Electrocardiography has progressed rapidly since it was first employed in the early 1900s. The electrocardiogram (ECG) is one of the most vital monitors of a patient"s cardiovascular status and is used today in nearly every clinical setting. ECG is used to measure the electrical activity of the heart as it changes over time: as action potentials within each myocyte propagate throughout the heart during the cardiac cycle. By utilizing the resultant electrical field present in the body, electrodes can be placed around the heart to measure potential differences as the heart depolarizes and repolarizes.

An ECG can be broken into several components. The P wave represents atrial depolarization, because there is little muscle in the atria the deflection is small. The QRS wave represents the depolarization of the ventricles and the repolarization stage of the atria, however since there is more muscle in the ventricles you only see the electrical activity of the ventricles. The T wave represents repolarization of the ventricular myocardium after systole is complete.