NOTE: This is an advanced ECG post that supplements my ECG Video-9. The essential concepts to master were presented
in the Video. But for those who want more — this post looks closer
at how to tell which beats in a
tracing with 2nd-dgree (but not complete)
AV block are likely to be conducted vs which beats represent a non-conducting escape focus. Spoiler Alert: What follows assumes you have already seen
our ECG Video-9. We start with the
Answer to that case — and then explore in more detail how we got
there ...
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In ECG Blog #110 ( = ECG Video-9) — We presented the case
of a 75-year old woman with syncope and dyspnea who presented to the ED (Emergency Department) with the simultaneously-recorded 3-lead rhythm
strip shown in Figure-1.
Although health care providers on the scene initially diagnosed this rhythm as complete AV block —
there is
ample evidence against this
diagnosis!
- QUESTION: Can you think of at least 3 reasons why we know that beats #1-thru-5 represent an escape focus, and — that beats #6-thru-9 in Figure-1 are being conducted?
 |
| Figure-1: Simultaneously-recorded 3-lead rhythm strip from a 75-year old woman with syncope. Red arrows in lead II highlight regular atrial activity. Although there is AV dissociation early on — this rhythm is not complete AV block. Can you think of at least 3 reasons why we know that beats #1-thru-5 represent an escape focus, and — that beats #6-thru-9 are being conducted? NOTE — Enlarge by clicking on Figures — Right-Click to open in a separate window. |
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ANSWER: The KEY to interpreting this rhythm strip (and to developing the complex laddergram
that we work through in ECG Video-9) — lies with recognizing
that the QRS complexes for beats
#1-thru-5 represent a ventricular
escape focus.
To arrive at this conclusion — We approach this arrhythmia in the same way we
approach any complex rhythm = Begin with what you know!
- NOTE: Use of calipers is of invaluable assistance for interpreting this rhythm strip! To facilitate discussion — We substitute Figure-2 for Figure-1, in which we add in our caliper measurements of critical intervals.
 |
| Figure-2: Our caliper measurements of key intervals have been added. How do these numbers support our contention that beats #1-thru-5 represent an escape focus? (See text). |
Why then do we say that beats #1-thru-5
represent a ventricular escape focus?
- Starting with what we know — the constant PR interval (of 195 msec.) for the last 4 beats on this tracing confirms sinus conduction. It should be emphasized that there is 2:1 AV block during these last 4 beats — but the fact that the previously variable PR interval suddenly becomes constant ( = 0.195 second) tells us beyond doubt that sinus conduction is at least intermittently occurring.
- Note that the QRS complex manifests an rS configuration in lead V1 (small initial positive r wave; deep negative S wave) for these 4 sinus-conducted beats. In contrast — each of the preceding 5 beats manifest a multiphasic (rsR’s’) complex in lead V1. Since there is no apparent reason for aberrant conduction during beats #1-thru-5 (since the ventricular rate isn’t overly fast) — the most logical reason for this change in QRS morphology is the presence of an escape focus for beats #1-thru-5 (especially since QRS morphology changes beginning with beat #6, which is the first of 4 consecutive beats to conduct with the constant PR interval of 195 msec.).
- Finally — the R-R interval shortens prior to beat #6. In our experience, in the presence of AV dissociation — one of the most helpful clues that intermittent conduction is occurring — is recognition of unexpected shortening in the R-R interval. This is admittedly subtle in Figure-2. Nevertheless, preceding R-R intervals for the first 5 beats on the tracing are between 1.22-to-1.24 second — whereas the R-R interval preceding the change in QRS morphology (that begins with beat #6) = 1.20 second. Additional slight shortening of the R-R interval is seen preceding the next 3 sinus-conducted beats. Admittedly, underlying subtle variation in the rate of both the escape rhythm and the sinus rate complicate assessment of R-R interval duration — but the fact that all sinus-conducted beats (#6-thru-9) manifest a shorter R-R interval than all non-conducted beats clearly supports our contention that beats #1-thru-5 represent an escape focus.
- We surmise that the escape focus is below the AV node — because QRS morphology of beats #1-thru-5 significantly differs from QRS morphology for the 4 conducted beats (#6-thru-9). That said — the fact that QRS duration of these 5 escape beats (#1-thru-5) is no more than minimally widened suggests that the site of escape is still within the conduction system — probably within the bundle branch sysem.
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CONCLUSION: Full discussion of this case (with illustrative laddergram) is presented in our ECG Video-9. While recognition
of all subtleties described above is clearly not needed to correctly interpret
this rhythm — Our hope is that these more advanced concepts provide additional insight for optimal assessment
of the AV blocks.
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- NOTE: For full review on the Basics of AV Blocks — Please check out our 58-minute ECG Video-8 .
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Acknowledgment — My appreciation to the following individual:
- Dr. Jenda Stros (Liberec, Czech Republic) — for providing me with the case for my ECG Blogs #110, 111, and for Video-Blog #9.
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