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Pericardial Effusion in Dogs and Cats
Published: April 30, 2022
Illustration by Tamara Rees

Pericardial effusion is a somewhat uncommon condition, but when it is diagnosed it can be serious for your pet. As more veterinary practices adopt new technologies like ultrasound, diagnosis of pericardial effusion has become more common. If your pet has been diagnosed with this condition, this article will include some information to supplement what your veterinarian has told you.

The Basics

The pericardial sac is formed by a thin membrane (the pericardium), and normally holds a very small amount of fluid. Pericardial effusion refers to an abnormal buildup of fluid in this sac. When extra fluid builds up, it’s like a straightjacket around the heart. Imagine trying to go about your daily life wearing a shirt that’s four sizes too small. The heart is unable to fill with blood and the extra pressure prevents it from pumping life-giving blood and oxygen around the body. When blood and oxygen can’t be delivered, the body goes into shock. Death is possible if the effusion can’t be removed before too much time passes. “Too much time” depends on many factors: how severe the lack of oxygen is, how long it goes on, and other issues.

In more chronic cases, the pressure buildup is gradual, and this causes the heart to fail more slowly. For these patients, fluid can back up behind the heart into the abdomen, leading to a pot-bellied appearance (a condition known as ascites) due to right-sided heart failure.

While there may normally be a teaspoon or two of thin, clear fluid in the pericardial sac, in several abnormal conditions it can build up to life-threatening amounts. Sometimes this can happen rapidly, and sometimes it can build up over a longer period. In either case, you may only notice symptoms when they become severe, which may make it seem like a sudden occurrence. Dogs and cats are experts in hiding illness, so something that may seem sudden to you may have been brewing unnoticed for a long time.

What Causes the Fluid to Build Up?

There are usually three types of fluid that can build up: blood, pus (a mix of dead white blood cells and bacteria), and a clear fluid that is similar to the normal fluid found in the pericardium, except in larger quantities.

We’ll start with blood as that is the most common fluid found in dogs (cats with pericardial effusion are rare). In the vast majority of cases, the blood leaks from a small tumor forming on the heart. Most of these are a type of aggressive malignancy (cancer) called a hemangiosarcoma. Due to the beating of the heart and the fact that hemangiosarcomas are made of blood-forming tissue, they can bleed excessively, even if quite small. This blood then builds up inside the sac around the heart and leads to shock and the heart not being able to pump blood. Other types of tumors can cause bleeding in the pericardium, but hemangiosarcoma is, unfortunately, the most common.

Certain types of rat poison can affect the body’s ability to clot blood and pericardial effusion can result. This can usually be detected by a blood clotting test, which many veterinary hospitals can do in-house. Any rat poison on the property should be reported to your veterinarian to help with the diagnosis, even if you think your pet can’t get to it.

Blood can also leak from a tear in one of the chambers of the heart due to long-standing heart disease. The chamber walls become thinned and can easily be ruptured by the normal beating of the heart. This is relatively uncommon.

Pus can collect around the heart in rare cases, usually due to a migrating foreign body like a porcupine quill, grass stem, or splinter. This is also an uncommon situation.

Fluid that is clear, not bloody or only a little bloody may be caused by inflammation, an autoimmune disease (like lupus), or, in the case of cats, Feline Infectious Peritonitis (FIP). A condition known as idiopathic pericardial effusion in which fluid builds up without a definable cause may be present. In these cases, the fluid is clear and does not return in about 50% of cases after removal.

If it is possible to sample the fluid (which can also temporarily relieve shock and buy some time) your veterinarian can help guide you through what may be causing the buildup. Samples may be sent to a pathologist for analysis and culture, and additional tests, such as ultrasound or CT scan, are often used to identify the source of the fluid. In some cases, the cause cannot be identified and repeated tests may be needed.

What are the Clinical Signs?

How fast your pet has symptoms depend on many factors. Symptoms often seem to come on suddenly, even if the fluid has built up over time. In some cases, the patient becomes very weak very suddenly or even collapses. Many will have pale gums or have a hard time breathing. This is a medical emergency and should be evaluated by a veterinarian as soon as possible, day or night. Pericardial effusion should be a consideration for any older pet with sudden weakness/collapse and an inability to get up. German Shepherd Dogs, Golden Retrievers, and Labrador Retrievers seem to have pericardial effusion more often than other breeds, although any dog can be subject to it.

In some cases, the onset can be less obvious and may just seem like a gradual “slowing down.” Many owners will also mistake a slower onset of pericardial effusion as age-related, or even weight gain as the abdomen slowly fills with fluid. Any older pet who can’t keep up on the usual walks or seems to be eating less or lying around more should prompt a veterinary evaluation.

How is Pericardial Effusion Treated?

If your pet has collapsed, the first steps in the hospital or ER will be to supply oxygen and promote circulation. Usually, IV fluids are administered rapidly, and blood samples are drawn. Do not be shocked if your pet is taken “to the back” for treatment. Time is vital if the patient is in shock.

In some cases, a quick, bedside ultrasound (echocardiography) can confirm the presence of pericardial fluid. Even if ultrasound is not available, valuable clues can be found in the history, on physical examination, on radiographs (X-rays), or other tests that can clue your veterinarian in that there may be fluid around the heart.

Rapid decompression of the pressure around the heart (often with ultrasound guidance) can be quickly performed with a long needle, which improves circulation and oxygen delivery, often quite fast. For some of these patients, improvement is often dramatic and can seem miraculous, even if the improvement doesn't last very long. Drawing off the fluid (called pericardiocentesis) can also provide samples for lab analysis and culture. Many patients do not need any sedation for pericardiocentesis, or only a local anesthetic at the site of needle entry.

In most cases, once the fluid is removed and the pressure on the heart relieved, they feel much better. IV fluids and other medications may need to be given, and oxygen will be given to patients with trouble breathing. Patients are usually watched for several hours to ensure that the fluid does not return and there are no complications. EKG monitoring is often needed, as cardiac rhythm disturbances can sometimes happen. For more stable patients where cancer is highly suspected (and there is no pain or trouble breathing), discharge for hospice care at home can be considered. This is usually a painless disease in most cases. Death, if it occurs, can be delayed for hours or even days.

Hospitalization for accompanying disorders may be needed. For some patients, a procedure known as a pericardiectomy, where the sac is surgically removed, can be considered, but this is uncommon and must be performed by an experienced surgeon.

What is the Prognosis?

Unfortunately, the prognosis is not good for many dogs with pericardial effusion. There are exceptions, however. For those in which a bloody fluid is found after pericardiocentesis, hemangiosarcoma is often the working diagnosis. Hemangiosarcoma is a very aggressive type of cancer, and survival is often measured in weeks to months after diagnosis. (Other types of tumors can have different survival times, so consultation with a veterinary oncologist should be a strong consideration).

An ultrasound of the heart is often part of the workup, even if it was initially used to diagnose and relieve the pericardial effusion. Echocardiography with a cardiologist, internist, or radiologist should be considered, if possible, to ensure the highest chance of finding the cause. Even if a mass is not found on ultrasound initially, it can sometimes be found after a few weeks have passed.

Sending the fluid to a lab is also often done and is a necessity if pus or clear/blood-tinged fluid is recovered on pericardiocentesis. Fluid that appears to be straight blood may not yield results, but it is sent to the lab in many cases.

For those uncommon cases where clear or blood-tinged fluid is recovered, the prognosis improves a bit (except in cats, where this might indicate Feline Infectious Peritonitis, which is difficult to treat.)

Clear fluid may indicate pericardial effusion for which the cause is unknown and may clear on its own, or with medication. In these cases, merely removing the fluid can be curative in about half of the cases.

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