In this episode, we discuss the concerns of QTc prolongation, which can cause a fatal arrhythmia called torsades de pointes (TdP). We cover the difference between QT and QTc, how to interpret a QTc (and when it is inaccurate), common medications that prolong QTc, and how pharmacists can evaluate the risk of QTc/TdP in patients who are receiving QTc-prolonging therapies.

Key Concepts

1. The QTc interval is the QT interval that has been “corrected” for heart rate. In nearly all cases, when describing a QT interval, it should be expressed as the QTc.
2. Although a prolonged QTc is usually defined as a QTc exceeding 450-480 msec, the risk of torsades de pointes (TdP) begins to become concerning when the QTc is more than 500 msec, 15-20% longer than baseline, or if the QTc has increased by more than 60 msec.
3. Vaughan-Williams Class III antiarrhythmics are most implicated in QTc prolongation and TdP risk. These therapies include sotalol, dofetilide, and dronedarone. Although amiodarone is a class III antiarrhythmic, its risk of TdP is quite low despite the fact that it often substantially prolongs the QTc.
4. When pharmacists are assessing the risk of QTc prolongation and TdP, multiple factors (not just the QTc itself) should be considered. Risk scores, like the Tisdale Risk Score, as well as considering the risks/benefits of switching drug therapy, should be evaluated.

References

• CredibleMeds.com. Arizona Center for Education and Research on Therapeutics. http://crediblemeds.org.
• Noel ZR, See VY, Flannery AH. Walk the Line-The Importance of Well-Informed Interpretation of QT Prolongation. Ann Pharmacother. 2021;55(1):123-126. doi:10.1177/1060028020934718
• E14 Clinical Evaluation of QT/QTc Interval Prolongation and Proarrhythmic Potential for Non-Antiarrhythmic Drugs. Food and Drug Administration (FDA). https://www.fda.gov/regulatory-information/search-fda-guidance-documents/e14-clinical-evaluation-qtqtc-interval-prolongation-and-proarrhythmic-potential-non-antiarrhythmic-0