Introduction

• 2:1 second-degree AV block occurs when every other P wave is not conducted to the ventricles

• As there is always only one PR interval before a dropped beat, it is not possible to differentiate between Mobitz I and Mobitz II type of second-degree AV block

• 2:1 AV block frequently does not persist and when the AV conduction ratio changes, the other forms of AV block should then become apparent

ECG features

• every other P wave is not followed by a QRS complex
• QRS complexes may be narrow or wide depending on the site of the AV block

ECG 1  AV block 2:1 with wide QRS complexes

• every other P wave is not followed by a QRS complex – dropped beat
• ventricular rate 34 bpm, atrial rate 68 bpm
• QRS complexes are wide with a duration of 200 ms with LBBB morphology – infranodal block

ECG 2  AV block 2:1 with wide QRS complexes

• every other P wave is not followed by a QRS complex
• ventricular rate 42 bpm, atrial rate 84 bpm
• QRS duration 150 ms – probable infranodal block

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Causes

• Sustained 2:1 and 3:1 AV block with a wide QRS complex, block occurs in the His-Purkinje system in 80% of cases and 20% in the AV node

• If the QRS is narrow and normal appearing, the level of the block is most likely in the AVN (which is more benign)

• The site of the block can be demonstrated with change in the vagal tone by rhythm monitoring during exercise (standing, walking) or by atropine use
  - block within the AV node – decrease in vagal tone should improve conduction
  - block below the AV node – may worsen, resulting in slowing of ventricular rate

Management

• Atropine use is not recommended for suspected blocks in the His-Purkinje system, because it may worsen the block

• Depending on the level of block, approach to management follows recommendations and guidelines for Mobitz types I or II second-degree AVB

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. Atrioventricular Block With 2:1 Conduction—Discussion. JAMA Intern Med. 2013;173(5):336–337. doi:10.1001/jamainternmed.2013.3182b

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