This is a repost of part of one of the most viewed posts ever.

This patient with little past medical history presented feeling moderately ill.  He had an ECG recorded:   

QRS duration = 102 ms.  What do you think?

Due to the peaked T-waves, the residents were immediately concerned for hyperkalemia and sought an old ECG, which they immediately found in the medical record.  It was recorded with a K of 4.5 mEq/L: 

QRS 82 ms.

The residents interpretation was that these were identical.  After viewing this previous ECG, and knowing that the K was 4.5 at the time it was recorded, the residents believed that the peaked T waves in ECG #1 were this patient’s baseline.  

Are they identical?  

Inspect lead V4 more closely, here for a side by side comparison:

On the left, the ST segment is nearly flat and rises abruptly.  On the right, it is not flat (though nearly so) and rises slightly less abruptly

It is true that early repolarization has tall and relatively peaked T waves, but not to the extent seen in ECG #1. Without seeing them side by side, it is hard to appreciate the difference. 

This patient, then, did not get immediate treatment for hyperK. 

I saw these ECGs at a slightly later time than the resident, recognized the difference and, worried about the patient, started toward his bedside.  As I was approaching the patient, he had a v fib arrest.  He was immediately resuscitated, then his K returned at 7.0 mEq/L.   This was a presumed hyperkalemic arrest.  

Some say you don’t need to treat hyperK unless there is QRS widening.  They claim that peaked T-waves are not enough.  This is only one case, and anecdotal, but we found no other etiology of arrest in this patient.  I always treat immediately if the ECG is affected by hyperK. 

There is a definite difference, with EKG 1 pathognomonic for hyperkalemia.