Physiology, Cardiac Repolarization Dispersion and Reserve

Excerpt

The heart carries out the vital function of pumping oxygenated blood around the body, for which it has to contract and relax in a coordinated fashion. This contraction process is preceded by electrical excitation, which under normal conditions is initiated by the SA node as an action potential. An action potential is the rapid sequence of changes in the membrane potential, resulting 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 two main classifications of cells in the heart to be considered: cardiomyocytes and pacemaker cells. Each of these individual cell types has a distinct pattern of action potentials divided into several distinct phases. A shared characteristic common to both cell types is the third phase, designated as repolarization. Repolarization defines the resetting of the electrochemical gradients of the cell to prepare for a new action potential. The action potential (AP) of the working myocardium lasts for several hundreds of 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 one current takes over if another one should fail. The time required for repolarization to occur can vary between cardiac myocytes. This heterogeneity, termed dispersion, can be a sign of pathology, especially when the heart cannot perfuse the body due to disturbances in cardiac output.