ECG Interpretation: ECG Blog #477 — Is One or Both 3rd-Degree?

Compare these 2 tracings.

Is one or both of them 3rd-Degree (Complete) AV Block?
If not — Why not?

Figure-1: Is one or both of these tracings 3rd-Degree?

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Let's look at these rhythms one at a time — starting with Rhythm-B:

Rhythm B (which I have reproduced in Figure-2):

As always — I favor the Ps, Qs, 3R Approach for assessing rhythm disorders, as I find this the most time-efficient way to narrow down my differential diagnosis of arrhythmias (See ECG Blog #185):

NOTE #1: Lead MCL-1 is a right-sided monitoring lead (providing a comparable view to what we typically see in right-sided lead V1 on a 12-lead ECG).
NOTE #2: It does not matter in what order you assess the Ps,Qs,3Rs. I often vary the sequence I use depending on which parameter is easiest to assess in the tracing I am looking at.
NOTE #3: If you have a moment of time (ie, If the patient in front of you is not about to have a cardiac arrest) — then using calipers will save you time (and increase your accuracy) for interpreting challenging rhythms.

Figure-2: This is Tracing-B from today's case.

Applying the Ps, Qs, 3Rs to Rhythm-B:

P waves are present. We see lots more P waves than QRS complexes (I've labeled with RED arrows in Figure-3 those P waves that we clearly see).
The QRS complex is wide ( = 0.15 second). I find it easiest to measure QRS duration by selecting a complex that either begins or ends on a heavy ECG grid line (In Figure-3 — Beat #1 ends on a heavy grid line).

And — The 3Rs (ie, Is the rhythm Regular? — the Rate? — and are P waves Related to neighboring QRS complexes?):

The R-R interval in Figure-2 is Regular — and measures just under 8 large boxes in duration (and 300 ÷ a bit less than 8 = a Rate slightly less than 40/minute).

Are P waves Related to neighboring QRS complexes?

Look at the PR interval in front of each QRS in Figure-3. Doesn't the PR interval continually change?

Figure-3: The QRS in Tracing-B is clearly wide. I've labeled those P waves that we clearly see with RED arrows.

QUESTION:

Is the atrial rhythm in Figure-3 regular?

Given the regularity of the RED arrows that we do see in Figure-3 — Wouldn't it make more sense for the rest of the underlying atrial rhythm to be regular, instead of all-of-a-sudden dropping several P waves?
PEARL #1: Use calipers to answer the above Question! (See the PINK arrows in Figure-4).

Figure-4: I've added PINK arrows to show where my calipers suggest that on-time P waves are likely to be hiding.

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ANSWER:

Although impossible to prove from the short single-lead tracing we show in Figure-4 — it makes much more sense for the underlying atrial rhythm to be regular (instead of having to postulate sudden and unpredictable onset of some unusual type of intermittent SA block).

Setting your calipers to the P-P interval between any 2 consecutive RED arrows in Figure-4 — allows us to predict the probable location of underlying on-time sinus P waves ( = PINK arrows in Figure-4).
PEARL #2: As shown in ECG Blog #344 — it is common in patients with 2nd-degree or 3rd-degree AV block to manifest slight variation in the P-P interval of underlying sinus P waves. This slight variation in the rate of sinus P waves is known as "ventriculophasic" sinus arrhythmia — and is easy to account for when postulating the probable location of hidden "on-time" sinus P waves.

Putting It All Together for Tracing-B:

The rhythm in Tracing-B is 3rd-Degree (Complete) AV Block.

PEARL #3: Complete (or 3rd-degree) AV block — is said to be present when none of the on-time sinus impulses from above are able to conduct to the ventricles despite having an adequate opportunity to do so.
KEY Point: It is this last requirement (ie, the need for "adequate opportunity" to conduct) — that is so commonly overlooked. There may be AV dissociation, with none of the on-time P waves in the small segment of the rhythm you are looking at being able to conduct — but unless the ventricular rate is slow enough (usually less than 55-60/minute) — and, unless the duration of monitoring is long enough — it may be that there has not yet been enough of a monitoring period to demonstrate that the degree of AV block is complete.

I illustrate the above concept in more detail in ECG Blog #191 (in which I also review the 3 types of AV Dissociation, of which AV block is only one of these types). For now, the "Take-Home" Point is that Tracing-B in today's case does show 3rd-Degree (Complete) AV Block. Note the following features of this rhythm in Figure-5:

The ventricular rhythm is slow, regular — and with a wide QRS complex.
The atrial rhythm is regular (RED arrows — showing no more than slight variation in the R-R interval, consistent with ventriculophasic sinus arrhythmia).
There is complete AV dissociation — because none of the on-time sinus P waves are related to neighboring QRS complexes (ie, The PR interval in front of each QRS complex continually changes).
More than just AV dissociation, there is complete AV Block — because on-time sinus P waves occur at all points within the R-R interval, and fail to conduct despite having more than adequate opportunity to do so.
The level of this AV block is below the AV Node (and therefore arising from the ventricles) — because there is a slow idioventricular escape rate (ie, with QRS widening — and an "escape" rate of just under 40/minute).

Figure-5: Tracing-B shows complete AV block.

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What about Tracing-A in today's case? (reproduced in Figure-6):

Figure-6: This is Tracing-A from today's case. (To improve visualization — I've digitized the original ECG using PMcardio).

My First Impression of Tracing-A:

The 12-lead ECG in Tracing-A is obviously abnormal. The marked T wave inversion in leads V1,V2 is impossible to overlook!

The rhythm is extremely slow.
Looking at just the last 2 beats (that occur in simultaneously-recorded leads V1,V2,V3 — and in V4,V5,V6) — I could not initially tell which beats might be supraventricular (ie, with RBBB conduction in lead V1?) — vs ventricular (producing the wide QS complexes in V4,V5,V6?).
NOTE #4: In order to complete my interpretation — I would clearly need to determine which of the 4 beats in this tracing are ventricular vs supraventricular (and whether any of these 4 beats might be conducted?).
NOTE #5: As I turned my attention to the rhythm — it was also clear that using calipers would be needed.

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Applying the Ps, Qs, 3Rs to Rhythm-A:

The QRS complex looks to be wide (about 3 little boxes = 0.12 second in duration).
As noted — the Rate is very slow (barely over 30/minute — such that we only see 4 beats). The R-R interval looks to be the same between the first 3 beats — but beat #4 looks like it occurs early (It also appears that QRS morphology of beat #4 is different from that of the first 3 beats).
P waves are present. That said — it is hard to tell from simple inspection of the rhythm if the atrial rate is (or is not) regular.
As to whether P waves are Related to neighboring QRS complexes? — We need to look in front of each of the 4 QRS complexes. It certainly appears that the PR interval in front of each of the 4 beats in the long lead II rhythm strip is different.

My Initial Impression of Rhythm-A:

I was not certain about the etiology of this rhythm.

Confession: Even after having had LOTS of time to contemplate Rhythm-A — I am still not sure what is going on. Obviously — some form of severe heart block is present. Pacing is likely to be needed unless something "fixable" is quickly found. That said — I admittedly had more questions than answers about this rhythm.

Looking closer ...

Does beat #4 occur early? To answer this question — I carefully measured each R-R interval in the long lead II rhythm strip (See Figure-7).

Figure-7: Caliper measurements of each R-R interval.

ANSWER: Beat #4 clearly does occur early.

PEARL #4: Most of the time when there is significant AV block — the BEST clue that a beat may be conducted is if a beat occurs earlier-than-expected. As a result — I wondered if beat #4 might be conducted by the P wave that appears in front of this beat?
NOTE #6: It's important to realize that we have no idea of how long the pause was before beat #1 — nor do we have any idea of when beat #5 occurred (or if beat #5 occurred at all?).

Next QUESTION:

Are P waves regular in Rhythm-A?

In Figure-8 — I show my attempt to answer this question ...

Figure-8: I've labeled those P waves that we clearly see in Rhythm-A with RED arrows.

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ANSWER: As for PEARL #1 that we applied when problem-solving Rhythm-B — Using calipers facilitates (and expedites) predicting where additional P waves might be hiding.

PINK arrows in Figure-9 suggest logical "hiding spots" for the 3 missing P waves in Rhythm-A. Therefore, it is likely that the atrial rhythm is regular — and that 2 of the on-time sinus P waves are hidden within the QRS complexes of beats #1 and 3, with a 3rd P wave accounting for slight increase in the volume of the T wave of beat #2.

Figure-9: I've added PINK arrows to show where my calipers suggest that on-time P waves are likely to be hiding.

Putting It All Together for Tracing-A:

I admit to not being certain of the etiology of Rhythm-A.

PEARL #5: As disappointing as my admission of not being certain about the interpretation of Rhythm-A might be — there is an important lesson in this admission: The etiology for every rhythm will not always be evident from a single short period of monitoring (ie, We only see 4 beats in Rhythm-A — so it is understandable that we might not yet appreciate what is going on).
Awareness that more information (ie, a longer period of monitoring) is needed — is an important concept to become comfortable with.

What We DO Know about Rhythm-A:

The above said — We do know a lot about Rhythm-A:

At the least — there appears to be high-grade 2nd-Degree AV Block in Rhythm-A, with a very slow "escape" rate (just over 30/minute).
Beat #4 occurs early and manifests a different QRS morphology compared to the first 3 beats. As per PEARL #4 — the earlier-than-expected occurrence of beat #4 usually suggests that this beat may be conducted. That said — 2 findings are present in Rhythm-A that are against beat #4 being conducted. These findings are: i) That the PR interval preceding beat #4 in the long lead II rhythm strip looks surprisingly shorter than I would expect being able to conduct given how severe the AV block otherwise seems; and, ii) QRS morphology in simultaneously-recorded leads V4,V5,V6 for beat #4 looks much more like ventricular than conducted beats (ie, the wide QS complexes).
BUT — None of the P waves before beat #4 appear to have any relation to neighboring QRS complexes.

BOTTOM Line for Tracing-A: Whether beat #4 is conducted — or whether instead there is complete AV block with slightly early "escape" from another ventricular focus in the form of beat #4 is not apparent from this single tracing. What is known:

There is at-the-least high-grade 2nd-degree AV block with a very slow "escape" rate. As a result — pacing will be needed unless some "fixable" etiology for this slow rate can be found.
Although wide — QRS morphology for the first 3 beats looks like it is arising from somewhere within the conduction system (perhaps from the His — with RBBB conduction based on the QR pattern with predominant positivity in lead V1).
The deep Q in lead V1 — and the very deep T wave inversion in anterior leads suggests anterior infarction at some time in the recent past may be the reason for the severe AV block.
More information (and a longer period of monitoring) is needed to say more.

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Acknowledgment: My appreciation to Adem Ahmed (from Nouakchott, Mauritania) for the case and this tracing.

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Additional Relevant ECG Blog Posts to Today’s Case:

ECG Blog #185 — Use of a Systematic Approach to Rhythm Interpretation.
ECG Blog #191 — Is AV Block Complete? (Assessing AV Dissociation).

Other Posts on Assessing for AV Block:

— NOTE: There are even more cases relevant to assessment for AV Block on this Blog, but those below provide some “practice” for those in search of some example cases.

ECG Blog #164 — On Mobitz I.
ECG Blog #187 — Some Form of Wenckebach.
ECG Blog #189 — Dual-Level Wenckebach.
ECG Blog #192 — AV Dissociation by Default (vs Usurpation).
ECG Blog #62 — 2:1 AV Block: Mobitz I or II?
ECG Blog #61 — Vagotonic AV Block.
ECG Blog #58 — AV Dissociation by “Default”.
ECG Blog #66 — Group Beating not Due to Wenckebach.
ECG Blog #154 — Group Beating with Acute Inferior MI.

ADDENDUM:

What follows below is a 7-page excerpt from my ACLS-2013 Arrhythmias (Expanded Version) book, in which I review the distinction between AV dissociation vs complete AV block.

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Today’s ECG Media PEARL #19a (6:45 minutes Audio) — Why is this Not Complete AV Block? This recording suggests a few Quick-Things-To-Do that help to rule in or rule out Complete AV Block (P.S. — I updated this Audio Pearl on 10/12/2021).

ECG Media Pearl #8 (8:20 minutes Video) — ECG Blog #191 — Distinguishing between AV Dissociation vs Complete AV Block (2/6/2021).