I have a two-lead ICD and my doctor wants to put in a three-lead ICD. Why?

You'll have to ask that doctor of yours to be sure, but let us take an educated guess. If you have congestive heart failure or low pumping efficiency, your heart may benefit from pacing both ventricles (the bottom chambers). The third lead is often placed in the left ventricle. This allows for better coordination of the contraction of the heart and improve circulation.