What does the QRS complex represent in an ECG?

The QRS complex in an ECG represents the depolarization of the ventricles, which is a critical phase in the cardiac cycle. This phase occurs just prior to ventricular contraction, leading to the pumping of blood from the ventricles to the lungs and the rest of the body.

During the QRS complex, the electrical signals originating from the atrioventricular node travel rapidly through the bundle of His, bundle branches, and Purkinje fibers, resulting in a synchronized contraction of the ventricular muscles. This is essential for ensuring that the heart effectively pumps blood. The shape and duration of the QRS complex can provide valuable information about the health of the heart's electrical conduction system and can indicate abnormalities such as bundle branch blocks or ventricular hypertrophy.

In this context, other options refer to different phases of the cardiac cycle. While the contraction of the atria occurs earlier in the cardiac cycle (represented by the P wave), preparation for the next heartbeat involves several cardiac events, including relaxation and filling of the chambers, typically reflected in other aspects of the ECG. Lastly, relaxation of the heart mostly correlates with the T wave, which signifies ventricular repolarization. Hence, the QRS complex is specifically associated with ventricular depolarization, making it