ECG Blog #344 — Mobitz I, Mobitz II or Neither?

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 — See ECG Blog #404 — for a Video presentation of this case!

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How would YOU interpret the lead II rhythm strip shown in Figure-1?

• Is the rhythm Mobitz I or Mobitz II 2nd-degree AV Block? 
• Or — Is it “something else”?

Figure-1: How would YOU interpret this lead II rhythm strip?

MY Initial Approach to the Rhythm in Figure-1:

Unfortunately — We are not provided with any history, nor with the 12-lead tracing for this patient. That said — the ECG in Figure-1 provides us with enough information to interpret the rhythm. As always — I favor the Ps, Qs, 3R Approach (See ECG Blog #185).

• PEARL #1: It does not matter in what sequence you look for the Ps, Qs and 3Rs — and I often vary the sequence I use depending on specifics of the tracing at hand. Most of the time when it looks like some form of AV block may be present (ie, because it seems like a number of on-time P waves that should conduct are not conducting) — I find it easiest to start by identifying P waves. The simple step of using calipers to “walk out” the atrial rhythm allows me to establish within a few seconds whether or not there is an underlying regular atrial rhythm (RED arrows in Figure-2). 

Figure-2: Using calipers — I marked out a regular atrial rhythm (RED arrows).

Applying the Ps, Qs and 3Rs to Today’s Rhythm:

• P Waves: As highlighted by the RED arrows in Figure-2 — the atrial rhythm is fairly regular with similar looking P waves that are upright in this lead II rhythm strip. There is therefore slight sinus arrhythmia — with an atrial rate that varies between ~85-95/minute.
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• PEARL #2: As is evident for many of the examples of AV block that have appeared in this ECG Blog — it is extremely common for there to be a “ventriculophasic" sinus arrhythmia in association with 2nd or 3rd degree AV block. Much of the time (as is the case in Figure-2) — the shorter P-P interval is the one that “sandwiches” a QRS complex (the theory being that perfusion improves following ventricular contraction — with resultant shortening by a slight amount the P-P interval that contains a QRS).
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• QRS Width: The QRS complex in Figure-2 looks narrow (ie, not more than 0.10 second = not more than half a large box in duration). 
• NOTE: In order to be 100% certain that the QRS is truly narrow — We would need to see a 12-lead ECG (since on occasion — a part of the QRS may lie on the baseline in the 1 lead that you are looking at). That said, for practical purposes — the very narrow appearance of all QRS complexes in Figure-2 suggests that more likely than not, the QRS is narrow. 

And the 3 Rs:

• Rate and Regularity of the Rhythm: It is good to keep in mind that when a 1:1 relationship between P waves and QRS complexes is not present — that we need to assess the “rate” and “regularity” of both the atrial and ventricular rhythms. In Figure-2 — after beat #2, the ventricular rhythm is fairly regular at a rate just under 50/minute. As already stated — the underlying atrial rhythm is a minimally irregular “ventriculophasic” sinus arrhythmia at between ~85-95/minute.
• Related? (ie, Are P waves “married” to the QRS?): To assess whether P waves are (or are not) related to neighboring QRS complexes — I look in front of each QRS complex on the tracing. The fact that each QRS complex in today’s tracing is preceded by a P wave — and the PR interval in front of most of these QRS complexes is the same — confirms that there is at least some conduction!

QUESTION: Note in Figure-3 — that I have labeled each of the P waves in today's tracing with letters. 

• Which of these P waves are definitely not conducted?
• Are you uncertain about any of the P waves in Figure-3?

Figure-3: I’ve labeled each of the P waves from Figure-2. Which P wave(s) would you say are definitely not conducted?

Which P Waves are Not Conducted?

• We have established that each of the P waves preceded by the same PR interval in Figure-3 — are conducted (ie, the P waves labeled a, d, f, h, j, l are all conducted).
• Note that the PR interval for each of these 6 P waves that we know are conducted — is prolonged (ie, to about 0.31 second in duration). As a result — an underlying 1st-degree AV block is present.
• Those P waves in Figure- 3 that we know are not conducted — are the ones that occur after a beat, but which are not then followed by a QRS (ie, P waves c, e, g, i and k are not conducted).
• This leaves us with P wave "b" — about which we might not yet know whether this P wave is (or is not) conducted — because the PR interval of "b" is different (longer) than the PR interval for the above beats that we know are conducted.

Putting It All Together:

Thus far — We have established the following:

• There is an underlying ventriculophasic sinus arrhythmia (RED arrows in Figure-3).
• The rhythm appears to be supraventricular (at least according to this single monitoring lead — the QRS looks to be narrow).
• Focusing on that part of the rhythm that we are certain about (ie, forgetting for the moment about beats #1 and 2 — and focusing on what happens beginning with beat #3 — it is only every-other-P-wave that is conducted). This defines the rhythm as some type of AV block!

PEARL #3: The KEY to recognizing that the rhythm in Figure-3 represents some type of AV block (instead of non-conducted PACs) — is that the atrial rhythm is regular (or at least almost regular).

• Note that we have ruled out complete (ie, 3rd-degree) AV block — because the constant PR interval proves that at least some beats are conducted.
• Therefore — starting with beat #3, there is 2nd-degree AV block with 2:1 AV conduction.

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NOTE: The above concepts are easier to visualize when illustrated by use of a laddergram. 

• Continuing to focus on what happens beginning with beat #3 — the laddergram in Figure-4 shows that every-other-P-wave is conducted to the ventricles. 
• P waves highlighted by WHITE arrows are blocked (they do not make it out of the AV node). 
• Thus, beginning with beat #3 — there is 2:1 AV block.

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Figure-4: Laddergram illustration of the rhythm in Figure-3. Focusing on the rhythm beginning with beat #3 — RED arrows after this point indicate P waves that are conducted to the ventricles (albeit with 1st-degree AV block). On-time P waves that do not make it out of the AV node are highlighted by WHITE arrows (NOTE: For review on how to read and/or draw laddergrams — Please check out ECG Blog #188).

PEARL #4: As reviewed in detail in the ADDENDUM below — there are 3 Types of 2nd-degree AV block. These are:

• Mobitz I, 2nd-degree AV block ( = AV Wenckebach) — in which the PR interval increases until a beat is dropped.
• Mobitz II — in which the PR interval remains constant for consecutively conducted beats until one or more beats are non-conducted.
• 2nd-degree with 2:1 AV block — in which because of the fact that we never see 2 consecutively conducted beats — We are not able to tell if the PR interval would lengthen before dropping a beat IF given the chance to do so.
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• BOTTOM Line: If the rhythm strip in Figure-4 began with beat #3 (and we only saw beats #3-thru-7) — then we would not be able to tell if this rhythm represents the Mobitz I or Mobitz II form of 2nd-degree AV block. KEY Point: Distinction between Mobitz I and Mobitz II is important — because the clinical course of Mobitz I is often fairly benign — whereas patients with Mobitz II are much more likely to need a permanent pacemaker.

PEARL #5: There are a number of ECG and clinical Clues that may help to determine IF the type of 2nd-degree AV block you are looking at is more likely to be a Mobitz I or Mobitz II block. These clues include the following:

• Mobitz II is rare. In my experience — well over 90-95% of all 2nd-degree AV blocks will turn out to be Mobitz I.
• The QRS complex will most often be wide with Mobitz II. In contrast — the QRS is most often narrow with Mobitz I (although an exception may exist if there is preexisting bundle branch block in a patient with Mobitz I).
• Mobitz I is most often associated with recent or acute inferior infarction. In contrast — Mobitz II is usually associated with anterior infarction.
• The PR interval of conducting beats with Mobitz I is often prolonged. In contrast — the PR interval of conducting beats with Mobitz II is often normal.
• Finally — It is unlikely to switch back-and-forth from Mobitz I to Mobitz II (or vice versa). Therefore, IF on reviewing additional monitoring on your patient you see clear evidence of Mobitz I (ie, progressive PR interval lengthening until a beat is dropped) — then it is very likely that the 2:1 AV block you are looking at also reflects Mobitz I.

Applying the Clues from PEARL #5:

Take one last look at the laddergram in Figure-4.

• QUESTION: What is happening at the beginning of this tracing?

ANSWER:

The completed laddergram in Figure-5 answers this question.

• The P wave in front of beat #1 in Figure-5 — is conducted with the same prolonged PR interval as is seen preceding conducted beats #3,4,5,6,7 (RED arrows).
• The P wave in front of beat #2 is also conducted (BLUE arrow) — but with a further increase in the PR interval.
• The WHITE arrow P wave seen after beat #2 is not conducted.
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• Putting It All Together: The first 2 beats in Figure-5 form a typical Wenckebach cycle (ie, progressive increase in the PR interval until a beat is dropped). This strongly suggests that the 2:1 AV block that begins with beat #3 also represents Mobitz I, 2nd-degree AV block. In support of this conclusion (as per Pearl #5) are: i) Statistics (ie, Mobitz I is so much more common than Mobitz II); ii) The narrow QRS complex for all beats in today's tracing; and, iii) The prolonged PR interval seen for all conducted beats.

Figure-5: I've completed the laddergram from Figure-4 (See text).

CASE Conclusion:

The above deductions illustrate how we arrive at a definitive diagnosis of 2nd-degree AV Block, Mobitz Type I with 3:2 and 2:1 AV conduction for today's tracing. Although I unfortunately lack specific follow-up for this case — We emphasize that clinical decision-making would be based on the following concepts.

• The overall ventricular rate in today's tracing is ~50/minute. At this overall heart rate — the patient may or may not be symptomatic. Symptoms may be subtle (ie, a vague "sense" of fatigue). Clearly, treatment recommendations will be greatly influenced by symptoms that result directly from bradycardia.
• The longterm prognosis of Mobitz I may be benign — especially if associated with recent inferior infarction that is successfully treated (ie, It is common for Mobitz I to spontaneously resolve in such patients over a period of days to 1-2 weeks).
• It is possible that this patient's conduction defect might improve IF — some other "fixable" precipitating factor(s) is present (ie, electrolyte disorder, sleep apnea, use of a rate-slowing medication). Permanent pacing will not be needed IF you can "find and fix" the precipitating cause of the rhythm.
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• On the other hand — If the degree of AV block progresses — or the ventricular rate slows more and/or the patient develops persistent bradycardia-related symptoms — then permanent pacing will be needed, even though the conduction defect is "only" Mobitz I.

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Acknowledgment: My appreciation to Hafiz Abdul Mannan Shahid (from Lahore, Pakistan) for the case and these tracings.

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ADDENDUM (11/13/2022):

• I've included below an Audio Pearl — a Video Pearl — and links for download of PDFs reviewing the ECG diagnosis of AV Blocks.

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ECG Media PEARL #4 (4:30 minutes Audio): — takes a brief look at the AV Blocks — and focuses on WHEN to suspect Mobitz I.

My GOAL in the 15-minute ECG Video below — is to clarify ECG diagnosis of the 2nd-Degree AV Blocks, of which there are 3 Types:

• Mobitz I ( = AV Wenckebach).
• Mobitz II.
• 2nd-Degree AV Block with 2:1 AV conduction.

This 15-minute ECG Video (Media PEARL #52) — Reviews the 3 Types of 2nd-Degree AV Block — plus — the hard-to-define term of "high-grade" AV block. I supplement this material with the following 2 PDF handouts.

• Section 2F (6 pages = the "short" Answer) from my ECG-2014 Pocket Brain book provides quick written review of the AV Blocks (This is a free download).
• Section 20 (54 pages = the "long" Answer) from my ACLS-2013-Arrhythmias Expanded Version provides detailed discussion of WHAT the AV Blocks are — and what they are not! (This is a free download).

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Related ECG Blog Posts to Today’s Case: 

• ECG Blog #185 — Review of the Ps, Qs, 3R Approach for systematic rhythm interpretation.
• ECG Blog #188 — Reviews how to read and draw Laddergrams (with LINKS to more than 50 laddergram cases — many with step-by-step sequential illustration).
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• ECG Blog #164 — Which reviews step-by-step the diagnosis of a Mobitz I 2nd-degree AV block (with sequential laddergram illustration).
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• ECG Blog #168 — A complex dual-level AV Wenckebach (Laddergram).
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• ECG Blog #154 and ECG Blog #55 and ECG Blog #224 and ECG Blog #232 — Acute MI with AV Wenckebach.
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• ECG Blog #63 — Mobitz I with Junctional Escape Beats.