Posts Tagged ‘slow -slow 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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AV nodal reentrant tachycadia(AVNRT) is the commonest mechanism of SVT. It is divided into slow-fast, fast-slow, slow-slow , representing the two limbs of he circuit.

Slow -Slow circuit is  the rarest  type of AVNRT.  It should be appreciated  ,  the scientific validity of  slow-slow circuit is  applicable  only in relative terms . A virtually  similar antegrade and retrograde limbs with identical conduction velocity and refractory  properties  , can neither  initiate  nor  sustain an AVNRT.

Caveat in the definition of slow -slow AVNRT.

Even though ,  we call it   a  slow-slow  tachycardia , one of the limbs need to be faster than the other.  So , every slow -Slow AVNRT in reality will have  two types

  • Slow- Slow ( Still , faster than antegrade slow) mimic a slow-fast physiology
  • Slow( Faster than retograde slow )  -Slow closely mimic typical  fast slow .

Implication for electrophysiologists  and   points of contention for the ablationist !

  • In Slow -Slow AVNRT ablation we do not know exactly ,  which of the slow pathway is being ablated , unless we specifically  analyse  the post ablative  data.
  • Very often it is not done.Every one in the lab is happy , for breaking the tachycardia circuit. Only after the procedure is over , we may realise the tachycardia is not really killed as it finds an alternate highway to complete  the short circuiting of heart.
  • We need to  suspect this type of AVNRT   prior to the  procedure .Electrophysiologist  shall  spend little   more time and a wide area ablation done , in the vicinity  of coronary sinus ostium can be attempted. .

It is not a smart practice to advocate  wide area ablation as a routine protocol in all AVNRT

as it directly  increase the rate of complication >

Final message

A   hurriedly  done slow pathway ablation  which  may  temporarily terminate the AVNRT ,only to recur later as  the retrograde  slow pathway may again form  a substrate  .The area of slow conduction  acts as a turnaround gateway and capture  the  retrograde fast  pathway which  could be  available in plenty in the anterior aspects of AV node  .   (Note : The unablated  slow pathway  now  form the antegrade  circuit )

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