Posts Tagged ‘decremental conduction’

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 .

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