What does the QRS complex indicate in an electrocardiogram?

The QRS complex in an electrocardiogram (ECG) represents ventricular depolarization, which is a critical phase in the cardiac cycle. This phase involves the electrical activation of the ventricles, leading to their contraction and subsequent pumping of blood to the lungs and the rest of the body.

During this process, the electrical impulses generated by the sinoatrial (SA) node travel through the atrioventricular (AV) node and then spread through the His-Purkinje system, causing the ventricular muscle fibers to contract. The QRS complex is characterized by a rapid and sharp spike in the ECG trace, reflecting this swift depolarization.

In contrast, the other choices refer to different aspects of the cardiac cycle. Atrial contraction occurs during the P wave, which signifies atrial depolarization. The repolarization of the heart is represented by the T wave in the ECG, where the ventricles recover after contraction. Atrial repolarization is represented during the QRS complex but is not visually distinct on the ECG due to its simultaneous occurrence with ventricular depolarization. Therefore, the correct interpretation of the QRS complex is that it specifically denotes ventricular depolarization.