Nomenclature, categorization and usage of formulae to adjust QT interval for heart rate

Abstract

Assessment of the QT interval on a standard 12 lead electrocardiogram is of value in the recognition of a number of conditions. A critical part of its use is the adjustment for the effect of heart rate on QT interval. A systematic search was conducted to identify studies that proposed formulae to standardize the QT interval by heart rate. A nomenclature was developed for current and subsequent equations based on whether they are corrective (QTc) or predictive (QTp). QTc formulae attempt to separate the dependence of the length of the QT interval from the length of the RR interval. QTp formulae utilize heart rate and the output QTp is compared to the uncorrected QT interval. The nomenclature consists of the first letter of the first author's name followed by the next two consonance (whenever possible) in capital letters; with subscripts in lower case alphabetical letter if the first author develops more than one equation. The single exception was the Framingham equation, because this cohort has developed its own "name" amongst cardiovascular studies. Equations were further categorized according to whether they were linear, rational, exponential, logarithmic, or power based. Data show that a person's QT interval adjusted for heart rate can vary dramatically with the different QTc and QTp formulae depending on the person's heart rate and QT interval. The differences in the QT interval adjustment equations encompasses values that are considered normal or significant prolonged. To further compare the equations, we considered that the slope of QTc versus heart rate should be zero if there was no correlation between QT and heart rate. Reviewing a sample of 107 patient ECGs from a hospital setting, the rank order of the slope - from best (closest to zero) to worst was QTcDMT, QTcRTHa, QTcHDG, QTcGOT, QTcFRM, QTcFRD, QTcBZT and QTcMYD. For two recent formulae based on large data sets specifically QTcDMT and QTcRTHa, there was no significant deviation of the slope from zero. In summary a nomenclature permits easy reference to QT formulae that adjust for heart rate. Twenty different formulae can produce discordant calculations of an adjusted QT interval. While the formulae developed by Bazett and Fridericia (QTcBZT and QTcFRD respectively) may continue to be used clinically, recent formulae from large population studies specifically QTcDMT and QTcRTHa appear to be better to adjust QT for heart rate in clinical practice.

Increased iron stores prolong the QT interval – a general population study including 20 261 individuals and meta‐analysis of thalassaemia major

“…In the meta‐analysis of eight thalassaemia major studies (Table SIV), the random effects estimate of the raw mean difference of QTcB interval was 19·5 ms (95%CI = 8·6–30·3; P < 0·001) (Fig ) with heterogeneity Q‐value of 50·6 (degrees of freedom = 7, P < 0·001) and I 2 of 85. QT intervals were corrected for heart rate by Bazett's formula(Rabkin & Cheng, ) in seven out of eight studies; Oztarhan et al () did not mention which formula was used for correction. Subgroup analyses made by excluding one study at a time showed robustness of the random effects estimate (Figure S1).…”

“…Since the QT interval is highly heart rate dependent, the corrected QT (QTc) interval was determined by heart rate using Fridericia's formula (Fridericia, ). Bazett's formula for heart rate correction of QT interval (Rabkin & Cheng, ) was used only in comparison with the meta‐analysis, when the variable will be referred to as QTcB.…”

Effects of atypical antipsychotic drugs on QT interval in patients with mental disorders

“…Eligible interventions included atypical antipsychotics when compared with placebo or other antipsychotic medications. Eligible outcomes included change in QT Interval, clinically important prolongation of QT corrected to RR interval ≥450 msec in men ≥480 msec in women, and QTc ≥500 msec associated with increased risk of life-threatening torsades de pointes ventricular tachycardia (9).…”

Measurement of the QT Interval: Lessons from Thirty-Two Animal Species for the Correction of the QT Interval by Heart Rate

“…A key issue in the utilization of the QT interval, is the need to appropriately adjust or correct for the impact of heart rate on the QT interval. Many formulae have been proposed to correct for the impact of heart rate on the QT interval so that the underlying QT interval can be assessed (for a review see [6]). The approach of Bazett, that the impact of heart rate can be corrected by dividing the QT interval by the square root of heart rate [7] and the approach of Fridericia that the QT interval needs to be divided by the cubed root of the heart rate (RR interval) [8], are still in use today.…”

“…Mathematical equation -linear, exponential, cubic etc. have been applied to ECGs from patients and large populations in an attempt to adjust for the effect of heart rate on the QT interval [6]. Testing for the validity of approaches to adjust or correct for the impact of heart rate on the QT interval has been conducted within or between individuals and mainly in humans.…”