Posts Tagged ‘ventricular extra systole’

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.

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What will be the pulse rate in a patient who has ventricular bigeminy in ECG with a heart rate of 90 ( 45 sinus beats 45 VPDS/minute ) ?

A.Exactly Same as HR , ie 90/mt

 B.Exactly half of HR , ie  45/mt

C.Can be anything between 45 to 90/mt

D.Any of the above can be true

 The  answer is D . 

I have  noted  ,this simple question in cardiology resident examinations cause great anxiety among students .

Why is it difficult to arrive at an easy answer to this question ?

Traditionally , ventricular ectopic beat were also called extrasystole , implying every ectopic beat shall produce a peripheral pulse .Since ,  we learnt this is not true , we started refering them as VPDs.(Simple ventricular depolarisation which may or may not have a mechanical activity ) So , in a patient whose alternate beat is a VPD  , things become little complicated.

What determines a VPD to acquire  mechanical  energy  or simply  remain as an  electrical event ?

  •  Timing of the VPD* .
  • LV residual volume(LVEDV ) at the onset of  VPD
  • Force of contractility of LV( Of course ,  it is directly related to LVEDV)
  • Temporal relation to  aortic valve opening**

If  the VPD is too early or too late it can not have a mechanical activity . It should be optimally timed midway between two sinus beat to have a good mechnically active VPD. Some refer this as an interpolated VPD .Here, the VPD  becomes a  true extra systole for that individual. So , in patient with ventricualr bigeminy in ECG the pulse rate is usually half , can be same as HR when the coupling interval is optimal or it can be totally irregular as someof the  VPDS gain a mechanical activity and some do not (as often occurs multifocal VPDs. )

* Among the above  four factors timing of the VPDS is the most crucial as it can influence all the other three factors.

** Whatever be the timing or force of contraction aortic valve should be opened to generate a pulse wave. If for some reason this does not happen  there can be intermittent mechanial activity what  we refer to as pulse deficit .

Read a related phenomenon:  Ventricular  paired pacing

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