- Participants with a history of congenitally prolonged QTc, a first degree relative with unexplained sudden death under 40 years of age, or a measured QTcB (Bazett s correction) longer than 480 msec on ECG. ECGs should be performed after correction of electrolyte abnormalities. Participants with a prolonged QTcB should have a repeat ECG twice, at least 24 hour apart, and the average of the 3 QTcBs should not exceed 480 msec. History of QT prolongation associated with other medications that required discontinuation of that medication.