Virtual Electrode, Graded Responses, Reentry, Breakthrough, and Undetermined:

The Puzzle of Defibrillation: Putting the Pieces Together

“…They also used pigs, $ but a different preparation; they isolated the right ventricle and recorded optically from both the epicardial and endocardial surfaces. Several mechanisms contributed to their cocktail of diverse postshock responses: reentry after make or break excitation, 10–12 propagated graded responses, 13–15 and breakthrough. Graded responses and breakthrough occurred after an isoelectric window, but reentry did not.…”