The ST segment is the part of the ECG from the end of the QRS to the beginning of the T wave. The transition point from the QRS to the ST segment is called the J point.

It usually appears isoelectric, but can be influenced by the presence of Ta waves, similar to how the PR segment is affected. Notice how the prominent Ta waves in the examples below can cause the ST segment to look slanted or bent.

Physiology of the ST Segment

The ST segment represents the time when the ventricular myocardium is in the early stages of repolarization – specifically, when it’s in Phases 1 and 2 of the myocardial action potential.

During the ST segment, the cell membrane potentials remain relatively constant, and that’s why it is flat (excluding the effect of Ta waves). There is a large influx of calcium ions into the cardiomyocytes, which resultantly undergo contraction via excitation-contraction coupling. 

Therefore, to summarize, the ST segment represents the plateau of the myocardial action potential, which represents the phase during which an influx of calcium ions causes myocardial contraction.

There are a few benign causes of ST deviations that we will discuss here:

• Ta waves

• Benign early repolarization (BER)

Ta waves

Below are two examples of how Ta waves can appear to be ST changes. It is important to trace Ta waves through to rule out false ST changes.

Below, you have an example of a true ST segment depression on top of a prominent Ta wave. See how the Ta wave can be traced through to help identify this ST change.

Benign early repolarization (BER)

BER can cause apparent ST elevation, as shown below on the left. It is commonly seen in young, healthy people and is not pathologic. The physiologic basis is not well understood. 

On the right, an alternate BER morphology, which appears as J point notching (or “J waves”) is shown.

There are some morphologic features that increase the likelihood of BER and reduce the likelihood of pathologic causes of ST elevation (specifically the STEMI).

In BER, the upstroke of the T wave is usually concave, the ST elevation is generally <2 mm in the precordial leads and <0.5 mm in limb leads, and the ST elevation is <25% of the height of the T wave.

A more worrisome morphology of ST elevation is a convex T wave with ST elevation >25% of the T wave height, as seen in STEMIs.