Calcium's main effect on the ECG appears to be on the duration of the ST segment, such that: 1. Hypercalcemia shortens the ST segment, producing also a short QTc. 2. Hypocalcemia prolongs the ST segment, producing also a long QTc. As an aside, there are only three conditions in which a short QTc is typically noted: hypercalcemia, digitalis toxicity, and a recently described syndrome that causes sudden death--"the short QT syndrome" (in which the QTc may be < 300ms...that's REALLY short!). As another aside, there are only two conditions that prolong the QTc via prolongation of the ST segment--hypocalcemia and hypothermia.
