Epicardial ST depression in acute myocardial infarction

The presence of electrocardiographic ST depression in acute infarction rema ins controversial and poorly explained. A combined animal and modeling stud y was performed to evaluate the source of ST changes in acute infarction; I n anaesthetized sheep, small infarcts showed uniform ST elevation over the infarction whereas larger infarcts showed marked ST depression over the nor mal myocardium in addition to the ST elevation. These findings were replica ted by bidomain models of the heart. A hollow sphere was used to model a gr adually increasing infarct, and this showed that there was a decrease in th e ratio of ST elevation to ST depression as the infarct was increased. The current flowing out of the heart must be identical to the current flowing b ack into the heart. This means that any infarction will produce ST depressi on as well as ST elevation, the ratio between the two being related to the size of the infarction, Small infarction is associated with a small region of ST elevation and minor ST depression of the remaining myocardium, and as the infarct region increases, the amplitude of the epicardial ST elevation falls and the amplitude of the ST depression increases. Infarction size is proportional to both the height of the ST depression on the epicardium and the strength of the epicardial ST segment dipole.