QTc Calculator
- Comprehensive explanation and clinical context
The QT interval measures ventricular depolarisation and repolarisation on the electrocardiogram. Correcting the QT interval for heart rate yields QTc. Several correction formulae exist. Bazett's formula divides QT by the square root of the RR interval in seconds. Fridericia's formula divides QT by the cube root of the RR interval. Framingham's formula adjusts QT by adding a linear term based on RR. Hodges's formula adjusts QT by adding a term based on heart rate. Each formula has strengths and weaknesses. Bazett's formula overcorrects at high heart rates and undercorrects at low heart rates. Modern evidence suggests Fridericia or Framingham provide better mathematical correction across a range of heart rates but clinical practice continues to use Bazett in many settings. When assessing a prolonged QTc consider sex based cutoffs age concurrent drugs electrolyte disturbances structural heart disease and the presence of conduction disease. A QTc equal to or greater than 500 ms is associated with a higher risk of life threatening ventricular arrhythmia. Repeat ECG confirmation and a clinical review of medications and electrolytes are recommended when QTc is prolonged.
References
1. Determination and interpretation of the QT interval. Circulation 2018. American Heart Association scientific statement. See AHA Circulation 2018 for detailed recommendations.
2. Which QT correction formulae to use for QT monitoring. Journal of the American Heart Association 2016. This comparative work discusses Fridericia and Framingham performance.
3. Drug induced QT prolongation and torsades de pointes. Review article. This review summarises risk thresholds and clinical implications including the association of QTc equal to or greater than 500 ms with increased risk.
4. Practical ECG resource summarising QTc cutoffs and clinical use. ECG library LitFL.
5. Recent comparisons of correction formulae and recommendations to prefer Fridericia or Framingham for rate correction. Scientific reports and systematic reviews.
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