The TP Segment

The TP segment is the segment from the end of the T wave to the beginning of the p wave.

Barring the presence of a U wave, this entire segment is truly isoelectric, as the entirety of the myocardium should be at resting membrane potential (i.e. Phase 4 of the action potential).

Additionally, when the ECG machine sets the baseline, it uses the TP segment as a reference point. Therefore, out of the PR, ST, and TP segments, the only true isoelectric baseline is the TP segment.

There are no diseases that elevate or depress the TP segment.