The heart carries out the vital function of pumping oxygenated blood around the body, and it must contract and relax in a coordinated fashion. This contraction process is preceded by electrical excitation, which under normal conditions is initiated by the sinoatrial node as an action potential. An action potential is the rapid sequence of changes in the membrane potential that results in an electrical impulse. This electrical impulse then travels down through the heart's electrical conduction system to cause myocardial contraction, followed by relaxation in an orderly fashion. There are 2 main cell types in the heart to consider: cardiomyocytes and pacemaker cells. Each of these cell types has a distinct pattern of action potentials, divided into several phases. A shared characteristic common to both cell types is the third phase, designated as repolarization. Repolarization refers to the resetting of the cell's electrochemical gradients to prepare for a new action potential. The action potential (AP) of the working myocardium lasts for several hundred milliseconds, with the delayed repolarization securing a refractory state for new excitations throughout the entire contraction phase. Delayed repolarization in the human myocardium relies mainly on the vast diversity of cardiac potassium channels, but also on a particular redundancy in the heart known as the "repolarization reserve," in which 1 current takes over if another fails. The time required for repolarization can vary among cardiac myocytes. This heterogeneity, termed dispersion, can be a sign of pathology, especially when cardiac output is impaired.