Monday, March 18, 2013

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?

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).

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.

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.

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).

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 -


  1. Hello...
    Nice explained... Little advanced concepts.. Apart from the history and non ecg related factors that suggests it to be mobitz 1;and strictly scrutinizing ecg only, we do not see a gradually prolonging PR interval which is required for mobitz 1..
    So can we still call it mobitz 1?or 2nd degree AVB with 3:1 conduction (or may be 2nd degree AVB with 2:1 conduction as its interrupted by junctional escape beat so excluding the extra non conducted beat.)?? Since no prolonging PR interval noted why cant it be even mobitz 2? What's the full interpretation of the rhythm...

  2. @ MG — Great questions you ask! As you note, this IS an advanced tracing — and we need at apply intuition. I don't know if there is any way to be 100% certain from this single tracing alone that this is Mobitz I — but by using probabilities and intuition, we can be >95% certain. First, although I've never seen a study of this — in my experience, Mobitz I AV block is MUCH, MUCH more common than Mobitz II (I'd estimate >95-98% of all 2nd-degree AV blocks are Mobitz I. Given that we have group beating here — a narrow QRS — and a 1st degree AV block for beats #2 and #4 that are conducting — the odds are already well over 98% that this is Mobitz I. I show my proposed mechanism in the Laddergram of Figure 3. And, finally, Mobitz I "does this". That is, this pattern of Mobitz I with junctional escape beats IS one that you will see. So I hope the above will satisfy your request for more explanation of my thought process! — :)