Jason's Blog: ECG Challenge of the Week for March 3rd - 10th.

I'm struggling to explain this one. There is an underlying sinus rhythm with LBBB, but there are premature beats with normal conduction. The premature beats are preceded by P waves which are different in shape from the sinus rhythm P waves and associated with a shorter PR interval, so I assume they are of atrial origin, but why is the associated QRS narrow? Could it be because of supernormal conduction in the left bundle branch? Is such a thing even possible? Following the atrial premature beat, the next sinus P wave is delayed slightly, as is usually the case, and the QRS is again of normal width. This is easy to explain by assuming that the LBBB is rate-dependent and that, given enough time, such as is provided by a post-ectopic pause, the LBB is capable of conducting.

So my initial guess is sinus rhythm with rate-dependent LBBB and atrial premature beats with supernormal LBB conduction. However, I may yet change my mind!

Two things I can usually count on from this excellent ECG Weekly Challenge:

1. A superb tracing from Jason.
2. An astute and concise explanation offered by Dave Richley!

I was about to post my interpretation of this when I saw Dave's first. I agree completely:

• Underlying sinus rhythm.
• Rate-related LBBB. Note that the P waves preceding beats #3,4,5 - 7,8,9 - 11,12,13,14 are identical in morphology AND in PR interval. They are NOT premature - therefore this proves the QRS widening is the result of rate-related LBBB.
• Beats #6 and 10 are premature. These beats are preceded by early-occurring P waves with a different morphology - therefore they are PACs! The ONLY explanation I can think of for the narrow QRS is supernormal conduction!
• These PACs reset the sinus node and result in the slight pause between beats #2-3; #6-7; #10-11. As a result of the pause - there is time for the left bundle branch to recover - and as a result beats #3,7,11 conduct normally.

I'd LOVE to see this patient's ECG when the PACs stop. It may well show "persistent" LBBB at such time (if the baseline sinus rate is at all a bit fast) - when in fact the patient really has rate-related LBBB.

NOTE: A Key point about rate-related BBB is that rate of onset is not necessarily the same as rate of offset. That is - you may develop rate-related BBB at a rate of 80 or 90/minute - but may need to drop back down to a rate under 70 in order for it to disappear. As might be imagined - this makes diagnosis of rate-related BBB that much more challenging! GREAT case !!!

Again, beautiful EKG. Seriously Jason, do you have EKG's with ALL the possible EKG findings in a folder? 

I didn't see many EKG's showing this anomaly (like Dave and Ken, I too believe we're looking at a possible super normal conduction case), so I got excited when I saw it, but then I read the comments and I saw everything nicely explained already... 

I might add that the LBBB (likely rate related) is somewhat expected as the patient has a constant LAFB (looks like all beats have it regardless of their rate/prematurity). So basically this is just an intermittent LPFB (probably rate related), but because of the underlying LAFB, the QRS length/morphology changes so evidently (it becomes a full blown LBBB). There's also LAE noted on this EKG, but the highlight is obviously the "super normal conduction" in the posterior fascicle seen on beats 2,6 and 10. BTW, is there an explanation of the physiology behind this interesting phenomenon?

P.S. Gotta love Ken's subject line