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, and
six electrodes are placed at
defined locations on the chest.
These electrode leads are
connected to a device that
measures potential differences
between selected electrodes to
produce thecharacteristic 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.