Abstract
QT interval in an electrocardiogram (ECG) is interpreted after correction (QTc) by various formulas. This study aimed to compare the QTcs calculated by nine formulas. Sinus rhythm ECG reports of 1140 anonymous subjects showed uncorrected QT interval of 388.49 ± 42.74 ms. The QTc calculated by Bazett (443.96 ± 57.58 ms), Fridericia (424.37 ± 50.1 ms), Dmitrienko (433.59 ± 53.37 ms), Framingham (422.59 ± 45.55 ms), Schlamowitz (433.89 ± 48.05 ms), Hodges (421.6 ± 46.4 ms), Ashman (434.33 ± 54.05 ms), Rautaharju (427.75 ± 47.4 ms), and Sarma (429.22 ± 48.67 ms) showed a significant difference F (8, 10251) = 22.78 p < 0.0001. Hence, ECG should contain the formula for proper reporting and ease of interpretation by clinicians.
Keywords: Electrocardiography, Heart rate, QT interval
1. Introduction
The QT interval in an electrocardiogram (ECG) indicates the electrical changes that occur during ventricular activation and recovery. The QT interval should be corrected or adjusted for heart rate before clinical interpretation and comparison with reference values.1
Several formulas are available for correcting the QT interval. Some commonly used formulas are Bazett, Fridericia, Dmitrienko, Framingham, Schlamowitz, Hodges, Ashman, Rautaharju, and Sarma. The details of these formulas are shown in Table 1. These formulas use the raw QT, heart rate (HR), the interval between two peaks of R (RR), and some mathematical constants to correct the QT for interpretation.2
Table 1.
Formula for calculation of corrected QT interval.
| Title | Formula for calculating QTc |
|---|---|
| Bazett | QT/RR^(1/2) |
| Fridericia | QT/RR^(1/3) |
| Dmitrienko | QT/RR^(0.413) |
| Framingham | QT+0.154 × (1−RR) |
| Schlamowitz | QT+0.205 × (1−RR) |
| Hodges | QT+0.00175 × (HR−60) |
| Ashman | QT/log10(10 × (RR+0.07)) × log10(10.7) |
| Rautaharju | QT+0.24251−0.434 × e^(–0.0097 × HR) |
| Sarma | QT−0.04462 + 0.664 × e^(–2.7 × RR) |
The raw QT, RR interval are measured in seconds. Heart rate (HR) is measured in beats per minute. The value of e (Euler's number) is an mathematical constant and irrational number; for an idea, it is approximately 2.71828.
Previous studies have reported differences in QTc between the Bazett and Fridericia formulas,3 as well as among the Fridericia, Hodges, and Framingham formulas,4 the Bazett, Fridericia, Hodges, and Framingham formulas,5, 6 and the Bazett, Fridericia, Framingham, Hodges, and Rautaharju formulas.7 Other heart rate adjustment formulas have also been compared.8 However, several newer formulas have been proposed for correction, incorporating more extensive calculation methods.2
In this context, this study was conducted to find the variation and correlation of QTc calculated from different formulas. The results would inform clinicians about the differences when using various formulas.
2. Methods
This cross-sectional study was conducted with anonymous ECGs of adult patients. The ECGs were collected from the digital storage of the electrocardiograph from June 2019 to May 2021. From the ECG reports, we collected anonymized data on age, HR, and QT (from lead II). We did not collect the QTc provided in the report. Any ECG of a patient aged over 18 years with sinus rhythm was included in the data analysis. A total of 1140 data points were collected for final analysis.
The QT and HR were used to calculate the QTc according to the formulas shown in Table 1. The data were first tested by the Shapiro–Wilk test for normality and were found to be normally distributed. Descriptive statistics were used to find the central tendency and distribution of the data. To compare the variance of QTc obtained by different formulas, we used a one-way Analysis of Variance (ANOVA). After obtaining the ANOVA results, we conducted Tukey's post hoc test to determine the significance of variance between pairs. Correlation coefficients were calculated among the QTc values obtained from various formulas to observe the relationships. Statistical tests were conducted using GraphPad Prism 6.01 (GraphPad Inc, USA) and Microsoft Excel® 2010 (Microsoft Inc, USA). A P value < 0.05 was considered statistically significant.
3. Results
A total of 1140 ECGs were analyzed. The mean age of the overall sample was 44.23 ± 15.41 years. There were 722 (63.33 %) men with a mean age of 43.71 ± 15.21 years and 418 (36.67 %) women with a mean age of 45.12 ± 15.72 years. In the overall sample, the mean HR was 78.92 ± 12.25 bpm and the uncorrected QT interval was 388.49 ± 42.74 ms. There was no gender difference in the distribution of age, HR, and uncorrected QT [Table 2].
Table 2.
Overall and gender wise age, heart rate, and uncorrected QT interval.
| Variable | Overall (n = 1140) |
Men (n = 722) |
Women (n = 418) |
P value |
|---|---|---|---|---|
| Mean ± SD | ||||
| Age (years) | 44.23 ± 15.41 | 43.71 ± 15.21 | 45.12 ± 15.72 | 0.15 |
| HR (bpm) | 78.92 ± 12.25 | 79.32 ± 12.66 | 78.22 ± 11.5 | 0.22 |
| QT (ms) | 388.49 ± 42.74 | 389.45 ± 41.84 | 386.86 ± 44.27 | 0.12 |
SD: Standard deviation, HR: heart rate, QT: QT interval in ECG, P value is of unpaired t-test to compare values of males and females.
The QTc calculated by Bazett (443.96 ± 57.58 ms), Fridericia (424.37 ± 50.1 ms), Dmitrienko (433.59 ± 53.37 ms), Framingham (422.59 ± 45.55 ms), Schlamowitz (433.89 ± 48.05 ms), Hodges (421.6 ± 46.4 ms), Ashman (434.33 ± 54.05 ms), Rautaharju (427.75 ± 47.4 ms), and Sarma (429.22 ± 48.67 ms) showed a significant difference (ANOVA, F (8, 10251) = 22.78, p < 0.0001). Fig. 1 shows the differences in a box plot. In a post hoc analysis, among 36 pairs, 20 pairs showed significant differences.
Fig. 1.
Corrected QT interval according to different formulas.
All the QTc values showed a significant and very strong positive correlation among them, with correlation coefficients (r) ranging from 0.983 to 0.998.
4. Discussion
With the aim of finding the variation and correlation of QTc calculated from different formulas, namely Bazett, Fridericia, Dmitrienko, Framingham, Schlamowitz, Hodges, Ashman, Rautaharju, and Sarma, we discovered that although the QTc values from these formulas showed an excellent level of correlation, the variance was significantly different. Currently, ECG reports include calculated parameters like HR, RR, PR, QT, and QTc. However, the formula used to calculate QTc from QT and RR or HR is not commonly mentioned in the report. Stakeholders may need to emphasize transparent reporting to help doctors decide whether to use the provided QTc or to calculate it using another formula.
Molnar et al compared the Bazett, Framingham, Fridericia, and Sarma formulas and found no differences in mean squared residuals, concluding that Bazett's formula was a good choice over regression-based calculations like the Framingham or Sarma formulas.9 In contrast, the Bazett formula is often questioned as it provides undercorrection at low heart rates and overcorrection at high heart rates.10 A study by Aytemir et al showed that in otherwise healthy individuals, QTc calculated by the Bazett, Hodges, and nomogram formulas may yield prolonged QTc intervals, while the Framingham formula may yield shortening of QTc intervals during exercise.11 Luo et al also analyzed the classical Bazett, Fridericia, Framingham, and Hodges formulas and found that normal ECGs from US hospitals showed a similar distribution of QTc obtained from the Fridericia, Framingham, and Hodges formulas, whereas Bazett's formula showed a significantly wider distribution.6 In our study, we found the highest variance of residuals in Bazett's formula and the lowest variance in the Framingham formula.
Several online calculators help clinicians calculate the QTc from QT, HR, or RR. Some of the calculator providers, available methods of calculation, required input, and web addresses are listed in Appendix 1. Clinicians might consider creating their own calculator using spreadsheet software like Microsoft Excel or Apache OpenOffice Calc. We created one such calculator according to the formulas available in Table 1, and readers can obtain it from the corresponding author via email.
5. Conclusion
When QT is corrected using different formulas, the calculated QTc may show significant differences across these formulas. Therefore, ECG reports should include information about both the QT and QTc values, along with the title and details of the formula used. This transparency enables clinicians to have a clear understanding of the correction method. If needed, they can recalculate the QTc from the QT and heart rate of the patients using an alternative formula.
Source(s) of support
Nil.
Details of contribution
1) Himel Mondal: Concept; Design; Data collection; Data analysis; Literature search; Drafting manuscript, Editing manuscript, Supervision, 2) Shaikat Mondal: Concept; Design; Administration; Data collection; Data handling; Data analysis; Data interpretation; Literature search, Editing manuscript, 3) Bhagyajyoti Priyadarshini: Concept; Design; Data Collection; Literature search; Editing manuscript.
All the authors approved the final version and take responsibility of the manuscript and its data and intellectual contents.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank all the operators who provided the data for this study. We are thankful to Dr. Siba Sai Swaroop, Department of Community Medicine, SLN Medical College, Koraput, Odisha and Prof. (Dr.) Joshil Kumar Behera, Head of the Department, Department of Physiology, Nagaland Institute of Medical Sciences and Research, Nagaland, India for their valuable comments and support during preparation of the manuscript and presentation materials for INSIGHT 2022, AIIMS Delhi.
Footnotes
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ihj.2024.07.006.
Appendix A. Supplementary data
The following is the Supplementary data to this article:
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