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Posts Tagged ‘accessory pathway’

WPW syndrome is the prototype of cardiac pre- excitation . The accessory  AV pathway short circuits the ventricle .Since  there are two options  available   for the  incoming  atrial  impulse  to reach ventricle ,  often  times  the qrs is contributed by both .Hence a  fusion  occurs  within qrs complex and stretches it wide   ,  it also  generates a delta wave and short PR interval .

The complexities of  conduction   properties and refractionaries of AV node and  accessory  pathways determine the degree of pre- excitation. When an optimally timed  APD  gate crashes  into the  accessary pathway it gets blocked ,  only to recover little late ,  unfortunately  invites AV nodal impulse  from below  . This facilitates a  re- entry circuit from ventricle to atria and result in classical AV reciprocating tachycardia .

Antegrade conduction through AV node is  physiological and  benign as it inherently checks the heart  rate . Antegrade conduction  occurring through the  accessory pathway  (which  constitutes the pathological  component  ), is   potentially  dangerous  as it lacks the  electrical breaks (Technically called decremental conduction )

What  is the  specific  ECG evidence for  antegrade conduction thorough accessory pathway  in ECG ?

Delta  waves

So,  what does it mean if there is absent delta waves  in WPW syndrome ?

It can mean three things

  1. Concealed pathway
  2. Manifest pathway , but intermittently  blocked pathway.
  3. It is not WPW syndrome at all .

We know concealed  pathways are  safe* as it allows only retrograde conduction. ( Safe  regarding   risk  of  sudden cardiac death ,  still unsafe for AVRT !)

Intermittent WPW

Intermittent pathways are equally  safe  as intermittent absence of  pre-excitation   indicate  the  presence   of naturally occurring     breaking system within accessory pathway . Are these  accessory pathways blessed with some AV nodal cells ?  May be !  . Histological studies do suggest that .This explains   intermittent missing of delta waves  which is  electro-physiologically a good sign

(We also know   there are exclusive slowly conducting accessory pathways like  Mahim and variants  )

If  one is lucky to observe this phenomenon in ECG  it can be termed as  a poor man’s  EP study  . ( Which requires specialized methods to document the refractory period of accessory pathway  to be   < 250 msec)

Techniques to  screen for or / unmask this concept.

Whenever  we  diagnose  WPW one has to look   ,  whether the patient  harbors  this phenomenon .

  • Holter monitoring has a useful role in this regard .
  • If there is nocturnal   disappearance of pre- excitation it would  suggest a safe  accessory pathway.
  • Similarly , if pre- excitation disappear during exercise  stress  testing it  would indicate a  type of intermittent WPW syndrome.

Final message

An astute cardiologist shall  look for this intermittent nature of delta waves  and  help avoid a costly and  potentially harmful EP study !

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