causes a sudden drop in blood output. Chronic, slower fluid buildup instead raises pressure inside the heart chambers over time, eventually producing signs of heart failure like fluid retention in the abdomen.
Cancer: The Most Common Cause
Tumors are the leading cause of pericardial effusion in dogs. Two types account for the majority of cancer-related cases.
Hemangiosarcoma is an aggressive cancer of blood vessel cells that most commonly grows on the right atrium and right auricle of the heart. Because it’s a blood vessel tumor, it tends to bleed, filling the pericardial sac with blood. German Shepherds and Golden Retrievers are particularly predisposed. Hemangiosarcoma carries a very poor prognosis. In one study published in JAVMA analyzing 46 dogs with pericardial effusion, the median survival time for dogs with hemangiosarcoma was just 16 days.
Heart base tumors (chemodectomas or aortic body tumors) are the other major type. These grow as single masses or multiple nodules near the base of the heart, inside the pericardial sac. They compress the atria or the large veins returning blood to the heart, triggering fluid buildup and right-sided heart failure. Brachycephalic (flat-faced) breeds like Boxers and Boston Terriers are predisposed, possibly because their airway anatomy causes chronic low oxygen levels that stimulate these tumors. They occur more often in older male dogs. Heart base tumors tend to grow slowly, which means survival times are significantly longer than with hemangiosarcoma. Dogs treated with surgical removal of the pericardium survived a median of about 661 days, compared to roughly 129 days with medical management alone.
Mesothelioma, a cancer of the pericardial lining itself, is less common but also causes effusion. Dogs with mesothelioma had a median survival of 13.6 months in the same JAVMA study, far longer than those with hemangiosarcoma.
Idiopathic Pericardial Effusion
In many dogs, no underlying cause is ever found. This is called idiopathic pericardial effusion, and it’s diagnosed by ruling out tumors, infections, and other identifiable causes. It tends to carry a much better outlook. The median survival time for dogs with idiopathic effusion was 15.3 months in the JAVMA analysis. Some of these dogs experience a single episode that resolves after the fluid is drained, while others have recurrent episodes that eventually require surgery to remove part or all of the pericardial sac.
Other Causes
Bacterial and fungal infections can inflame the pericardium and cause fluid accumulation, though infectious pericarditis is far less common than cancer or idiopathic cases. Heart failure itself, from any cause, can lead to fluid accumulating in the pericardial space as part of widespread fluid retention. Trauma, blood clotting disorders, and toxins that cause bleeding can also fill the sac with blood. In rare cases, a left atrial tear from severe heart disease allows blood to leak directly into the pericardial space.
Signs You Might Notice
The symptoms depend on how quickly the fluid builds up. A sudden accumulation, even a small amount, can cause collapse because the pericardium doesn’t have time to stretch. A slow buildup allows the sac to gradually expand, so dogs may tolerate larger volumes before showing signs. Common things owners notice include lethargy, reduced appetite, a swollen or distended abdomen (from fluid backing up into the liver and belly), rapid or labored breathing, weakness, and collapse. Some dogs faint during exercise or excitement. Because the abdomen often swells before the dog seems obviously “sick,” owners sometimes mistake pericardial effusion for weight gain or bloating.
How It’s Diagnosed
An ultrasound of the heart (echocardiogram) is the primary diagnostic tool. It clearly shows fluid surrounding the heart and can reveal whether the heart chambers are being compressed. In cardiac tamponade, the right atrium visibly collapses inward on the ultrasound image. Echocardiography can also detect masses on or near the heart, though small tumors are sometimes missed.
When fluid is drained from the pericardial sac, it’s typically sent for analysis. However, distinguishing cancerous cells from reactive (inflamed but non-cancerous) cells in pericardial fluid is notoriously difficult. Even specialist pathologists struggle to tell the difference. This means a fluid sample that looks “clean” doesn’t guarantee there’s no tumor present, and finding abnormal cells doesn’t always confirm cancer. In many cases, a tissue biopsy obtained during surgery is needed for a definitive answer.
Treatment and What to Expect
The immediate priority is draining the fluid. A procedure called pericardiocentesis involves inserting a needle or catheter through the chest wall and into the pericardial sac to withdraw the fluid. This often produces rapid, dramatic improvement because it relieves the pressure on the heart. Many dogs go from near-collapse to walking around within hours.
What happens next depends on the underlying cause. For a first episode of idiopathic effusion, your veterinarian may take a wait-and-see approach after draining the fluid, since some dogs never have a recurrence. If the fluid keeps coming back, surgical removal of the pericardium is the definitive treatment for idiopathic cases. Without the sac, fluid can no longer become trapped around the heart. Surgery is also used to obtain biopsies and remove accessible masses.
For heart base tumors, removing the pericardium serves as palliative treatment that significantly extends survival, from a median of about 4 months to nearly 22 months. For hemangiosarcoma, the prognosis remains poor regardless of treatment because the cancer has typically already spread by the time pericardial effusion develops. Treatment decisions at that point focus on quality of life and comfort.