Tuesday, April 2, 2013

ECG Interpretation Review #64 (AV Block – Mobitz I – AV Wenckebach – Junctional Escape – Mobitz II – AV Dissociation)


Interpret the 2 rhythm strips below using the Ps,Qs,3R Approach. The patient is hemodynamically stable in each case.
  • Is there AV block or not? If so – Describe the type of AV block present.
  • HINT: A similar mechanism is operative in each tracing.
Figure 1: Tracings A and B were each obtained from an asymptomatic young adult. Is there AV block? HINT: A similar mechanism is operative in each tracing (Figure reproduced from ACLS-2013-ePub)NOTE – Enlarge by clicking on Figures – Right-Click to open in a separate window (See text).
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ANSWER to Tracing A in Figure 1:
The first 4 beats in Tracing A are sinus conducted. The QRS complex is narrow throughout – although there is slight change in QRS morphology beginning with beat #5.
  • Note that there is gradual slowing in the rate of the sinus bradycardia seen for the initial 4 beats. That is, the R-R interval between sinus beats (red arrows in Figure-2) increases from 6.2 large boxes (between beats #1-2) – up to an R-R of 7.0 large boxes (between beats #3-4).
  • No P wave precedes beat #5. This QRS complex is narrow (albeit slightly different in morphology compared to the QRS for sinus conducted beats). Beat #5 is a junctional escape beat.
  • Note that the R-R interval preceding beat #5 is a bit over 7 large boxes in duration. Thus, it is appropriate (and downright fortunate) that this AV nodal escape beat occurred – since the underlying sinus bradycardia rhythm has continued to slow down. Note also that the R-R interval for each of the junctional escape beats in Tracing A manifests the same R-R interval (7.4 large boxes) – which corresponds to an appropriate AV nodal escape rate at ~40/minute.
  • Finally – note that each junctional escape beat (beats #5,6,7) is followed by a negative P wave in this lead II (blue arrows). This reflects retrograde atrial conduction from junctional beats that continually resets the sinus node and serves in this way to perpetuate the junctional escape rhythm.
Figure 2: Addition to Tracing A of red arrows (denoting sinus beats) – blue arrows (denoting retrograde atrial conduction from junctional escape beats) – and measurement of R-R intervals (See text).
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BOTTOM Line Regarding Tracing A in Figure 1:
There is no evidence of any AV block at all in Tracing A. On the contrary, in view of the fact that this tracing was obtained from a presumably healthy and otherwise asymptomatic young adult – there is not necessarily any abnormality at all. We simply see progressive sinus bradycardia with an appropriate AV node escape rhythm arising once the sinus rate drops below 40/minute.
  • Marked sinus bradycardia as seen here could be normal for this patient IF this young asymptomatic individual was an endurance athlete. In this case – no intervention would be needed. On the contrary – sinus bradycardia to this degree with need for an AV nodal escape rhythm to arise would be cause for concern IF the patient was an older adult with a history of weakness or syncope. Clinical correlation is everything.
  • There is no evidence of any AV dissociation in Tracing A. This is because all P waves seen on this tracing are related to neighboring QRS complexes (either appearing before the QRS and conducting – or appearing after the QRS reflecting retrograde AV conduction from junctional escape beats).
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Review of Tracing B in Figure-1:
Figure 3: Addition to Tracing B of vertical red arrows (denoting sinus beats) – a blue arrow (denoting retrograde atrial conduction following beat #3) – and slanted red arrows denoting nonconducting P waves that precede the QRS complex of beats #6-thru-9 (See text).
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ANSWER to Tracing B:
The underlying rhythm in Tracing B is sinus – as determined by the presence of an upright P wave with fixed PR interval preceding beats #1 and #2.
  • Beat #3 is premature. It is slightly wider and quite different in morphology compared to other beats on this tracing. Beat #3 is not preceded by a premature P wave. We suspect that beat #3 is a PVC (although acknowledge that it could be a fascicular escape beat given that it is not overly wide).
  • Regardless of the origin of beat #3 – this premature beat is followed by a retrograde P wave (blue arrow). Just like the situation in Tracing A – retrograde atrial conduction resets the sinus node. As a result – there is no P wave preceding beat #4. Given how similar QRS morphology of beat #4 is to the first two sinus beats – this defines beat #4 as a junctional escape beat.
  • The remaining beats in Tracing B (beats #5,6,7,8,9) represent a fairly regular albeit slightly accelerated junctional escape rhythm at ~65/minute.
  • Sinus node activity gradually returns toward the end of the tracing (slanted red arrows preceding beats #6,7,8 with a PR interval too short to conduct).
NOTE: We are uncertain if the P wave preceding beat #9 is or is not conducting. We suspect that it is not, because the PR interval preceding it appears to be slightly shorter than the PR preceding sinus beats #1 and #2. Clinically – it does not matter if the P wave preceding beat #9 is conducting or not – since the “theme” of this rhythm remains unchanged. In either case – Our Interpretation for Tracing B is the following:
  • Underlying sinus rhythm (beats #1 and #2). One PVC is seen (beat #3).
  • Resultant AV dissociation by default beginning with beat #4 – with a fairly regular albeit slightly accelerated junctional escape rhythm. That said – there is no evidence of any AV block in Tracing B, since P waves never fail to conduct when given a chance to do so. The PR interval preceding beats #6-thru-9 is simply too short to conduct.
  • Given that Tracing B was obtained from a presumably healthy and asymptomatic young adult – no intervention is needed.
Presumably sinus rhythm is restored in Tracing B. If this has not yet occurred by beat #9 – it looks like it will occur soon thereafter.
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2 comments:

  1. Hi Ken, and thanks again for your wonderful work. I have 2 questions:
    1)In the first tracing, the sinus rate goes down until a junctional focus lights up. Is it normal that we are not seeing sinus p waves, but only the retrograde conduction ? I mean, is a junctional focus meant to suppress sinus activity if the sinus node goes slower than that? Or are slower P sinus wave hidden behind the QRS? Would not we have to refer a "sinus arrest with junctional escape"?
    2) In tracing 2, we are not seeing sinus activity until beat 6. Should we refer a "sinus arrest", or is the junctional activity suppressing the slower sinus node even in this case?
    Thanks a lot,
    Niko G

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  2. Thanks for you comment Niko. In the 1st ECG — beat #5 arose because of increasing sinus bradycardia. Since no P wave preceded the QRS of beat #5 — it is not abnormal or unusual that this impulse that begins from the AV node conducts back to the atria. Given underlying sinus bradycardia — this retrograde conduction back to the atria may be enough to result in delay of the next sinus node impulse by an amount more than the R-R interval of the junctional escape rhythm. This is not necessarily pathologic. So it may become (for a little while at least) — a self-perpetuating process that we have junctional escape beats that conduct retrograde and don’t allow the next SA nodal impulse resulting in more junctional escape beats. Most of the time if the patient is otherwise healthy, the sinus node will “wake up” and begin to fire fast enough to take over the rhythm again. We cannot tell that though from simply looking at this tracing (without knowing more history).

    Regarding ECG #2 — the term, “sinus arrest” implies that NOTHING comes out of the SA node. That is not the case here. Instead, we merely have a PVC (beat #3). Similar to ECG #1, this PVC conducts retrograde — thereby delaying the next sinus beat — thereby allowing emergence of a “protective” junctional rhythm until such time as the SA node recovers and takes over the rhythm again.

    I hope that explains your questions.

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