Overview
- Pattern of AV block in which there are constant PR intervals with intermittent dropped beats
- Typically due to failure of at the level of the His-Purkinje system (i.e. below the AV node), which is incapable of significant variations in conduction time
- Often due to structural heart damage to the conducting system (e.g. infarction, necrosis, fibrosis); on the other hand, Mobitz I is typically from functional suppression of AV conduction (e.g. medications, reversible ischemia)
- Not uncommon for patients to have a pre-existing left bundle branch or bifascicular block with the 2nd degree AV block Mobitz II as a result of intermittent failure of the remaining fascicle to conduct
- In most cases (~75%), the conduction block is distal to the His bundle, resulting in wide QRS complexes; otherwise, the conduction block is within the His bundle, resulting in narrow QRS complexes
- Secondary AV block Mobitz II is considered an “all-or-nothing” phenomenon in which the cells of the His-Purkinje system suddenly and unexpectedly fail to conduct a supraventricular impulse; in comparison, Mobitz I is a result of progressive fatigue and consequent failure of the AV nodal cells to conduct a supraventricular impulse
ECG Features
- Intermittent non-conducted P waves with constant PR intervals
- No progressive prolongation of the PR interval as seen with second-degree AV block Mobitz I (Wenckebach)
- P waves “march through” at a constant rate
- Constant PR interval in the conducted beats
- RR interval surrounding the dropped beat is an exact multiple of the preceding RR interval
- There may be a fixed relationship between the P waves and QRS complexes (e.g. 2:1 block, 3:2 block, 3:1 block) or no pattern to the conduction blockade
Causes
- Cardiac: anterior MI (due to septal infarction with bundle branch necrosis), idiopathic fibrosis of the conducting system (Lenegre’s or Lev’s disease), cardiac surgery (e.g. surgery near the septum, mitral valve repair), infiltrative myocardial disease (e.g. amyloidosis, haemochromatosis, sarcoidosis), inflammatory conditions (e.g. rheumatic fever, myocarditis, Lyme disease), autoimmune conditions (e.g. lupus, systemic sclerosis)
- Other: medications (e.g. beta-blockers, calcium channel blockers, digoxin, amiodarone), electrolyte disturbance (e.g. hyperkalaemia)
Clinical Significance
- Greater risk of hemodynamic instability, severe bradycardia, and progress to 3rd-degree (complete) AV block compared to Mobitz I
- Hemodynamic instability can occur suddenly and be unexpected, leading to syncope (e.g. Stokes-Adams attacks) or even sudden cardiac death
- ~35% annual risk of asystole if left untreated
- Patients require admission with cardiac monitoring along with temporary and eventual permanent pacemaker placement
ECG Example
Patient: 74-year-old male with coronary artery disease and recent surgical mitral valve repair present with chest pain
Interpretation: Normal sinus rhythm with second-degree AV block (Mobitz II), left atrial enlargement, complete right bundle branch block, acute anteroseptal infarct
– Ventricular rate: 62 BPM
– PR interval: 168 ms
– QRS duration: 128 ms
– QT/QTc interval: 340/345 ms
– P-R-T axes: +57 -29 +88
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