What is TP segment on the ECG?

TP segment is the isoelectric interval on the electrocardiogram (ECG). It is the region between the end of the T wave (ventricular repolarization or electrical inactivation) and the next P wave (atrial depolarization or electrical activation). It represents the time when the heart muscle cells are electrically silent. TP interval shortens when the heart rate increases and vice versa. Ideally, elevation and depression of ST segment and PR segment are measured with reference to the TP segment. But when the heart rhythm is fast, a good TP segment may not be seen as the T wave of once cycle merges with the P wave of the succeeding cycle. This often occurs in the setting of an exercise test and in such situation, the E point which is at the beginning of the QRS complex can be taken as the reference.