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Jason's Blog: ECG Challenge of the Week for Dec. 23rd - 30th.

jer5150's picture

I performed this ECG earlier this year in an outpatient clinic.

Patient's clinical data:  85-year-old white man; asymptomatic. 

(1.)  What rhythm is this tracing showing?
(2.)  What clue/sign is used to differentiate between the two anomalous beats (i.e., 4th and 6th beats).  HINT:  It's named after a part of a certain animal's anatomy.

** *** NOTE *** **:  For the sake of determining QRS morphology and axis, I've also included a simultaneous single-channel rhythm strip of Lead I directly below the 12-lead ECG. 

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Dave Richley's picture

i think this is sinus rhythm with an atrial prenmature beat showing RBBB aberrant conduction (4th beat) and a ventricular premature beat (beat 6) There is also an  atrial premtaure beat at the end of the ECG, this time with normal conduction, probably because the slightly longer coupling interval allows the RBB time to recover before the atrial impulse reaches it. The VPB has an RBBB-ish apeparance but it is not a typical RBBB pattern because the 1st R wave is taller than the second. I think this is something to do with rabbits' ears but I can't remember what the connection is! Also, of course, the 1st anomalous QRS is preceded by a P wave, wheras the 2nd one isn't, and this helps to distinguish between their different origins.


Dave R's picture

I'll add to discussion by Jason - and Dave Richley's excellent answer. Additional points include the following:

  • We KNOW (100%) that beat #6 is a PVC. As per Dave (and as per Jason's nice Figure above) - this 6th beat has a taller left rabbit ear - which is highly specific for ventricular ectopy. But in addition - this QRS is VERY wide - and it is not preceded by a premature P wave.
  • Another reason we KNOW (100%) that beat #6 is a PVC is its morphology in lead V6 - which is not only ALL negative - but ALSO manifests a slow descent for its QS complex.
  • In contrast - we KNOW that beat #4 is a PAC with aberrant conduction (100%) because: i) it is preceded by a PAC (which is the most definitive indication!); ii) it manifests classic RBBB morphology with initial r- S that descends below baseline- and tall, slender R' in right-sided V1 - PLUS (!!!) - if you look in simultaneously obtained left-sided leads I and V6, you see a wide terminal S wave for this aberrant beat that confirms typical RBBB morphology.
  • In addition - we have classic "cycle-sequence comparison" that further supports aberrant conduction. By this I mean - that based on coupling intervals and preceding R-R intervals - there is a REASON for beat #4 to be more aberrant than the last (10th) beat in this tracing. Remember that the length (duration) of the absolute and relative refractory periods (ARP and RRP) depends on the preceding R-R interval. This preceding R-R interval is LONGER for the R-R between beats #2-3 than it is for beats #8-9 - THEREFORE - in addition to the fact that the PAC that precedes beat #4 has a shorter coupling interval than the PAC that precedes beat #10 - beat #4 is also more likely to conduct with aberration because it has a preceding longer R-R interval.
  • There IS some (albeit minimal) aberrant conduction for beat #10. Note that the QRS is not as tall as normally conducted beats in lead aVF. 

GREAT example by Jason of some essential points in the distinction between ventricular ectopy and aberrant conduction.

  • Moral - Look not only for "rabbit ears" in V1 - but also look at other leads on the tracing (lead V6 in particular often provides additional key clues that elucidate whether an anomalous beat is ectopic ventricular vs aberrant).
  • For anyone interested - My summary (and illustrative figure) on use of QRS morphology for PVC vs aberrancy in leads V1 and V6 is at:,V6%20-%20Main%20HEADING.

Ken Grauer, MD   [email protected] 

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