Participants with a history of congenitally prolonged QTc, a first degree relative with unexplained sudden death under 40 years of age, or a measured QTc (Bazett s correction) longer than 480 msec on ECG. ECGs should be performed after correction of electrolyte abnormalities. Participants with a prolonged QTc should have a repeat ECG at least 24 hour after the first, and the mean of the 2 QTcs should not exceed 480 msec.