Dr.S.Venkatesan MD

Failure of  enocardardial  cushions  to separate and reach the  predesignated destination  ie  right and left AV valve is the basic embryological feature in AV canal defect .This brings whole AV ring  down and stretches the distance between  the semi-lunar valve (especially aortic)  with that of LV , thus elongating  the LVOT into a classical  goose neck deformity.The defect  has a profound  impact on how the AV node and its branches penetrate the ill-formed AV junctional tissue and fan out into the ventricle. There are  four basic issues  that are responsible for the various conduction defects in AV canal defect.

1. Postero- inferior  displacement of the A-V node is the key abnormality .
2. Hence  AV node penetrates the ventricle at the level of crux which is abnormal .This results in short his bundle  (AV node short of compressed with His  early direct origin of the left bundle branching)
3.  Left bundle branching system by itself is also abnormal  with hypoplasia   left anterior bundle branches.
4.  Right bundle branch is relatively long and elongated

Physiological effects

1. Prolonged PR interval (50%)
2. QRS  axis shift can be extreme right or left , but superior direction is a rule .Typically its around -180 . Left axis deviation is distinct in downs syndrome (Counter-clock wise rotation q in lead 1 and  AVL ) .It should be learnt , the ECG features (due to  anatomical defects in AV conduction system  ) can be be  easily modified by the hemodynamic stress of  ventricles  due to associated conditions and classical pattern may non exist )
3. Surprisingly high grade AV blocks are rare (“viz a viz” LTGV )

Electro-physiology

Short HV interval is documented  in AV canal defects inspite of prolonged PR due to small his bundle length.

A large Inlet VSD , simply takes over the place meant for the conducting system and its pushed down and out

Reference

Robert Feldt from Mayo clinic did excellent work about this issue and published in Circulation, Volume XLII, September 1970