Second-Degree AV Block - ECG

Second-degree AV block is usually accompanied with bradycardia (heart rate below 60/min). There exists two main types of the second-degree AV block – Mobitz I (Wenckebach) second-degree AV block and Mobitz II second-degree AV block. The Mobitz I (Wenckebach) is much less severe finding than the Mobitz II.

Mobitz I (Wenckebach) means a gradual lengthening of the PR interval that in the end results in a complete block of atrioventricular transmission with resulting absence of a single QRS complex. There is a P wave not followed by a QRS complex. After that, the above-mentioned process repeats.


Second-Degree AV Block - ECG
Mobitz I (Wenckebach) – The best finding is in lead II (long curve at the bottom). There are four regular QRS complexes. The P wave moves gradually away of the QRS complexes and then suddenly we see an alone P wave that is not followed by the QRS. After this loss the whole process repeats.

Mobitz II means repeated losses of QRS complexes without changes of the PR interval. The major danger of this arrhythmia lies in risk of progression into the third-degree AV block.


Second-Degree AV Block - ECGMobitz II – The finding is again best visible in the long recorded lead (this time V1 lead). We see two P waves between every two QRS complexes. One P wave normally precedes the QRS complex and the second is alone – its QRS complex is missing.

Conclusion: From the clinical point of view, the Mobitz I is a less important finding ad it is often just monitored. It is usually asymptomatic but sometimes it may be accompanied by collapses with short periods of unconsciousness.  The second-degree AV block subtype Mobitz II is dangerous by its possible progression into the third-degree AV block and its solution is the permanent pacing.


Jiri Stefanek, MD  Author of texts: Jiri Stefanek, MD
 Sources: basic text sources