- Patients with QTc interval > 450 msecs or other factors that increase the risk of QT prolongation or arrhythmic events (e.g., heart failure, hypokalemia, family history of long QT interval syndrome); this may be determined by either Bazett's correction (QTc =QT/RR0.5) or Friderica's correction (QTc = QT/RR0.33); QTc exclusion > 450 msec requires calculation according to both formulas