AV Block: 2nd degree, Mobitz II (Hay block)

Definition of Mobitz II block (Hay Block)

A form of 2nd degree AV block in which there is intermittent non-conducted P waves without progressive prolongation of the PR interval

ECG Mobitz II Hay AV Block 1
Arrows indicate “dropped” QRS complexes (i.e. non-conducted P waves)
Other features:
  • 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, triple for two dropped beats, etc)

ECG Mobitz II Hay AV Block 2
Mobitz type II rhythm strip demonstrating 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 (Lenègre-Lev 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



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Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

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