Illustration copyright 2002 by Nucleus Communications, Inc. All rights reserved. http://www.nucleusinc.com

An electrocardiogram (EKG, ECG) translates the heart's electrical activity into line tracings on paper. The spikes and dips in the line tracings are called waves.

The P wave represents the electrical activity in the atria. The Q, R, and S waves—called the QRS complex—represent the electrical activity in the ventricles. The T wave represents the electrical recovery, or repolarization, of the ventricles.