Electrocardiography (EKG, ECG) measures your heart's electrical signal as it triggers each of your four heart chambers to pump (contract). Small pads (electrodes) attached to the surface of your skin detect the electrical signals. The electrodes are attached with wires to a machine that draws a graph of the electrical signals.

A comprehensive measurement of your heart's signals requires 6 electrodes on your chest and 4 distributed across your arms and legs. Each electrode measures your heart's electrical activity from a different angle, which the EKG machine displays as 12 separate readings. Your doctor gets the equivalent of a three-dimensional perspective of your heart's electrical function from these readings.

See an illustration of electrode placement.

The different "waves" on the EKG graph show the signal triggering different parts of your heartbeat. By looking at these EKG waves, your doctor can detect problems with your heart's rhythm. An EKG can also show evidence of other problems with your heart.

See an illustration of an EKG rhythm strip.