ECG Blog #226 (42) — A Variable Form of Complete AV Block?

Today's case was sent to me — with the only information that I was provided being the opinion that the ECG represented "a variable form of complete AV block".

• How would YOU interpret the ECG in Figure-1?
• What is the rhythm?

 

Figure-1: ECG obtained from a middle-aged woman with new-onset chest pain (See text).

 

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NOTE: My detailed answer for this case appears below this 11-minute Video PEARL. 

• For those who want a "quicker answer" — you can find it below in Figures-2, -3 and -4.

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Today’s ECG Media PEARL #42 (11:00 minutes Video) — Reviews the 12-lead ECG and long lead II rhythm strip shown in Figure-1. Does the rhythm represent, "a variable form of complete AV block?"

 

 

The "Quicker" Answer to Figure-1

The only ECG I was given is shown in Figure-1.

• I am not a "fan" of 12-lead ECGs that only show a single, unattached complex, as is seen in this figure. One reason why, is that lead II in the 12-lead may suggest a sinus rhythm (as we see here in Figure-1) — but the long lead rhythm strip tells us a very different story.
• I discuss the rhythm in detail below (in Figure-2 and Figure-3). For now, what is important for interpretation of the 12-lead ECG — is that the QRS complex is narrow. Therefore, the rhythm is supraventricular — which means that we can assess this ECG for acute or recent ST-T wave changes.

 

The 12-lead ECG in Figure-1 shows what appears to be a recent inferior MI. Note the following:

• Hyperacute ST-T waves in each of the inferior leads (II, III, aVF) — with a prominent peaked T wave ending in terminal negativity. ST elevation is most marked in lead III, and probably still present in lead aVF — but not seen in lead II.
• Reciprocal ST depression in lead aVL — in which the ST-T wave in lead aVL shows the exact mirror-image opposite picture to the elevated ST-T wave in lead III (See ECG Blog #184 for more on this "magical" lead III-aVL relationship).
• A lesser degree of reciprocal ST-T wave depression in lead I.
• Hyperacute-looking ST-T waves in anterior leads V1, V2 and V3 — which end in taller and "fatter"-than-they-should-be T waves. In the context of the recent inferior MI that we have just diagnosed — these anterior lead changes suggest recent posterior MI that is now showing evolution of the event with reperfusion ST-T waves.
• Poor R wave progression. That said — there is an initial positive deflection (r wave) in leads V2 and V3 — so these complexes do not qualify as Q waves, and we cannot diagnose anterior infarction. An obviously fragmented QRS complex is seen in lead V3 (with a lesser amount of fragmentation seen in the downstroke of the S wave in lead V2).
• Half-standardization in leads V4,5,6 — which means that the increased R wave amplitude (~30 mm in V5; ~25 mm in V6) qualifies for LVH. The abnormal ST-T waves in these lateral chest leads probably reflect a combination of LV "strain" and ischemia from the recent event.
• IMPRESSION of the 12-Lead ECG: Without information on the nature or time frame of symptoms — it is not possible to estimate the onset of this event. But the 12-lead ECG in Figure-1 strongly suggests there has been a recent infero-postero MI, that could have begun as recently as a few hours earlier — or — perhaps a day or 2 earlier. The minimal amount of ST elevation, together with terminal T wave negativity in the inferior leads + terminal T wave positivity in lead aVL + the voluminous anterior T waves — combine to suggest evolution of this recent infero-postero MI, with this ECG now showing what appears to be reperfusion ST-T wave changes.

 

Moving on to the Long Lead II Rhythm Strip:

• By the Ps, Qs & 3Rs Approach (reviewed in ECG Blog #185) — Sinus P waves (ie, upright P waves in lead II) are present — the QRS complex is narrow — and (regarding the 3Rs) — although the atrial rhythm is regular (RED arrows in Figure-2) — the slow, ventricular rhythm is not regular. A number of P waves are clearly not conducted — and, the PR interval preceding QRS complexes does not stay the same. 

Figure-2: I've labeled P waves from the long lead II rhythm strip in Figure-1 (RED arrows).

PEARL #1: Despite variation in the PR interval — I instantly suspected that this rhythm was not complete AV Block, because the R-R interval was clearly irregular. 

• Because the rate of an "escape" rhythm tends to be regular (or at least almost regular) — the finding of definite irregularity of the ventricular rhythm (as seen in Figure-2) suggests that AV block is not complete. 
• If some regularly-occurring P waves are not conducted — but the rhythm in question is not 3rd-degree (ie, complete) AV Block — then we are dealing with some form of 2nd-degree AV block.

PEARL #2: In a patient with evidence of recent inferior MI — the overwhelming majority of 2nd-degree AV blocks will turn out to be some form of AV Wenckebach (ie, Mobitz I). This is especially true when the QRS complex is narrow (as it is in Figure-2).

PEARL #3: The fact that the pause in this short rhythm strip ends with a QRS complex having a normal PR interval — suggests that beat #4 is conducted. 

• The fact that the PR interval preceding beat #4 looks to be the same as the PR interval preceding beat #1 — suggests that at least 2 of the 4 QRS complexes in this tracing are probably conducted.

PEARL #4: The mechanism of the rhythm in Figure-2 is obviously complex. But IF we can construct a plausible laddergram — then we might be able to work out a plausible mechanism for the rhythm. 

• Taking into account the points brought out in PEARLS #1, 2 and 3 (above) — plus — awareness that AV Wenckebach may occur at more than a single level within the AV Node — suggested to me the likelihood of dual-level Block out of the AV Node.

 

 

Drawing the LADDERGRAM:

Step-by-step review of my proposed Laddergram that I show in Figure-3 is illustrated in the above Video PEARL. Brief summary follows below:

• The horizontal dotted black line divides the 2 levels of block within the AV Nodal Tier.
• To facilitate understanding — I've labeled with RED arrows those P waves in Figure-3 that I believe do make it through the upper level within the AV Node. I've used BLUE arrows to indicate those P waves that do not make it through this upper level. Note that my theory proposes that every-other-P wave makes it through the upper AV Nodal level.
• Of those P waves in RED that do make it through to the lower AV Nodal level — Note that there is progressive prolongation of the time needed to traverse the lower AV Nodal level — until the 4th RED P wave is non-conducted. The next P wave (ie, the 5th RED P wave in Figure-3) is then able to conduct — and the cycle begins again.
• IMPRESSION: The rhythm in Figure-3 is 2nd-Degree AV Block with dual-level Wenckebach out of the AV Node. There is 2:1 AV conduction out of the upper AV Nodal level — and, 4:3 Wenckebach conduction out of the lower AV Nodal level.

Figure-3: Laddergram illustration of my proposed mechanism for the rhythm in today's case (Please see ECG Blog #188 for review of how to read and/or draw Laddergrams).

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Acknowledgment: My appreciation to Abdullah Al Mamum (from Dhaka, Bangladesh) for allowing me to use this case and this tracing.

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Related ECG Blog Posts to Today’s Case: 

• ECG Blog #185 — Systematic Approach to Rhythm Interpretation.  
• ECG Blog #188 — Reviews HOW to Read and Draw Laddergrams. 
• ECG Blog #184 — Reviews the "Magical" Lead 3-Lead aVL Relationship with acute OMI (Occlusion-based Myocardial Infarction). 
• ECG Blog #224 — Reviews a case in which Group Beating + Acute Inferior MI strongly suggested Mobitz I as the cause.