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.

~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.


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.


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.


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

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