Fluid in the pericardial space, the gap between the two layers of the pericardium, is common and usually harmless. The pressure it exerts becomes dangerous when it is high enough to stop the heart filling, and the same volume can be harmless or fatal depending on how fast it arrived and how much the pericardium can stretch.
Pericardial effusion
A pericardial effusion is an accumulation of fluid in the pericardial space. Fluid arrives faster than usual in several situations: inflammation or infection of the pericardium itself, bleeding after trauma, cardiac surgery or during anticoagulation, and non-inflammatory conditions that shift fluid into the space — raised pulmonary hydrostatic pressure in congestive heart failure, increased capillary permeability in hypothyroidism, and reduced plasma oncotic pressure in cirrhosis. Fluid also leaves more slowly when the thoracic duct is obstructed by tumour or damaged during surgery.
The composition of the fluid follows its cause. Viral pericarditis produces serous or serofibrinous fluid, bacterial infection produces suppurative (purulent) fluid, malignant effusions are typically haemorrhagic, and a chylous effusion, one made of lymphatic fluid, points to thoracic duct disruption.
Whether an effusion causes symptoms depends on three things: the volume of fluid, the rate at which it accumulates, and the compliance of the pericardium itself, meaning how readily it stretches. A small volume arriving quickly — 100–200 mL of blood after trauma, for example — can cause tamponade, because the pericardium has had no time to stretch. A much larger effusion accumulating slowly in heart failure or hypothyroidism may be tolerated without any haemodynamic change (any change in blood flow or pressure) at all. Volume alone is therefore a poor guide to danger.
Small effusions do not disturb haemodynamics. Large ones compress neighbouring structures and produce a recognisable but uncommon set of symptoms: dysphagia from compression of the oesophagus, hoarseness from compression of the recurrent laryngeal nerve, hiccups from diaphragmatic irritation, and dyspnoea from pleural inflammation or a pleural effusion. Dyspnoea is the symptom that usually brings the patient in; the others are memorable but rare.
Imaging and the electrocardiogram (ECG) support the diagnosis without confirming it. The chest radiograph shows an enlarged, water-bottle-shaped cardiac silhouette once the effusion is large. The ECG may show low voltage, because fluid dampens the electrical signal, and electrical alternans — beat-to-beat variation in QRS amplitude as the heart swings within the fluid-filled sac. Both signs are specific but insensitive, so their absence means little. Echocardiography is the imaging test of choice: it shows the fluid, estimates its size and distribution, and looks for signs of tamponade.
Cardiac tamponade
Cardiac tamponade is fluid under pressure compressing the cardiac chambers until diastolic filling fails. It is the complication that kills, and it comes in two forms. Acute tamponade follows trauma or left ventricular rupture, where the effusion may be small but accumulates within minutes because the pericardium cannot stretch. Gradual tamponade develops from a large effusion of any cause, often over weeks: the pericardium stretches as the fluid collects, until it reaches the limit of its compliance, after which haemodynamic compromise appears suddenly. The general rule behind both is that the faster fluid accumulates, the smaller the volume needed.
At the bedside, tamponade presents with dyspnoea and tachycardia, a falling blood pressure, a raised jugular venous pressure, and muffled heart sounds. The classic triad of hypotension, distended neck veins and muffled heart sounds is Beck’s triad. Two further signs are worth knowing because they are distinctive. Pulsus paradoxus is an exaggerated inspiratory fall in systolic blood pressure, greater than 10 mmHg; it is present in most patients with tamponade but disappears with atrial fibrillation and severe aortic regurgitation, so its absence does not exclude the diagnosis. Kussmaul’s sign is a paradoxical rise in jugular venous pressure during inspiration.
The haemodynamic signature on right heart catheterisation is equalisation of diastolic pressures: right atrial, right ventricular diastolic, pulmonary artery diastolic and pulmonary capillary wedge pressures converge within about 5 mmHg of each other, because the compressed heart cannot generate a gradient to fill against. Because the right-sided chambers are the low-pressure ones, they are compressed first, which is why jugular venous pressure rises early.
Echocardiography adds right atrial and right ventricular collapse, respiratory variation in mitral and tricuspid inflow velocities, and a dilated inferior vena cava that does not collapse with inspiration.
Draining the fluid is the treatment; pericardiocentesis is performed urgently under echocardiographic guidance, while surgical drainage is needed when the fluid is blood or clot. Intravenous fluids and vasopressors may support the circulation for the minutes to hours before drainage, but they do not treat the cause. Recurrent malignant effusions may be managed with a pericardial window.
Tamponade is the acute end of the range of pericardial disease. At the other end, a pericardium scarred by long-standing inflammation stiffens the heart from the outside without any fluid under pressure.