<P> Electrocardiography (ECG or EKG) is the process of recording the electrical activity of the heart over a period of time using electrodes placed on the skin . These electrodes detect the tiny electrical changes on the skin that arise from the heart muscle's electrophysiologic pattern of depolarizing and repolarizing during each heartbeat . It is a very commonly performed cardiology test . </P> <P> In a conventional 12 - lead ECG, ten electrodes are placed on the patient's limbs and on the surface of the chest . The overall magnitude of the heart's electrical potential is then measured from twelve different angles ("leads") and is recorded over a period of time (usually ten seconds). In this way, the overall magnitude and direction of the heart's electrical depolarization is captured at each moment throughout the cardiac cycle . The graph of voltage versus time produced by this noninvasive medical procedure is an electrocardiogram . </P> <P> During each heartbeat, a healthy heart has an orderly progression of depolarization that starts with pacemaker cells in the sinoatrial node, spreads out through the atrium, passes through the atrioventricular node down into the bundle of His and into the Purkinje fibers, spreading down and to the left throughout the ventricles . This orderly pattern of depolarization gives rise to the characteristic ECG tracing . To the trained clinician, an ECG conveys a large amount of information about the structure of the heart and the function of its electrical conduction system . Among other things, an ECG can be used to measure the rate and rhythm of heartbeats, the size and position of the heart chambers, the presence of any damage to the heart's muscle cells or conduction system, the effects of cardiac drugs, and the function of implanted pacemakers . </P> <P> The etymology of the word is derived from the Greek electro, because it is related to electrical activity, kardio, Greek for heart, and graph, a Greek root meaning "to write". </P>

Explain to john the difference between an ecg and an echocardiogram