Tuesday, February 2, 2021

Another diagnostic ECG of a potentially deadly condition

 Written by Pendell Meyers


A middle aged woman with no significant past medical history presented with epigastric abdominal pain with vomiting off and on for the past few days. Today her symptoms returned and intensified, so she came to the Emergency Department. Her vital signs were only significant for mild bradycardia.

What do you think? It is basically pathognomonic.

Here is her triage ECG:







Here is her baseline ECG on file from several months prior:







The presentation ECG shows sinus bradycardia with a normal QRS complex followed by diffuse down-sloping ST depression with extremely long down-up T waves. This is diagnostic of hypokalemia, of course with additional potential contributing factors of hypomagnesemia, medications, etc., but the morphology here is (I believe) quite specific for hypokalemia as the most significant single contributor.

Notice the fact that this diffuse STD is maximal in V4-V6 and lead II, just as all diffuse, nonfocal STD should be. 

The computer calculates the heart rate as 52 bpm, and the QT interval as 637 ms.

When I measure the QT interval myself, I get approximately 680-690 ms.

Notice that the "half the R-R interval" rule of thumb becomes more and more dangerous below 60 bpm. In this case, the QT interval is either just at or just beyond the halfway point, with massively prolonged QT.

Also note that one should not even correct the QT interval for heart rates below 60 bpm. 

See very detailed discussion here for the reasons why in the article by Smith and others: 

This is in agreement with the Chan et al. nomogram approach (which is derived based on predicting torsades de points in acute drug-induced long QT), see the nomogram below and notice that the slope is zero below 60 bpm, meaning no correction for heart rate less than 60, and a fixed QT cutoff of around 485ms.

 
https://academic.oup.com/qjmed/article/100/10/609/1523194



Red lines show the duration of the QT interval.



Potassium and magnesium repletion were begun before labs returned.

Potassium = 2.4 mEq/L
Mg = 1.8 mEq/L



8 hours later, when potassium = 3.0 mEq/L:



Next day (potassium = 4.5 mEq/L):





Teaching Points:

This morphology is believed to be specific for severe hypokalemia.

The computer is often wrong when calculating the QTc when it matters most.

The "half the RR interval" rule of thumb should not be used when the HR is less than 60.

The QT interval should not be corrected for heart rates less than 60 bpm, and instead a fixed QT cutoff of 485 ms can be used.

The ECG often contains important information that can be obtained much sooner than lab studies.


Here are more cases to lock in this pattern and its variants:

A pathognomonic ECG you should recognize instantly


Diffuse ST depression, and ST elevation in aVR. Left main, right?



3 comments:

  1. Great case. And by the way the patient may be congenital long QT case. Most normal patients dont have that large of a Qt prolongation with a 2,4 K level. Many (about 30%) LQTS patients have a normal baseline Qtc, but have an abnormal prolongation with an acute insult such as in this case. You should consider referring these cases to a genetic cardiology consultant.

    ReplyDelete
  2. Hi Pendell!
    beautiful case.. is this also "salvador Dali"-ish? ie, possible digoxin toxicity?
    thank you once again

    ReplyDelete
    Replies
    1. tom, QT is far too long for Dig, which has a very short QT

      Delete

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