The ECG in patients with acute pericarditis goes through 4 stages. The classic findings are widespread STsegment elevation and PR depression. These changes are present during the acute phase, which has an onset within hours and may last a few days. The ST and PR segments usually normalize during the transitional second stage. The third stage is marked by diffuse T wave inversions, while the fourth is represented by either normalization of the the ECG or permanence of the t wave inversion. This temporal evolution of ECG changes is highly variable, and with disease progression, other nonspecific abnormalities may develop as well.

Widespread elevation of the STsegment above the isoelectric line is the most common finding of acute pericarditis. Unlike with myocardial infarction, the ST segment elevation is typically concave upward. While STsegment elevation can occur in most leads, it is usually normal or depressed in leads AVR and V1. Depression of the PR segment below the isoelectric line is a more specific finding for acute pericarditis than ST elevation. It is also widespread, except in leads AVR and V1 in which it can be elevated.

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