AV Block: 2nd degree, Mobitz II

Mobitz 2

Definition

  • Intermittent non-conducted P waves without progressive prolongation of the PR interval (compare this to Mobitz I).
  • The PR interval in the conducted beats remains constant.
  • The P waves ‘march through’ at a constant rate.
  • The RR interval surrounding the dropped beat(s) is an exact multiple of the preceding RR interval (e.g. double the preceding RR interval for a single dropped beat, treble for two dropped beats, etc).

Example of Mobitz II

mobitz 2

  • Arrows indicate “dropped” QRS complexes (i.e. non-conducted P waves)

Mechanism

  • Mobitz II is usually due to failure of conduction at the level of the His-Purkinje system (i.e. below the AV node).
  • While Mobitz I is usually due to a functional suppression of AV conduction (e.g. due to drugs, reversible ischaemia), Mobitz II is more likely to be due to structural damage to the conducting system (e.g. infarction, fibrosis, necrosis).
  • Patients typically have a pre-existing LBBB or bifascicular block, and the 2nd degree AV block is produced by intermittent failure of the remaining fascicle (“bilateral bundle-branch block”).
  • In around 75% of cases, the conduction block is located distal to the Bundle of His, producing broad QRS complexes.
  • In the remaining 25% of cases, the conduction block is located within the His Bundle itself, producing narrow QRS complexes.
  • Unlike Mobitz I, which is produced by progressive fatigue of the AV nodal cells, Mobitz II is an “all or nothing” phenomenon whereby the His-Purkinje cells suddenly and unexpectedly fail to conduct a supraventricular impulse.
  • There may be no pattern to the conduction blockade, or alternatively there may be a fixed relationship between the P waves and QRS complexes, e.g. 2:1 block, 3:1 block. 

Causes of Mobitz II

  • Anterior MI (due to septal infarction with necrosis of the bundle branches).
  • Idiopathic fibrosis of the conducting system (Lenegre’s or Lev’s disease).
  • Cardiac surgery (especially surgery occurring close to the septum, e.g. mitral valve repair)
  • Inflammatory conditions (rheumatic fever, myocarditis, Lyme disease).
  • Autoimmune (SLE, systemic sclerosis).
  • Infiltrative myocardial disease (amyloidosis, haemochromatosis, sarcoidosis).
  • Hyperkalaemia.
  • Drugs: beta-blockers, calcium channel blockers, digoxin, amiodarone.

Clinical Significance

  • Mobitz II is much more likely than Mobitz I to be associated with haemodynamic compromise, severe bradycardia and progression to 3rd degree heart block.
  • Onset of haemodynamic instability may be sudden and unexpected, causing syncope (Stokes-Adams attacks) or sudden cardiac death.
  • The risk of asystole is around 35% per year.
  • Mobitz II mandates immediate admission for cardiac monitoring, backup temporary pacing and ultimately insertion of a permanent pacemaker.

Related Topics

Further Reading

Author Credits

References

  • Hampton, JR. The ECG in Practice (5th edition), Churchill Livingstone 2008.
  • Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.

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