Case B6. Second Degree AV Block, 2 :1 Block. St Emlyn’s ECG Library

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The ECG was recorded from a patient who was admitted for insertion of permanent pacing system.


Rhythm: There are P waves present at a rate of approximately 64 per minute; QRS complexes are present at a rate of 32 per minute. All P waves in any one lead are of uniform morphology as are the QRS complexes. Alternate P waves are conducted to the ventricles and are followed by a QRS complex. Alternating P waves are not conducted and a dropped beat results. There is a constant PR interval in the conducted beats. This variety of second degree AV block is known as 2 :1 AV (heart) block.

Morphology: The QRS complex width is greater than .10 seconds and is therefore prolonged. There are no septal Q waves in V5 or V6 and the ECG shows left bundle branch block.


There are dropped beats so this is clearly second degree AV block. In this case alternate beats are conducted to the ventricles so 2 : I block is present. It is not possible to say with certainty from the surface ECG whether the block is of Mobitz type I or type II. In Mobitz type I (Wenckebach) block the conduction delay is usually situated proximally within or near tha AV node. In Mobitz type II block the delay is usually situated more distally in tissues below the AV node. The ECG here shows the appearance of left bundle branch block in conducted beats; it is quite possible that the block occurs because of intermittent failure of conduction in the right bundle and the block is of the type II variety. However it could also arise because of the failure of the AV node to conduct alternate beats in the presence of established LBBB and therefore be of the type I variety- it is not possible to make the distinction from this ECG. Further clues can be obtained if extended observation shows the block to change from 2 : 1 to 3 : 2 or 4 : 3 etc. Under these circumstances the characteristic prolongation of the PR interval that occurs with type I block may become evident while if the PR interval remains constant the block is of the type II variety.

Cite this article as: Simon Carley, "Case B6. Second Degree AV Block, 2 :1 Block. St Emlyn’s ECG Library," in St.Emlyn's, October 29, 2018,

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