ECG Blog #63 — Escape from Mobitz I

Interpret the rhythm below using the Ps,Qs,3R Approach. The patient is hemodynamically stable.

• Is there AV block or not? If so — Describe the type of AV block present.
• HINT #1: Look first at this tracing from a little distance away.
• HINT #2: The patient was admitted with recent inferior infarction.

Figure-1: Rhythm strip obtained from a patient with recent inferior infarction. He was hemodynamically stable at the time this tracing was recorded. Is there AV block? If so — What type of AV block? 

ANSWER to Figure-1:

Full interpretation of the rhythm strip shown in Figure-1 is indeed challenging. That said — interpretation of the essential points is not nearly as difficult as it may seem IF one keeps in mind the basic concepts for interpreting any cardiac arrhythmia.

• The overall rhythm is not regular. Looking first at the tracing from a little distance away — there appears to be group beating. That is — there are alternating short-longer cycles (beats #1-2; 3-4; and 5-6).
• Awareness that an arrhythmia manifests “group beating” — should at least suggest the possibility of some type of Wenckebach conduction disturbance. This is especially true in a patient with recent inferior infarction — since 2nd degree AV block, Mobitz Type I (AV Wenckebach) often occurs in this setting.
• We emphasize that not every rhythm with group beating will be the result of Wenckebach. For example, atrial or ventricular bigeminy or trigeminy (every 2nd or every 3rd beat a PAC or a PVC) — will also manifest group beating. But recognition that there is group beating may prove invaluable for honing our diagnostic assessment — as we will see momentarily!

Continuing with the Ps,Qs,3R Approach for the rhythm in Figure-1 — we note the following:

• The QRS complex is narrow for all beats on the tracing (albeit some QRS complexes look slightly different from neighboring QRS complexes). The fact that the QRS complex is narrow confirms that the mechanism of this rhythm is supraventricular.
• The atrial rhythm is at least fairly regular (red arrows in Panel B of Figure-2). There is slight variability in the P-P interval (due to underlying ventriculophasic sinus arrhythmia) — but all P waves are upright and manifest similar morphology in this lead II.
• The most challenging aspect of this arrhythmia is determining IF there is any relationship between P waves and neighboring QRS complexes?

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NOTE: To facilitate detection of whether any P waves may be conducting — We have added arrows and circles to the original rhythm strip (Figure-2).

• PEARL: Whenever you are confronted with a challenging rhythm strip — START with those aspects of the tracing about which you can be certain. Therefore, in Figure-2 – we know that the P wave preceding beat #5 is not conducting. It can’t be — since the PR interval preceding beat #5 is simply too short to conduct! Since the QRS complex of beat #5 is narrow and not preceded by a P wave that conducts — it must be a junctional escape beat.
• There are other P waves in Figure-2 that are not conducting. These include the P waves occurring just after the T waves of beats #2 and #4.
• On the other hand — some P waves do conduct. Note in Panel B of Figure-2 that the PR interval preceding beats #2 and #4 is identical — and that both of these beats end a short cycle. This is because beats #2 and #4 are sinus-conducted beats. These beats are conducted with 1st degree AV block (since the PR interval preceding beats #2 and #4 clearly exceeds one large box in duration).
• Since the atrial rhythm is at least fairly regular — and some beats are conducted while others are not – the rhythm disturbance in Panel B of Figure-2 must represent a form of 2nd-degree AV block.
• Given that the QRS complex in Panel B is narrow — and that there is group beating in this patient with recent inferior infarction — the odds overwhelmingly favor Mobitz I (which is so much more common than the Mobitz II form of 2nd degree AV block). In further support of Mobitz I — is the long PR interval preceding the 2 beats that we know are conducting.

Figure-2: Panel A reproduces Figure-1. Labels have been added in Panel B. There is underlying sinus arrhythmia (red arrows). Second degree AV block is diagnosed by the fact that at least some P waves are conducting (the P waves preceding beats #2 and #4) — while others are not. Group beating is present (alternating short-long cycles) — and the QRS complex is consistently narrow. The longer cycles (7.8 large boxes in duration) are ended by junctional escape beats (highlighted by blue circles). The conduction disturbance is almost certain to be Mobitz I (See text).

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Beyond-the-Core:

IF your interpretation of the rhythm in Figure-2 was simply 2nd degree AV block of some sort with group beating – therefore possible Mobitz I — We would be ecstatic. Exploring this premise further — We make the following advanced-level observations:

• Despite the fact that there are many nonconducted beats in Figure-2 — the conduction disturbance for this rhythm is clearly not 3rd degree AV block. We can recognize at a glance that this is unlikely to be 3rd degree AV block — because the ventricular rhythm is not regular. Instead — beats #2 and #4 each occur earlier-than-anticipated, and are preceded by the same PR interval. These beats are conducted.
• Note the subtle-but-real difference in QRS morphology among various beats on this tracing. This is not artifact. Compared to beats #2,4,6 — beats #1,3,5 all manifest slightly taller R waves and less deep S waves (blue circles). We have already established that beat #5 is a junctional escape beat — since the PR interval preceding beat #5 is clearly too short to conduct. Recognition that each of the beats highlighted by the blue circles in Panel B (beats #1,3,5) manifest similar QRS morphology strongly suggests that beats #1 and #3 are also junctional escape beats. This is supported by the finding that beats #3 and #5 are each preceded by the same R-R interval (of 7.8 large boxes in duration) — which corresponds to a junctional escape rate of ~40/minute.

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We can now make sense of the complex events that occur in Figure-2:

• Beat #2 begins a Wenckebach cycle. This beat is conducted with a long PR interval.
• The P wave occurring just after the T wave of beat #2 is nonconducted.
• The P wave just preceding beat #3 was about to begin the next Wenckebach cycle — but before it could do so, 7.8 large boxes in time (1.56 second) had elapsed. As a result – a junctional escape beat (beat #3) arises.
• Beat #4 is conducted and begins the next Wenckebach cycle.
• The P wave immediately following the T wave of beat #4 is nonconducted.
• The P wave just preceding beat #5 was about to begin the next Wenckebach cycle — but before it could do so, 7.8 large boxes in time has elapsed, which results in another junctional escape beat (beat #5).
• The rhythm strip ends with beat #6. Despite the relatively short PR interval preceding beat #6 — We suspect that beat #6 is conducted because it manifests the same QRS morphology as the other 2 beats on the tracing that we know are conducted (beats #2 and #4).

BOTTOM Line: This is not an easy rhythm strip to interpret! Do not be concerned if you did not follow all aspects of our explanation. The point to emphasize is that 2nd degree AV block is present — and that in this patient with recent inferior infarction, a narrow QRS complex plus group beating most likely represents a form of AV Wenckebach.

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ADDENDUM: For those with an interest — We add the following laddergram illustrating the sequence of events in this tracing (Figure-3).

Figure-3: Laddergram of events occurring in Figure-2 (See text).

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- For more information – GO TO:

• See Section 20.0 (from ACLS-2013-ePub) – on the Basics of AV BLOCK – 
• See Section 20.25 for the specific part on Laddergrams. We also walk you through the process of constructing a Laddergram in ECG Blog #69 -

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ECG Interpretation Review #62 (AV Block - Mobitz I - AV Wenckebach - Mobitz II - 2-to-1 AV Block)

Imagine the ECG tracings shown below were obtained from a patient with syncope.

• How would you interpret the rhythm in Tracings A and B. Is AV block present? If so — what kind?
• Has the degree of AV block worsened between Tracing A and Tracing B?

Figure 1: Sequential tracings obtained from a patient with syncope.

Interpretation of Tracing A:

There is group beating. The QRS complex is narrow. Similar-looking P waves occur at a slightly irregular rate of between ~60-70/minute (ventriculophasic sinus arrhythmia). The PR interval gradually increases within groups until a beat is nonconducted (the P waves that occur after beats #1 and #3 are not conducted). This is 2nd-degree AV block, Mobitz Type I (AV Wenckebach). Of interest – Note the following:

• The 2 pauses that contain dropped beats are of equal duration (1.78 second for the interval between beats #1-2 and between #3-4).
• Each pause ends with a P wave that conducts with the shortest PR interval (0.23 second for the PR interval preceding beats #2 and #4 in Tracing A). Progressive PR lengthening then begins anew.

Interpretation of Tracing B:

The QRS complex is again narrow. The ventricular rhythm is fairly regular at a rate just under 40/minute. A slightly irregular sinus rhythm is again present — but this time at a faster rate (range between 80-85/minute). Every-other-P wave conducts — as determined by the presence of a fixed (albeit prolonged) PR interval preceding each QRS complex on the tracing. This is 2nd-degree AV block with 2:1 AV conduction.

• Although fewer beats are conducted and the overall ventricular rate in Tracing B is slower — there has not necessarily been any “worsening” in the degree of AV block between Tracing A and Tracing B.
• Instead — it may simply be that at the slightly faster atrial rate (of 80-85/minute) — fewer impulses are able to penetrate the AV node.

Brief Review of the 2nd Degree AV Blocks:

It is important to remember that there are 3 Types of 2nd-Degree AV block: i) Mobitz I (AV Wenckebach); ii) Mobitz II; and, iii) 2nd-degree AV block with 2:1 AV conduction. This distinction is important because the clinical course and recommended management for the various types of 2nd-degree AV block is quite different.

• HINT: Use of a pair of calipers is invaluable for understanding and facilitating diagnosis of the AV blocks. Your ability to recognize the various AV blocks (and distinguish them from “mimics” of AV block) will be instantly enhanced the moment you use calipers ...
• Mobitz I is recognized by the presence of Wenckebach Periodicity. Features of this phenomenon include: i) Progressive lengthening of the PR interval until a beat is dropped; ii) Group beating; iii) A regular (or almost regular) atrial rate; and iv) The pause that contains the dropped beat is less than twice the shortest R-R interval. These characteristics are evident in Tracing A. Note that the pause containing the dropped beat (the pauses between beats #1-2 and 3-4) is less than twice the shortest R-R interval (which is the R-R interval between beats #2-3 and #4-5).
• Mobitz II is recognized by QRS widening and a constant PR interval for consecutively conducted beats — until one or more beats are dropped.
• By far — the most common form of 2nd-degree AV block is Mobitz I. Because Mobitz I 2nd-degree AV block usually occurs at the level of the AV node — the QRS complex is typically narrow — the conduction defect may respond to Atropine (especially if due to acute inferior infarction and treated within the first few hours of onset) — and a permanent pacemaker is usually not needed.
• In contrast — the less common Mobitz II form is more likely to occur at a lower level in the conduction system (below the AV node) — often in association with large anterior infarction. As a result — the QRS complex is likely to be wide — Atropine will usually not work — and permanent pacing is much more likely to be needed.
• The reason for addition of a 3rd category ( = 2:1 AV conduction) – is to acknowledge that because you never see 2 consecutively conducted beats in a row, you can not tell IF the PR interval would progressively lengthen if given a chance to do so. As a result — one cannot be certain if 2nd-degree AV block with 2:1 AV conduction represents Mobitz I or Mobitz II simply from looking at a rhythm strip such as Tracing B.

Clinical PEARLS regarding the 2nd-Degree AV Blocks:

The 2 sequential tracings in Figure-1 serve to highlight the following important points regarding the 2nd-degree AV blocks:

• The atrial rate should be regular (or almost regular) when there is AV block. It is common to see slight variation (known as ventriculophasic sinus arrhythmia) in the setting of 2nd- or 3rd-degree AV blocks (as is evident in Tracings A and B). However, marked P wave irregularity — change in P wave morphology — or prolonged sinus pauses all suggest some phenomenon other than AV block is operative (blocked PACs; escape rhythms without AV block; sick sinus syndrome; sinus arrest).
• The KEY for diagnosing Mobitz II as the type of 2nd-degree AV block is that the PR interval remains constant for consecutively conducted P waves. Because you never see 2 consecutively conducted complexes in Tracing B — you can not tell IF the PR interval would increase if given a chance to do so. Therefore – you can not diagnose Mobitz II for Tracing B — but instead should classify this rhythm as 2nd-degree AV block with 2:1 AV conduction.
• That said — we can make an educated guess that Tracing B is almost certain to represent Mobitz I 2nd-degree AV block because: i) Mobitz I is so much more common than Mobitz II; ii) The QRS is narrow; iii) The PR interval of conducting beats is prolonged (far more common in Mobitz I); and, iv) Tracing A is definitely Mobitz I — and it is rare to go back-and-forth between Mobitz I and Mobitz II in the same patient.
• Knowing that Tracing B is almost certain to be Mobitz I would support a trial of Atropine as an initial intervention IF this patient was symptomatic. Whether or not pacing would eventually be needed would be determined by other factors including the cause of the conduction disturbance — how slow the ventricular response remained — degree of symptoms.
• Although Atropine may be highly effective during the early hours of vagally induced Mobitz I 2nd-degree AV block — this treatment is not bengin. In addition to improving AV conduction, Atropine may speed up the sinus rate. This in fact could be what happened between Tracing A and Tracing B — in which case this sequence of tracings would illustrate a potential paradoxical response from appropriate use of Atropine. That is, by increasing the atrial rate — use of Atropine may sometimes result in slowing of the overall ventricular response.
• PEARL: Remember to assess the atrial rate whenever evaluating patients with AV block. Forgetting to do so may result in overlooking the reason for a change in AV conduction ratios (as occurs here between Tracing A and Tracing B).

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- For more information – GO TO:

• See Section 20.0 (from ACLS-2013-ePub) – on the Basics of AV BLOCK – 

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