Introduction

The S wave is a component of the QRS complex in an electrocardiogram (ECG). The QRS complex represents ventricular depolarization, a crucial phase in the cardiac cycle where the ventricles contract to pump blood.

Understanding the S wave’s origin helps in interpreting ECG readings accurately, especially when diagnosing cardiac conduction abnormalities.

What Causes the S Wave in an ECG?

• After initial depolarization of the interventricular septum and the main mass of the ventricles (represented by the Q and R waves), the S wave represents the depolarization of the upper portions of the ventricular walls, particularly the late depolarization of the ventricular walls.

• This terminal wave is characterized by a negative deflection following the R wave, occurring as the wave of depolarization moves back toward the atrioventricular (AV) junction (base of the heart).

• The vector of this depolarization is directed away from the positive ECG electrode, resulting in the negative deflection seen as the S wave.

Why Other Choices Are Incorrect

1. Septal and left ventricular depolarization: This primarily contributes to the Q wave and early R wave, not the S wave.

2. Left to right septal depolarization: This is the initial septal movement represented by the small Q wave or the small r wave in some leads.

3. Repolarization of atrium: Atrial repolarization usually occurs during the QRS complex and is not distinctly seen as a separate wave like the S wave.

Summary of Ventricular Depolarization Phases in ECG

Conclusion

The S wave on a normal human ECG originates due to:

(2) Late depolarization of the ventricular walls moving back toward the AV junction.