VERY interesting tracing, that as Dawn mentions doesn’t quite follow the “usual rules” we follow for assessment of AV blocks. I’d add a 3rd possible explanation to the 2 that Dawn mentions (alternating conduction through dual AV nodal pathways; supernormal conduction) — with this 3rd possibility being that may there are 2 consecutive non-conducted beats with the beat ending the relative pause reflecting junction escape that is then followed by a single conducted beat. I have no idea which of these 3 possibilities is operative (or whether there may be some other mechanism operating that I’ve not thought of ... ). What can be said, is that there appears to be severe conduction system disease. The PR interval is prolonged, and there is underlying LBBB. In addition, there are primary ST-T wave changes in at least lead V6 — which should not manifest an upright T wave with simple LBBB. Wish we had clinical follow-up on what happened to this patient!

Dawn...

Thanks so much for finding and posting this complex dysrhythmia. And I certainly agree with you about electrocardiograpy education in the US. People are given textbook examples of dysrhythmias or infarcts (some of which are drawn - not captured!) and once learned, the student feels that he or she is interpreting at an intermediate to an advanced level. WRONG!

This particular dysrhythmia exemplifies one of my favorite topics - concealed conduction. Let me tell you, when concealed conduction is a favorite topic of yours, you will see this exact dysrhythmia over and over - though in various disguises.

I have uploaded a hand-drawn laddergram of part of the lower line. Although the lower line of this ECG represents four different leads, the recording is continuous.

Let's begin with P wave #2. The PR interval that follows is most likely the true PR interval for this patient. It conducts - first down the right bundle branch and then across the septum to activate the left ventricle. The QRS interval is not all that wide for a left bundle branch block, so I would assume that the impulse crossing the septum from right to left manages to enter the left bundle branch pretty quickly rather than discharge the entire left ventricle by traveling from cell to cell.

The next P wave, P wave #3 appears too early to conduct to the ventricles. Is the block in the AV node? Maybe - maybe not. Remember: we have a pre-existing LBBB here which means the right bundle branch is initiating conduction in the ventricles. Also recall that the right bundle branch typically has a longer refractory period than the left bundle branch. In this case, however, whether the block is in the AV node or proximal right bundle branch, with a pre-existing LBBB both would look essentially the same.

P wave #3 runs into refractory tissue that is either low in the AV node or the right bundle branch and can go no further. However, its arrival there has added to the resolving refractoriness, thus prolonging it beyond what would be expected. We know this because P wave #4 encounters this refractoriness. However, due to the delay caused by the P-P interval, P wave #4 finds the lower AV node / proximal right bundle branch in its RELATIVE refractory period. Conduction is slowed and the PR interval is lengthened, but P wave #4 does manage to conduct on through. Remember that the PR interval is not just the delay in the AV node - it represents the time from the exit of the impulse from the sinus node to its arrival at the first working ventricular myocyte.

If the block is in the AV node, then it is physiologic and not necessarily life-threatening. However, if it is in the right bundle branch, there is a very serious problem here.

The prognostic value of an AV block does not lie in the DEGREE of block. The prognosis depends on the LOCATION of the block. A Mobitz I block in the infrahisian conducting system carries the same prognosis as a Mobitz II block.

I would not expect this to be a case of escape-capture bigeminy because I would expect the PR interval of the first QRS to be shorter. This would be the escape beat and it could not appear if the PR interval were normal. (If it did, it would be a fusion beat and would appear morphologically different.)

I hope this helps.

Keep up the good work, Dawn.

Jerry W. Jones, MD FACEP FAAEM

     NICE laddergram Jerry. I like your explanation! It is obvious from the underlying fragmented LBBB with primary ST-T wave changes (upright T wave in V6) — that in addition to whatever type of AV block is operative, that this patient has severe conduction disease. Given concealed conduction (as you wonderfully illustrate) — might this likely be Mobitz II (with infrahisian block) in which a beat is dropped (beat #3) after consecutively conducted complexes (beats #1,2) — with the reason why the PR intervals preceding beats #1 and 2 simply being the concealed conduction you illustrate on your laddergram? THANKS again for your laddergram and explanation!

     Some of our Italian colleagues have also commented (on the ECG Guru Facebook page) about this tracing. Always good to expand our discussion internationally!