Interval QT

The QT interval is inversely proportional to heart rate:

·       The QT interval shortens at faster heart rates

·       The QT interval lengthens at slower heart rates

·       An abnormally prolonged QT is associated with an increased risk of ventricular arrhythmias, especially Torsades de Pointes

·       Congenital short QT syndrome has been found to be associated with an increased risk of paroxysmal atrial and ventricular fibrillation and sudden cardiac death

Normal QTc values

·       QTc is prolonged if > 440ms in men or > 460ms in women

·       QTc > 500 is associated with an increased risk of torsades de pointes

·       QTc is abnormally short if < 350ms

·       A useful rule of thumb is that a normal QT is less than half the preceding RR interval

Causes of a prolonged QTc (>440ms)

·       Hypokalaemia

·       Hypomagnesaemia

·       Hypocalcaemia

·       Hypothermia

·       Myocardial ischemia

·       ROSC Post-cardiac arrest

·       Raised intracranial pressure

·       Congenital long QT syndrome

·       Medications/Drugs

Causes of a short QTc (<350ms)

·       Hypercalcaemia

·       Congenital short QT syndrome

·       Digoxin effect

https://litfl.com/qt-interval-ecg-library/

·       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 Sep 15;70(7):797-801

·       Gaita F, Giustetto C, Bianchi F et al. Short QT Syndrome: a familial cause of sudden death. Circulation. 2003: 108: 965-70.

·       Chan A, Isbister GK, Kirkpatrick CMJ, Dufful SB. Drug-induced QT prolongation and torsades de pointes: evaluation of a QT nomogram. QJM: An International Journal of Medicine. 2007. 100(10)609-615

·       Taggart NW, Haglund CM, Tester DJ, Ackerman MJ. Diagnostic miscues in congenital long-QT syndrome. Circulation. 2007 May 22;115(20):2613-20

·       Vetter VL. Clues or miscues? How to make the right interpretation and correctly diagnose long-QT syndrome. Circulation 2007;115:2595–8.

·       Viskin S. The QT interval: too long, too short or just right. Heart Rhythm. 2009 May;6(5):711-5. Epub 2009 Mar 3