ECG Blog #465 — A Tale of Syncope & 2 Rhythms

The ECG in Figure-1 was obtained from an older woman who presented to the ED (Emergency Department) because of a syncopal episode. She was asymptomatic at the time this ECG was recorded.

QUESTIONS:

• How would YOU interpret the ECG in Figure-1?
•   Is there AV block?  If so — What kind?

Figure-1: The initial ECG in today's case. (To improve visualization — I've digitized the original ECG using PMcardio).

MY Thoughts on Figure-1:

As always — I favor the Ps, Qs, 3R Approach as an optimally time-efficient way to assess any arrhythmia, including the AV blocks (See ECG Blog #185).

• As shown in the long lead II rhythm strip at the bottom of Figure-1 — the Rhythm is regularly irregular (ie, there is group beating, with repetitive short-long cycles). As a result — the Rate of the rhythm is not constant, although the overall ventricular rate is not overly rapid.
• P waves are present.
• The QRS is wide (ie, more than half a large box in duration). Looking at the 12-lead ECG that appears above the long lead II rhythm strip — QRS morphology appears to be consistent with LBBB (Left Bundle Branch Block) conduction, in that the QRS is all upright in left-sided leads I,V6 — and predominantly negative in right-sided lead V1, as well as in other anterior chest leads.

This leaves us with needing to assess the 5th Parameter — which is determining whether those P waves that are present, are Related to neighboring QRS complexes?

• To answer this question — I’ve labeled the P waves that we see in the long lead II rhythm strip with RED arrows (Figure-2).
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• PEARL #1: To facilitate determining if any of the P waves that you see are conducting — Label the P waves you identify! Doesn't this simple step of labeling P waves make it easier to determine which P waves are (or are not) Related to neighboring QRS complexes?

Figure-2: I've labeled P waves in today's initial ECG with RED arrows.

Are P Waves Related to Neighboring QRS Complexes?

Focus on QRS complexes that end each of the pauses in Figure-2 (ie, on beats #3,5,7,9 and 11).

• Isn't the PR interval that precedes beats #3,5,7,9,11 constant? This tells us that each of these beats is being conducted to the ventricles. (NOTE: Although we do not see far enough in front of beat #1 to assess the PR interval — by its similarity to the overall pattern in this tracing, we can presume that beat #1 is also being conducted to the ventricles).
• Next — Focus on the 2nd beat in each group (ie, on beats #2,4,6,8,10,12). Although somewhat difficult to tell because the P wave in front of each of these beats is partially hidden within the preceding T wave — Doesn't it appear that the PR interval before beats #2,4,6,8,10,12 is also constant, as well as being equal to the PR interval before beats #1,3,5,7,9,11?

For clarity in Figure-3 — I've drawn a laddergram of the long lead II rhythm strip from today's initial ECG. Note the following:

• Group beating (repetitive 2 beat groups throughout the tracing).
• A fairly (albeit not completely) regular atrial rhythm.  
• QRS widening (that we determined is consistent with LBBB conduction).
• All 12 QRS complexes in Figure-3 are preceded by P waves that manifest the same constant PR interval (ie, the slope of the RED lines within the AV Nodal Tier remains constant). 
• The PR interval of conducted beats is not prolonged.
• Every 3rd P wave is not conducted (ie, every 3rd P wave fails to make it out of the AV Nodal Tier).

Conclusion: The finding of a fairly regular atrial rhythm with failure of one or more on-time P waves to conduct to the ventricles defines today's initial rhythm as a form of 2nd-degree AV block.

• QUESTION: Which type of 2nd-degree AV block? 

Figure-3: Laddergram of the long lead II rhythm strip from Figure-2.

PEARL #2: 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 QRS complexes 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: By the above definitions — the rhythm in Figure-3 appears to be the Mobitz II form of 2nd-degree AV block because: i) The QRS is wide; ii) On-time sinus P waves are not conducted to the ventricles (which happens with every 3rd P wave in Figure-3); — and, iii) The PR interval remains constant for consecutively conducted beats.
• KEY Point: Distinction between the Mobitz I and Mobitz II forms of 2nd-degree AV block 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 #3: 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. As a result — Mobitz II is very unlikely if the QRS is not wide. In contrast — the QRS is most often narrow with Mobitz I (although exceptions exist IF in a patient with Mobitz I — there is preexisting bundle branch block).
• Mobitz I is most often associated with recent or acute inferior infarction. In contrast — Mobitz II is most often associated with anterior infarction.
• The PR interval for beats that conduct with Mobitz I is often prolonged (sometimes markedly so). In contrast — the PR interval of conducting beats with Mobitz II is more often normal, or no more than minimally prolonged.
• It is unlikely to switch back-and-forth from Mobitz I to Mobitz II (or vice versa). Therefore, IF on review of 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 likely that all tracings on that patient (including those with 2:1 AV block) — are also Mobitz I.

Regarding Today's CASE ...

• Because of the implication that pacing will be needed if today's rhythm is Mobitz II — We want to be as certain as possible that the PR interval is not increasing with consecutively conducted beats. As noted earlier — it is difficult to be certain of this in Figure-2, because the onset of the 2nd P wave in each 2-beat group is partially hidden within the preceding T wave.
• The 12-lead ECG that is seen in Figure-2 does show LBBB — but there is no clear sign of acute, recent or previous infarction.
• Conclusion: I strongly suspect the rhythm is Mobitz II — but I cannot be 100% certain this from this single ECG.

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The CASE Continues:

Today's patient had another syncopal episode. Her ECG at that time is shown in Figure-4.

QUESTIONS:

• How would YOU interpret the ECG in Figure-4?
•   Is there AV block?  If so — What kind?

Figure-4: Repeat ECG obtained following another syncopal episode.

MY Thoughts on Figure-4:

The rhythm in Figure-4 is now slower, but regular. The R-R interval is ~8 large boxes in duration — corresponding to a rate a bit less than 40/minute. P waves are seen. The QRS is obviously wide.

• As per PEARL #1 — Assessment of whether P waves are (or are not) related to neighboring QRS complexes is most easily accomplished by labeling P waves — which I have done in Figure-5.

Figure-5: I've labeled the P waves in Figure-4.

Are P Waves in Figure-5 Related to the QRS?

In Figure-5 — I've labeled those P waves that we can readily identify with RED arrows. Although there is slight variation in the P-P interval — this type of ventriculophasic sinus arrhythmia is common with 2nd- and 3rd-degree AV blocks.

• Given that it makes more sense for the underlying atrial rhythm to remain regular (rather than to all-of-a-sudden drop several beats) — I added PINK arrows in Figure-5 where I expected to find 2 additional P waves. (Slight distortion at the beginning of the QRS of beat #3 — and at the end of the T wave of beat #3 — strongly suggests that P waves do lie below these 2 PINK arrows).

Putting Together what we've determined in Figure-5:

• The QRS is wide.
• The ventricular rhythm is essentially regular (with minor variation in the R-R interval due to ventriculophasic sinus arrhythmia). The ventricular rate is just under 40/minute.
• An almost regular atrial rhythm is present (colored arrows in Figure-5). Focusing on the P waves before each of the 6 beats in the long lead II rhythm strip — the PR interval continually changes. Thus, P waves “are marching through” the QRS — such that there appears to be complete AV dissociation (ie, None of the P waves in Figure-5 are being conducted to the ventricles).

PEARL #4: Clearly, there is at least 2nd-degree AV block in Figure-5 — because many of the on-time P waves are not being conducted to the ventricles (ie, There are many more P waves than QRS complexes in this tracing). 

• As discussed in ECG Blog #405 — the KEYs for determining if complete (3rd-degree) AV block is present are: i) Whether there is an underlying regular (or almost regular) atrial rhythm; and, ii) Whether all P waves fail to conduct despite having an adequate opportunity to conduct. To satisfy these conditions — the rhythm strip must be long enough for P waves to occur during all parts of the R-R interval, and still fail to conduct.
• PEARL #5: Escape rhythms from the AV Node, the His or the ventricles — tend to be regular. As a result — the BEST clue for suggesting that AV block is not complete — is if the ventricular rhythm is not regular. The occurrence of one or more QRS complexes earlier-than-expected usually means that those earlier beats are being conducted.
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• Conclusion: As a result, I strongly suspect that the rhythm in Figure-5 is complete AV block because: i) The QRS is wide; ii) The ventricular rate is slow and regular; iii) The atrial rate is regular; — and, iv) None of the P waves appear to be conducting to the ventricles despite many of these on-time P waves occuring in parts of the cycle during which we would expect P waves to be able to conduct.
• Beyond-the-Core: Technically, we can not rule out the possibility that some P waves might be able to conduct if "given the chance" — since we do not truly see P waves occurring over all parts of the R-R interval. To see this — we would probably need another 20-to-30 seconds of monitoring. That said — the failure to conduct consecutive P waves at many points in this 10-second rhythm strip suggest at the very least, that there is high-grade (if not complete) AV block. 

QUESTIONS:

Take another LOOK at the 2 ECGs in today's case. To facilitate comparison — I have placed these tracings together in Figure-6. 

• Did YOU notice how different the QRS complex looks in each tracing? — WHY is this so?
• What about the appearance of ST-T waves in ECG #2?

Figure-6: Side-by-side comparison of the 2 ECGs in today's case. 

 

ANSWER: QRS morphology in ECG #1 (TOP tracing) is consistent with LBBB conduction (ie, all positive in left-sided leads I and V6 — but predominantly negative in the anterior chest leads). The constant PR interval preceding each beat in ECG #1 tells us that each of the 12 QRS complexes in this tracing is being conducted — although every third P wave is not. Thus, the rhythm in ECG #1 is 2nd-degree (not 3rd-degree) AV block.

• In contrast — the QRS complex is wider in ECG #2, with a very different QRS morphology (ie, resembling RBBB conduction in lead V1 — albeit with marked right axis in the limb leads). In association with the much slower ventricular rate and the constantly changing PR interval throughout this tracing — this suggests there is now a ventricular "escape" focus (which supports our assumption that the rhythm is now complete AV block).
• If any of the P waves in ECG #2 were to be conducting — we would expect to see a return to the LBBB conduction morphology that we saw in ECG #1.
• The laddergram in Figure-7 schematically illustrates failure of all atrial impulses to conduct in ECG #2 — because there is now complete AV block (dotted line in the AV Nodal Tier) — with resultant slow ventricular "escape".

Figure-7: Laddergram showing complete AV block in ECG #2.

To conclude the case — Take another LOOK at ST-T wave morphology in ECG #2 (BOTTOM tracing in Figure-6). Although none of the QRS complexes in this tracing are being conducted — Doesn't ST-T wave morphology look abnormal? (ie, There is ST segment coving with deeper-than-expected T wave inversion in leads V1-thru-V4).

• It's possible that the reason for the slower heart rate and progression to complete AV block is the result of a recent event. Serial troponins and ECGs are indicated to rule out an acute or recent MI.

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Summary of Today's CASE:

The patient in today's case is an older woman who presented to the ED for a syncopal episode. Although she was asymptomatic at the time her initial ECG was recorded — this initial tracing (shown in Figure-1) showed frequent non-conducted P waves as a result of 2nd-degree AV block of the Mobitz II type.

• As emphasized — clinical implications of Mobitz II are clearly more worrisome than for the much more common Mobitz I type of 2nd-degree AV block. For this reason — permanent pacing will often be needed when the rhythm is Mobitz II.

Today's patient then had a 2nd syncopal episode. Her repeat ECG at this time (shown in Figure-4) — now showed complete AV block, with a slow ventricular "escape" rhythm.

• Final Follow-Up: A recent event was ruled out. Thus, this case showed progression of Mobitz II to complete AV block. The patient received a permanent pacemaker.

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Acknowledgment: My appreciation to Rajeesh R Pillai (from Kollam, Kerala, India) for the case and this tracing.

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ADDENDUM (1/18/2025): 

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

—  —

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 — Reviews the Ps, Qs and 3R Approach to Systematic Rhythm Interpretation.
• ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.
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• ECG Blog #188 — Reviews how to read and draw Laddergrams (with LINKS to more than 100 laddergram cases — many with step-by-step sequential illustration).
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• ECG Blog #256 — and Blog #342 — Review  of SSS (Sick Sinus Syndrome).
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• ECG Blog #295 — Reviews the concept of bradycardic-induced BBB ( = Phase 4 block). This is discussed near the bottom of the page (ie, in Pearl #5 — that appears just under Figure-6).
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• 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).
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• ECG Blog #192 — The 3 Causes of AV Dissociation.
• ECG Blog #191 — Reviews the difference between AV Dissociation vs Complete AV Block.
• ECG Blog #389 — ECG Blog #373 — and ECG Blog #344 — for review of some cases that illustrate "AV block problem-solving".
• ECG Blog #251 — Reviews the concepts of Wenckebach periodicity and the "Footprints" of Wenckebach.
• ECG Blog #164 — Reviews a case of typical Mobitz I 2nd-Degree AV Block (with detailed discussion of the "Footprints" of Wenckebach). 
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• ECG Blog #236 — for an ECG Video Pearl on the 3 Types of 2nd-degree AV block.
• ECG Blog #344 — thoroughly reviews the Types of 2nd-degree AV block (Mobitz I vs Mobitz II vs 2:1 AV Block).
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• ECG Blog #63 — Mobitz I, 2nd-degree AV block with junctional escape.
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• ECG Blog #405 — ECG Video presentation that reviews the distinction between AV Dissociation vs Complete (3rd-degree) AV Block (For a LINKED Contents to this ECG Video — Click on MORE in the Description under the video on YouTube).