Diagnosis, Wenckebach?

aka ECG Exigency 016.1

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?

I will open this one up to the floor. Post your answers below people…

For those of you who are struggling, learn more about Wenckebach here


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  1. says

    1. Lengthening PR (progressive are characteristic of Wenckeback

    2. P-P interval is also lengthened / reset after third QRS (atypical for 2nd degree AV block)

    3. Is this a Type II SA block with Wenckebach?

  2. Sally says

    1. Lengthening PR segment is present.
    2. P wave without a QRS complex is absent. P waves occur at varied distance from T waves.
    3. P wave could be hidden in the T wave or could be complete heart block

  3. Sammy says

    Dropped P’s not QRS’s, however progressively lengthening PQ-intervals.

    Concurrent AV-Wenkebach and SA-Hay? Hypervagotonia?

    • Andrew says

      Wouldn’t a sinus exit block have a gap in P waves that is a multiple of the R-R interval? As ours doesn’t appear to be I think it’s a sinus pause/arrest. The constant R-R interval before the missing P wave rules out Wenckeback SA block, too, I think.

      - Wenckeback AV block with 3:1 sinus pause.

  4. Staffan says

    1) Yes there is gradual lengthening of the PR-interval.
    2) At a first look there doesn’t seem to be any P-waves that doesn’t get conducted through to the ventricles
    3) At a closer look it seems the PR-interval of complex 4 and 7 is too short. It is under 0,12s and thus the P and QRS cannot be connected. For diagnosis I would go for second degree SA block with wenckeback and also complex 4 and 7 being junctional beats.

  5. Rob says

    Don’t agree that it is a Wenckebach.
    Long PR check. Progressive lengthening check.
    But no dropped beats… So can’t be a 2nd degree block at all.

    If I had to call it, I’d say a first degree block on a sinus arrhythmia.

  6. Ant says

    1. Progressive prolongation of PR interval present.
    2. No dropped QRS -- therefore definitely not Mobitz 1 -- Wenckebach.
    3. Deliberate mistake? Could that be that the last PR interval isn’t as exactly delineated as it should be? Seems to me your marker is off by 6-12ms (ie. 1/2 to 1 small square).

    My conclusion: sinus exit block.

  7. Jan Štros says

    I think that the best thing for me to do is to just cite Jason Roediger from EKG club on Facebook. He actually brought up this ecg there in his “misinterpreted ECG of the day”. So check out this EKG Club on FB!

    Jason Roediger:
    “The T-waves are identical in height and shape. A P-wave this prominent would obviously distort a T-wave. If you march the P-waves out with calipers, you would not expect to see the P-wave overlapping the T-wave at all but rather it would land slightly after the T-wave. I agree with Raed that this meets the criteria for S-A block and not for A-V block. There is potential for A-V block but it appears to be interrupted by a concomitant S-A block. Dr. Marriott published similar mechanisms in at least two of his textbooks.”

  8. Zsombor Kovacs says

    SR/ variable rhythm with regular pattern
    Ventricular rate 72/100bpm
    group beating: gradual PR prolongation folowed by absent P not unconducted P
    tall and wide Ps / suggest atrial disease/enlargement

    Wenkebach AV conduction defect with sinus block
    Typical second degree block Mobitz1. not likely because
    the monomorphic large Ps could not be buried in Ts

  9. says

    Thanks for all the great comments guys.

    My impression of the ECG (by no means the “correct” answer) 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:
    - Grouping of the QRS complexes
    - 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 Wenckeback SA block!

    Maybe we should call it Wenckebach squared…