ST Segment

The ST segment is the segment between the S and T waves and is usually isoelectric. It represents the period between ventricular depolarization and ventricular repolarization. This segment is mostly important for evaluation for myocardial infarction. OMIs that require emergent cath lab evaluation and treatment mostly produce ST elevation (there is some debate here). Most NOMIs will produce ST depression, though not always. However, numerous conditions besides myocardial infarction can produce ST deviation, be that elevation or depression. 

The morphology of the ST segment elevation or depression should also be evaluated. ST elevation that is convex or straight should almost always be considered an OMI. ST elevation that is concave may be an OMI or one of the other (usually more benign) conditions previously listed. The morphology of the ST depression should be evaluated as well. Horizontal or downsloping ST depression is much more specific for ischemia than upsloping ST depression. OMIs commonly cause ST depression in parts of the EKG that are reciprocal (in leads opposite of) to the ST elevation. 

The actual measurement of ST segment deviation should occur at the J point. The J point is the junction between the QRS complex and the ST segment. In most EKG's, this is easy to determine; however, in EKG's with ST elevation or terminal QRS distortion, this is not always simple. In leads with J waves (positive deflections at the end of the QRS), the J point falls after the J wave.