Which part of the ECG is the flat, isoelectric line between the QRS complex and T wave, representing the early part of ventricular relaxation?

• A. P Wave
• B. ST Segment
• C. QRS Complex
• D. T Wave

Answer: (B)

The ST segment is the flat, isoelectric portion that lies between the end of the QRS complex and the start of the T wave. It represents a moment when the ventricles are fully depolarized and during the plateau phase of their action potential, so there’s little net electrical activity and the tracing sits at baseline. This period corresponds to the early part of ventricular relaxation, just before the ventricles begin to repolarize (which is shown by the T wave). In contrast, the P wave reflects atrial depolarization, the QRS complex reflects ventricular depolarization, and the T wave reflects ventricular repolarization (relaxation).