Tuesday, September 21, 2021

ECG Blog #252 (66) — Mobitz I? Mobitz II? or Not?

The ECG in Figure-1 was obtained from a middle-aged patient who was thought to have advanced AV block.



  • Is the rhythm Mobitz I? — Mobitz II? — or not?

Figure-1: 12-lead ECG and long lead II rhythm strip from a patient thought to be in AV block (See text).


  — The Case Continues BELOW today's Audio Pearl ... —




NOTE: Some readers may prefer at this point to listen to the 11:15-minute ECG Audio PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to refer to My Thoughts on this tracing (that appear below ECG MP-66).


Today's ECG Media PEARL #66 (11:15 minutes Audio) — Delves into the theme that "Common Things are Common". This recording is full of PEARLS for expediting recognition of blocked PACs — Atrial Flutter — AFib — VT — Mobitz I vs Mobitz II 2nd-degree AV block — Left Anterior vs Posterior Hemiblock — and SSS (Sick Sinus Syndrome).




My Sequential Approach to Today's Case:

There is a temptation to look at the rhythm in Figure-1 — and immediately suspect some form of 2nd-degree AV block. I'll suggest a different approach!

  • My 1st impression on seeing the long lead II rhythm strip in today's tracing was that there is group beating in a supraventricular (ie, narrow QRSbigeminal pattern!


PEARL #1: As discussed in ECG Blog #232 — recognition of a bigeminal supraventricular pattern when the 1st beat in each pairing is conducted, should suggest the following differential diagnosis:

  • Sinus rhythm with atrial or junctional bigeminy (ie, every-other-beat is a PAC or a PJC).
  • Sinus rhythm with atrial trigeminy — in which every-third P wave is a PAC that is "blocked" (non-conducted).
  • Some form of SA ( = Sino-Atrial) Block.
  • Mobitz I, 2nd-Degree AV Block ( = AV Wenckebach) with 3:2 AV conduction.
  • Mobitz II, 2nd-Degree AV Block.


PEARL #2: Another way to look at the bigeminal pattern of group beating that we see in Figure-1 — is that each group of 2 beats is separated by a short pause.

  • As emphasized above in today's Audio Pearl — by far, the commonest cause of a pause is a blocked PAC. In clinical practice — the finding of blocked PACs is far more common than any form of AV block.
  • Therefore — within seconds of seeing today's tracing — my thoughts were that statistically, the most likely cause of the pauses seen in Figure-1 is blocked PACs. As a result — I knew that my assessment would need to focus on confirming or excluding this possibility. 



At this point, I took a closer look at the rhythm — applying the systematic Ps, Qand 3Rs approach (as discussed in detail in ECG Blog #185):

  • P waves — are present (RED arrows in Figure-2 highlight upright sinus P waves in lead II).
  • The QRS is narrow — which confirms that the rhythm is supraventricular.
  • The rhythm in Figure-2 is not "Regular" — so the Rate varies. That said — there is group beating, with a repetitive pattern of alternating short-long intervals. P waves are Related to neighboring QRS complexes — in that each QRS complex in this tracing is preceded by a sinus P wave with a fixed PR interval.

Figure-2: I've labeled sinus P waves from Figure-1 with RED arrows. I've also measured the R-R interval for the shorter and longer R-R intervals ( = 720 and 1560 msec., respectively).


PEARL #3: Now that we have systematically assessed the 5 parameters in the Ps, Qs & 3R approach — it's time to consider the entities in the differential diagnosis that we suggested earlier in Pearl #1.

  • The rhythm in Figure-2 is neither Mobitz I nor Mobitz II 2nd-degree AV block. The reason we can easily rule out 2nd-degree AV block — is that the atrial rhythm is not regular! (RED arrows in Figure-2). With rare exceptions, for there to be AV block — the P-P interval should be at least fairly regular, and this is definitely not the case in this tracing. That is — no P wave is seen near the mid-point of each pause, as would be expected if some form of 2nd-degree AV block was present.
  • Additional reasons why the most common form of 2nd-degree AV block ( = Mobitz I) is not present in Figure-2 are that: i) The PR interval is not increasing: — and, ii) Wenckebach periodicity is not seen (ie, the pause [1560 msec.] is not less than twice the shortest R-R interval [720 msec.] — as discussed in ECG Blog #164).
  • The rhythm in Figure-2 is neither atrial nor junctional bigeminy — because similar-looking sinus P waves with a constant PR interval precede each QRS complex on the tracing. Morphology for the P waves preceding beats #2, 4, 6 and 8 should look different if atrial or junctional bigeminy was present.
  • SA block is unlikely because: i) In my experience — this conduction disturbance is extremely rare in clinical practice; — andii) Wenckebach periodicity should be present IF there was Type I SA block — and — the longer R-R intervals in Figure-2 should be almost exactly twice the duration of shorter R-R intervals if there was Type II SA block (and 1560 msec. is clearly more than 2 X 720 msec.).
  • By the process of elimination — we can now focus on determining IF the cause of the group beating in Figure-2 is indeed the result of blocked PACs.


HOW to Assess for the Possibility of Blocked PACs:

As emphasized above in today's Audio Pearl — the commonest cause of a pause is a blocked PAC. Sometimes the blocked PAC(s) will be obvious — and, sometimes not. That said — I guarantee that once you begin to regularly look for blocked PACs, you will begin to find them with surprising frequency!

  • The KEY to identifying a specific ECG finding — is to know when to look for it. The secret is to look for blocked PACs whenever you encounter any unexpected pause in the rhythm.
  • HOW to Look: Carefully examine the ST segment and T wave at the onset of the pause. Compare this ST segment and T wave at the onset of the pause — with the ST-T wave of all normally conducted sinus beats on the tracing. Is there any difference?
  • NOTE: Detecting blocked PACs can be challenging! One has to distinguish between minor variations that naturally occur from beat-to-beat in the ST-T wave — from notches or deflections that are the result of a premature P wave buried within (and therefore deforming) the ST-T wave.
  • IF in doubt — Look at simultaneously-recorded leads! 12 leads are better than one! Sometimes "tell-tale" signs of a hidden non-conducted PAC may only be evident in some (but not all) of the leads on a 12-lead tracing.
  • For practice — examples of blocked PACs can be found in ECG Blog #33  Blog #57   Blog #66 — and Blog #147, among others.


CONCLUSION to Today's Case:

The rhythm in today's case is exceedingly subtle — because the blocked PACs are extremely well hidden. The "normal" ST-T wave appears after the 1st beat in each pair (ie, the ST-T waves for beats #1, 3, 5 and 7 in Figure-3).

  • I've labeled with YELLOW arrows those leads in today's tracing in which the T wave appears to be noticeably more peaked than the normal T wave that precedes it. Perhaps the easiest lead for noticing this difference is for beats #5 and 6 in lead V1 — in which the YELLOW arrow clearly highlights deepening of the inverted T wave caused by the blocked PAC, that is not present for the T wave of beat #5.
  • Ideally, we would have additional monitoring on this patient to confirm the appearance of the "normal" ST-T wave in each of the 12 leads. That said, despite near identical appearance of all QRS complexes on this tracing — the 2nd T wave in each group is consistently a little bit different in appearance than the 1st T wave in multiple leads. This strongly supports my impression that the rhythm is Atrial Trigeminy, in which every-third-beat is a blocked PAC
  • Interpretation of the remainder of this 12-lead ECG is unremarkable. There are no acute changes.
  • BOTTOM LINE: The patient does not have any form of AV block. Instead, the rhythm is sinus — with atrial trigeminy, in which each PAC is "blocked" (because it occurs very early in the absolute refractory period). Clinically — attention should focus on potential causes of PACs (ie, caffeine, alcohol, dehydration, anxiety — or other underlying medical issues) — but overall, the rhythm diagnosis is much less worrisome than would be the case if there was AV block.

Figure-3: I've labeled with YELLOW arrows those leads in today's tracing that suggest where blocked PACs may be hiding (and subtly deforming the peak of the T wave).



Acknowledgment: My appreciation to Praneet Manekar (from Amritsar, India) for the case and this tracing.




Related ECG Blog Posts to Today’s Case: 

  • ECG Blog #185 — Reviews the Ps, Qs and 3R Approach to Systematic Rhythm Interpretation.
  • ECG Blog #232 — Reviews the concept of a Bigeminal Rhythm (which may be due to Atrial or Ventricular Bigeminy, Wenckebach conduction — or other causes — Listen to Audio Pearl #47).
  • ECG Blog #164 — Reviews a case of Mobitz I 2nd-Degree AV Block, with detailed discussion of the "Footprints" of Wenckebach.
  • ECG Blog #33 — Reviews a case showing blocked and aberrantly-conducted PACs. 
  • ECG Blog #66 — Reviews a case showing blocked and aberrantly-conducted PACs. 
  • ECG Blog #147 — Reviews a case showing blocked PACs. 
  • ECG Blog #57 — Reviews a case showing atrial bigeminy with blocked PACs.



Related ECG Blog Posts to Today’s AUDIO Pearl: 

  • The July 5, 2018 post in Dr. Smith's ECG Blog — (Please see My Comment at the bottom of the page for Review on the ECG diagnosis of Sick Sinus Syndrome). 

  • ECG Blog #196 — Reviews "My Take" on assessing the Regular WCT (Wide-Complex Tachycardia) — with tips for distinguishing between VT vs SVT with either preexisting BBB or aberrant conduction. 
  • ECG Blog #220 — Reviews the differential diagnosis for List #1: Causes of a Regular WCT Rhythm without clear sign of atrial activity (Media Pearl #37 in this blog post is an Audio that reviews assessment to determine IF the patient is Hemodynamically Stable).

  • ECG Blog #164 — Reviews a case 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 ABlocks (including "high-grade" AV block). 
  • ECG Blog #186 — Reviews when to suspect 2nd-Degree, Mobitz Type I.

  • ECG Blog #229  Reviews distinction between AFlutter vs ATach (and WHY AFlutter is so commonly overlooked — Listen to Audio Pearl #45)
  • The November 12, 2019 post in Dr. Smith's ECG Blog — Reviews another case of a Regular SVT rhythm.
  • The October 16, 2019 post in Dr. Smith's ECG Blog — My Comment (at the bottom of the page) reviews my approach to another case of a Regular SVT rhythm.
  • ECG Blog #199 — Reviews the ECG diagnosis of MAT (Watch Video Pearl #16).
  • ECG Blog #203 — Reviews a "user-friendly" approach to the Hemiblocks (LAHB/LPHB) and Bifascicular BlocksWatch Video Pearl #21).

Thursday, September 16, 2021

ECG Blog #251 (65) — HOW Does the Cycle End?

The ECG in Figure-1 was obtained from an 85-year old man who presented with "dizziness".



  • Can you explain what is happening with each beat on this tracing?


Figure-1: 12-lead ECG and long lead II rhythm strip from a 85-yo man with dizziness (See text).



My Sequential Approach to Today's Case: This is indeed a challenging tracing! I allowed myself a brief look at the 12-lead ECG before delving into details of the complex lead II rhythm strip — since I wanted to first ensure that the cause of the rhythm was not an acute event.

  • All of the beats on this tracing are narrow — therefore supraventricular! This means that the "usual" criteria for chamber enlargement and ST-T wave assessment can be applied.
  • Several criteria for LVH are clearly satisfied — based on very deep S waves in V3,V4 (Peguero Criteria) — and very tall (overlapping) R waves in V5,V6  (For more on LVH — See ECG Blog #245 — with attention to the Audio Pearl and Figures in the Addendum for specifics).
  • There is modest T wave inversion in a number of lateral leads (ie, leads I, aVL and V6) that is consistent with LV "strain" and/or some ischemia — but nothing that looks like an acute cardiac event! 


My Sequential Approach to the Rhythm:

As always — I began my systematic approach to the rhythm with assessment of the PsQs and 3Rs (as discussed in detail in ECG Blog #185).

  • P waves — A fairly (but not completely) regular atrial rhythm is seen (RED arrows highlighting sinus P waves in Figure-2).
  • As previously stated — the QRS is narrow — which confirms that the rhythm is supraventricular.
  • The rhythm is obviously not completely Regular — so the Rate varies. Regarding the 3rd R (ie, the "Relation" between P waves and neighboring QRS complexes) — it looks as if most of the sinus P waves (RED arrows) may be related to neighboring QRS complexes — but we need to look closer!



Figure-2: I've added RED arrows for the sinus P waves that were seen in Figure-1.


Additional QUESTIONS:

  • Take another look at Figure-2. In addition to the sinus P waves (RED arrows) — Are there other signs of atrial activity in this tracing?
  • Of all of the P waves that are seen in Figure-2  WHICH ones do you suspect are (and are not) being conducted?



The Case Continues:

In additon to the sinus P waves (RED arrows) — I've labeled the signs of other atrial activity with a YELLOW and a BLUE arrow (Figure-3). The challenge is to figure out the relationship (if any) to neighboring QRS complexes for each of the P waves in Figure-3.

  • PEARL #1: When faced with a complex rhythm that manifests a number of different features (such as the rhythm strip shown in Figure-3) — it is sometimes easiest to begin by looking to see IF there are any features that I can definitely explain. I then look to see if there is an "underlying rhythm" — and, I save for last the interpretation of those features that I am not yet certain about.
  • In Figure-3 — Although a sinus P wave does precede beat #1 — this PR interval is clearly too short to conduct. Since QRS morphology of beat #1 is narrow and virtually identical to the QRS morphology of all other beats on this tracing — beat #1 is not sinus-conducted. Note the long pause that precedes beat #1. This suggests that beat #1 is probably a junctional escape beat.
  • No QRS complex follows the negative P wave highlighted by the YELLOW arrow in Figure-3. This suggests that there is no "forward conduction" of this negative P wave (that looks very different in shape than the sinus P waves).
  • The P wave highlighted by BLUE arrow also looks very different than the sinus P waves (RED arrows) in this tracing. As a result — beat #6 is not sinus-conducted. Note the pause (ie, of 7.5 large boxes = 1.5 second) between beat #6 and the preceding QRS complex. Given that QRS morphology of beat #6 is narrow and virtually identical to the QRS morphology of all other beats on this tracing — this suggests that beat #6 is most probably either an ectopic atrial or junctional escape beat

PEARL #2: The most common form of 2nd-degree AV block is Mobitz Type I ( = AV Wenckebach— which is diagnosed by the finding of a progressively increasing PR interval until a beat is dropped. There follows a brief pause — after which the next Wenckebach cycle begins with a PR interval that is shorter than the longest PR interval that was seen right before the pause.

  • What is not commonly appreciated — is that there can be other ways that a Wenckebach cycle ends (See the Audio Pearl below).


Figure-3: I've labeled the additional signs of atrial activity with a YELLOW and a BLUE arrow.


  — The Case Continues BELOW today's Audio Pearl ... —



NOTE: Some readers may prefer at this point to listen to the 6:20-minute ECG Audio PEARL before reading My Thoughts regarding the Laddergram that I derive below.



Today's ECG Media PEARL #65 (6:20 minutes Audio) — Reviews the different ways that an AV Wenckebach cycle may end (as well as reviewing the "Footprints" of Wenckebach).




Deriving the LADDERGRAM:

The complex mechanism of today's case is best explained by step-by-step derivation of a Laddergram (See ECG Blog #188 for review on how to read and/or draw Laddergrams).

  • NOTE: Today's case provides an example in which I needed to construct a valid laddergram explanation in order to "solve" the arrhythmia.
  • Sequential legends over the next 7 Figures illustrate my thought process as I derived this laddergram.



Figure-4: It is usually easiest to begin a laddergram by marking the path of sinus P waves through the Atrial Tier (RED lines drawn directly below the onset of each of the sinus P waves — as shown by the large GREEN arrows). Note that these RED lines in the Atrial Tier are nearly vertical — since conduction of sinus P waves through the atria is rapid. Note also that the P-P interval between successive sinus P waves (vertical RED lines) is equal!


Figure-5: The most challenging part of most laddergrams is construction of the AV Nodal Tier — so I generally save that for last. Therefore, after drawing sinus P waves into the Atrial Tier — I prefer to next add indication of all narrow QRS complexes into the Ventricular Tier. The large GREEN arrows show that my landmark for QRS complexes in the laddergram is the onset of the QRS. Note that the RED lines in the Ventricular Tier are also nearly vertical — since conduction of these narrow QRS complexes through the ventricles is rapid.


Figure-6: It's time to begin "solving" what we can in the laddergram. I do this by connecting those P waves in the Atrial Tier that might logically be conducting to narrow QRS complexes in the Ventricular Tier (slanted BLUE lines within the AV Nodal Tier)Doesn't it appear as if the angle of slant for these BLUE lines in the AV Nodal Tier is increasing?


Figure-7: Caliper measurement of the PR intervals for beats #2-thru-5 confirms progressive increase in the PR interval (from 340 to 440 msec.). The PR interval shortens before beat #7 (to 340 msec.) — and then lengthens again, to 410 msec. before beat #8. This is Wenckebach conduction! But unlike a typical Mobitz I block — the Wenckebach cycle from beats #2-thru-5 does not end with an on-time non-conducted sinus P wave. 


Figure-8: It's time to assess the "missing parts" of our laddergram. As we previously suggested — the PR interval of the sinus P wave preceding beat #1 is clearly too short to conduct. This strongly suggests that beat #1 is a junctional escape beat. We schematically represent this with a BLUE circle within the AV Nodal Tier that conducts retrograde (dotted BLUE line), thereby preventing forward conduction of this first sinus P wave. Since beat #6 is preceded by a dfferent-looking (negative) P wave with a short PR interval — this suggests beat #6 is a low atrial escape beat (BLUE circle within the Atrial Tier).


Figure-9: The remaining "missing part" of our laddergram relates to the small-amplitude negative P wave deflection highlighted by the YELLOW arrow. I introduced the concept of Echo beats in ECG Blog #239. The dotted BLUE lines propose retrograde conduction originating from the path of the sinus P wave that precedes beat #5 (as it passes through the AV Nodal Tier) as the source of this negative P wave. This is a logical proposal — because Echo beats are most likely to originate following PR interval prolongation (as discussed in detail in ECG Blog #239).


Figure-10: For clarity — I've labeled the unusual elements in this tracing. The rhythm strip begins with a junctional escape beat (JE), which is beat #1. There follows a Wenckebach cycle for beats #2-thru-5, with progressively increasing PR intervals. Instead of ending with a non-conducted on-time P wave — this short run of Mobitz I, 2nd-Degree AV Block is terminated by an Echo beat (YELLOW arrow). Presumably — this Echo beat resets the SA Node — so that instead of a sinus P wave, the 1.5 second pause between beats #5-and-6 ends with a low atrial escape beat (AE). This finally allows enough time for the SA Node to recover (RED arrow preceding beat #7) — as the next Wenckebach cycle begins (ie, with beats #7 and 8).


Conclusion of Today's Case:

In addition to Mobitz I 2nd-Degree AV Block — this 85-year old man with dizziness had significant bradycardia with pauses on additional monitoring. A permanent pacemaker was placed.




Acknowledgment: My appreciation to 林柏志 (from Taiwan) for the case and this tracing.




Related ECG Blog Posts to Today’s Case: 

  • ECG Blog #185 — Reviews the Ps, Qs & 3R Approach to Systematic Rhythm Interpretation.
  • ECG Blog #164 — Reviews a case 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 #239 — Reviews the concept of Echo Beats, and its clinical applications (showing another case of Wenchekbach terminated by Echo beats).
  • ECG Blog #188 — Reviews how to understand (and how to drawLaddergrams!