ECG Blog #252 — 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.

 

QUESTION:

• 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 ... —

 

 

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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).

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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 QRS) bigeminal 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, Qs and 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; — and, ii) 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).

 

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

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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).
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• ECG Blog #164 — Reviews a case of Mobitz I 2nd-Degree AV Block, with detailed discussion of the "Footprints" of Wenckebach.
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• 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 AV Blocks (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).
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• ECG Blog #203 — Reviews a "user-friendly" approach to the Hemiblocks (LAHB/LPHB) and Bifascicular Blocks — Watch Video Pearl #21).