Electrocardiography

Electrocardiography (ECG) is the practice of measuring and interpretating the electrical signals produced by the heart and traverse through the skin surface of the chest. With this exam, the rythm and regularity of the cardiac activity can be measured and deriving abnormalities can be diagnosed. It is a safe, fast, and non-invasive (no surgical cuts are needed) practice.

When the heart muscle depolarizes during every heartbeat, ions traverse through the cell membranes causing a potential diffrence, which can be measured by electrodes placed in the skin surface. The electrodes usually consist of a conducting gel, embedded in the middle of a self-adhesive pad onto which cables clip. Sometimes the gel also forms the adhesive.

The simplest layout of the electrodes needed are in the arms (avoiding thick muscle,RA,LA), in the lateral calf muscle of the left leg (LL) and some times in the same position in the right leg (RL).

This results in a signal that if passed to an osciloscope, will give us the well known wave form of the electrocardiogram. Although the really interesting part will be when we digitalize the signal and pass it to a computer. There, programmers like me, can do anything we want with it.
Also look at this video wich will help you interpret and understand the form of the output signal