ST Depression

There are numerous causes of ST depression: secondary repolarization abnormalities (ie LVH, LBBB), digoxin effect, hypokalemia, demand ischemia, myocardial infarction, and digoxin effect. The morphology of the ST segment can indicate a more likely cause of the depressions, with horizontal or downsloping ST depressions usually indicating myocardial infarction. To meet the definition for an NOMI, the ST depressions need to be at least 0.5mm in at least 2 contiguous leads. The overall amount of ST depressions (both the number of leads and amount of depressions) with an NOMI indicates a worse prognosis; in fact, ST depressions at least 2mm in depth indicate a very poor prognosis, with a mortality rate of 35% at 30 days. The depression with ACS do not localize (unless they are reciprocal depressions) and are commonly found throughout the anterolateral leads. If there is localized ST depressions, one should carefully scrutinize the reciprocal leads for signs of OMI. ST depressions associated with an NOMI may or may not have associated T wave inversions. ST depressions related to a posterior OMI are maximal in V1-V4 and usually have upright T waves and large R waves in those leads as well. The ST depressions with an NOMI are usually maximal in V4-V6. The ST depressions are often dynamic, worsening with worsening ischemic symptoms and improving or resolving with nitrates. 

Demand depressions are commonly upsloping and usually related to tachycardia. They also may be found for a period (usually 15-30 minutes) after cardiac arrest or cardioversion. They do not necessarily indicate primary cardiac disease (coronary artery disease). However, if a patient has persistent ST depressions or ischemic symptoms after resolution of the tachydysrhythmia, one should pursue an evaluation for ischemic heart disease. 

Examples:

Further Reading:

https://litfl.com/st-segment-ecg-library/

https://litfl.com/myocardial-ischaemia-ecg-library/

https://hqmeded-ecg.blogspot.com/search/label/Non-Occlusion%20MI%20%28NOMI%29?updated-max=2020-01-31T08:58:00-06:00&max-results=20&start=20&by-date=false

https://hqmeded-ecg.blogspot.com/search/label/NonSTEMI