First and second year — the floor first, then every step
Size the effusion, then answer the real question — is it compressing the heart (tamponade) or is the pericardium thick and the filling restricted (constriction)?
Orient first
Tamponade is physiology, not size: a small rapid effusion can tamponade, a large slow one may not.
Constriction is a thickened, stiff pericardium limiting filling; the septum bounces and the atria and veins distend.
Normal pericardium is up to about 2–3 mm thick on CT and MRI (verify against the reference used).
Acquire the study
Subxiphoid and parasternal views with a curvilinear or phased-array probe; M-mode across the right ventricle.
The manoeuvre
Measure the largest diastolic separation between the pericardial layers in mm on the subxiphoid view.
Right atrial systolic and right ventricular diastolic collapse (tamponade) — time them against the cardiac cycle on M-mode.
IVC diameter in mm and its respiratory variation — a plethoric IVC supports tamponade.
Septal position compared between inspiration and expiration — a bounce suggests constriction.
What confirms it
Tamponade: chamber collapse or reflux signs with an effusion. Constriction: thick or calcified pericardium with interdependence.
What licenses you to exclude it
A normal-thickness pericardium does not exclude constriction.
The classic misread
Calling anterior epicardial fat an effusion.
Reference values
Each value carries the caveat that keeps it from being misused. Normal limits and diagnostic criteria are kept apart on purpose: a disease cut-off read as a normal range is the more dangerous mistake.
Diagnostic criteria
Pericardial effusion · Size grading — echo versus CT
a named distinction: echocardiography grades by centimetre strips in standard views; CT reports depth at a stated location (commonly a few millimetres is physiological, over about 2 cm is large) — the two scales are not the same measurement
A moderate CT effusion can be tamponading if it accumulated fast, and a large chronic one may not. Report depth, distribution (circumferential vs loculated) and mass-effect, and leave tamponade to the bedside.
CT · USG · MRI
Pericardium · Pericardial thicknessup to about 2 mm
ECG-gated CT or MRI, measured over the right heart border where it is best seen.
⚠️ THICKENING IS NEITHER NECESSARY NOR SUFFICIENT FOR CONSTRICTION — constriction occurs with a normal-thickness pericardium, and thickening occurs without it. The physiological signs (septal bounce, ventricular interdependence on real-time cine, IVC dilatation) carry the diagnosis.
CT · MRI
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.