- Patients with potassium <3.0 mmol/L at study entry, magnesium <0.4 mmol/L at study entry, calcium <1.75 mmol/L at study entry, family history of long QT syndrome, and concomitant medications known to prolong the QT interval. If the electrolyte abnormalities are corrected prior to study commencement, the patient may become eligible for the trial.