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Conduction disorders. Indications for pacemaker implantation

1. ATRIOVENTRICULAR BLOCK

First degree AV block

  • Simple prolongation of PR interval to>0.22S
  • Every atrial depolarisation is followed by conduction to the ventricles but with delay

Second degree AV block – see pic

  • Occurs when some P waves conduct and others don’t

Mobitz I block – due to block in AVN (atrioventricular block)

  • AKA Wenckeback block
  • Progressive PR interval prolongation until a P wave fails to conduct
  • The PR interval before the blocked P wave is much longer than the PR interval after it

Mobitz II block – due to infranodal block e.g. in His bundle

  • Occurs when a dropped QRS complex is not preceded by progressive PR interval prolongation
  • QRS is normally wide >0.12s

2:1 or 3:1 block

  • Occurs when every 2nd or 3rd P wave conducts to the ventricles

Management

  • Mobitz I – usually only requires monitoring
  • Mobitz II – higher risk of progression to complete heart block
    • Pacemaker indicated

Third degree (complete) AV block

  • Occurs when all atrial activity fails to conduct to the ventricles
  • Spontaneous escape rhythms maintain life
  • Etiology –

Narrow complex escape rhythm – QRS <0.12s

  • Imply that it originates in His bundle – so region of block is in AVN
  • Escape rhythm is 50-60bpm – reliable
  • IV atropine – for recent onset narrow complex AV block (NCAVB)
  • Permanent pacing – for chronic NCAVB

Broad complex escape rhythm – QRS >0.12s

  • Implies that escape rhythm originates below His bundle
    • So block lies in His-Purkinje system
  • Escape rhythm is slow (15-40bpm) – unreliable
  • CF – Stokes-Adams attacks (dizziness + blackouts)
  • Permanent pacing
  • ICD

 

 

2. BUNDLE BRANCH BLOCK (BBB) https://upload.wikimedia.org/wikipedia/commons/3/32/Left_and_right_bundle_branch_block.png

  • His bundle gives rise to right and left bundle branches
  • Left bundle subdivides into anterior and posterior divisions

Etiology

Bundle branch conduction delay

  •  Produces slight widening of QRS complex – incomplete BBB

Complete BBB

  • Associated with a wider QRS >0.12s
  • Shape of QRS depends on whether the right or left bundle is blocked

Right BBB– best seen in V1 (rSR)

  • Produces late activation of the RV
  • I + V6 – broad S wave
  • V1 – tall late R wave, seen as rSR
    • Second R wave here is due to the late depolarisation of the RV compared to the LV

Left BBB – best seen in V6 (M shape due to broad QRS with notched top)

  • I + V6 – tall late R wave
  • V1 – broad S wave
  • In LBBB septum becomes depolarised from right to left – causes abnormal Q wave in V1 and R wave in V6

Hemiblock

  • Delay/block in the divisions of the LBB
    • Produces a swing in the direction of depolarisation (the axis)
  • Left anterior hemiblock
    • means the LV is activated from inferior to superior
    • produces LAD (left axis deviation)
  • Left posterior hemiblock
    • Produces RAD (right axis deviation)

Bifasciular block

  • Combination of a block of any 2 of the following
    • RBB, Left anterior division, Left posterior division

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