The P waves march … slowly past Occam

I’m pretty sure I’ve brought up Occam’s principle in the past, which I most commonly hear summarized as “the simplest explanation is the most most likely.”  Usually this is meant to imply that you’re looking for one cause of the patient’s problems, not two.  I’m honestly not sure whether it’s violated in this EKG or not, but either way there’s a lot going on in it.

60’s y.o. woman with weakness and near-syncope.

Beginning as always with rate, we immediately get into some complexity as it’s irregular.  There’s 9 complexes so it’s averaging in the mid 50’s (9 complexes/EKG x 6 10second standard EKG strips/minute = 54 complexes/minute).

What is the rhythm.  Let’s look at the irregularity.  There’s pretty consistently about 6 big boxes between most of the complexes.  There’s also 2 places on teh EKG where there’s about 4 big boxes between complexes.  Let’s zoom in on the lead II rhythm strip.


There appear to be P waves before the QRS’s that come much earlier than the other beats on the EKG.  The QRS complexes are the same width between the beats that may have a  P wave and those that do not.  The QRS complexes are narrow.  This argues that most (or all) of the complexes are coming from the junction; they’re not ventricular escape beats at any rate.  Look carefully at the PR interval of the yellow dotted complex and the blue-dotted complex.


To my eyes the PR is slightly longer with the blue beat than the yellow.  Let’s come back to the significance of that and ask if we see any other potential P waves.


There are P waves and they are marching through.  Does that mean it’s a 3rd degree heart block?  Note that the P waves are a pretty consistent 7.5 big boxes apart.   The P-P interval is actually wider than the average R-R interval of the junctional escape rhythm.  The ventricles aren’t actually getting a chance to respond to the atria since the P waves are so slow in coming that the automaticity of the junction is taking over and most of the P waves are falling in the ST segment.  Technically this is more of an SA node failure with Junctional Escape than it is 3rd degree heart block.  I’m not really sure what to make of the slightly different PR intervals between the blue and the yellow dot beats, this likely does imply that there is an element of heart block present; a person would really need a 5 or 6 page rhythm strip to really know for sure.

The patient was admitted for near-syncope and potential need for a pacemaker.  The (either calcium channel or beta-blocking) agents were held overnight and the bradycardia resolved in 24 or 36 hours.

Take-homes from this EKG?  First of all, P waves can be subtle.  The other take-home is that I think we sometimes try to over-categorize an EKG.  Some EKG’s don’t fit specific labels and this is a difficult one to try to shoehorn into a narrow choice of categories.  If the patient needs to be admitted then they need to be admitted, you will definitely see rhythms change and people go back and forth between different degrees of block.

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