The electrocardiogram can play a significant role in suggesting or supporting the diagnosis of pericardial effusion. In common with most other lesions, pericardial effusion leaves no pathognomonic imprint on the tracing but produces three signs that are highly suggestive: low voltage, ST-segment elevation, and electrical alternans. Electrical alternans, especially if it involves all complexes ("total alternation"), is most diagnostic1-3; and the combination of these signs as a triad offers strong evidence of a substantial accumulation of exudative fluid in the pericardial sac.
Generalized low voltage of all waves (Figure, top) results from the short-circuiting action of the fluid and is the most constant sign of pericardial effusion. It is generally due to the accumulation of fluid alone, since in most cases it disappears after tapping or spontaneous clearing of the effusion. When low voltage fails to disappear after pericardiocentesis, it may be due to the insulating action
Nizet PM, Marriott HJL. The Electrocardiogram and Pericardial Effusion. JAMA. 1966;198(2):169. doi:10.1001/jama.1966.03110150117031