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

Definition of Mobitz II block (Hay Block)

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)

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 (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

Related Topics

• AV block: 1st degree
• AV block: 2nd degree, Mobitz II
• AV block: 2nd degree, “fixed ratio blocks” (2:1, 3:1)
• AV block: 2nd degree, “high grade AV block”
• AV block: 3rd degree (complete heart block)
• Eponymythology: History of Second-degree AV block. 2020
• Eponym: Karel Frederik Wenckebach (1864 -1940). LITFL 2020
• Eponym: Woldemar Mobitz (1889 – 1951). LITFL 2020
• Eponym: John Hay (1873 – 1959). LITFL 2020

Advanced Reading

Online

• Wiesbauer F, Kühn P. ECG Yellow Belt online course: Become an ECG expert. Medmastery
• Wiesbauer F, Kühn P. ECG Blue Belt online course: Learn to diagnose any rhythm problem. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.

Textbooks

• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Hampton J. The ECG Made Practical 7e, 2019
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography 1e, 2009
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• Mattu A, Brady W. ECG’s for the Emergency Physician Part I 1e, 2003 and Part II
• Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004

LITFL Further Reading

• ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
• ECG A to Z by diagnosis – ECG interpretation in clinical context
• ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
• 100 ECG Quiz – Self-assessment tool for examination practice
• ECG Reference SITES and BOOKS – the best of the rest