Ischemia induced depression of the ST segment on the body surface electrocardiogram occurs in two settings:
- During acute transmural ischemia in leads that are not overlying the ischemia region. This represents the reciprocal of the ST segment elevation that is, or would have been, recorded in leads placed over the ischemic area. The depression of the ST segment that occurs in leads V1-V3 as the result of posterior wall ischemia exemplifies this situation.
- As a manifestation of non-transmural ischemia, also referred to as sub-endocardial ischemia.