Tuesday, April 20, 2021

ECG Blog #216 (ECG MP-33) — Is AV Block Complete?

PLEASE NOTE: Before I begin — there are lessons to be learned from this tracing. Although I immediately recognized the basic conduction disturbance when this tracing was first sent my way — it was only several months later (as I was preparing this ECG Blog post) — that I realized my initial interpretation was not correct.

  • We all live and learn daily!
  • This is a challenging tracing! Are YOU up for the challenge? (NOTE: Even if you don’t completely get the interpretation of this tracing — There is still LOTS to be learned.).




The long lead II rhythm strip in Figure-1 was obtained from a 65-year-old man, who came in for a “routine” check-up. A slow and irregular heart rhythm had been heard on auscultation — which prompted an ECG. The 12-lead tracing showed an incomplete RBBB — but no acute changes.


QUESTION: How would you interpret the rhythm shown in Figure-1?

  • Is complete (ie, 3rd-degree) AV block present?


Figure-1: Long lead II rhythm strip, obtained from an asymptomatic 65-year-old man. Is there complete AV block?



NOTE #1: Some readers may prefer at this point to listen to the 7:00 minute ECG Audio PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to review to My Thoughts on this tracing (that appear below ECG MP-31).


Today’s ECG Media PEARL #33 (7:00 minutes Audio) — Explores what to do when you are not certain of the Rhythm Diagnosis. How often does this really matter?




MY Thoughts on Figure-1:

There is slight angulation of this tracing — therefore some distortion. That said — I believe the quality of this tracing is still adequate for accurate rhythm assessment. By the “Ps, Qs & 3R Approach” that I favor (Reviewed in ECG Blog #185):

  • P waves are present (To be discussed in detail below!).
  • The QRS complex is of borderline duration (ie, about 0.10 second). This is consistent with the incomplete RBBB that was seen on the 12-lead ECG. KEY Point: The rhythm seen in Figure-1 is supraventricular!


Regarding the 3 Rs:

  • Regularity of the Rhythm? — The R-R interval in Figure-1 is not Regular! (NOTEThis is a very important observation — that the R-R interval is not regular!).
  • Rate? — The rate of the ventricular rhythm is slow. Since the R-R interval varies between 7-to-8 large boxes in duration — the ventricular rate is in the low 40s.
  • Related? (ie, Are any P waves related to neighboring QRS complexes?) — This is another KEY Question. Do YOU think any of the P waves in Figure-1 are related to neighboring QRS complexes?


QUESTION: Is the atrial rhythm in Figure-1 regular?

  • My answer appears in Figure-2.

Figure-2: I’ve added RED arrows to Figure-1 to highlight sinus P waves (See text).

ANSWER: I believe that a regular atrial rhythm at ~100/minute is present in Figure-2 (RED arrows).

  • Although tiny in amplitude, upright sinus P waves are regularly seen in this lead II rhythm strip — except for the 2 places where I added RED question marks.
  • Given consistent equal-spacing for the arrows that I’ve drawn — it seems logical to intuit that regularly-occurring P waves continue throughout the entire rhythm strip (and that 2 P waves are almost certainly present and hidden within the QRS complex in the 2 places where I’ve put question marks).
  • NOTE: Were this not the case — We’d have to postulate a highly unusual form of SA block. Common things are common — and — Life is far simpler IF we presume that regular sinus P waves continue throughout the entire tracing (even though we do not see clear indication for 2 of the 18 P waves that I believe are present in Figure-2).

PEARL #1: Using calipers is essential for assessment of complex arrhythmias — especially those involving challenging AV block rhythms. 

  • I am continually amazed at how helpful the simple step of labeling P waves has been to me as an aid for facilitating visualization of even complex P-to-QRS relationships (which is why I added the RED arrows in Figure-2).


PEARL #2: With rare exceptions, one KEY to recognizing 2nd- or 3rd-degree AV Block — is that the atrial rhythm should be regular (or at least almost regular)

  • Awareness that the atrial rhythm should be at least fairly regular when there is 2nd- or 3rd-degree AV Block — is important for ruling out entities such as sinus pauses, sinus arrest, SA block, and blocked PACs as a potential cause of the rhythm disturbance.
  • Therefore — Since the atrial rhythm in Figure-2 is regular — and, since it is obvious that many of the P waves highlighted by the RED arrows are not being conducted to the ventricles (despite having adequate opportunity to be conducted) — some form of 2ndor 3rd-degree AV Block is present.

KEY QUESTION: Is the degree of AV Block in Figure-2 complete?

  • HINT #1: How does the fact that the R-R interval is not regular help to answer this question?
  • HINT #2: Take another LOOK at all of the P waves in Figure-2. Do any of the PR intervals repeat? (ie, Do YOU see the same PR interval in more than 1 place in this rhythm strip?).

PEARL #3: One KEY to recognizing 3rd-degree (ie, complete) AV Block — is that most of the time, the ventricular rhythm should be regular (or at least almost regular). The reason this is true in most cases of complete AV block — is that escape rhythms arising from the AV node, the His or the ventricles tend to be regular.

  • As a result — one of the most helpful clues that one or more beats in an AV block rhythm are being conducted — is IF one or more of the QRS complexes occur earlier-than-expected.
  • It should be apparent in Figure-2 that beats #4 and 8 both occur earlier-than-expected (IF this is not apparent to you — please measure with calipers the R-R intervals between beats #3-4 and #7-8 — and compare these to all of the other R-R intervals in this tracing). The reason beats #4 and 8 occur earlier-than-expected — is that both of these beats are almost certainly being conducted.

Do ANY of the PR Intervals in Figure-2 Repeat?

  • My answer appears in Figure-3.

Figure-3: I have color-coded my theory for which of the P waves in Figure-2 are conducting (See text).

MY Explanation of Figure-3: Unfortunately — there is some slanting of this rhythm strip recording, and this distorts certain measurements (especially toward the end of the tracing). That said — I believe accurate interpretation is still possible.

  • PEARL #4: One of the most helpful clues that certain P waves are conducting — is the finding of one or more identical PR intervals elsewhere on the tracing. While possible by chance for a given PR interval to be repeated elsewhere on the tracing — when you find multiple PR intervals that repeat — this becomes very unlikely to be the result of chance — and it strongly suggests that there is conduction!
  • It is clear that many of the P waves in Figure-3 are not conducting. I chose BLACK arrows to represent those P waves that I believe are not conducting.
  • The 2 RED arrows in Figure-3 highlight P waves I believe are conducting with normal and equal PR intervals.
  • Although extremely long — the 2 YELLOW arrows in Figure-3 highlight P waves that have an identical PR interval. Therefore — I thought these YELLOW P waves were conducting.
  • Even longer, but nevertheless equal — are the PR intervals associated with the 2 BLUE arrow P waves. The reason I strongly suspect that these 2 BLUE arrow P waves are conducting — is that the R-R intervals that contain these 2 BLUE P waves are equal and shorter than all other R-R intervals on this tracing! (As per Pearl #3 — beats #4 and 8 occur earlier-than-expected — and therefore these beats are likely to be conducted).
  • NOTE: Although the P waves immediately preceding beats #4 and 8 appear to manifest similar PR intervals — these PR intervals are quite short — so I thought it less likely that these P waves were conducting (which is why I used BLACK arrows to highlight these P waves).
  • At this point — I was not sure about whether the P wave highlighted by the WHITE arrow was or was not conducting.

Putting IAll Together: At this point — I did not have a definitive diagnosis for the cardiac rhythm in Figure-3. Nevertheless — I had established the following:

  • The QRS was narrow.
  • The atrial rhythm was regular. Some P waves were conducting — but other P waves were not conducting (despite having "adequate opportunity" to conduct — as suggested by their occurrence at many different points within the R-R interval). Therefore — some type of AV block was present.
  • The fact that: i) The ventricular rhythm was not regular; and, ii) Several PR intervals repeated — told us that the rhythm was not 3rd-degree AV block. Since some form of AV block other than 3rd-degree was present — the rhythm in Figure-3 had to represent some type of 2nd-degree AV block.
  • Since the overall ventricular rate was in the low 40s — the effect of the conduction defect was clearly significant. So, despite the fact that this 65-year-old man was asymptomatic at the time he was seen — unless some reversible cause of his conduction defect could be identified and corrected, it seemed probable that a permanent pacemaker would be needed in the near future. Clinical correlation would help to sort this out.

The Laddergram: This case provides an excellent example of how I was uncertain about the specific mechanism of this arrhythmia until I was able to work things out in a laddergram.

  • The reason I was uncertain about a precise etiology — is that the mechanism of this arrhythmia is unusual and complex. I believe there is a dual-level block occurring within the AV Node (represented by the horizontal BLACK line within the AV Nodal Tier in Figure-4).
  • I reviewed basics for reading and drawing laddergrams in ECG Blog #188. That said — I admittedly had to try a number of different possible mechanisms before arriving at a proposed mechanism that seemed to work (Figure-4). This wasn’t a simple case.
  • KEY POINT: Learning to to read laddergrams is not difficult (guarantee anyone reading this blog post can quickly gain confidence and competence in reading laddergrams after Review of the material in ECG Blog #188). However — it does take time and application to learn how to draw laddergrams. That said — You do not have to know how to draw laddergrams to be able to competently manage this case! (as demonstrated by my analysis above under, “Putting It All Together”).

Figure-4: My proposed laddergram for the rhythm in today’s case (See text).


Explanation of My Laddergram:

  • The atrial rhythm is regular at ~100/minute (regularly-spaced arrows in Figure-4).
  • There is a dual-level block occurring within the AV Node.
  • Every-other-P wave makes it through the upper level of the AV Node (ie, there is 2:1 Wenckebach block out of this upper level within the AV Node).
  • At this point — it’s easiest to follow the path of the P wave highlighted by the 1st RED arrow. This 1st RED arrow P wave is conducted to the ventricles with a normal PR interval ( = beat #2).
  • The 1st YELLOW arrow highlights the next P wave that was able to make it through the upper level of the AV Node. This 1st YELLOW arrow P wave also makes it through the 2nd level within the AV Node — albeit requiring additional time to do so (resulting in the increased PR interval that precedes beat #3).
  • The P waves highlighted by the 1st BLUE arrow — and then the WHITE arrow also make it through both levels of the AV Node, albeit with a progressively increasing PR interval (to produce beats #4 and 5) — until finally, the P wave highlighted by the BLACK arrow occurring at the end of the 5th QRS complex fails to make it out of the lower AV Nodal level. This results in a 5:4 Wenckebach cycle out of the lower level of the AV Node (as seen by the progressively increasing PR interval that precedes beats #2, 3, 4, and 5).
  • This dual level Wenckebach block then repeats for the last 3 beats in Figure-4. Note how the pattern of RED, YELLOW and BLUE P waves repeats for these last few beats in the tracing.
  • PEARL #5: I am often asked, “How long can the PR interval be and still conduct?” The P wave highlighted by the WHITE arrow in Figure-4 illustrates that it is possible for the PR interval to be longer than 1.0 second and still conduct.

BOTTOM LINE for Today’s Case: There are lessons-to-be-learned from today’s case for all interpreters, regardless of their level of experience. Even though I did not initially interpret this tracing correctly — the points that I highlight under, “Putting It All Together” form the message I hoped to convey by presenting this challenging case.



Acknowledgment: My appreciation to Shomi Ganguly and Prabal Chakrabarty (from Bangladesh) for the case and this tracing.

  • My THANKS on this case also extend to David Richley (Scarborough, UK) — who is well known to many of my readers, for his always astute insights on complex arrhythmia interpretation. My thought process and Dave’s were similar on this tracing — both regarding our initial misdiagnosis, as well as on our revised (hopefully correct) laddergram interpretation.




Relevant ECG Blog Posts to Today's Case: 

  • ECG Blog #185 — Reviews my Systematic Approach to Rhythm Interpretation using the “Ps, Qs & 3R Approach”.
  • ECG Blog #188 — Reviews the essentials for reading (and/or drawingLaddergrams, with LINKS to numerous Laddergrams I’ve drawn and discussed in detail in other blog posts. 
  • ECG Blog #186 — Reviews some basics on AV Blocks, and WHEN to suspect Mobitz I ( = AV Wenckebach) 2nd-degree AV Block. 
  • ECG Blog 189 — More on determining What TYPE of AV Block is present?

Saturday, April 17, 2021

Blog #215 (ECG-MP-32) — At Least 4 Notable Findings

The ECG shown in Figure-1 was obtained from a 60-ish year old man who presented with new-onset chest pain.

  • HOW would you interpret this ECG?
  • HINT: There are at least 4 remarkable findings that should be noted ...


Figure-1: ECG obtained from a 60-ish year old man with new-onset chest pain (See text).



NOTE #2: Some readers may prefer at this point to listen to the 7:30 minute ECG Audio PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to review to My Thoughts on this tracing (that appear below ECG MP-29).


Today’s ECG Media PEARL #32 (7:30 minutes Audio) — reviews the Cabrera ECG Format — and doing ECGs at a recording speed of 50 mm/second.



The First 2 Findings that should be Noted:

Depending on where you live and work — the ECG in Figure-1 may have appeared unusual. The first 2 findings that should be noted relate to technical aspects of the ECG format used.

  • The recording speed for each of the leads in the 12-lead tracing was 50 mm/second — or twice as fast as the standard (25 mm/second) recording speed used in the United States, and throughout much of the world. Germany, Sweden, and some other countries favor the faster 50 mm/second speed. 
  • DID YOU NOTICE the notation of “50 mm/sec” that appeared in the upper left of the 12-lead tracing in Figure-1(enclosed within the RED rectangle in Figure-2).
  • The other technical finding that should be noted in Figure-1 — is the difference in limb lead sequencing (compared to the standard lead sequencing format used in the United States, and throughout much of the world). The ECG in Figure-1 was recorded in Germany — and it was recorded using the Cabrera Lead Format (Figure-2).


Figure-2: I’ve labeled the technical findings from Figure-1 that should be noted. These are: i) A recording speed of 50 mm/second for leads in the 12-lead tracing (within the RED rectangle in the upper left corner); and, ii) The Cabrera Format for limb lead sequencing (See text).


Editorial NOTE:

As an American-trained physician who practiced exclusively in the United States — I had not been exposed to ECG formats other than the standard format used in the United States. This changed for me in 2010 — at which time I began active participation in multiple ECG internet forums, many with a huge international audience. 

  • Even for those clinicians not participating in internet ECG forums — the internet has changed the world. Because of international travel — the chances are that all clinicians at one time or another will encounter in medical records or elsewhere, different ECG formats from countries other than their own.
  • Two of these differences are seen in today’s case: i) Use of the Cabrera Format for limb lead sequencing; andii) Use of a 50 mm/second recording speed.



The Cabrera Format:

Figure-2 highlights the difference in limb lead sequencing with the Cabrera Format:

  • Instead of beginning the limb lead display with standard lead I — the Cabrera format begins with the highest lateral lead ( = lead aVLcorresponding to an electrical viewpoint perspective obtained from -30 degrees).
  • The right augmented lead is displayed using reversed polarity ( = -aVRcorresponding to an electrical viewpoint perspective obtained from +30 degrees).
  • As suggested by the Axis Insert in Figure-2 — the Cabrera Format offers a more logical display of limb lead sequencing, in that there is a gradual progression of equally spaced (at increments of 30 degrees) limb leads, beginning with lead aVL (at -30 degrees) — and continuing until the most rightward frontal plane lead = lead III (at +120 degrees).
  • In contrast, the standard U.S. format is not sequential — it views the right augmented lead from the remote perspective of the right shoulder — and it uses unequal spacing between neighboring leads.
  • NOTE: The chest leads are unchanged with the Cabrera format.



Potential Advantages of the Cabrera Format:

The more logical limb lead sequencing used in the Cabrera Format offers a number of potential advantages:

  • It simplifies calculation of the frontal plane axis. One can tell at a glance which of the 6 limb leads manifests the greatest net QRS deflection.
  • The addition of lead -aVR provides a transition lead between lateral and inferior frontal plane location. Having 6 equally spaced frontal plane viewpoints facilitates appreciation of the location and relative amounts of ST segment elevation and reciprocal ST depression in the 6 limb leads, making it easier to assess acute ischemia and/or infarction in these infero-lateral leads.
  • Availability of 6 equally-spaced, sequential limb leads renders comparison of serial tracings easier and more accurate.



Correcting for the 5mm/second Recording Speed:

The format used in recording the ECG shown in Figure-2 provides a single complex in each of the 12 leads(recorded at 5mm/second) — and a long lead II rhythm strip at the bottom of the tracing (recorded at 25 mm/second).

  • Confession — My brain is “programmed” to interpreting 12-lead ECGs and rhythm strips at the 25 mm/second speed that is standard in the United States. After 4+ decades of interpreting tens of thousands of tracings — there is an instant (automatic) process of “pattern recognition” that I find takes place in my brain, even before I begin systematic interpretation of any given tracing. This process is invalidated by the unfamiliar appearance of wider-than-expected complexes that are produced when a 50 mm/second recording speed is used.

PEARL #1: For readers like me who find themselves “visually challenged” by the sight of an ECG recorded at 50 mm/second speed — I will offer the quick and easy way I’ve developed for “visually correcting” the wider appearance produced by a doubling of recording speed to 50 mm/second.

  • To compensate for a 50 mm/second recording speed — I upload the ECG to Power Point and perform these 2 Steps: i) Reduce the width of the ECG by 50% (ie, from 10 inches to 5 inches); andii) Make SURE that I uncheck the “Keep Proportions” ( = “Lock aspect ratio” box) in the Format Pane under Size options in Power Point before reducing the tracing’s width. Carrying out these 2 Steps narrows the width of the ECG by 50% without affecting the height. 
  • NOTE: When you perform these 2 Steps — each R-R interval on ECG paper will contain twice as many little boxes (which must be considered when assessing rates and interval duration) — but QRS complexes and ST-T waves will now "look normal" to my eye.



Figure-3 shows the result of applying the procedure I describe in Pearl #1 — with end result of reducing the width of QRST complexes by 50% (but not affecting the height of QRST complexes) for the 12-lead ECG shown in Figure-2.

  • For clarity — I have added an insert of Cabrera limb lead sequencing to the upper left part of Figure-3.
  • Visually — QRST complexes in Figure-3 now “look normal” to my eye (although you’ll note the reduced width of little boxes, compared to the wider small boxes in the long lead II rhythm strip that was recorded at 25 mm/second).


Figure-3: Application of the procedure described in Pearl #1 results in a 50% reduction in QRST complex width — with end result that QRST complexes in the 12-lead ECG now “look normal” to my eye (See text).



Interpreting the 12-Lead ECG in Figure-3:

I now found it much easier to interpret the ECG in today’s case. Assessment of QRST complexes in the 12-lead tracing shown in Figure-3 suggests acute RCA (Right Coronary Artery) Occlusion. I noted the following findings:

  • There is marked ST segment elevation with hyperacute T waves in each of the inferior leads.
  • Inferior lead Q waves are seen. Although small in size — given dramatic ST-T wave changes in each of these leads with Q waves — it is likely that these Q waves reflect ongoing infarction.
  • There is terminal QRS distortion (T-QRS-D) in each of the leads that manifest acute ST-T wave changes. By this, I mean that none of the 4 inferior leads with ST elevation have either J-point notching or an S wave. Although usually reported as a finding supportive of acute infarction when seen in anterior leads — the impressive T-QRS-D seen in Figure-3 is clearly an additional manifestation of ongoing acute RCA occlusion.
  • High lateral leads I and aVL show marked reciprocal ST depression.
  • PEARL #2: Note how nicely the Cabrera limb lead sequencing seen in Figure-3 displays gradual progression of ST elevation that begins with transitional lead -aVR (at +30 degrees) — and then maximizes in leads IIaVF and III. As expected from the Cabrera limb lead insert in this Figure — maximal ST elevation in leads II, aVF and III is countered by maximal reciprocal ST depression in lead aVL (much more so than the modest ST depression seen in lead I)Cabrera sequencing localizes the vector of maximal ST elevation “rightward” (ie, to about +90 degrees) — which supports our suspicion of the RCA (rather than the Left Circumflex) as the “culprit” artery.
  • In addition to acute inferior infarction — maximal ST depression in leads V2V3 and V4 strongly suggests associated acute posterior MI. Presumption of posterior MI is supported by the finding of taller-than-expected anterior R waves (with a dominant R wave already seen by lead V3).
  • NOTE: ST depression extends across the entire precordium. How much of this ST depression that extends to leads V5 and V6 is a result of extensive posterior MI — vs lateral ischemia — vs LV “strain” is uncertain from this single tracing (NOTEAlthough subtle due to overlap of QRS complexes — R wave amplitude in lead V5 is very large, therefore consistent with LVH).
  • In Addition — there is terminal T wave positivity in leads V2-thru-V5. This is one more finding consistent with acute ongoing/evolving infarction.
  • Finally — there are small and narrow q waves in leads V5 and V6. These are of uncertain significance.



Interpreting the Long Lead II Rhythm Strip:

For facility of discussion — I intentionally deferred assessment of the rhythm in today’s case until last (Figure-4).

  • Unfortunately — the tracing in today’s case is curved, and therefore somewhat distorted. That said — accurate interpretation of the rhythm in Figure-4 is clearly possible.
  • Note that the recording speed for the long lead II rhythm strip is 25 mm/second — so no adjustment in tracing width was made.
  • The QRS complex is narrow — therefore the rhythm originates within the conduction system.
  • Although curvature of the long lead II rhythm strip makes precise measurement and comparison of intervals problematic — I believe the ventricular rhythm is regular (or at least, almost regular) — with an R-R interval of ~6 large boxes, corresponding to a ventricular rate of ~50/minute.
  • RED arrows in the long lead II rhythm strip highlight sinus P waves that we know are present.
  • I strongly suspect additional P waves are present under the YELLOW arrows because: i) Given how regular the RED arrows seem to be — we would logically expect P waves to also be present under the YELLOW arrows (ie, there would have to be a highly unusual form of SA block if you didn’t postulate regular sinus P waves continuing throughout the tracing); andii) I believe there is a slight deformity under each of the YELLOW arrows, corresponding to partially hidden P waves.
  • Note that none of the PR intervals repeat in this 9-beat rhythm strip. This suggests there is complete AV dissociation. But despite the slow ventricular rate (ie, of ~50/minute) — and despite P waves having more than adequate opportunity to conduct, yet failing to do so (ie, P waves occur at virtually all points in the cardiac cycle) — there is no indication that any of the P waves conduct. Therefore — there is complete AV block.


Figure-4: I’ve labeled P waves in today’s case with colored arrows (See text).



Putting It All Together:

In summary — the notable findings for the ECG in today’s case include the following:

  • Use of the Cabrera Format.
  • A recording speed of 50 mm/second for the 12-lead ECG.
  • Strong suggestion of acute RCA occlusion in this 60-ish year old man with new-onset chest pain — with an ongoing, acute infero-postero STEMI.
  • Complete AV Block — with an AV nodal escape rhythm at ~50/minute.
  • LVH and strain and/or ischemia.




Acknowledgment: My appreciation to Tim Hellmann (from W├Ârth am Rhein, Germany) for the case and this tracing.




  • ECG Blog #114 — Reviews another case using the Cabrera format (from Romania) — and explores a bit more potential advantages of this more logical anatomical sequencing.
  • The September 26, 2018 post in Dr. Smith’s ECG Blog (Please scroll down to the BOTTOM of the page for My Comment regarding this interesting ECG submitted from Sweden).
  • The October 26, 2020 post by Dr. Pendell Meyers, in Dr. Smith’s ECG Blog (Please scroll down to the BOTTOM of the page for My Comment regarding this fascinating ECG case of dynamic shark fin” morphology, submitted from Norway).
  • The November 14, 2019 post in Dr. Smith’s ECG Blog (Please scroll down to the BOTTOM of the page for My Comment and illustration of the phenomenon of T-QRS-D = Terminal QRS Distortion).