Myocardial ischemia and infarction are the terms used to describe the sequence of events that occur when blood supply to the myocardium is inadequate to meet its metabolic demands. This imbalance between metabolic demand and supply leads to a series of metabolic, ionic and neuro-hormonal events that cause changes in the electrical and mechanical properties of the cells and fibers within the ischemic region. The electrical changes that occur during repolarization and during diastole (phases 3 and 4 of the action potential) are reflected on the body surface electrocardiogram by changes in the TQ and ST segments and by changes in the T wave. The TQ and ST segment changes are usually transient and resolve when ischemia is relieved or the cells die and/or become electrically uncoupled. The T wave changes usually persist for a longer duration, but usually eventually resolve.
The electrical changes that occur during depolarization are reflected by changes in the QRS complex. They indicate that the cells in the ischemic region are no longer excitable, usually the result of cell death. These changes are usually irreversible although they may evolve over time.
In this chapter, we will discuss the electrophysiologic causes of the electrocardiographic changes that accompany myocardial ischemia and infarction and how these changes help to identify the involved myocardial region(s) and, in some situations, the location of the obstruction(s) within the coronary circulation. We will also discuss the situations in which electrocardiographic changes that simulate ischemia and infarction may occur in the absence of these events.