What an electrocardiogram can (and can't) tell you - Part 1: how and why it's done

The electrocardiogram, also called an EKG or ECG, is a test that records electrical activity within the heart.  With each heartbeat, an electrical signal is conducted through specialized nerves from the upper to lower chambers of the heart.  This causes the heart muscle to contract in a synchronized manner and pump blood throughout the body.  These electrical signals are detected by an EKG