EKG

K cuts both ways

We’ve spent our share of time on hyperkalemic EKG’s, and for good reason.  Recognition of hyperkalemia is an opportunity to save a life.   But hypokalemia can cause problems as we’ll see in this case.

20171024_
~70 y.o. man recently started on amio and zaroxyln as inpatient, presents today with weakness

Starting as always with the rate, there’s about 8 big boxes between QRS’s, by 300 150 100 80 70 60 50 rule we get about 50; using the 7 complexes x6 rule we get a little over 40.  So clearly slow.  There does appear to be a P before every QRS though interestingly the P runs in to the QRS on the initial complex.  The QRS in that complex is a little wider than the other QRS’s so I suspect that’s a junctional escape beat that happens to start mid-P wave there.  Otherwise this is sinus brady.

The QRS is narrow.  There is some ST depression in II.

What to make of the T waves.  Let’s zoom in on V4 and V5.

20171024_v

Do you see how there’s multiple inflection points in the repolarization?  Remember, extra inflection points in the repolarization often mean extra waves.  We’ve looked at lost P waves in the past, however, in this case the waves aren’t superimposed upon each other, they come sequentially as below.

20171024_v2

What comes after a T wave (in green)?  A U wave (in brown).  I didn’t realize until I read about this case that U waves can actually be taller than the T wave when hypokalemia is severe (in this case 2.5).

Per Chou’s Electrocardiography in Clinical Practice, the 3 features to look for in hypokalemia is ST depression, T wave flattening, and U waves  (1).  We tend to remember the last one and forget the first two.  Chou reviews a paper by Surawicz et al.  These authors reviewed the presence of 3 EKG features on EKG’s with K<2.7 (ST depression, U wave >1mm, and U wave taller than T wave in the lead with the tallest U wave).  78% of patients in their series had all 3 present.

Two other things to know about U waves:  they can be somewhat superimposed on the T wave such that the only way you can see them is a change in the terminal slope of the T wave, i.e. a very asymmetric one.  U waves can also get lost if the patient is tachycardic as the P wave will project over the the top of them.

This patient doesn’t have any premature beats, but they become more common as hypokalemia worsens; you can easily see looking at the above EKG how a premature beat could fall in the repolarization complex and lead to a V tach.

References

  1. Chou’s ELectrocardiography in Clinical Practice, 5th ed. pp 522-6

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