This 70 year old woman with a history of left bundle branch block presented with 10 hours of dyspnea.
Notice there is QRS widening with a wide upright R wave in lead V6. That the QRS is not similar to V6 in leads I and aVL makes this an atypical LBBB. Nevertheless,, the same rules of concordance and discordance apply: the ST-T complex should go the opposite, or discordant, direction of the majority of the QRS.
Notice there is concordant ST elevation in inferior leads.
In addition, there is less discordant ST elevation in leads V1--V3 than is normal for LBBB. A normal amount of discordant ST elevation almost always produces an ST to S ratio of less than 0.20. The mean of normal LBBB is 0.09 to 0.11, but ratios of 0.07 to 0.15 may be seen. Thus, with a 26 mm S-wave in V2, the minimum amount of normal ST elevation is 26 x 0.07 or 1.8 mm. However, there is only 1 mm of ST elevation in V1 because of relative ST depression in leads V1 to V3.
This is infero--posterior STEMI in the context of old atypical LBBB.