Why do biomedical (ekg) use 12 leads?

12 leads are used to evaluate the heart (electrically) in two planes:  the frontal plane and the horizontal plane.  Each lead "looks" at the heart from a different angle.  In EKG nomenclature, lead 1 is comparing the voltage swings of the left arm to the right arm; lead 2 is comparing the voltage swings of the left leg to the right arm and lead 3 is comparing the voltage changes of the right leg compared to the left arm.  These form a horizontal plane, each 120 degrees apart.  By definition, lead 1 is given a value of zero degrees, lead 2, 60 degrees and lead 3, 120 degrees.  For evaluation from different angles, different combinations of the "limb leads" (leads 1, 2 and 3) are combined to look at voltage changes at -30 degrees, -150 degrees and 90 degrees (called avL,avR and avF, also 120 degrees apart).  Finally, the leads placed in the horizontal plane are called V1 through V6.  These are placed across the sternum and left side of the chest.  In some cases, they are placed on the right side in a mirror image, and called RV1 through RV6.  In some cases, with cardiac research, the other vertical plane (a plane facing either the right or the left) can be evaluated.  

Jonathan Richard, M.D.