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Aberrant conduction is most often due to a premature atrial impulse – except when it isn’t! This is an unusual example of the “Ashman phenomenon”. The sinus P waves are regular @ 70/min. with left axis due to a prior inferior M.I. There is repetitive 3:2 Wenckebach A-V block, which results in a lengthy pause. This, by the Ashman effect, prolongs the refractory period after the first conducted beat and the second is conducted aberrantly. (RBBB)
The refractory period of the rt. bundle is a little longer than that of the lt. bundle, so the most frequent pattern of aberrant ventricular conduction is RBBB. In this patient, every sinus beat is wedded to a premature atrial impulse. So, the four words are “aberrantly conducted atrial bigeminii”…..But let’s add – with RBBB aberration. What descriptive term is frequently used to identify this variety of QRS abnormality?
If the refractory period of the lt. bundle is abnormally long, premature stimuli will find it unresponsive and be conducted with a pattern of LBBB. The four words are the same –”aberrantly conducted atrial bigeminii” --- but now let’s add – with LBBB aberration.