Basic ECG theory, recordings, and interpretation

An electrocardiogram (ECG; in German, the electrokardiogram, EKG) is a measure of how the electrical activity of the heart changes over time as action potentials propagate throughout the heart during each cardiac cycle. However, this is not a direct measure of the cellular depolarization and repolarization with the heart, but rather the relative, cumulative magnitude of populations of cells eliciting changes in their membrane potentials at a given point in time; it shows electrical differences across the heart when depolarization and repolarization of these atrial and ventricular cells occur.