An Electrocardiogram (ECG)
The heart is a beating muscle that causes blood to be pumped through the circulatory system. Each heart beat is stimulated by electrical activity. An electrical signal passes through the heart muscle causing it to contract. A doctor can monitor this electrical activity with an electrocardiogram or ECG, also sometimes called an EKG.
Electrodes are placed on the chest, and sometimes arms and legs. These electrodes measure the electrical activity. A heart monitor can then display and print a graph of the electrocardiogram. The above figure shows a typical electrocardiogram for a heart beat cycle. The different line graphs such as the red, blue and green lines in this figure are created by different electrode positions. They each show a different activity of the heart.
The electrical signal follows a set pattern, repeated over and over again with each heart beat. Variations to the normal electrical pattern could indicate damage to the heart, such as a heart attack or heart disease.