Expansion
Hypercalcaemia shortens the QT interval; hypocalcaemia prolongs it
Expansion
Calcium influx sustains phase 2, the plateau, so it sets the duration of repolarisation and therefore the QT interval.
- Hypercalcaemia - shortened ST segment and QT interval; in severe cases bradycardia and heart block
- Hypocalcaemia - prolonged ST segment and QT interval, with a risk of torsades de pointes
Note the contrast with potassium, which chiefly affects the T wave and QRS because it governs the resting potential and repolarisation reserve.
Calcium also has an inotropic effect: raising extracellular calcium increases the trigger for calcium-induced calcium release and so increases contractility. This is why intravenous calcium transiently improves contractility, and why citrate-induced hypocalcaemia after massive transfusion can cause hypotension.