===Phase 4 Aberration===
Phase 4 aberration only occurs after prolonged pause. During such a pause (e.g. in second degree AV block) the Purkinje fibers can 'hyper'-depolarize spontaneously. As their membrane potential becomes more and more negative the conduction velocity decreases and can even be blocked altogether. This also requires an upward shift of the threshold membrane potential and a change in membrane responsiveness, so it is rarely seen in normal hearts.