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).


  1. Very good explanation as always sir!
    David White

  2. Thank you for your detailed interpretation which is very interesting and informative, especially the way you confirm and exclude the causes of the pause in this ECG. I learn a lot from this. I also have 2 queries which need your help, your explanation, those are : (1) why the PP interval containing the nonconducted PAC (# 1560 ms) is longer than 2 basic PP intervals ( 2x 720= 1440 ms) . Because, as usual, in case of PAC ( regardless of conducted or nonconducted) the PP interval containing the PAC is almost less than 2 basic PP intervals ( if the PAC resets the SA node) or equal to 2 basic PP ( if the PAC does not reset the SA node) and (2) how can we rule out the probability that this is AVB2 Mobitz 2 with 3:2 ratio underlying trigeminal nonconducted PAC ?

  3. THANKS for your question. I do review this in detail on the blog post. Ideally, there would be additional monitoring of this patient to prove beyond a doubt that my theory is correct (it wasn't my case — so all I have is this single tracing) — BUT — the fact that there is NOT "Wenckebach periodicity" (because the pause is more than twice the shortest R-R interval) is strongly againsts Type I AV or SA block — and the P-P isn't regular (as it should be if there was Mobitz II ... ).

    Although it IS possible for PACs that reset the sinus node to have relatively shorter pauses — when they are longer-than-expected — you have to think of either sinus pauses of non-conducted PACs.

    The KEY is the VERY CAREFUL assessment of ST-T wave morphology for BOTH beats in EACH of the PAIRS (ie, looking at all 12 leads). YELLOW arrows illustrate this in Figure-3.

    BIGGEST CLUE: The title of my Audio Pearl for this episode is, "Common Things are Common" — and that title provides a KEY CLUE as to what we should immediately be looking for WHENEVER we see a rhythm like this one in today's case, in which there are PAUSES.

    Thanks again for your interest! — :)