A very complex patient presented critically ill.  This was his initial ECG:

There are pacer spikes.  Is this a simple paced rhythm?

There is a paced rhythm with a sine wave, diagnostic of hyperkalemia.  The potassium was 6.8mEq/L.

He was aggressively treated for hyperkalemia.  Another ECG was recorded 42 minutes later:

The QRS is still very wide (about 250 ms), but substantially narrower.

Here is a recent baseline ECG:

This has a rather long baseline QRS at about 180 ms, but not nearly as long as with the high K.

A paced
rhythm with a duration of greater than 180 ms is very long and, if it is chronically long, it is associated with poor prognosis.  

When the QRS in paced rhythm is acutely prolonged, as in this case, all of the etiologies of prolonged QRS should be on the differential: especially hyperkalemia and sodium channal blocking drugs.