Monday, January 15, 2018

ECG Blog #147 (AV Block - PACs - Mobitz I, II).

The long lead II rhythm strip shown in Figure-1 was diagnosed as showing 2nd-Degree AV Block, Mobitz Type II.
  • Do you agree with that assessment? If not — What is your diagnosis?
NOTE: This is a difficult arrhythmia to interpret. That said, we present numerous Pearls on arrhythmia interpretation throughout our discussion that should be of value for interpreters of any level. Are you up for the challenge?

Figure-1: Long lead II rhythm strip that was interpreted as showing Mobitz Type II 2nd-Degree AV Block. Do you agree? 

KEY Clinical Points: Our systematic approach for interpreting any cardiac rhythm is to, “Watch your Ps and Qs, and the 3 Rs”. That is, systematically evaluate each and every tracing you encounter for the following 5 Key Parameters:
  • i) Are there P waves?or, if no clear P waves, then are there signs of atrial activity (such as “fib waves” or atrial flutter)?
  • ii) Is the QRS wide? — for which we accept anything more than half a large box in duration (ie, >0.10 second) as qualifying as a “wide” QRS.
And the 3 Rs:
  • iii) Rate? — What is the ventricular (and the atrial) rate?
  • iv) Regularity? — Is the ventricular (and atrial) rhythm regular?
  • v) “Related”? — Is there is a specific relationship between QRS complexes and neighboring atrial activity?

NOTE: It does not matter in what sequence you assess the Ps, Qs & 3Rs — as long as you always assess each of these 5 key parameters. We often vary the sequence in which we address the Ps, Qs and 3Rs — depending on whether atrial activity, QRS width, and/or rhythm regularity is easier or harder to evaluate in the particular rhythm we are looking at.
  • PEARL: By remembering to always “Watch Ps, Qs and the 3Rs” — you have at your fingertips an easy-to-recall method to ensure that you are always systematic (as well as time-efficient) in your approach and, that you never forget to assess each of these 5 essential elements. By religiously applying the “Ps, Qs & 3R Approach” to every arrhythmia you encounter — even if the specific etiology of the rhythm remains elusive — you will have narrowed down diagnostic possibilities, and clarified which specific parts of the rhythm you are still uncertain about.

Additional Suggestions: To facilitate interpretation of more complex rhythms — we offer the following additional suggestions:
  • Start with what you know! If there are easier parts to interpret in a tracing (as well as more complicated parts) — Begin with the easier parts! Is there is an underlying rhythm? If there are a number of sinus-conducted beats on the tracing — it is often easiest to first identify these sinus beats, and to leave for later interpretation of the more difficult portions of the arrhythmia.
  • Use Calipers! You’ll be amazed at how much “smarter” you instantly become the moment you begin to regularly use calipers for the interpretation of challenging arrhythmias. Your colleagues will marvel at how much more focused you become by regular application of this simple measure. Even the experts do better when they use calipers! Remember: The cardiologist who does not use calipers to interpret complex arrhythmias — is a cardiologist who will not always come up with the correct interpretation.
  • It often helps to label P waves on the tracing! If you are teaching others and/or if discussing a tracing with colleagues — it also often helps to number the beats, as this is the most time-efficient way to ensure that all participants in the discussion immediately know the specific part of the tracing being addressed. (Note: It is best not to mark up an original tracing — so please try to use a copy before you label.)

With these points in mind, we now proceed with our systematic interpretation of the rhythm in Figure-2:

Figure-2: We have numbered beats and labeled sinus P waves from Figure-1.

Interpretation: Imagine the patient in question is hemodynamically stable. We begin our interpretation with assessment of the “Ps, Qs & 3 Rs”. Note the following:
  • The QRS complex in Figure-2 is narrow. Although ideally we would have access to all leads on a 12-lead ECG before committing to comment on QRS duration — the QRS complexes in this tracing clearly look to be narrow and supraventricular.
  • The ventricular rhythm is not completely Regular. That said, there is a pattern to this rhythm — in that “group beating” is present, with 3 groups comprised of 3 beats each in a repetitive pattern.
  • Sinus P waves are evident (RED arrows) — as recognized by the presence of upright P waves with similar morphology in this long lead II rhythm strip. The P-P intervals appear to be constant, with the exception of 2 short pauses that occur at the end of each group (ie, after beat #3 and after beat #6).
  • The Rate of the rhythm varies — but it is neither excessively fast, nor excessively slow.
  • There does appears to be a consistent Relation between a number of sinus P waves and neighboring QRS complexes. That is, the PR interval preceding beats #2,3; #5,6; and #8,9 appears to be constant (albeit slightly prolonged).

Next Step: Now that we’ve addressed each of the 5 key parameters — Let’s see what conclusions might be drawn:
  • Although this rhythm is complex — there does appear to be an underlying sinus rhythm — because an upright P wave with fixed PR interval precedes no less than 6 of the 9 beats on this tracing (ie, beats #2,3; #5,6; and #8,9).
  • A much shorter, but still constant PR interval precedes the 1st beat in each of the 3 groupings. We need to explain WHY this is so. We also need to explain why the rate of sinus P waves is not constant throughout this tracing — and why short pauses punctuate each of the groups.

Diagnostic Possibilities: Two clinical entities should come to mind as possible explanations for the ECG findings described above. These are: i) some form of AV block; and ii) Blocked PACs. Let’s consider each of these possibilities in turn.
  • There is NO 2nd- or 3rd-Degree AV Block in Figure-2. Despite the apparent increase in PR interval between the 1st and 2nd beats in each grouping — the rhythm in Figure-2 is not AV Wenckebach. This is because the premise of AV Wenckebach (which is also known as the Mobitz I form of 2nd-degree AV block) — is that there should be an underlying regular sinus rhythm throughout the tracing. The PR interval with AV Wenckebach progressively increases, until one or more of the regularly occurring sinus P waves is not conducted. However, RED arrows in Figure-2 show that regular sinus P waves do not continue throughout this tracing. For similar reasons, this rhythm does not represent the Mobitz II form of 2nd-degree AV block. First, the PR interval does not remain constant (as it should if Mobitz II was present) — and second, the P-P interval of sinus P waves does not remain constant throughout the rhythm strip as it almost always does for virtually any form of AV block. Finally, this rhythm cannot be complete (ie, 3rd-degree) AV block — because there is conduction of a number of sinus beats (ie, beats #2,3; #5,6 and #8,9 are all conducted with a constant PR interval).

PEARL: The most common cause of a pause is a blocked PAC! This phenomenon occurs far more often than is generally appreciated. Blocked PACs are a much more common cause of pauses than any form of AV block. The challenge diagnostically, is that blocked PACs may be extremely subtle and easy to overlook. The secret is to look for blocked PACs whenever you encounter any unexpected pause.
  • 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 (ie, the ST-T wave of beats #3 and #6 in Figure-2) — with the ST-T wave of all normally conducted sinus beats on the tracing. Is there any difference?
  • NOTE: We fully acknowledge that detecting blocked PACs may 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.
  • For Practice: We illustrate detection of blocked PACs in our ECG Blog #33 and Blog #57. Once you begin to routinely look for blocked PACs whenever you see an unexpected pause — I guarantee that you will find them with surprising frequency!
Test yourself looking for signs of blocked PACs in Figure-2. Look carefully in at the base of the T wave at the onset of each pause — paying special attention to the T wave after beat #6.
  • Be sure you have magnified the tracing by clicking on it to view in a separate window!
  • We illustrate our answer below in Figure-3.

Figure-3: We have added a RED-BLACK arrow at the base of the T wave near the beginning of each pause (See text).

Answer: Note subtle angulation at the base of the T waves of beats #3 and #6 at the onset of each pause (RED-BLACK arrows in Figure-3). This subtle deformity is not present in the T waves of all other beats on this tracing.
  • We strongly suspect this angulation at the base of these T waves is due to blocked PACs. These blocked PACs then reset the SA node — and this accounts for the brief pause that follows beats #3 and #6.

Final Question: Why is the PR interval at the end of each pause shorter than the PR interval of normally conducted sinus beats?

Answer: The reason beats #1, 4 and 7 all manifest a shorter PR interval than beats #2,3; #5,6; and #8,9 — is that beats #1, 4 and 7 are junctional escape beats that occur before the sinus P waves preceding them have a chance to conduct! From the consistent-length PR interval preceeding beats #2,3; 5,6 and #8,9 — we can see that sinus conduction in this tracing requires a bit more than 0.20 second. 
  • In Figure 3 — the junctional escape focus fires before the P waves preceding beats #1, 4 and 7 have enough time to conduct. 

Two findings support our theory that beats #1, 4 and 7 are not sinus-conducted, but are instead junctional escape beats:
  • Finding #1: The R-R interval preceding the 2 junctional beats is the same! (ie, the R-R interval between beats #3-4 and between beats #6-7 is identical). The reason these 2 R-R intervals are the same, is that this R-R duration corresponds to the junctional escape rate.
  • Finding #2: QRS morphology of the 3 junctional beats on this tracing (ie, beats #1,4,7) is slightly different than the QRS morphology of sinus-conducted beats. That is, the R wave of beats #1,4 and 7 is slightly taller — and the S wave slightly smaller — than the R and S waves for each of the sinus-conducted beats. Although this difference is exceedingly slight — it appears to be real, and provides an invaluable clue that beats #1, 4 and 7 are indeed junctional escape beats (and that the P waves preceding beats #1, 4 and 7 are not being conducted). This further supports our premise that this rhythm is not AV Wenckebach.
PEARL: Sometimes (not always) the QRS morphology of AV nodal beats will look different in some slight way from the QRS morphology of sinus-conducted beats. This is because one never knows from where within the AV junction a nodal beat arises (ie, junctional beats could arise from one or another marginal edge of the AV node) — in which case the “path” that this supraventricular junctional beat travels may be just a little bit different than the path traveled by normal sinus-conducted beat. Recognition of this consistent slight difference in QRS morphology when it occurs can at times provide an invaluable clue as to which beats on a rhythm strip are sinus conducted vs which beats arise from the AV node.

Comment: We fully acknowledge that additional monitoring of this patient would be needed to definitively prove our theory for the mechanism of this fascinating arrhythmia. And, it is true that on occasion a single definitive interpretation of a complex arrhythmia may simply not be possible from the surface ECG. That said — We feel the above discussion clearly provides a plausible explanation for all findings noted on this tracing.
  • For clarification — Figure-4 offers a laddergram illustration of our theory.

Figure-4: Laddergram illustration of our theory for the mechanism of this arrhythmia. The underlying rhythm is sinus with a long PR interval. Several PACs are seen. These occur with a short coupling interval — and are therefore non-conducted to the ventricles (RED Triangles). This resets the SA node, therefore delaying the next sinus P wave. Because of the resultant brief pause — junctional escape beats (#4 and #7) arise, and are conducted to the ventricles before the sinus P waves preceding beats #4 and #7 are able to conduct to the ventricles. Therefore, this arrhythmia entails sinus rhythm (with prolonged PR interval); blocked PACs; and appropriate emergence of several junctional escape beats.

Clinical Implications: There is no 2nd- or 3rd- AV block in this tracing. Clinical implications for this patient are the same as they would be for anyone having PACs and slight prolongation of the PR interval. This usually entails search for a cause of the PACs, with corrective measures (ie, caffeine restriction; treatment of heart failure or electrolyte disturbance, etc.) if/as clinically indicated. In the absence of other forms of heart disease — the isolated presence of 1st-degree AV block is usually of minimal clinical significance. The brief pauses terminated by junctional escape beats manifest a completely appropriate response to blocked PACs that reset the SA nodal pacemaker. In short — this is most probably a fairly benign arrhythmia.
  • NOTE: Even though the precise mechanism we postulate for this rhythm is advanced — the basic principles discussed in this blog post are within grasp of any clinical provider. Use of the Ps, Qs & 3R Principle should organize your approach and narrow your differential to the entities we consider. Mobitz I and Mobitz II forms of 2nd-degree AV block can easily be ruled out — because the atrial rate is not regular. Complete AV block is ruled out because there clearly is conduction of a number of beats. Appreciation of the clinical truism that “the most common cause of a pause is a blocked PAC” should then lead you to the correct diagnosis!

Acknowledgment: My thanks to Robert Drutel for allowing me to use this tracing and clinical case.

Additional Material: For Review on the Basics of AV Block — See my 58-minute ECG Video on this subject at
  • Please note that if you click on SHOW MORE on the You-Tube page under where this video appears — You’ll see a detailed linked Contents that will allow you to immediately find whatever key points you are looking for in this video.
NOTE: For a Primer on How to Draw a Laddergram — See my ECG Blog #69
For more on the recognition of Blocked PACs — Please see my ECG Blog #33 and Blog #57. With practice — you will begin to find blocked PACs with surprising frequency!


  1. Admittedly a difficult tracing (at least for me) and I read every line with great attention and interest! Ken, I'm in debt!!

    1. Thank you Mario. It's a compliment coming from you! — :)

  2. Replies
    1. First — SORRY for my delay in answering. SSS ( = Sick Sinus Syndrome) is a syndrome made up of a consistent history (most often an older individual, in whom rate-slowing medication, hypothyroidism, sleep apnea nor ischemia/recent infarction is NOT the cause of bradyarrhythmias. SSS is then a complex of bradyarrhythmias including things like sinus bradycardia/arrhythmia, sinus pauses/arrest, slow junctional escape — and often tachyarrhythmias followed by prolonged sinus recovery time. We just do NOT have enough information to make that diagnosis in this case — :)

  3. I have my calipers with me at all times. Excellent strip and explanation.