The fundamental difference between accessory pathways (APs) and AV nodal tissue is the former lacks decremental properties . That is , APs continue to conduct whatever the impulse it receives. (Unlike the AV node which has a filtering mechanism , A heart rate sinker / Dampener) . This is what we were taught and we believe in that .
If it is true , every episode of atrial fibrillation should conduct with 400-600 ventricular responses . In reality it does not happen . The usual ventricular rate in AF with WPW is 250-300 /mt .
What happens to the rest of atrial impulses ?
I am sure it must get blocked in APs . Of course it is possible the block need not be in a fixed ratio .It changes in a dynamic manner with reference to the refractory period . (Please note , blocks and increased refractory periods can be used inter changeably in most physiological situations .
Final message
All APs are not dangerous .They do have a restrictive mechanism in place .This is evident in every patient with AF and WPW syndrome with a fairly controlled ventricular response . Hence one can conclude APs in WPW syndrome do have a physiological block in most episodes of Antidromic AF . The cut off for safe refractory period is defined empirically as > 250 ms.
Coming to the title question , Is there a physiological 2 : 1 block in accessory pathway during AF and WPW syndrome ?
Yes . It seems so ! A WPW patient who has just recovered from a well tolerated AF , is sort of a natural screening test which effectively rules out a future SCD .(Unless of course he has multiple APs with varying RPs , one for AF other for VF !)
Is that a correct way of reasoning ? Experts may provide further input .
Dr.S.Venkatesan MD 