Why do we correct the QT interval for heart rate?

  Since heart rate is the major modifier of ventricular action potential, the QT interval changes with heart rate. This is the basis for QT interval correction. To compare the QT intervals between different individuals, they must be corrected according to the heart rate.

  Heart rate is INVERSELY related to the QT interval:
As the heart rate increases, the QT interval decreases. As the heart rate decreases, the QT interval increases.



Proposed formulae for QT interval correction





Linearity of the QT interval correction formulae

  Nonlinear formuale
- Bazett
- Fridericia

  Linear formuale
- Framingham
- Hodges



Which QT interval correction formula is the best?

  The most frequently used one is the Bazett's formula. It has been reported that the current Bazett's correction formula first appeared in a paper titled "The Duration of the Electrical Systole (Q-T) in Acute Rheumatic Carditis in Children" by Leo M Taran and Nelly Szilagyi in the American Heart Journal in 1947 -27 years after Bazett’s paper-. In order to normalize QT measurements for heart rate, they proposed that the QT interval be divided by the square root of the R-R interval and called it Bazett’s Formula.

  However Bazett's formula is also the most criticized formula. Bazett's formula overcorrects the QT interval at fast heart rates, undercorrects the QT interval at slow heart rates.

  Hodges formula is suggested to be the best one.



References

  Clinical Arrhythmology and Electrophysiology: A Companion to Braunwald's Heart Disease. 2nd edition. 2012.

  J Am Coll Cardiol 2008;51:2291-2300.

  J Electrocardiol 2018;51:1009-1010.

  Am Heart J 1947;33:14-26.

  H. C. Bazett, An Analysis of the Time-Relations of Electrocardiograms Heart 1920;7:353-370.. DOI: 10.1111/j.1542-474X.1997.tb00325.x