Pericardial Effusion

Pericardial effusion is an abnormal accumulation of fluid within the pericardial space, most often from inflammation of the pericardium but arising from a wide range of causes. Echocardiography has been the first-line imaging test for pericardial effusion since the earliest days of clinical ultrasound, and it remains the fastest, most accurate way to detect it, describe it, and — in a separate assessment — judge whether it is hemodynamically significant.

Normal Anatomy and Why It Matters for Imaging

The pericardium is a two-layered fibroelastic sac: an outer fibrous pericardium, fused to an outer (parietal) serous layer, and an inner (visceral) serous layer that is synonymous with the epicardium. Between these layers, a physiologic amount of fluid — under roughly 50 mL by most estimates, though some sources cite as little as 5 to 10 mL — is normally present as a plasma ultrafiltrate, believed to originate from the visceral pericardium itself. This small physiologic volume is what’s seen as a thin echo-free space visible only in systole, typically first appreciated posterior to the left ventricle.

Two anatomic points explain findings discussed throughout this page:

• The pericardial reflection around the pulmonary veins limits how far fluid can track behind the left atrium. This is precisely why a fluid collection seen behind the left atrium is more likely pleural than pericardial — the pericardium simply doesn’t extend far enough posteriorly in that location to contain much fluid there.
• The oblique and transverse sinuses, and their recesses, are true extensions of the pericardial space where effusion can localize even when the main pericardial cavity looks clear. The oblique sinus lies posterior to the left atrium between the four pulmonary veins; the transverse sinus lies between the origins of the great arteries and the atria. Fluid confined to the transverse sinus and its recesses (the superior and inferior aortic recesses, the pulmonic recesses) can be mistaken for other pathology, such as a type A aortic dissection, if the anatomy isn’t recognized.

Why Rate Matters More Than Volume

The physiologic effect of pericardial fluid depends on the pericardial pressure-volume relationship, which is markedly nonlinear. Early on, the curve is flat — the pericardium stretches to accommodate added volume with little pressure change. Past a certain point, the curve turns steeply upward, and small additional volumes produce large pressure increases. A slowly accumulating effusion can exceed 1000 mL with only a mild rise in intrapericardial pressure, because the pericardium has time to remodel and stretch; a rapidly accumulating effusion of as little as 50 to 150 mL can raise intrapericardial pressure sharply and produce tamponade physiology, because the pericardium is exposed to the acute volume before it can stretch. This is why abrupt bleeding from trauma, aortic dissection, free wall rupture after myocardial infarction, or a complication of an invasive cardiac procedure can cause hemodynamic collapse from what would otherwise be a modest volume of fluid — and why effusion size on its own is a poor predictor of hemodynamic significance.

Etiology

A wide range of processes can produce a pericardial effusion, often grouped as:

• Idiopathic — the most common cause of a minimal-to-small effusion in developed countries, frequently presumed viral.
• Infectious — viral, bacterial (including tuberculosis), and fungal.
• Autoimmune and rheumatologic — systemic lupus erythematosus, rheumatoid arthritis, and other connective tissue diseases.
• Neoplastic — metastatic disease (most commonly lung, breast, and lymphoma) or, less often, a primary pericardial tumor.
• Radiation-induced — following mediastinal radiotherapy.
• Uremic — from advanced renal disease.
• Post-myocardial infarction — including Dressler syndrome (a delayed, immune-mediated pericarditis).
• Traumatic — penetrating or blunt chest trauma.
• Iatrogenic and post-procedural — after cardiac surgery, electrophysiology procedures, or pacemaker implantation; common in the immediate postoperative period.
• Drug-induced — certain medications, including hydralazine and isoniazid.
• Aortic dissection — producing hemopericardium when a proximal dissection ruptures into the pericardial space.
• Endocrine or metabolic — notably hypothyroidism (myxedema).
• Volume overload states — congestive heart failure, cirrhosis.

A rough pattern by size is worth knowing, though it’s a heuristic, not a rule: minimal or small effusions in the setting of acute pericarditis are most often idiopathic or viral; moderate-sized effusions have a broader range of medical causes; and large effusions more often raise suspicion for malignancy, tuberculosis, or hypothyroidism, particularly when the effusion is initially asymptomatic.

Clinical Findings

• Chest pain — classically sharp or pleuritic, relieved by sitting up and leaning forward, when there is concurrent pericarditis. An effusion by itself doesn’t reliably cause pain.
• Dyspnea — particularly with large effusions, from mechanical compression of adjacent structures rather than the effusion pressing on the heart alone.
• Mechanical compression symptoms — esophageal compression can cause dysphagia, bronchial or tracheal compression can cause cough, phrenic nerve compression can cause hiccups, recurrent laryngeal nerve compression can cause hoarseness, and lung compression or atelectasis can cause dyspnea.
• Pericardial friction rub — may or may not be present; best heard at the left sternal border, and reflects concurrent pericarditis rather than the fluid itself. Purulent pericarditis with effusion can also accompany infective endocarditis, particularly when a periannular abscess extends into the pericardial space.
• Often entirely asymptomatic — many effusions, including large ones that accumulate slowly, are discovered incidentally on imaging obtained for another reason.

ECG

• Reduced QRS voltage — from electrical damping by the surrounding fluid.
• Diffuse ST-segment elevation (with reciprocal changes, classically PR depression) — reflects concurrent pericarditis rather than the effusion itself.
• Electrical alternans — beat-to-beat variation in QRS amplitude, associated with large effusions and the swinging motion of the heart within a large fluid collection.

Chest X-Ray

• Cardiomegaly — a globular or “flask-shaped”/“water-bottle” cardiac silhouette with large effusions, from the shape the fluid-distended pericardial sac takes on a frontal projection.

Echocardiographic Findings

How Fluid Actually Accumulates

Fluid, when trivial or small, characteristically appears first posterior to the left ventricle in the oblique sinus, seen on parasternal long-axis as an echo-free space present only in systole. As the volume increases past roughly 100 mL, the fluid becomes circumferential, with an echolucent space appearing both anteriorly and posteriorly, and persisting throughout the cardiac cycle rather than systole alone. Knowing this sequence matters for grading: a small collection seen only posteriorly and only in systole is expected early behavior, not evidence that something has been missed.

A View-by-View Approach

See TTE Views and Tips for the standard windows this builds on.

• Parasternal long-axis. The primary view for detecting the earliest, smallest amount of fluid, seen posterior to the left ventricle. This is also the key view for the pericardial-versus-pleural distinction below, since it shows the relationship between the fluid and the descending thoracic aorta directly.
• Parasternal short-axis and apical views (four-chamber, two-chamber, long-axis). Used together to define the lateral, medial, and apical extent of the effusion, and to identify localized or loculated collections that a single view can miss. In the apical four-chamber view specifically, an isolated echo-free space seen superior to the right atrium is more likely to represent pleural fluid than a pericardial effusion.
• Subcostal (subxiphoid) view. Shows fluid between the diaphragm and the right ventricle, and is particularly valuable for planning echo-guided pericardiocentesis — this is its genuine, sourced strength, rather than a general claim that it’s the single most sensitive view for small effusions. It’s also a reliable fallback window when parasternal and apical access is limited, such as in mechanically ventilated or recently postoperative patients.
• Multiple acoustic windows, used together, are what actually excludes a loculated effusion reliably — no single view is sufficient on its own for this purpose.
• TEE. A helpful adjunct for posterior loculated effusions, particularly after cardiac surgery, where transthoracic windows may be limited and posterior collections are common.
• 3D echocardiography. Not needed routinely, but genuinely useful for loculated effusions or a suspected hematoma, and capable of a more quantitative estimate of pericardial fluid volume than 2D linear measurement alone.

M-Mode

The classic M-mode sign is a persistent echo-free separation between a relatively flat, immobile posterior pericardial echo and the moving epicardial echo, present through both systole and diastole — this pattern, rather than a vague sense of “reduced pericardial motion,” is what’s diagnostic. A separation seen only during systole represents a trivial or physiologically insignificant amount of fluid; separation present in both systole and diastole is the M-mode correlate of a effusion in the small range and above.

Grading: One Current Standard, and Why Older Conventions Differ

The current, converging standard — used by both ASE Comprehensive Echocardiography and the 2025 ESC guideline — grades effusion size by the maximal end-diastolic distance of the echo-free space, in millimeters:

Grade	Size (end-diastolic)
Mild / trivial	Under 10 mm
Moderate	10–20 mm
Large	Over 20 mm

Older texts use centimeter-based conventions with different cutoffs from each other, and it’s worth recognizing both when reading older reports or literature: one common version defines small as under 0.5 cm, moderate as up to 1 cm, and large as over 1 cm (with estimated volumes of under 100 mL, 100–500 mL, and over 500 mL respectively); another defines small as under 0.5 cm, moderate as 0.5 to 2 cm, and large as over 2 cm. These aren’t interchangeable — a report using one convention’s “moderate” may correspond to a different tier in another. Beyond a rough size category, more granular quantitative measurement of effusion volume is rarely needed clinically.

Size and distribution are separate questions, not one scale. The current ESC classification frames pericardial effusion along four independent axes, worth documenting separately rather than folding into a single descriptor:

• Onset — acute (≤4 weeks), subacute (>4 weeks to ≤3 months), or chronic (>3 months)
• Size — by the grading above
• Distribution — circumferential or loculated
• Composition — transudate or exudate (echo can suggest this from echogenicity, discussed below, but can’t reliably distinguish the two on its own)

When reporting an effusion, document the extent and location of the measurement itself, not just a single number — this is what allows a meaningful comparison on follow-up imaging.

What the Fluid Looks Like: Echogenicity as a Clue

Pericardial effusions aren’t always uniformly echolucent. Varying degrees of echogenicity — from fibrin stranding to frank clot — can be seen, and the character of the fluid is worth describing in every report, since it can point toward a cause and can influence whether drainage, if needed, is better approached percutaneously or surgically.

• Fibrinous stranding, seen within the fluid or along the epicardial surface, is common in recurrent or long-standing pericardial disease, and is itself a recognized precursor to future constrictive physiology.
• Hematoma has an echodensity similar to myocardium itself — genuinely easy to overlook or misinterpret if not specifically considered, since it doesn’t have the classic echolucent appearance of simple fluid.
• A malignant effusion can be difficult to distinguish from benign fibrinous stranding on echo alone, but specific features that favor malignancy include a nodular appearance, evidence of extension into the myocardium, and a supporting clinical context.
• Air in the pericardial space (pneumopericardium) inhibits ultrasound imaging of the heart entirely — worth knowing as a genuine technical limitation rather than a failed study.

Mimics and Look-Alikes

Epicardial Fat

Epicardial fat sits inside the pericardium, preferentially along the coronary arteries and in the atrioventricular groove, and is most prominent anteriorly — but it is not confined to the anterior region alone, so “it’s posterior, so it can’t be fat” is not a reliable rule on its own. It’s distinguished from pericardial fluid by texture and behavior rather than location: fat has a relatively bright, heterogeneous or finely stippled echogenicity (in contrast to the echolucent appearance of fluid) and moves in concert with the heart, rather than remaining a static fluid collection. Additional fat outside the pericardium (mediastinal fat, often anterior to the right heart) shouldn’t be mistaken for a loculated effusion either. Epicardial fat is more common with increasing age and in obesity, particularly in obese women.

Pleural Effusion

The single most reliable distinguishing feature is the relationship to the descending thoracic aorta on parasternal long-axis: pericardial fluid tracks anterior to the descending aorta (contained by the oblique sinus, which lies between the aorta and the left atrium), while a left pleural effusion extends posterolateral to it. This is the anatomic consequence of the pericardial reflection around the pulmonary veins described above, which limits how far pericardial fluid can track behind the heart. When a large left pleural effusion is present, it can sometimes be used to advantage — cardiac images can occasionally be obtained with the transducer placed on the patient’s back, using the effusion itself as an acoustic window.

Two Genuine Look-Alikes Worth Naming Specifically

• The coronary sinus and the descending thoracic aorta themselves can be mistaken for an echo-free space if they aren’t specifically identified during the parasternal sweep — worth deliberately confirming their identity rather than assuming any tubular echolucent structure near the heart is fluid.
• A loculated effusion confined to the atrial region can be mistaken for a normal cardiac chamber, precisely because it sits where a chamber is expected — a pitfall specific to loculated fluid in this location.

Pericardial Cyst

An uncommon, congenital, fluid-filled sac, usually adjacent to the right heart. Pericardial cysts are usually missed on echocardiography and are better characterized by chest CT or cardiac MRI — but when they are seen on echo, they can be mistaken for a loculated pericardial or pleural effusion.

Pseudoaneurysm

Worth distinguishing conceptually from a simple effusion when the clinical setting (myocardial infarction, cardiac procedure, or trauma) raises the possibility of a contained rupture. A pseudoaneurysm is a saccular structure communicating with a ventricle, with walls composed of pericardium — a rupture site “contained” by pericardial adhesions rather than a genuine loculated effusion. This is anatomically distinct from a true aneurysm, whose walls are made of thinned, scarred myocardium. With effusion from aortic dissection or cardiac rupture, the actual entry site into the pericardium is rarely visualized directly on echo, so a high index of suspicion in the right clinical setting matters more than expecting to see the defect itself.

Loculated Effusion Deserves Its Own Attention

Loculated fluid is common after surgical or percutaneous procedures, and in patients with recurrent pericardial disease, where adhesions localize the effusion to a small area or separate it into several distinct pockets. Recognizing a loculated effusion is genuinely important, not just descriptively complete, for two reasons: hemodynamic compromise can occur from a small, strategically located collection despite the modest total volume, and drainage of a loculated effusion is not always achievable by a percutaneous approach — a surgical approach is sometimes needed instead. Reliable exclusion of a loculated effusion requires examination from multiple acoustic windows; TEE improves detection and characterization of loculated collections after cardiac surgery, especially when they’re located posteriorly.

How to Diagnose It: A Practical Sequence

1. Image from parasternal long-axis first, since this is where the earliest fluid appears — posterior to the left ventricle — and where the pericardial-versus-pleural relationship to the descending aorta is most directly seen.
2. Sweep through parasternal short-axis and all three standard apical views to define the lateral, medial, and apical extent of the fluid and to screen for a loculated distribution.
3. Add the subcostal view, both to confirm findings from a different window and, if the effusion is significant, to plan a safe pericardiocentesis approach.
4. Confirm true pericardial fluid rather than a mimic: check the fluid’s relationship to the descending aorta (pericardial anterior, pleural posterolateral), specifically identify the coronary sinus and descending aorta as themselves rather than fluid, and distinguish epicardial or mediastinal fat by its brightness and cardiac-synchronous motion.
5. Grade the size using the current millimeter-based convention (or note explicitly if using an alternate cm-based system), and separately describe distribution (circumferential vs. loculated) and onset if known from the clinical history.
6. Characterize the fluid’s echogenicity — echolucent, fibrin-stranded, or frankly echodense/hematoma-like — and note any nodularity or myocardial extension that would raise concern for malignancy.
7. If a loculated effusion is suspected but not clearly excluded on transthoracic imaging, or if a posterior collection is suspected after cardiac surgery, proceed to TEE; consider 3D imaging for volumetric assessment or further characterization of a loculated collection or hematoma.
8. Separately and deliberately assess hemodynamic significance — this is a distinct evaluation from grading the effusion’s size; a large effusion can be hemodynamically insignificant while a small, rapidly accumulating or strategically loculated one can cause tamponade physiology. See Cardiac Tamponade for the full echocardiographic and Doppler approach to that assessment.

When to Image

Cardiac imaging is recommended whenever pericardial effusion is suspected clinically, since physical examination and chest x-ray alone cannot make a definitive diagnosis. This includes chest pain consistent with pericarditis or aortic dissection, a chest x-ray showing an enlarged or flask-shaped cardiac silhouette, systemic disease associated with effusion alongside jugular venous distension, and new hypotension or hemodynamic instability after myocardial infarction or an invasive cardiac procedure. It’s worth remembering that acute pericarditis has an entirely normal-appearing transthoracic echocardiogram in 40 to 50% of first episodes — the absence of an effusion never excludes pericarditis as a diagnosis. Conversely, new or worsening pericardial effusion is itself one of the accepted diagnostic criteria for pericarditis, so the echocardiographic and clinical pictures are meant to inform each other rather than one simply confirming the other.

Clinical Importance

Pericardial effusion spans an enormous range, from a physiologic finding to a life-threatening emergency, and the size of the effusion alone tells you surprisingly little about where on that range a given patient sits. The discipline that actually matters is separating what the fluid is (true pericardial fluid, correctly distinguished from its several genuine mimics), how it’s distributed (circumferential vs. loculated, since loculated fluid carries its own specific risks), and what it’s doing hemodynamically — three genuinely different questions that a single millimeter measurement can’t answer on its own.