abnormal heart electrocardiography waveform feature abnormal ST segment 2026-05-04T17:36:28Z MP:0031731 MPheno.ontology any anomaly in the ECG section between the end of the S wave (the J point) and the beginning of the T wave; it represents an electrically neutral area of the complex between ventricular depolarization (QRS complex) and repolarization (T wave) The ST segment is normally flat and at the same level (isoelectric) as the PR and TP segments. It corresponds to the plateau phase (phase 2) of the cardiac action potential during which the ventricular myocardium is refractory to further stimulation.