I’m sorry, but there was an error in the previous post and I had to remove it.  Depending on information received, I may be able to repost it.

A dialysis patient presented with vomiting.  He has  known baseline left bundle branch block.  Here is his initial ECG:

There is left bundle branch block, with a QRS duration of 220 ms according to the computer analysis.

Here is his previous ECG, recorded with normal postassium:

Here, the QRS duration was 160 ms.  So the previous one is widened by 60 ms.

A widened QRS should always make you suspicious of hyperkalemia.  Indeed, the potassium was 7.3 mEq/L.  After therapy, the widening resolved.

Here is a similar case involving right bundle branch block.

What is the normal QRS duration in LBBB?

In this study of consecutive patients with LBBB who were hospitalized and had an
echocardiogram, a QRS duration less than 170 ms (n = 262), vs. greater than 170 ms (n = 38), was associated with a significantly better ejection fraction (36% vs. 24%).  Only 13% had a QRS duration greater than 170 ms, and only 1% had a duration greater than 190 ms.

Obviously, if you have a previous ECG for comparison, you can diagnose prolonged QRS in the setting of LBBB easily.  But it is clear that if your patient with LBBB has a QRS duration greater than 190, then there is more going on.