I was sent this tracing — and told that providers thought that the rhythm was 3rd-degree (ie, complete) AV block.
- Do YOU agree?
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| Figure-1: The initial ECG in today’s case. |
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NOTE: The tracing in Figure-1 is an 8-lead rhythm strip (and not a complete 12-lead ECG). This offers the advantage of providing 8 leads with simultaneously-recorded viewpoints of the same 7 beats that we see in this tracing.
- Note also that the 3rd lead from the top is lead -aVR (and not +aVR) — with this negative perspective of lead aVR providing an electrical viewpoint of +30 degrees (which corresponds to an electrical viewpoint located between leads I and II ).
- The reason we see lead -aVR in today's tracing, is that this ECG is provided by Dr. Magnus Nossen — who is from Norway, where the Cabrera Format is the standard for ECG recordings (Please check out ECG Blog #365 for potential advantages of the Cabrera Format).
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MY Thoughts on the Rhythm in Figure-1:
Regarding the clinical question posed by today's case — I immediately recognized that ECG #1 was unlikely to represent complete AV block — simply because the ventricular response is not regular.
- PEARL #1: While exceptions exist, most of the time when the rhythm is complete AV block — the escape rhythm will be regular (or at least fairly regular). This truism generally holds regardless of whether the escape rhythm originates from the AV Node — from the His — or from the ventricles.
- PEARL #2: My favorite follow-up to PEARL #1 — is that the best clue in a tracing with AV block that at least some on-time P waves are being conducted — is if one or more of the otherwise regular QRS complexes occur earlier-than-expected (which strongly suggests that the reason this QRS occurs earlier-than-expected — is that this QRS is being conducted to the ventricles). As a result, rather than "complete" AV block — this tells us that some form of 2nd-degree AV block is almost certain to be present.
The “Quick Answer” to Today’s Rhythm:
Even without calipers, as we look at the rhythm in Figure-1 — it should take no more than seconds to recognize that there are P waves that look to be too far away from neighboring QRS complexes to conduct — BUT — that at least beat #4 occurs a little bit earlier than expected. This suggests that beat #4 is probably conducted (albeit with a long PR interval) — such that this rhythm is unlikely to be complete AV block.
- To Emphasize: Regardless of whether the rhythm in Figure-1 represents 2nd- or 3rd-degree AV block — the overall ventricular rate is slow (in the 40s/minute) — so that IF this patient is symptomatic and no “fixable” cause of the bradycardia is found — pacing might still be needed.
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The "Longer" Answer: = What then is going on?
As always — I favor systematic assessment of the cardiac rhythm by the Ps, Qs, 3R Approach (See ECG Blog #185 for review).
Returning to Figure-1 — I look for the Ps, Qs and 3Rs:
- P waves are clearly present in today's rhythm. Although small in amplitude — these P waves are fairly well seen in lead II.
- In Figure-2 — I've highlighted with RED arrows those sinus P waves that we definitely see.
- Awareness of this feature helps to exclude potential mimics of AV block, such as non-conducted PACs (in which case the atrial rhythm will clearly be “off” — much more obviously than by the limited irregularity of a ventriculophasic sinus arrhythmia).
- HINT: Using calipers facilitates (and expedites) the search to determine if an underlying regular atrial rhythm is present.
LOOK again at Figure-2. Considering the RED arrow sinus P waves in this tracing that we know are definitely present:
- Don’t the small extra deflections under the 2 PINK arrows (occurring just after the QRS complex of beats #5 and #7) correspond to a location where you’d expect to see P waves if there was an underlying regular sinus rhythm?
- Isn’t the T wave of beat #2 slightly more peaked than all other T waves in the long lead II rhythm strip (under the WHITE arrow)?
- Therefore — Haven’t we just established that it is almost certain that an underlying regular sinus rhythm is present in Figure-2?
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| Figure-2: Identifying the partially hidden P waves. |
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Continuing with the Ps, Qs, 3Rs ... ( = Looking at the QRS):
- PEARL #4: "More leads are better than one!"
Although I generally favor starting with lead II in rhythm analysis — the QRS complex does not look much different from one beat-to-the-next in this lead.
- Confession: I was in fact initially misled in my interpretation of today's rhythm — beause I forgot to apply PEARL #4 ...
- Then I looked more closely at lead V5.
In Figure-3 — I highlight the most helpful CLUE for solving today's complex arrhythmia — which is the changing QRS morphology for the 7 beats in Figure-3:
- Overall — The QRS complex is clearly wide (at least 3 little boxes, or ≥0.12 second in duration).
- The most logical reason for the changing QRS morphology that is best seen in lead V5 — is that some beats are conducted (albeit with some form of conduction defect) — while other beats arise from a ventricular "escape" focus.
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| Figure-3: The KEY to solving today’s arrhythmia — is to appreciate that the changing QRS morphology is most easily seen in lead V5. I’ve added RED arrows in both leads II and V5 to highlight the underlying regular sinus rhythm in both of these leads. |
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The Next STEPS …
From this point on — I focus on the long lead V5 rhythm strip that I’ve labeled in Figure-4:
- RED arrows show the fairly regular underlying sinus rhythm (albeit with some ventriculophasic sinus arrhythmia) — that we worked out above in Figure-2.
- 3 different QRS shapes are seen in Figure-4: i) Beats #1,3,5,7 look similar (with a widened RSr’ complex); — ii) Beats #2 and 4 look similar (each with a fragmented RR’ complex); — and, iii) Beat #6 resembles beats #2 and 4 — but lacks the distinct, wide terminal S wave that the other 2 beats have.
- The last KEY finding results from application of PEARLS #1 and 2 — for which we need to carefully measure all R-R intervals (as I’ve done in msec. in Figure-4).
QUESTION:
- Which QRS complexes in Figure-4 are most likely to represent escape beats?
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| Figure-4: Focusing on the lead V5 rhythm strip — RED arrows highlight the underlying sinus arrhythmia — with R-R intervals measured in msec. |
Which Beats in Figure-4 represent Ventricular Escape?
Ventricular escape beats are most easily recognized by the presence of: i) A different QRS morphology compared to sinus-conducted beats; — ii) A longer preceding R-R interval; — and, iii) The absence of a preceding P wave with a PR interval that is likely to conduct.
- The 3 QRS complexes in Figure-4 with the longest preceding R-R intervals are beats #3, 5 and 7 (with preceding R-R intervals of 1400 msec. — 1410 msec. — and 1400 msec., respectively).
- Beat #3 is preceded by a P wave with a PR interval that looks too short to conduct. Beats #5 and 7 are both preceded by P waves with PR intervals that look too long to conduct.
- QRS morphology of each of these 3 beats is the same (an RSr’ complex) — which suggests that beats #3,5,7 are all ventricular escape beats with a preceding R-R interval of ~7 large boxes (corresponding to a ventricular escape rate of ~43/minute).
- Beat #1 is also most probably a ventricular escape beat — because it manifests the same RSr’ morphology as do beats #3,5,7.
- Advanced Point: Even though beat #1 in Figure-4 is preceded by a P wave with a seemingly reasonable PR interval (of ~0.18 second) — this PR interval is probably still too short to conduct because of the degree of underlying AV block in today’s tracing.
Which Beats in Figure-4 are probably Conducted?
Applying PEARL #2 — beats #2 and 4 both occur significantly earlier-than-expected (ie, with preceding R-R intervals of 1340 msec. and 1280 msec., respectively). This suggests that both of these beats are conducted.
- PEARL
#5: Knowing that beats #2 and 4 are conducted allows us to deduce 2 more important points: i) That the QRS morphology of conducted beats looks like the RSr’ complex of beats #2 and 4; — and, ii) That despite their markedly prolonged PR intervals — the P waves preceding beats #2 and 4 are conducting because of how early these beats occur — which implies a significant degree of AV block is present.
This leaves us with beat #6 as the only unaccounted QRS complex:
- Note that QRS morphology of beat #6 is intermediate between the QRS morphology of escape beats #1,3,5,7 — and sinus-conducted beats #2 and 4. This suggests that beat #6 is a Fusion beat (See ECG Blog #128 — for more on fusion beats).
- Support that beat #6 truly is a fusion beat — is forthcoming from the finding of its relatively longer preceding R-R interval (of 1380 msec.) — and — a preceding P wave with a PR interval that would seem likely to be able to conduct.
Putting Together the “Longer” Answer:
By the Ps, Qs, 3R Approach — the rhythm in Figure-4 is slow (average rate ~40/minute) — slightly irregular — with an underlying sinus arrhythmia and several different widened QRS morphologies — for which some P waves are conducting, but others are not.
- The rhythm in Figure-4 — therefore represents some form of 2nd-degree AV block, with significant bradycardia.
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What about the Other Leads?
As noted earlier — today's tracing is not a 12-lead ECG. That said, this rhythm recording shows all 6 limb leads and 2 of the chest leads — such that enough leads are present to provide insight to the clinical situation.
- It's important to rule out an acute event.
In Figure-5 — I focus on QRST morphology of the 2 conducted beats in this tracing ( = beats #2 and #4) — since it is much more difficult to assess for acute changes with escape or fusion beats.
- The rsR' complex in lead V1 — in association with wide terminal S wave in lead I, and to a lesser extent in lead V5 — is consistent with underlying RBBB (Right Bundle Branch Block).
- The predominantly negative QRS complex in lead I — in association with the presence of a qR pattern in inferior leads II,III,aVF — is consistent with LPHB (Left Posterior HemiBlock) — so that there is bifascicular block (RBBB/LPHB).
- Extra notching in multiple leads (ie, especially in leads I,II,V5) indicates fragmentation, which is often a sign of previous scarring with significant underlying heart disease.
- The above said — assessment of ST-T wave changes in the 8 leads shown in Figure-5 does not suggest acute changes.
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| Figure-5: Assessing today's tracing for ECG findings (including whether or not there was recent infarction). To facilitate assessment — I focus on the 8 leads that we see in the 2 sinus-conducted beats ( = beats #2 and #4). Doing so suggests there is RBBB/LPHB — but there are no acute ST-T wave changes. |
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The LADDERGRAM:
For clarity over the next 5 Figures — I work out what I believe is the most plausible mechanism for today's complex arrhythmia:
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| Figure-6: It is usually easiest to begin a laddergram by filling in the Atrial Tier — which I've done with near-vertical lines for the 10 P waves in this tracing. I have also filled in the information we derived regarding the Ventricular Tier — in that beats #2 and #4 are conducted — beats #1,3,5,7 are ventricular escape beats — and beat #6 is a fusion beat. = = = = = = = = = = = = = Now the challenge begins — as we try to "solve" the laddergram by figuring out which of the P waves in the Atrial Tier are being conducted to the ventricles. |
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| Figure-7: Since beats #2 and 4 are conducted — I've drawn in slanted BLUE lines connecting those P waves presumed to be conducting to the ventricles (albeit these P waves are conducting with very prolonged PR intervals). Since beat #6 is a fusion beat — I've also connected the P wave before it to represent the supraventricular component of this fusion. |
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| Figure-8: This leaves us with a series of P waves about which we don't know if AV block prevents conduction to the ventricles — or — if those P waves would have conducted had there not been escape beats blocking their path of conduction. |
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| Figure-9: I thought it most logical to postulate that the RED butt ends in the AV Nodal Tier were the result of AV block preventing conduction — whereas the BLUE butt ends indicate P waves that probably would have conducted, had it not been for escape beats #1,3,5,7 blocking their path to the ventricles. |
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| Figure-10: Finishing my proposed laddergram (and color-coding the P waves in the lead V5 rhythm strip) — RED arrow P waves represent sinus conduction — YELLOW arrow P waves are non-conducted because of AV block — and PINK arrow P waves are accompanied by a question mark because I can not prove these P waves would have been able to conduct had it not been for the ventricular escape beats. = = = = = = = = = = = = = BOTTOM Line: This patient with bifascicular block (RBBB/LPHB) — has some form of 2nd-degree AV block, in which there is marked bradycardia (effective rate in the 40s) — and for which those P waves that do conduct, do so with marked 1st-degree AV block. Unless some "fixable" cause of this conduction disturbance can be found — the patient will probably need a pacemaker. |
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Acknowledgment: My appreciation to Magnus Nossen (from Fredrikstad, Norway) for the case and these tracings.
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Related ECG Blog Posts to Today’s Case:
- ECG Blog #185 — My Ps, Qs, 3R System for Rhythm Interpretation.
- ECG Blog #188 — Reviews how to read and draw Laddergrams (with LINKS to more than 100 laddergram cases — many with step-by-step sequential illustration).
- ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.
- ECG Blog #164 and ECG Blog #251 —Review of Mobitz I 2nd-Degree AV Block, with detailed discussion of the "Footprints" of Wenckebach.
- ECG Blog #236 — Reviews in our 15-minute Video Pearl #52 how to recognize the 2nd-Degree AV Blocks (including "high-grade" AV block).
- ECG Blog #186 — Reviews when to suspect 2nd-Degree, Mobitz Type I.
- ECG Blog #404 — Walks you through a step-by-step approach to this AV block case (with links to a VIDEO of this case, and to Blog #344 for more details).
- ECG Blog #352 — emphasizes that 1st-degree AV block with a very long PR interval may have hemodynamic consequences.
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ADDENDUM (12/1/2024) — RP/PR Reciprocity ...
I received a comment today on this tracing from David Richley, who is well known to most ECG enthusiasts who frequent any of the many ECG internet forums. Dave always offers the most astute commentary on complex arrhythmia interpretation.
- NOTE: What follows below goes beyond-the-core! But for those readers who love complex arrhythmia diagnosis — I think you'll find what follows is both fascinating and extremely useful in complex tracings such as the one in today's case!
Dave writes the following:
- Dear Ken — I enjoyed your latest ECG blog and I fully agree with your analysis. In addition to all the points you make, I think this is a great example of RP/PR Reciprocity — in which the PR interval of the conducted beats is inversely proportional to the preceding RP interval (Please see Figure-11 below).
- When I have presented similar ECGs in class, the question that is occasionally asked is: If these are conducted beats — How come the PR intervals are all different?
- The answer is RP/PR Reciprocity: The ventricular escape beats conduct retrogradely into (but not through) the AV node, and render it refractory to stimulation. When subsequently a sinus impulse arrives at the AV node — the speed with which it conducts depends on the relative refractoriness of the node.
- PEARL #6: The longer the RP interval — the more time the AV node has to recover, and the shorter the PR interval will be. Obviously, if the sinus impulse is very early (very short RP interval) — the AV node will be in its ARP (Absolute Refractory Period), and the impulse will fail to conduct at all.
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| Figure-11: Submitted by David Richley — in which he calculates the RP and PR intervals for the 3 conducted beats in today's tracing. Note that there is RP/PR Reciprocity — in that the shortest RP interval ( = 720 msec.) results in the longest PR interval ( = 640 msec.). And, the longest RP interval ( = 960 msec.) results in the shortest PR interval ( = 400 msec.). |
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PEARL #7: = the "Footprints" of Wenckebach:
David Richley's astute observation regarding today's case brings up a KEY concept known as the "Footprints" of Wenckebach — that I've discussed in many prior ECG Blog posts (You'll find many of these Wenckebach tracings with illustrative laddergrams in my ECG Blog #188).
- In addition to group beating (that many readers are familiar with) — there are a number of other characteristics that suggest a periodicity consistent with some form of Wenckebach conduction. Marriott has colorfuly labeled these characteristics as the "Footprints" of Wenckebach.
- To Emphasize: Wenckebach conduction does not always manifest each of these findings. That said — the recognition that several of these characteristics are present goes a long way toward suggesting the diagnosis! (and this is the reason I suspected within seconds — that some form of Wenckebach conduction was present in today's case).
These are the "Footprints" to look for:
- Group beating.
- Lengthening of the PR interval until a beat is dropped — after which the cycle resumes and the PR interval shortens.
- A regular atrial rhythm (ie, a regular, or at least fairly regular P-P interval).
- The pause that contains the dropped beat is less than twice the shortest R-R interval.
- Progressive shortening of the R-R interval within groups of beats, until a beat is dropped.
- RP/PR Reciprocity (as explained above by David Richley).
Today's case is extremely challenging — because none of the PR intervals of the conducting beats are the same. There is however group beating — a fairly regular atrial rhythm — and (as per Figure-11) — RP/PR Reciprocity!
- As always — My appreciation to David Richley for reminding me of the importance of RP/PR Reciprocity that Dr. Barney Marriott first taught me decades ago!
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Additional LINKS of Interest:
- ECG Blog #164 — the "Footprints" of Wenckebach.
- ECG Blog #236 — has a 15-minute ECG Video Pearl on the 2nd-degree AV blocks (I begin talking about the "Footprints" at ~1:20 in this video).
- ECG Blog #235 — in the ADDENDUM to this Blog #235 (at the bottom of the page) — David Richley adds more Pearls regarding advanced AV block diagnosis.
- ECG Blog #188 — in which I provide LINKS to over 115 laddergrams (many showing step-by-step analysis) — of which there are many examples of Wenckebach conduction.
- NOTE: I have added a LINK to facilitate finding this Laddergram page (See the TOP bar of the MENU just below my Blog name at the top of every page in my Blog! ).
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