;Criteria for posterior fascicular block:
:Right [[heart axis|axis
:Deep S in I;
:Small q in III;
Aberrancy can result from:
fastening of the heart rate that the bundles cannot conduct (phase 3 aberration)
#Retrograde concealed conduction
#A slow heart rate (phase 4 aberration)
Right bundle branch block is most common, because the right bundle has the
longest refractory period. Left bundle branch block accounts for about 1/3rd of cases.
===Phase 3 Aberration===
Phase 3 aberration occurs when conduction fibers receive a new impulse, before they have fully repolarized. This can sometimes be observed at the start of paroxysmal supraventricular tachycardias or in a long-short sequence
where the refractory period of the long sequence is prolonged.
===Retrograde Concealed Conduction===
===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
upwards shift of the threshold membrane potential and a change in membrane responsiveness, so it is rarely seen in normal hearts.