The ST segment

Following complete depolarization, the entire ventricular mass is negatively charged with each region having the same magnitude of charge. Thus, the electrodes measure no potential difference (voltage) between the different parts of the ventricles and this registers in the ECG as a flat line with 0 voltage (isoelectric line). This flat segment is called the ST segment.

The point where the QRS complex ends and the ST segment begins is called the J point.

Alterations of the ST segment and the J point are of great diagnostic value when analyzing an ECG. The ST segment seems to be an electrically inactive phase; however, minute repolarization activities do take place during this phase. The electrical activity is usually too weak to be measured by the ECG electrodes. The STT segment (The ST segment and the T wave together) represents the total repolarization of the ventricles.



 

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