EKG Standard Leads
 There are
three of these leads which are usually designated as I, II and III.
They are all bipolar (i.e., they detect a change in electric potential
between two points) and detect an electrical potential change in the frontal
plane.
Lead I is between the right arm
and left arm electrodes, the left arm being positive.
Lead II is between the right arm
and left leg electrodes, the left leg being positive.
Lead III is between the left arm
and left leg electrodes, the left leg again being positive.
A diagrammatic representation of these three leads is termed Einthoven's
triangle (shown in blue below), after the Dutch doctor who first described
the relationship. The central source of electrical potential in the triangle
is the heart.
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