This is an old one, but wasn't shared here. The 1st ECG in the ER. The patient transferred to CCU with a diagnosis of NSTEMI and the 2nd ECG was obtained. What is happening ?

Thanks for all replies. In the first ECG there is a lead fault in lead II (but it is not lead reversal, in this situation we would not see a perfectly flat line, generally see tiny notches corresponding to QRS complexes. Also other leads are not consistent).

The machine had an internal recording problem, but this was not the interesting part. The interesting part is that some ECG machines (not all) record only lead I and lead II and then calculate lead III by III=II-I, not by LF - LA. And then all augmented leads from I, II, III.

So, when II is zero, all other leads are derived from lead I. Here, lead III is -lead I (compare its QRS axis to that in 2nd ECG). aVR=-(I+II)/2, aVL=(I-III)/2, aVF=(II+III)/2. So, all of the frontal plane information actually comes from lead I. No 'inferior' lead here.

See here for more detailed discussion: https://pubmed.ncbi.nlm.nih.gov/20334873/ 

A similar example: