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→Mechanisms of aberrant conduction
Right bundle branch block is more common, because the right bundle has the longer refractory period. Left bundle branch block accounts for about 1/3rd of cases.
[[File:lbbb_phase3_phase4.svg|thumb|The image explains LBBB due to phase 3 and phase 4 aberration.]]
Phase 3 aberration occurs when conduction fibers receive a new impulse, before they have fully repolarized. A premature impulse is encroaching on the refractory period of the bundle branch. This is a physiological phenomenon. This can sometimes be observed at the start of paroxysmal supraventricular tachycardias or in a long-short sequence in which the refractory period of the long sequence is prolonged. This is also called Ashman phenomenon.
===Phase 4 Aberration or deceleration dependant===
Phase 4 aberration only occurs after prolonged pause. During such a pause (e.g. in second degree AV block) the Purkinje fibers can depolarize spontaneously. As their membrane potential becomes more positive, the conduction velocity decreases and can even be blocked altogether. This is usually a pathological response, but can be normal at very low heart rates (e.g. 40 bpm)
A small increase in rhythm resulting in aberrancy due to an abnormal response of tissue that has diminished excitability.
===Retrograde Concealed Conduction===
[[File:retrograde_concealed_conduction.svg|thumb|LBBB due to retrograde concealed conduction]]
This is the most common mechanism for sustained aberrancy during tachycardia. The sequence of QRS widening that is often observed is phase 3 aberration in the first premature beat. This can leave the left bundle (for example) refractory for the next complex. This next beat is conducted by the right bundle and once it reaches the apex, it is conducted retrograde by the left bundle. This can continue until a new premature ventricular complex causes a compensatory pause and 'resets' the system.