Understanding Electrocardiography lesson -2

Lead: It is the electrical picture of the heart.

The 12-lead ECG is generated from ten physical electrodes that are attached to the skin.
  (the right leg electrode acts as an earthing electrode).

They comprise three groups:
Three dipole limb leads, three augmented voltage limb leads and six unipole chest leads.
         Bipolar Lead:- I, II, III.
         Unipolar lead:-Chest lead (V1-V6) + aVL , aVR, aVF.
 Lead-I: left Chest wall,
Lead –II: left hip,
Lead-III: Right hip.
Lead-aVR: Right shoulder.
Lead-aVL: left shoulder.
Lead-AVF: Perineum.

The six chest leads are unipolar recordings obtained by electrodes in the following positions: lead V1, fourth intercostal space, just to the right of the sternum; lead V2, fourth intercostal space, just to the left of the sternum; lead V3, midway between V2 and V4; lead V4, midclavicular line, fifth intercostal space; lead V5, anterior axillary line, same level as V4; and lead V6, midaxillary line, same level as V4 and V5.

Leads- I,II, and aVL  indicates the  changes in left lateral surface of the heart.
Lead- III and aVF   indicates the inferior wall changes in the heart.
Lead : V1-V6 indicates the extensive anterolateral changes.
Lead: V1-V2  indicates the septal changes.
Lead:V3-V4  indicates the anterior changes.