A record of the variations in electric potential which occur in the HEART as it contracts and relaxes. Any muscle in use produces an electric current but, when an individual is at rest, the main muscular current in the body is that produced by the heart. This can be recorded by connecting the outside of the body by electrodes to an instrument known as an electrocardiograph. The patient is connected to the electrocardiograph by leads from either the arms and legs or different points on the chest. The normal electrocardiogram of each heartbeat shows one wave corresponding to the activity of the atria and four waves corresponding to the phases of each ventricular beat. Various readily recognisable changes are seen in cases in which the heart is acting in an abnormal manner, or in which one or other side of the heart is enlarged. This record therefore forms a useful aid in many cases of heart disease (see HEART, DISEASES OF). The main applications of the electrocardiogram are in the diagnosis of myocardial infarction and of cardiac ARRHYTHMIAS.