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The axis of an EKG lead is determined
by the placement of the lead patches and which patch is electrically positive
or negative. Practically, lead axis is equivalent to point of
view. An inferior lead "looks at" the inferior aspect of the
heart - the apex. A left lead "looks at" the left aspect of the heart
- the left ventricle. The limb leads are easy to remember:
they are numbered clockwise.
Imagine that the arrows representing these three leads pass through
the heart (seen from the front), and that when examining these leads on
an EKG you are looking down the shaft of the arrow from the positive end.
Lead I is a left lead; leads II and III are inferior leads.
In this diagram, the three augmented limb leads have been added.
They are easy to remember too: R means right, F means foot, and L means
This may make it easier to see the lead axes. Only the vector
from the center of the chest to the "point of view" of each lead is shown.
The six V-leads, also called precordial leads, are oriented in a different
plane. They look from the electrode patches on the anterior chest
wall in to the center of the chest. They provide good points of view
to examine the ventricular septum (V1) and areas in the left ventricle
(V2 through V6).