- QTcF interval > 450 msec (when calculating the QTc interval, Fridericia's equation [QT/RR0.33] will be used) 18. Patients with congenital QT prolongation. 19. Current use of Class 1A or Class 3 antiarrhythmic medications. 20. New York Heart Association Class III or IV heart failure. 21. Patients with clinical laboratory abnormalities suggesting clinically relevant underlying disease, or with any of the following abnormal results, at screening.