What Do You Think 29

Recently, I reported two Holter monitor tracings that stated, “Mobitz I and II AV block”.

Young patient. Nocturnal.

Middle aged. Nocturnal

Can both Mobitz I and II occur in the same patient?

What do you think?

The reporting of MobitzI and II in the same patient is not uncommon.

To answer our question, we need to review the history and basic electrophysiology of AV blocks.

Karel Wenckebach in 1898, saw a patient in her early 40’s with an irregular pulse and worked out the relationship between atrial and ventricular contractions, now referred to as Wenckebach AV block. A few years later in1906, Hay described type II AV block. Both reports described the conduction abnormalities before the use of the ECG and today are referred to as second degree AV block. In 1924, Mobitz using the ECG, classified the two abnormalities as Mobitz I and Mobitz II block and since then there has been utter confusion.                                                                                                  

Let us try and make sense of it.

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To summarize the ECG:

Wenckebach / Mobitz type I AV block

• Sequences of progressive lengthening of the PR interval terminating with a dropped beat.‍
• Predominately in the AV node

Mobitz type II

• PR interval remains ”constant”
• Dropped beat‍
• Distal conducting system

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The two ECG abnormalities usually occur in different parts of the conduction system and thus if they occur together, it is fortuitous.

Some electrophysiology facts:

Wenckebach AV block

• Very common; 4 to 6% of Holter monitor reports. Unusual with 12 lead ECG.
• Usually innocent. Frequent - young at night/vagal mediated. Less frequent in older patient/daytime and usually associated with 1^st degree AV block.
• Usually narrow QRS. If associated with wideQRS, then 75% are still in the AV node. Rest are intra-hisian or contralateral bundle branch.
• Variable block, but frequently sequences of4:3, 3:2 and 2:1. Occasionally very short runs of a higher block.
• AV nodal - AH prolongation and no AH with the dropped beat.
• Reversed with exercise and atropine.
• Occasional intra-hisian with split His potentials and no second potential with dropped beat. Worse with atropine and better with carotid sinus massage.

Mobitz type II AV block

• Uncommon ECG finding.
• 2:1 block or higher.‍
• Distal conducting system;20-35% bundle of His and remainder in the distal conducting system.
• Bundle of His - QRS narrow, but remainder wideQRS.
• Exercise, atropine worse.
• Associated with syncope, complete heart blockand ventricular standstill.

The clinical course and prognosis are, therefore, very different and thus misreporting the ECG can have serious consequences.

Why the confusion?

Wenckebach AV block although usually easy to report, has many atypical appearances, which may result in misdiagnosis.                                                                    

The three absolute foot prints that define Wenckebach AV sequences are:

Now let us return to the first of the ECGs.

There is 2:1 AV block (top and middle tracings). Because PR intervals only occur with alternate P waves, there is no opportunity for the PR intervals to lengthen and thus they remain constant (red highlight). However, with other sequences the PR intervals lengthen (yellow highlight) and the absolute criteria for Wenckebach AV block are fulfilled (blue highlight). Clearly this is Wenckebach or Mobitz type I AV block and there is no evidence of Mobitz type II AV block. The the rhythm is essentially innocent. The documentation of Mobitz type II block would have carried a much more sinister prognosis.

The second ECG has more subtle, but nevertheless diagnostic features of Wenckebach AV block.

Some of the PR intervals remain fixed (red highlight) but the absolute criteria for Wenckebach AV block have been fulfilled (blue highlight). This mixture of responses is frequently seen with Wenckebach AV block. Another feature of a typical Wenckebach AV block is sinus slowing during the sequence. This is vagal hypertonia. The sinus slowing is as a result of right vagal stimulation and the AV block, the left vagus (yellow highlight).

Here is another example of nocturnal vagal hypertonia with sinus slowing (blue arrows) and Wenckebach AV block (yellow highlight) with fixed PR intervals:

We now know how to recognise Wenckebach AV block.

How then do we recognise Mobitz II AV block?

Once again it is 2:1 AV block, but in the presence of distal conducting tissue disease.

2:1 AV block with left posterior fascicular block and right bundle branch block.

Remember AV nodal block can also be associated with distal conducting tissue disease and thus other features are important:

• No evidence of absolute Wenckebach AV block
• Clinical features such as syncope
• Presence of higher levels of AV block including ventricular standstill

2:1 AV block with bundle branch block progressing to ventricular standstill.

In my discussion, I have preferred the terms:

• Wenckebach second degree AV block or Wenckebach AV block (It is important to include “AV” as Wenckebach block occurs elsewhere).
• 2:1 AV block
• Mobitz type II second degree AV block or Mobitz type II block

Although widely used in the literature, I avoid the term “Mobitz type I AV block”. This is because in my reading and hearing people talk about Mobitz block, there is difficulty analysing in my mind which Mobitz they are talking about. The term Wenckebach is easier to envisage and hence less confusion. Also, use of 2:1 AV block correctly states what you see. Frequently we don’t know if it is types I or II.

Finally, can both forms of block occur in the same patient. Although, I am sure that it has been reported in the literature, I strongly believe that most examples are really Wenckebach AV block.

Look at this example:

The top tracing is clearly Wenckebach AV block (red highlight). The bottom tracing could be called Mobitz II AV block, but careful inspection identifies vagal hypertonia with sinus bradycardia, AV block and junctional escape and although the absolute features of Wenckebach AV block are not present, it is still identifiable as proximal block.

Look carefully at the tracings with AV block as misdiagnosis can lead to an unfortunate conclusion.