- 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
Arrows indicate “dropped” QRS complexes (i.e. non-conducted P waves)
- 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).
- Drugs: beta-blockers, calcium channel blockers, digoxin, amiodarone.
- 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.
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