Second degree atrioventricular block - ECG / Mobitz type I, type II

It consists of two types of blocks:
1) Mobitz type I block,
2) Mobitz type II block.

Mobitz type I

It is also called as Wenckebach pattern. In this condition, each stimulus from the atria appears to have more difficult time to pass through the AV junction. Finally one stimulus is not conducted through the defective AV node.

A characteristic ECG shows progressive lengthening of the PR interval until a beat is dropped. i.e. the P wave is not followed by a QRS complex. It is also important to note that the PR interval after the dropped beat is always shorter than that before the non conducted P wave. Also the R-R interval encompassing the non conducted P wave is less than twice the preceding R-R interval.

This ECG is also shows a Wenckebach pattern and we can clearly see at first glance that the narrow QRS complexes appear to be clustered and separated by a pause. This is called as group beating. If ever you find such a pattern, look out for the progressively increasing PR interval to make your diagnosis.
It is usually associated with inferior wall MI and does not progress to complete heart block.

Mobitz type II
It is a more serious condition and usually seen in cases of anterior wall MI. There is high risk of progression to complete heart block and there is usually indication to insert permanent pacemaker.
The ECG has the following characteristics:
1) the P-P intervals are constant
2) constant P-R intervals prior to a non conducted P wave.
3) intermittent failure of conduction of a P wave.

Above is a case of anterior wall MI complicated by a second degree Mobitz type II block. The P-P and P-R intervals are constant and the second P wave on the ECG is not conducted.

A 2:1 block suggests that for every 2 P waves that are conducted 1 P wave is not. The same holds true for 3:1 that means that we have 3 normally conducted P waves followed by QRS complexes while 1 P wave is not conducted.

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