What phase does the ST segment correspond to in the cardiac cycle?

What phase does the ST segment correspond to in the cardiac cycle?

• A. Initial (plateau) phase of ventricular depolarization
• B. Initial (plateau) phase of ventricular repolarization
• C. Ventricular systole
• D. Atrial contraction

Answer: (B)

The ST segment corresponds to the initial (plateau) phase of ventricular repolarization during the cardiac cycle. This phase is critical as it represents the period when ventricular muscle cells are in a state of plateau due to the prolonged influx of calcium ions, which creates a balance between depolarization and repolarization. The ST segment is seen on the electrocardiogram as a flat line following the QRS complex and preceding the T wave.

During this time, the heart muscle is not contracting (which occurs during the earlier phase - ventricular systole) but is rather in a state of electrical stability, preparing for the next phase of repolarization before the ventricles can relax and refill with blood. Understanding the significance of the ST segment is key in diagnosing various cardiac conditions, as deviations from the normal ST segment can indicate ischemia or other cardiac issues.