Diagnosis, Wenckebach Squared?

aka ECG Exigency 016.2

Thanks to all who contributed to solving the puzzle of Diagnosis, Wenckebach? in ECG Exigency 016.1.

Let’s recap:

The following tracing can be found in our ECG library, allegedly as an example of Wenckebach AV block. However, as one of our readers, Jan Štros has pointed out, there is something not entirely right about this ECG tracing…

Can you spot the “deliberate” mistake?

Mobitz I


Q1. What features of Wenckebach AV block are present on this ECG?
Q2. What features of Wenckebach are notably ABSENT?
Q3. What possible explanations could exist to explain this tracing?

Here is my impression of the ECG (by no means the “correct” answers) is as follows…

1. The presence of a progressively prolonging PR interval that abruptly shortens (i.e. resets) after a pause in the rhythm is strongly suggestive of Wenckebach AV block.

Explanation: The underlying process is progressive fatigue of dysfunctional AV node cells. Commonly, this culminates in a non-conducted P wave, as the AV node cells fatigue to the point where they are unable to conduct a further impulse. However, anything that interrupts the rhythm (e.g. a sinus pause or ventricular ectopic beat) will give the AV node cells chance to “rest”, following which they will conduct normally.

2. Notably absent is the lack of a non-conducted P wave. While it is tempting to assume (as I did) that the non-conducted P wave must be hidden in the preceding T wave, there is no evidence of this – the P waves are too tall to be superimposed on the T waves without producing an obvious “bump”.

As discussed above, the presence of a non-conducted P wave is not required to make the diagnosis of Wenckebach AV block.

3. The frequently dropped P waves occurring in a regular pattern are suggestive of second degree SA block (type I). Features that are supportive of this: (1) Grouping of the QRS complexes, and (2) The duration of the each sinus pause is less than double the preceding PP interval

Explanation: There are two types of cells in the SA node – the inner core of P cells that produce the impulse, and the outer layer of T cells that transmit the impulse out into the atrium. Analogous to Wenckeback AV block, progressive fatigue of the T cells produces second-degree SA exit block (type I), also known as Wenckebach SA block. This is the most common type of SA block detectable on the surface ECG and is characterised by grouping / clustering of the QRS complexes culminating in a dropped P wave.

(See our ECG library page on SA block for an explanation of the different types http://lifeinthefastlane.com/ecg-library/sa-exit-block)

Possible causes of this ECG pattern would include conditions that suppress both SA and AV node function, e.g.

  • Sick sinus syndrome
  • Increased vagal tone
  • Inferior MI
  • Myocarditis
  • Drugs: digoxin, beta-blockers, verapamil

So, the final diagnosis:

Wenckebach AV block with Wenckebach SA block!


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  1. Sam says

    What does “grouping” of QRS-complexes mean? The R-R interval is steady at 13,5 squares so I really can’t see any grouping.