Sunday, May 10, 2015

ECG Blog #111 (Addendum to ECG Video-9) - How to Distinguish between Escape vs Conduction?

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 ...
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.
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.
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.
  • NOTE: For full review on the Basics of AV Blocks  Please check out our 58-minute ECG Video-8 .
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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