What does hypocalcemia primarily affect with regard to the heart's electrical activity?

Hypocalcemia primarily affects the heart's electrical activity by prolonging the QT interval. The QT interval on an electrocardiogram (ECG) reflects the time it takes for the heart's ventricles to depolarize and then repolarize. When calcium levels are low, it affects the cardiac myocyte's ability to conduct electrical impulses effectively, leading to a delay in repolarization. This delay manifests as an elongated QT interval.

This prolonged interval can increase the risk of life-threatening arrhythmias, such as torsades de pointes, due to its impact on the heart's refractory period. Monitoring the QT interval is crucial in patients with hypocalcemia, as it provides insight into their electrical stability and risk for arrhythmias.

The other options, while related to cardiac function, do not directly reflect the primary effect of hypocalcemia on the electrical activity of the heart as accurately as the prolongation of the QT interval.