The ST segment on an electrocardiogram represents which physiological process?

• A. The distance between the QRS complex and the T wave
• B. The isoelectric line immediately after the P wave
• C. The ST segment represents early ventricular repolarization lasting from end of QRS to the beginning of the T wave
• D. The point where QRS and P waves meet

Answer: (C)

The ST segment is the flat, isoelectric interval that sits between the end of ventricular depolarization and the start of ventricular repolarization. After the QRS complex ends, the ventricles are depolarized and enter the plateau phase of their action potential, during which there is little net current flow detectable on the ECG. This makes the ST segment appear as a level line. It then marks the period just before the ventricles begin repolarizing, which is reflected by the T wave. So the ST segment represents early ventricular repolarization, lasting from the end of the QRS complex to the beginning of the T wave. Under normal conditions this interval is flat; deviations like ST elevation or depression can indicate pathology such as ischemia or injury.