CFM Cardiology Practice Test

Large amounts of fluid around the heart dampen the electrical signals as they travel to the chest leads, so the QRS complexes become globally smaller in amplitude. This diffuse reduction in QRS voltage is the classic ECG finding in pericardial effusion and can be seen in tamponade as well. In tamponade you might also encounter electrical alternans, where the QRS amplitude varies with each beat as the heart swings within the fluid, but the hallmark remains low voltage across multiple leads.

Diffuse ST elevations in all leads point to acute pericarditis, not tamponade. Tall R waves in V1 and a short PR interval are not typical features of pericardial effusion or tamponade.