Corrected QT Interval (QTc)
Why Use
A prolonged QT interval is associated with an increased risk of torsades de pointes. The QT shortens at faster heart rates and lengthens at slower heart rates; this calculator corrects the QT to what it would be at a heart rate of 60 bpm.
When to Use
Useful in the evaluation of patients with syncope. Important when assessing for QT prolongation in patients receiving multiple QT-prolonging medications.
Formula
Management
Management depends on the specific etiology of a prolonged or shortened QT interval. In general, minimize the use of QT-prolonging medications in patients with QT prolongation and only use them after a thorough risk-benefit assessment.
Advice
Ensure the QT interval measurement does not mistakenly include a U wave. If it does, re-measure, recalculate, and consider potential etiologies of U waves. If there is no U wave, consider common causes of a prolonged QT interval, including: Electrolyte abnormalities. Intrinsic cardiac causes. Central causes. Medications. If a short QT interval is observed, confirm measurement accuracy and consider short QT syndrome (s ee Facts & Figures ).
More Information
Long QT Syndrome: The 2022 ESC guidelines recommend a cutoff QTc ≥ 480 ms (regardless of symptom) or an LQTS diagnostic score >3, or ≥460 ms (in individuals with arrhythmic syncope without secondary causes for QT prolongation) for the diagnosis of long QT syndrome (including acquired long QT syndrome). Meanwhile, a 2013 expert consensus of the Heart Rhythm Society, European Heart Rhythm Association, and the Asia Pacific Heart Rhythm Society recommended the following diagnostic criteria for long QT syndrome: Long QT syndrome is diagnosed: In the presence of an LQTS risk score > 3.5 in the absence of a secondary cause for QT prolongation, and/or In the presence of an unequivocally pathogenic mutation in one of the LQTS genes, or In the presence of a QTc >500 ms in repeated 12-lead ECG and in the absence of a secondary cause for QT prolongation. Long QT syndrome can be diagnosed in the presence of a QTc between 480-499 ms in repeated 12-lead ECGs in a patient with unexplained syncope in the absence of a secondary cause for QT prolongation and in the absence of a pathogenic mutation. A longer QTc puts the patient at increased risk for torsades de pointes. A QTc >500 ms indicates especially high risk of torsades de pointes. Some causes of prolonged QT: Electrolyte abnormalities: Hypocalcemia. Hypokalemia. Hypomagnesemia. Intrinsic cardiac causes: Myocardial. ischemia. After cardiac arrest. CAD. Cardiomyopathy. Severe bradycardia, high-grade AV block. Congenital long QT syndrome. Central causes: Raised intracranial pressure. Autonomic dysfunction. Hypothyroidism. Hypothermia. Medications: Anti-arrhythmics. Psychotropic drugs. Other drugs. QT Nomogram: This tool is for evaluating patients at high risk of torsades de pointes – the QT interval here is the absolute / uncorrected QT interval. Patients with parameters plotted above the line are at high risk of torsades de pointes: Short QT Syndrome: In the rarer case of a short QT interval, the 2022 European guidelines recommend that short QT syndrome be diagnosed in the presence of a QTc ≤360 ms and one or more of the following: A pathogenic mutation. A family history of short QT syndrome. Survival from a ventricular tachycardia or fibrillation episode in the absence of heart disease. These same guidelines recommend short QT syndrome to be considered in the presence of a QTc ≤320 ms, and in the presence of a QTc between 320-360 ms with arrhythmic syncope and a family history of sudden death at age <40 years. Meanwhile, the aforementioned 2013 expert consensus recommended the following diagnostic criteria for short QT syndrome: Short QT syndrome is diagnosed in the presence of a QTc <330 ms. Short QT syndrome can be diagnosed in the presence of a QTc <360 ms and one or more of the following: Pathogenic mutation. Family history of SQTS. Family history of sudden death at age <40. Survival of a VT/VF episode in the absence of heart disease.