In acute pericarditis, the inflammatory process usually involves the entire epicardial surface. As a result, the epicardial cells exhibit changes similar to those induced by acute ischemia. The resting membrane potential of the epicardial cells becomes more positive (i.e. less negative) and the plateau potential becomes more negative than in the mid-myocardial and sub-endocardial cells. Thus, during diastole (phase 4 of the action potential), there will be injury currents directed away from the epicardial surface, and during systole (phases 2 and 3) they will be directed toward the epicardial surface. This will result in depression of the TQ segment and elevation of the ST segment in all leads placed on the body surface with the exception of lead aVR. aVR is the exception because its positive pole is located superiorly and to the right. Thus the electrical forces will be the in the reverse direction as those recorded by the left sided and more inferior leads