Posts Tagged ‘slow fat avnrt’

AV node is the  “Go slow” region in the cardiac highway .Every impulse is delayed  for about 120ms and then pursue its  onward journey to depolarize the ventricle.

Since  AV node  has inherently slow conduction properties , it is not  surprising  this zone  is vulnerable  for developing  AV block .We know AV junction and the adjacent his bundle  is the site  for many types of AV block. In  classical  Mobitz type  2 AV block ,  for every two or three supra ventricular impulse only one is  conducted and we call this as   2:1  or 3: 1 AV block ( More appropriately AV conduction  )

Can we have reverse of the above situation ?  That is , for each supra ventricular  impulse  can ventricles  fire twice or thrice   ?

Yes it can  ,  what looks like a funny situation ,  could be more common   .We are not recognising it often.

How is  this possible ?

This can happen only if there are two different  tracts of conduction from atrium to ventricle and  both of them conducting  fully to  reach ventricle and complete the depolarisation.

This situation can  occur in

  • Dual AV nodal pathway*
  • Triple nodal pathway**
  • Multiple AV accessory pathways (All contributing  AV conduction )

* Exact incidence in general population is not known ,but it could be higher than what  we believe !

** Very rare

 Some what  related  phenomenon , never the less , it   mimics 1:2 or 1 : 3 AV  conduction

  • AV nodal echo beats
  • Non sustained AVNRTs

How is simultaneous conduction possible in dual AV nodal  physiology  ?  Will ( it not  ! ) the first impulse make the ventricle refractory to the following impulse ?

Under normal physiological conditions simultaneous conduction*  is not possible .It happen if  . . .

  • The first impulse goes relatively fast  and activate the ventricles .
  •  The second component of the first impulse, ie  through  the slow path conduction   is sufficiently  slow ,  it  reaches the ventricle and  able to depolarize it , well after  the  first beat’s  refractory period .
  • A Further requirement is , the initial  fast response fails to block the incoming slow  response  by a retrograde   slow path block .

* It need to be further clarified , even in physiology ,  simultaneous conduction is possible , but it is  often incomplete . At best it can result in ventricular fusion beat as in pre -excitation beat or it can be a concealed one travelling halfway through the AV node or the bundle.

Why recognising this 1:2 conduction  is important ?

  • It is traditional  to  think  , an unexpected beat  occurring prematurely  in a given strip of ECG is always thought to be an ectopic beat .This is not the case. An  unexpected premature narrow QRS  complex  with out a  p wave , should  make us suspect   dual AV nodal conduction .
  • If  this  dual AV nodal  pathway  is intermittently  conducting or conducting with  varying velocities ,  it becomes  an     irregular narrow QRS  rhythm  .This  can ,  very well  be confused with  atrial fibrillation.
  • If  one of the paths in the dual AV pathway  is conducted aberrantly   it  mimics a  VPD.

Final message

1:2  AV conduction may not be rare . Cardiac physicians are encouraged to look  for this phenomenon whenever they encounter an abnormal  early  narrow  QRS  beat without preceding P waves. Apart from academic curiosity , it can  solve many mysteries in CCUs and EP labs .

 Reference :


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