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How will you differentiate a wenkebach’s Mobitz type 1 AV block from type 2 second degree AV block if the conduction ratio is 2:

Posted Nov 11 2009 10:01pm

It is the classical description of wenkebach AV block type 1 there is progressive prolongation of PR interval followed by a blocked p wave and hence a dropped qrs complex. The usual conduction ratio for wenkebach AV block is 3:2 or 4:3 .

It is well known wenkebach AV block is usually a benign form of AV conduction defect and it recovers spontaneously without any pacemaker support.The block is at the level of AV node and since the his purkinje conduction is intact and the prognosis is good. In type 2 AV block the disease is often(Not exclusively ) located in the infra hisian area .This makes this type of block very unstable and these patients have a high risk for going in for complete heart block and often require pacemaker implantation. The reason for the poor outcome in type 2 AV block is now more attributable to the more extensive myocardial damage these patients suffer than the location of the block itself .

So it is important for the physician to differentiate the two entities .

It is a simple task in most situations , but when the condition ratio is 2:1 one can imagine the difficulties as none of the classical criteria of wenkebach’s AV block are applicable .When alternate qrs complex is missing there is no question of progressive PR prolongation .

How common is 2:1 AV wenkebach ?

It is considered rare , but may not be recognised in surface ECG so real prevalence is under reported.

What are points to differentiate the two ?

  • The qrs width  : A normal qrs width suggests wenkebach .A widened qrs indicate the block is infra hisian.
  • The conducted basic PR interval  is usually normal in ttype 2 AV block. In wenkebach the PR interval is usually prolonged.(Not always though! )
  • Response to atropine *: Wenkebach tend to accelerate while type 2 AVblock tend to worsen.
  • Response to excercise:Wenkebach conduction ratio improves , type 2 does  not, some times worsens
  • Response to carotid massage: Wenkebach AV block worsens ,Non Wenkebach improves

*The principle behind the varying response is due to the fact that AV node is under the influence of vagal fibres than the his purkinje system.

**It should be noted the atropine effect on AV conduction  is a complex one .Atropine by it’s direct vagolytic action improves AV conduction , while it’s effect on sinus node accelerates the heart rate and make the AV node more prone for physiological AV delay (Decremental conduction: Increased refractionary period at fast hear rates) .This effect is in exact opposite of it,s direct action on AV node.So the net effect will be the balance between these two.Hence atropine effect on heart can be quite variable in both physiological pathological situations.

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