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Pericarditis
Inflammation of the pericardium will almost always produce an injury current. This injury current comes from all around the surface of the heart, so that it will cause ST segment elevation in all leads. This is different from the changes in infarction, when the ST segments are up in some leads and down in others. There will be progressive change of the ST-T complex, very like the changes of infarction, except that the QRS is never involved.

ELECTROCARDIOGRAM IN PERICARDITIS — Pericarditis can be caused by a number of factors including viral or bacterial infection, metastatic tumors, collagen vascular diseases, myocardial infarction, cardiac surgery, and uremia (show table 1). (See "Evaluation and management of acute pericarditis").

The ECG pattern of acute pericarditis resembles in some ways that seen with acute myocardial infarction [1,2].

ST segment elevation — The early phase of acute pericarditis is usually characterized by ST segment elevations due to alterations in ventricular repolarization (show ECG 1). This current of injury pattern results from concomitant inflammation of the epicardium. This explains why the ECG often shows no signs of pericarditis in uremic patients who have a fibrinous pericarditis that may not affect the epicardium. (See "Pericarditis in renal failure").

PR segment depression — Acute pericarditis also may affect repolarization of the atria, which starts during the PR segment (from the end of the P wave to beginning of the QRS) (show ECG 1). Acute pericardial inflammation often causes an atrial current of injury, reflected by elevation of the PR segment in lead aVR and depression of the PR segment in other limb leads and in the left chest leads V5 and V6. Thus, in acute pericarditis, the PR and ST segments typically change in opposite directions. In aVR, for example, the PR segment is elevated (often by only 1 mm or so) while the ST segment is usually slightly depressed. Other leads may show PR depression and ST elevation.

Chapter 70:
Acute Pericarditis: The electrocardiogram (ECG) is the most important laboratory test for diagnosing acute pericarditis (see Chap. 12) . The classic finding is diffuse ST segment elevation ( Fig. 70-3 ). The ST segment vector typically points leftward, anterior and inferior, with ST segment elevation in all leads except AVR and often V1. Thus the term “diffuse” is a slight misnomer.
Chapter 12:
Acute pericarditis, in contrast to acute myocardial infarction, typically induces diffuse ST segment elevation, usually in most of the chest leads and also in leads I, aVL, II, and aVF. Reciprocal ST depression is seen in lead aVR. An important clue to acute pericarditis, in addition to the diffuse nature of the ST elevation, is the frequent presence of PR segment elevation in lead aVR, with reciprocal PR segment depression in other leads, caused by a concomitant atrial current of injury (see Fig. 12-48 ). Abnormal Q waves do not occur with acute pericarditis, and the ST elevation may be followed by T wave inversion after a variable period.

FIGURE 12-48 Acute pericarditis is often characterized by two apparent injury currents, one atrial, the other ventricular. The atrial injury current vector (STa) is usually directed upward and to the right and produces PR segment elevation in aVR with reciprocal PR depression in II, V5, and V6. The ventricular injury current (STV) is directed downward and to the left, associated with ST elevation in II, V5, and V6 with reciprocal ST depression in aVR. This characteristic PR-ST segment discordance is illustrated in the bottom-most tracing. Note the diffuse distribution of ST segment elevation in acute pericarditis (e.g., I, II, and V2 through V6, with reciprocal changes in aVR and perhaps minimally in V1). Note the PR segment elevation in aV. LA=left atrium; LV=left ventricle; RA=right atrium; RV=right ventricle. (From Goldberger AL: Myocardial Infarction: Electrocardiographic Differential Diagnosis. 4th ed. St. Louis, Mosby-Year Book, 1991.)