QT interval is measured from the beginning of the QRS complex 
to the end of the T wave on the ECG and represents the period from 
the onset of depolarisation to the completion of repolarisation of the ventricular myocardium

QTc interval of 450 ms ( men ) and 460 ms ( women ) are generally 
accepted as upper limits of normal

Recognized risk factor for occurrence of cardiac arrhythmias
○ VT /TdP
○ syncope
○ sudden death Particularly torsades de pointes ( TdP )
○ polymorphic ventricular tachyarrhythmia
○ on ECG → continuous twisting of the vector of the QRS complex around isolelectric baseline
○ Marked feature of TdP is pronounced prolongation of the QT interval in the supraventricular beat preceding the arrhythmia
○ TdP can degenerate into ventricular fibrillation leading to sudden death Qt interval varies greatly and is affected by age , sex , sympathetic tone and diurnal pattern The magnitude of the ↑↑ in QT interval from baseline is also helpful in evaluating the risk

QTc ( QT corrected ) QT interval is inversely proportional to heart rate ie
QT interval increases as HR falls QTc is the interval corrected for HR @ 60 Several formulas exist to correct the QT interval Drug induced TdP is more commonly seen with bradycardias

Do not use QT prolonging drugs in patients with known LQTS-Consider the risk of QT prolongation when starting a new medication-Assess for risk factors for
 QT prolongation and overall
 risk of drug 
induced QT prolongation-If patient at ↑↑ risk of QT prolongation or already taking a drug that can ↑↑ QT consider an alternative drug which does not affect QT-Not practical to consider an 
ECG every time a QT
 prolonging medication is prescribed-When risk of QT prolongation
 is high consider baseline ECG before starting the medication and when the new drug 
reaches steady state 
( five half lives )-QTC of 470 – 500 ms in men
 QTc of 480- 500 ms in women
 An increase in QTc >= 60 ms


 QTC > 500 ms

Citalopram and 
Escitalopram are 
associated with dose dependent QT interval prolongation -> reason 
why the max dose was reduced

Brilliant website for all QT related issues https://crediblemeds.org
registration needed for list- free when checked last

References

 Drug Induced QT Interval Prolongation National Medicine Information Center Volume 21 Number 6 2015 Drug Induced QT Prolongation PostScriptExtra Issue 21 , December 2012 NHS Glasgow and Clyde Medicines Information Service The Clinical Evaluation of QT/QTc Interval Prolongation and Proarrhythmic Potential for Non-Antiarrhythmic European Medicines Agency November 2005 CHMP/ICH/2/04 Drug-induced QT interval prolongation : mechanisms and clinical management Ther Adv Drug Saf . 2012 Oct ; 3(5) : 241-253 Drug-Induced QTc Interval Prolongation : A Multicenter Study to Detect Drugs and Clinical Factors Involved in Every Day Practice Current Drug safety , 2016 , 11 , 86-98 QT Interval and drug therapy drug and Therapeutics Bulletin BMJ 2016 ; 353 :i2732 QT Interval Edward Burns from https://lifeinthefastlane.com/ecg-library/basics/qt_interval/ My Approach to the patient with a borderline long QT Interval Trends in Cardiovascular Medicine Roston Thomas MD October 1 , 2017 , Issue 7 . Pages 516-517 Long QT syndrome BMJ Best Practice Sudden Arrhythmia Death Syndrome : Importance of the Long QT Syndrome AAFP Drug and non drug – associated QT interval prolongation Br J Clin Pharmacol . 2010 Jul ; 70 ( 1) : 16-23 Website snapshot from https://crediblemeds.org