A 50 yo male presented with chest pain.  This ECG was recorded at 0415.

There is sinus tachycardia.  There is left bundle branch block (LBBB).  All ST-T complexes are discordant.  However, the ST-T in V1-V4 is excessively discordant: with 6 mm ST elevation, V3 meets criterion 3 of Sgarbossa’s criteria (giving 2 points, not enough for a diagnosis of “MI”).  By the Smith modification of Sgarbossa’s criteria, the ratio of the ST elevation at the J-point (6 mm) to the S-wave (24 mm) is 0.25.  Since this is greater than the cutoff of 0.20, it would be diagnostic not just of MI, but of LAD occlusion.

The occlusion was not appreciated by the treating physicians, and another ECG was recorded at 0457:

Now the R-wave in V3 is gone (QS-wave), the S-wave is “fragmented” (an indication of infarction analogous to Q-waves), and the ST elevation remains at 6mm, with an S-wave of only 12 mm, for a ratio of 0.50.  The injury is worsening.

The notching in V3 is also known as “Cabrera’s sign” (prominent notching of at least 40 msec in the ascending limb of the S-wave in any of leads V3-V5).

The patient was taken to the cath lab and a 100% LAD occlusion was opened.

Here is the ECG after reperfusion:

There is less tachycardia, as the stroke volume is now higher with improved myocardial function.  The ST elevation has mostly resolved.  There are now concordent T-waves diffusely, especially in the LAD territory.

Such T-wave inversion is a frequent sign of reperfusion even in LBBB, and when seen alone (without the preceding ECGs diagnostic of STEMI) is a common sign of NSTEMI.  T-wave concordance can be normal, so it is not a very specific nor sensitive sign of ischemia.  But it should raise your suspicion.  In the context of this case and the preceding ECGs, it is diagnostic of reperfusion and is definitely the result of ACS.