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 circuit——and 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.
Dr.S.Venkatesan MD 
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