Pericardial effusion:
what it means, when to worry and how it is treated

Article written and reviewed by Dr Luca Moretti

A pericardial effusion is an accumulation of fluid in the space surrounding the heart. It may be minimal and found incidentally during an echocardiogram, or it may become large and interfere with normal cardiac filling. The amount of fluid is important, but it is not the only factor to consider: the speed at which it accumulates, the cause and its hemodynamic effects are especially important.
A small stable effusion is therefore not equivalent to an emergency. By contrast, rapid accumulation can cause tamponade even without reaching enormous volumes, because the pericardium does not have time to adapt.

To understand the significance of an effusion, it is necessary to consider:

• how much fluid is present on echocardiography;
• whether the effusion is new, stable or increasing;
• whether symptoms are present;
• whether there are signs of compression of the heart chambers;
• the probable or documented cause;
• the presence of pericardial inflammation;
• any associated oncologic, infectious, autoimmune, renal or endocrine diseases.

The monograph on pericardial effusion examines classification, etiology and pathophysiology in depth. This guide is intended mainly to interpret the practical significance of the finding and explain when simple monitoring is sufficient and when urgent assessment is required.

What can cause fluid around the heart?

An effusion can occur in many conditions. A common cause is inflammation of the pericardium: in pericarditis increased vascular permeability can promote fluid accumulation. Other causes include malignancy, bacterial or tuberculous infection, advanced kidney failure, autoimmune diseases, hypothyroidism, trauma and complications of cardiac procedures or surgery.
In some patients, despite appropriate assessment, no precise cause is identified and the effusion is defined as idiopathic. Its significance changes substantially, however, if the fluid develops in a patient with cancer, high fever or suspected infection.

A hypothyroidism-related effusion, for example, often tends to accumulate slowly and may become large before causing major symptoms. Hemopericardium after trauma or vascular rupture may instead accumulate rapidly and compromise cardiac filling much sooner.
For this reason, the echocardiographic report should not be interpreted in isolation: the same measurement can have a completely different clinical significance in two different patients.

What mild, moderate or large effusion means

On echocardiography, effusion is often described semiquantitatively. Traditionally, an echo-free separation of less than about 10 mm is considered small, 10 to 20 mm moderate and more than 20 mm large. These categories are useful for communicating the extent of the finding, but they do not correspond directly to a precise fluid volume and do not by themselves establish the risk.
The echocardiographer also assesses whether the fluid is distributed uniformly or localized, whether it is increasing over time and whether it causes right atrial or right ventricular collapse, abnormal respiratory variation in flows or other signs compatible with increased intrapericardial pressure.

The speed of accumulation is crucial. If fluid increases slowly, the pericardium can stretch and tolerate relatively large amounts. If accumulation is rapid, even a smaller amount can sharply increase pressure and reduce cardiac filling.
For this reason, a "large effusion" is not automatically synonymous with tamponade, and an effusion that is not very large does not exclude a hemodynamic problem. The diagnosis of tamponade is clinical and echocardiographic, not simply a threshold in millimeters.

What symptoms it can cause

Many small effusions cause no symptoms and are discovered incidentally. When fluid increases, shortness of breath, reduced exercise tolerance, a feeling of chest pressure or discomfort, and tachycardia may occur. Symptoms result from pressure on the heart and, in very large effusions, also on the lungs and surrounding structures.
Symptoms may be more evident when lying down and improve when sitting up, but this feature is not specific. A patient with pericarditis may also have chest pain from inflammation regardless of the amount of fluid present.

The most concerning signs are significant or rapidly progressive dyspnea, low blood pressure, a feeling of faintness or syncope, marked weakness, confusion and persistent tachycardia. In this context, cardiac tamponade must be excluded.
Tamponade is an emergency because pressure in the pericardial sac impairs filling of the heart chambers and reduces cardiac output. If the patient is unstable, treatment cannot be delayed while waiting for the effusion to become "larger".

How it is investigated and when drainage is needed

The echocardiogram is the first-line test because it shows the fluid and allows real-time assessment of its hemodynamic effects. CT and cardiac magnetic resonance imaging are used when additional anatomical or tissue information is needed, for example in loculated effusions, pericardial masses or when complex pericarditis is suspected.
Blood tests are chosen according to the context: inflammatory markers, kidney function, thyroid tests and other investigations may help identify the cause. There is no identical panel for every patient.

According to the 2025 ESC guidelines, pericardiocentesis is recommended in cardiac tamponade, when bacterial or neoplastic pericarditis is suspected, and in symptomatic moderate or large effusions that persist despite medical therapy. Surgical drainage is preferred when pericardiocentesis is technically impossible or in purulent forms, where complete drainage is important.
A small asymptomatic effusion is therefore not drained simply to determine "what kind of fluid it is". In low-risk cases, observation and treatment of the cause are often more appropriate than an invasive procedure.

How a pericardial effusion is treated

Treatment depends first of all on the cause. If the effusion accompanies inflammatory pericarditis, anti-inflammatory therapy may lead to reduction of the fluid. If it is due to hypothyroidism, kidney failure, infection, malignancy or another disease, treatment must also target the underlying condition.
Not all effusions respond to NSAIDs or colchicine. These medications make sense when there is an inflammatory pericardial process, not as universal treatment for any fluid collection.

Follow-up is decided according to size, stability and cause. A newly developed or increasing effusion may require close follow-up; a minimal stable rim in an asymptomatic patient may be managed much more simply. The finding described as "minimal" or "mild" on echocardiography is discussed in more detail in the guide small pericardial effusion.
The priority is therefore to interpret the effusion in context and not automatically turn an echocardiographic measurement into a diagnosis of severity.

Frequently asked questions about pericardial effusion

What does it mean to have a pericardial effusion?
It means that a greater-than-normal amount of fluid has accumulated in the pericardial space.
Is it always dangerous?
No. Many small effusions can be monitored. Risk depends on the amount, speed of accumulation, cause and consequences for cardiac filling.
What symptoms can it cause?
It may be asymptomatic or cause shortness of breath, chest discomfort, tachycardia and reduced exercise tolerance.

How is it detected?
Echocardiography is the main test because it shows both the fluid and any effects on the heart chambers.
When does it need to be drained?
In tamponade, when bacterial or neoplastic causes are suspected, and in symptomatic moderate or large effusions despite medical therapy.