QT depression is seen in ?
In vitro, hypokalemia not only prolongs the QT interval in the absence of drug but also potentiates drug block of ion channels that results in QT prolongation. Drugs (i.e., certain antiarrhythmics and erythromycin) and electrolyte disorders (i.e., hypokalemia, hypocalcemia, hypomagnesemia) can prolong the QT' interval and predispose to torsades de pointes.
Hypercalcemia can result  in significant electrocardiographic changes, including bradycardia, AV block, and short QT interval; changes in serum calcium can be monitored by following the QT interval
Hyperkalemia, or drugs such as flecainide) tend to increase QRS duration. Conditions that prolong phase 2 (use of amiodarone, hypocalcemia) increase the QT interval.
Hypocalcemia typically prolongs the QT interval (ST portion), while hypercalcemia shortens it
Hypokalaemia
Hyperkalaemia
Hypocalcaemia
Hypercalcaemia