What does the ST segment of an EKG represent?

The ST segment on an EKG represents the period following ventricular depolarization and preceding ventricular repolarization. This segment reflects the time during which the ventricles are in a plateau phase, where they are in a state of uniform depolarization and are preparing to recover their resting state.

During the ST segment, there is no net electrical activity detected in the heart because all of the heart muscle cells in the ventricles have undergone depolarization, and the cells are transitioning to repolarization. This phase is critical for maintaining a coordinated heartbeat and ensuring that the ventricles properly refill with blood after contracting.

The other options provided do not accurately describe the function of the ST segment. The beginning of atrial contraction occurs during the P wave, the phase of heart rest between beats pertains to the intervals not occupied by depolarization (like the T-P interval), and atrial filling occurs during the diastolic phase but is not represented in the ST segment. Thus, the ST segment is specifically associated with the transition from depolarization to the upcoming repolarization of the ventricles.