Second-degree atrioventricular (AV) block - Mobitz Type II

Introduction

  • Mobitz type II AV block is a form of 2nd degree AV block in which there is intermittent non-conducted P waves without progressive prolongation of the PR interval

ECG features

  • The PR interval in the conducted beats is constant
  • Block of one or more P waves that fail to conduct to the ventricles
  • The P waves ‘march through’ at a constant rate 
  • In Mobitz type II the block is generally located below the AV junction in the His bundle (intra-His block) or lower in the His-Purkinje system (infra-His block). Because of this, the QRS complexes are typically wide or demonstrate bundle branch block morphology
  • Mobitz type I and Mobitz type II second degree AV blocks cannot be differentiated from the ECG when a 2:1 AV block is present. In this situation, every other P wave is non-conducted and there is no opportunity to observe for the constant PR interval that is characteristic of Mobitz type II second degree AV block

ECG 1  Second degree AV block – Mobitz II type

  • the PR intervals are constant, duration 184 ms

Causes

  • Mobitz type II second degree AV block is rarely seen in patients without underlying heart disease
  • It is associated with a high rate (>50%) of progression to complete heart block (third degree AV block), which may be sudden and unpredictable in onset
  • The block can be exacerbated by exercise and by drugs that increase AV nodal conduction and the sinus rate – like atropine
  • potentially reversible causes:
    - Myocardial infarction with ischemia of the conduction system (usually left anterior descending coronary artery)
    - Drugs that slow or block conduction in the His-Purkinje system (propafenone, flecainide, procainamide, quinidine, disopyramide)
  • other pathologic causes include:
    - Idiopathic intrinsic degenerative diseases (Lenegre-Lev disease)
    - Myocarditis – Lyme disease
    - Infiltrative disease (sarcoidosis, amyloidosis)
    - Hyperkalemia
    - Post-catheter ablation – when in proximity to AV node
    - Following transcatheter placement of valves

Management

  • In patients with acquired second-degree Mobitz type II atrioventricular block not caused by reversible or physiologic causes, permanent pacing is recommended regardless of symptoms 
  • It is important to exclude vagotonic block and Mobitz type I AV block, especially if the QRS complexes are narrow


References

  1. Brian Olshansky, Mina K. Chung, Steven M. Pogwizd, Nora Goldschlager, Chapter 2 - Bradyarrhythmias—Conduction System Abnormalities, Editor(s): Brian Olshansky, Mina K. Chung, Steven M. Pogwizd, Nora Goldschlager, Arrhythmia Essentials (Second Edition), Elsevier, 2017, Pages 28-86, ISBN 9780323399685, https://doi.org/10.1016/B978-0-323-39968-5.00002-0.
  2. CAMM, A. J., LÜSCHER, T. F., & SERRUYS, P. W. (2009). The ESC textbook of cardiovascular medicine. Oxford, Oxford University Press
  3. STRAUSS, David G., et al. Marriott's Practical Electrocardiography. Wolters Kluwer, 2021