Second-degree atrioventricular (AV) block - Mobitz Type I (Wenckebach phenomenon)

‍Introduction

• Mobitz type I AV block is one of the types of second-degree AV block and is also known as the Wenckebach phenomenon

• Second-degree AV block is characterized by periodic failure of conduction from atria to ventricles

• Mobitz type I / Wenckebach AV block typically has progressive prolongation of the PR interval until one P wave fails to be conducted and is not followed by a QRS complex

• Occurs in up to 6% of healthy individuals

ECG findings

• progressive prolongation of PR interval
  - shortest PR interval after dropped beat
  - longest PR interval before dropped beat

• constant P-P interval and changing R-R intervals with the cycle ending with a P wave not followed by a QRS complex
• the classic Wenckebach pattern occurs usually with ratios of 3:2, 4:3, or 5:4

ECG 1  Second degree AV block – Mobitz I type

• progressive prolongation of PR intervals with one non-conducted P wave
• constant P-P intervals

ECG 2  AV block type 2 Mobitz I

• progressive elongation of PR intervals 
• 2nd P wave is not followed by QRS complex – dropped beat
  

‍

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.

Causes

• Wenckebach AV block is usually caused by functional suppression of conduction in the AV node
• This type of AV block is more common in the older population, in which it may be an isolated finding
• Can also occur in individuals who have high vagal tone, such as younger persons or highly conditioned athletes at rest and is usually completely benign

Pathologic causes include:

• Myocardial infarction – anterior or inferior involving the conduction system
• Drugs slowing down AV node conduction – digoxin, beta blockers, calcium channel blockers, many antiarrhythmic agents
• Myocarditis – Lyme disease
• Hyperkalaemia
• Post-catheter ablation – when in proximity to AV node
• Following transcatheter placement of valves

Management

• Treatment for asymptomatic Mobitz type I is often not necessary, because progression to advanced or complete AV block or asystole is rare; In this case pacemaker implantation is not indicated
• All patients with Mobitz 1 block should be admitted and monitored
• Depending on the frequency of dropped ventricular beats, it may cause a symptomatic bradycardia with hypotension; this usually responds well to atropine, if not – pacing should be initiated for stabilization
• If the cause is found to be a medication directed at AV node, it should be reduced or discontinued.
• Permanent pacing is indicated if symptoms or hemodynamic compromise can be directly attributed to this rhythm

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
4. Mangi MA, Jones WM, Mansour MK, et al. Atrioventricular Block Second-Degree. [Updated 2020 Nov 20]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482359/