Contributor: Travis Barlock MD

Educational Pearls

The QT interval represents phases 2 and 3 of ventricular plateau and repolarization, respectively.

As the QT interval lengthens, more sodium and calcium channels are available and susceptible to action potentials.

Prolonged QT interval is more concerning in the setting of bradycardia.

This scenario increases the likelihood of R on T phenomenon.

R on T phenomenon occurs due to an early afterdepolarization event in which a premature ventricular contraction (PVC) occurs during the repolarization period (superimposed on the T wave), leading to an aberrant re-entry circuit.
The re-entry circuit leads to Torsades de Pointes (polymorphic ventricular tachycardia with prolonged QT) and subsequent ventricular fibrillation.
Treatment for Torsades de Pointes is 2g MgSO4.
The preferred antiarrhythmic for VTach is IV lidocaine 1.5 mg/kg over 2 minutes.

Avoid amiodarone due to risk of further QT prolongation.

A heart rate under 80 does not need QT correction

Corrected QT interval is used in the setting of tachycardia due to an abnormally small T wave

Correction for the QT interval in tachycardia:

472 ms for males vs. 482 ms for females

References

1. Banai S, Schuger C, Benhorin J, Tzivoni D. Treatment of torsade de pointes with intravenous magnesium. Am J Cardiol. 1989;63(20):1539-1540. doi:10.1016/0002-9149(89)90033-7

2. Gorgels APM, Van Den Dool A, Hofs A, et al. Comparison of procainamide and lidocaine in terminating sustained monomorphic ventricular tachycardia. Am J Cardiol. 1996;78(1):43-46. doi:10.1016/S0002-9149(96)00224-X

3. Liu MB, Vandersickel N, Panfilov A V., Qu Z. R-From-T as a Common Mechanism of Arrhythmia Initiation in Long QT Syndromes. Circ Arrhythmia Electrophysiol. 2019;12(12):1-15. doi:10.1161/CIRCEP.119.007571

4. Sagie A, Larson MG, Goldberg RJ, Bengtson JR, Levy D. An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study). Am J Cardiol. 1992;70(7):797-801. doi:10.1016/0002-9149(92)90562-D

5. Vandenberk B, Vandael E, Robyns T, et al. Which QT correction formulae to use for QT monitoring? J Am Heart Assoc. 2016;5(6). doi:10.1161/JAHA.116.003264

6. Zipes DP, Camm AJ, Borggrefe M, et al. ACC/AHA/ESC 2006 Guidelines for Management of Patients with Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death - Executive Summary: A Report of the American College of Cardiology/American Heart Association Task Force and the European Society of Cardiology Committee for Practice Guidelines. Vol 114.; 2006. doi:10.1161/CIRCULATIONAHA.106.178104

Summarized by Jorge Chalit, OMSII | Edited by Meg Joyce & Jorge Chalit, OMSII

Podcast 855: QT Intervals

Emergency Medical Minute