What does the QRS complex indicate in the cardiac cycle?

The QRS complex on an electrocardiogram (ECG) represents the electrical activation of the ventricles during the cardiac cycle. This complex reflects the depolarization of the ventricular myocardium, which leads to ventricular contraction. The QRS complex is typically a brief and sharp waveform, indicating that a significant electrical event is occurring as the electrical impulse travels rapidly through the His-Purkinje system, leading to the coordinated contraction of the ventricles.

Understanding this is crucial in interpreting ECG readings, as the QRS complex directly correlates with the efficiency and timing of the heart's pumping action. In contrast, the other choices relate to different phases of the cardiac cycle or other electrical events, such as atrial activity, which do not involve ventricular depolarization. This differentiation is essential for diagnosing various cardiac conditions.