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The fractured rule of thumb in localising VPD by ECG !

Posted May 30 2013 12:00am

We are taught in  medical schools  early in our career ,   ventricular premature  depolarization (VPD ) takes  LBBB morphology if it arise from right ventricle , and  RBBB morphology if it arise from left ventricle .This is a rough rule of thumb.

Why this rule is  unreliable ?

VPDs have a focus of origin–a short circuitand an epicardial  breakthrough . All these together influence the morphology. Within  the left ventricle , a deep endocardial focus  can  behave  vastly different  from superficial epicardial focus  . The  course of VPD is influenced by the myocardial status ( scars etc ) . Further,  the electrical  properties of  interventricular septum is shared  by both ventricles .

  • Generally – LBBB morphology  has  more localizing value .
  • Most RV focus have LBBB morphology (but not vice versa!)
  • LV focus can either have LBBB or RBBB

What happens to  a VPD  arising from  interventricular septum ?

IVS is  not only shared by both ventricles , it does  not have  true  epicardial  surface  (Both side  bordered by endocardium ) In most septal VPDs , breakthrough occur on either side of the ventricle  . However , It  keeps trying  to break through  epicardial surface  !  .  Hence , septal VPD  is like cat on wall situation .So the morphology varies quiet frequently.Further , the VPD can capture  the specialised conduction tissue occurs  more commonly with septal VPDs. This can alter both the width and morphology of QRS.

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