The differential for ST elevation is broader than ACS (although it would be nice if medicine were that easy). In EM, the maxim is always to think about the worst first. After you have determined the patient is likely not experiencing an acute coronary occlusion aka ST-Elevation Myocardial Infarction, what else could lead to ST elevation? Outside of acute coronary occlusion, there are also a few causes that are not so benign.
Hyperkalemia can be a cause of ST elevation, but distinguishing features include peaked T waves, widened QRS complexes, and decreased P wave amplitude.
Another dangerous cause of ST elevation is Brugada syndrome, where ST elevation is most prominent in V1-V3 and is accompanied by T wave inversions.
Chronic causes of ST elevation include Left Ventricular Hypertrophy, LBBB, & LV aneurysm. Of course, you could have an acute coronary occlusion leading to STEMI in patients who have baseline LVH, LBBB, or LV aneurysm. This is out of scope from this POTD, but modified Sgarbossa's criteria seeks to help identify STEMIs in patients who have underlying LBBB (or paced rhythms).
LV aneurysm morphology often has deep Q waves with persistent ST elevation and T wave inversions and is accompanied by anatomical aneurysms that can be visualized on echocardiogram.
LBBB is characterized by a wide QRS with dominant S wave in V1 and a broad notched R wave in V6.
There are many criterias for LVH, but morphology wise, ST elevations in LVH are usually confined to V1-V3. There can also be ST depressions in the lateral leads.
Two final causes of ST elevation include Benign Early Repolarization (BER) and pericarditis. These can often be tricky to distinguish from subtle STEMIs, especially subtle anterior STEMIs.
Classically in pericarditis, there is PR depression, diffuse ST elevation (without ST depressions), and ST elevation in lead II > lead III. In BER, there is again diffuse ST elevation without ST depressions, elevations are often most prominent V2-V5, and there can be J point notching.
Picture below showing some common causes of ST elevation
Sources:
https://litfl.com/left-ventricular-hypertrophy-lvh-ecg-library/
https://emergencymedicinecases.com/ecg-cases-st-elevations-mnemonic-occlusion-mi/
https://litfl.com/left-bundle-branch-block-lbbb-ecg-library/
http://hqmeded-ecg.blogspot.com/2020/07/subtle-ecg-findings-of-left-anterior.html
