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