As the heart undergoes depolarization and repolarization, electrical currents spread throughout the body because the body acts as a volume conductor . The electrical currents generated by the heart are commonly measured by an array of electrodes placed on the body surface and the resulting tracing is called an electrocardiogram (ECG, or EKG). By convention, electrodes are placed on each arm and leg (standard and augmented limb leads), and six electrodes are placed at defined locations on the chest (precordial leads). These electrode leads are connected to a device that measures potential differences between selected electrodes to produce the characteristic ECG tracings.

Some of the ECG leads are bipolar leads (e.g., standard limb leads) that utilize a single positive and a single negative electrode between which electrical potentials are measured. Unipolar leads (augmented leads and chest leads) have a single positive recording electrode and utilize a combination of the other electrodes to serve as a composite negative electrode. Normally, when an ECG is recorded, all leads are recorded simultaneously, giving rise to what is called a 12-lead ECG.