An electrocardiogram, also called an EKG or ECG, is a simple test that detects and records the electrical activity of the heart. It is used to detect and locate the source of heart problems.
Electrical signals in the heart trigger heartbeats. These signals start at the top of the heart in an area called the right atrium. The electrical signals travel from the top of the heart to the bottom. They cause the heart muscle to contract as they travel through the heart. As the heart contracts, it pumps blood out to the rest of the body.
An EKG shows how fast the heart is beating. It shows the heart’s rhythm (steady or irregular) and where in the body the heartbeat is being recorded. It also records the strength and timing of the electrical signals as they pass through each part of the heart.
An EKG is sometimes called a 12-lead EKG (or 12-lead ECG) because the electrical activity of the heart is most often recorded from 12 different places on the body at the same time.
See the How the Heart Works section for more details on the way the heart works.
What EKG Reveals
Many heart problems change the electrical signature of the heart in distinct ways. EKG recordings of this electrical activity can help reveal a number of heart problems, including:
Heart attack
Lack of blood flow to the heart muscle
A heart that is beating irregularly, or too fast or too slow
A heart that does not pump forcefully enough
EKG recordings can help doctors diagnose a heart attack that is happening now or has happened in the past. This is especially true if doctors can compare a current EKG recording to an older one. EKG recordings can also reveal:
Heart muscle that is too thick or parts of the heart that are too big
Birth defects in the heart
Disease in the heart valves between the different heart chambers
An EKG also reveals whether the heartbeat starts at the top right part of the heart like it should. It shows how long it takes for the electrical signals to travel through the heart.
Copy from: http://www.nhlbi.nih.gov/health/dci/Diseases/ekg/ekg_what.html
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