What does the S-T segment in an ECG represent?

What does the S-T segment in an ECG represent?

• A. The right and left ventricular plateau during full contraction.
• B. The start of atrial depolarization.
• C. The time from ventricular repolarization to atrial depolarization.
• D. The overall duration of the QRS complex.

Answer: (A)

The S-T segment corresponds to the Plateau phase of the ventricular action potential, during which the ventricles are fully depolarized and actively contracting in systole. It begins after the end of ventricular depolarization (the S wave) and ends just before ventricular repolarization (the start of the T wave). During this interval, the ventricular myocardium remains depolarized with a relatively steady electrical potential, so there’s little net current flow along the body surface, giving a flat (isoelectric) segment on the ECG. This is why the statement describing the right and left ventricular plateau during full contraction is the best way to describe what the S-T segment represents. For contrast, the start of atrial depolarization is shown by the P wave, the QRS duration reflects the entire ventricular depolarization, and the overall duration of the QRS complex is the whole depolarization period, not the S-T segment.

The S-T segment corresponds to the Plateau phase of the ventricular action potential, during which the ventricles are fully depolarized and actively contracting in systole. It begins after the end of ventricular depolarization (the S wave) and ends just before ventricular repolarization (the start of the T wave). During this interval, the ventricular myocardium remains depolarized with a relatively steady electrical potential, so there’s little net current flow along the body surface, giving a flat (isoelectric) segment on the ECG. This is why the statement describing the right and left ventricular plateau during full contraction is the best way to describe what the S-T segment represents. For contrast, the start of atrial depolarization is shown by the P wave, the QRS duration reflects the entire ventricular depolarization, and the overall duration of the QRS complex is the whole depolarization period, not the S-T segment.