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Left bundle branch block : (iFAQs)infrequently asked questions

Posted Aug 23 2010 11:02am

Why the qrs complex becomes wide and tall in LBBB ?

The qrs  complex is  wide , due to delayed conduction over non specialized fibres .The qrs  becomes are  tall due to temporal dissociation of RV  and  LV forces ,  which  leaves  the LV forces  unopposed , thus  a tall qrs  is inscribed  , without the neutralizing effect of RV forces.

Is muscle to muscle  conduction a hall mark of LBBB ?

No , it is not . Even though the left bundle is blocked , much of the conduction tend to occur in

specialized  conduction  system  . It depends upon the level of block of LBBB.

What is the mechanism and clinical significance of left axis deviation in isolated LBBB?

The mean qrs axis is surprisingly  not  altered greatly ,  in LBBB . If there is a significant left ward shift  it may imply associated organic LV pathology or involve ment of predominately  left anterior fascicle

What is  the impact of IVS contraction and timing in LBBB ?

In isolated LBBB, it is expected an abnormal septal motion due to altered sequence of septal activation. This results in an abnormal appearance of  septal motion in Mode (Septal beak immediately following qrs complex) .In fact , this sharp downward movement indicate good LV  function  .Absence  of which  is a  good clue  for a pathologcal LBBB due to structural heart disease

Why does the abnormal  septal motion in LBBB  ,  do not  desynchronize  the normal LV ?

CRT is the much fancied  treatment in patients with LBBB and cardiac failure. In normal ventricles LBBB do not destabilize LV function in spite of septal /free wall desynchronisation  .This is still a mystery how IVS is cope up with the totally unexpected  insult of asking to work in head over heal situation !In spite of  this the ventricle gets used to the altered conduction pattern and the contractile pattern.(Nature’s  at it’s best !)

What are the mechanical disadvantage of LBBB

  • Septal contraction is  ill timed
  • Mitral  reguritation

Most isolated chronic LBBBs  do not  confer  any hemodynamic  disadvantage  to LV  – why ?

LBBBs are dangerous looking ECG , but in most patients it is benign , in the absence  0f structural heart disease like valvular , myocardial or ischemic  disease.

Can there be a small r wave in V1 and V2 in LBBB ?

yes . Presence of small r in v1 to v3 dose not rule out LBBB. This depends upon hte orientation of septum to the recording electrode.

How do we explain concordant  pattern  of QRS  v1 to v6  in LBBB ?

We expect the qrs to  transit from QS  complex  to RS ,  at-least by lead  v5/v6 .Some times even V6  shows a RS complex.This is usually due to faulty lead  position or a grossly enlarged  LV,  ie  if we  record V 7 or V8 we will be able to pick up the qs complex.

What will be the morphology of a VPD that is arising  from LV in the presence of  LBBB ?

A premature beat arising  from a  ventricle which is having  a bundle block  is  sort of  electrical blessing !The VPD often bye  passes the block and makes  the conduction near normal  and a normal  qrs may be  recorded. So , when a patient with LBBB suddenly develops a normal qrs beat or  normal qrs tachycardia  one  should consider a VT arising from the  Left ventricle .

And a studious electro physiology fellow  should  be able to answer the following !

What will be the morphology of  VPD if it arises from RV and septum in the presence of  LBBB ?

Kindwall has tried answer  this question

What is the effect of  LBBB on S1 and  S 2 ?

The classical  description in LBBB   is

  • Paradoxical split of S2
  • Wide split of S1

You are supposed to hear  4 components in complete LBBB  !  In reality this does not happen . At best you can hear the reversed  split of  S2 with difficulty .

One  more reason  for the  non manifestation of these splits is  confounding factors like LV dysfunction , MR , PR interval etc .(Each one tend to pull or push  S1 and S 2 in different directions )

Do  patients with LBBB  , are at increased risk for developing  complete heart block   when

beta blockers , calcium blockers etc  are administered ?

Common sense would say yes. Scientific  sense has  no answer .

We know, ventricles are innervated by two bundles  .When only one bundle  is  functional, it means the ventricles  are experiencing  50 % power shutdown .   In CAD  , single vessel blood supply due to a CTO  is considered  dangerous but in electrical  flow it is not so !  In spite of the fact  that  ventricle has numerous  cell cell electromotive conduction   it is  always better to exercise caution  when administering  beta blockers, calcium blockers and digoxin in patients with LBBB . If it is a must periodic  monitoring is advised .(HV interval in isolated LBBB is slightly prolonged ) Never administer beat blocker in a patient with recent onset LBBB and ACS

Also read the related article  in this blog  Incomplete LBBB

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