What is an EKG?

What does that durned squiggle mean?

So what about those durned precordial leads, V1 through V6? Well, just like the V in aVF, aVL and aVR, the V in these leads corresponds to the idea that these are unipolar leads. The electrodes serve as the positive (and only) lead and the center is taken to be the center of the heart's electrical field, just as with aVF, aVL and aVR.

Why have precordial leads? Well, the heart is not two dimensional. There is a component of the potential at any given moment that is travelling from the heart towards or away from the chest wall (in the sagittal rather than coronal plane) and the limb leads cannot "see" or measure this. Another way of putting this is that at any given moment, there is a vector in the heart representing the potential and it has three coordinates in space plus it changes with time. The limb leads see two of these coordinates; the precordial leads help flesh out the third.