A 2020 review discussing the measurement of QT interval prolongation notes that while several correctional formulas exist, none have proven superior. An algorithm to determine which formula to use is also provided. If a patient does not show atrial fibrillation, and the QRS duration is broad (> 120 ms or 3 small squares on standard ECG paper), it is suggested to consider the modified Bogossian formula (QTc= QT - 0.5x [QRS duration]). If the QRS duration is narrow (<120 ms or 3 small squares on standard ECG paper) and the heart rate is <50, it is recommended to use the nomogram provided by the authors (nomogram figure pictured in the article) or the Fridericia formula. For patients with narrow QRS duration and heart rate between 50 to 70, it is suggested to use the nomogram, Fridericia or Bazett formula. Finally, for patients with a narrow QRS duration and heart rate > 70, it is suggested to use the nomogram. See Table 1 for a summary of methods for correction of the QT interval with their respective strengths, limitations, and recommendations for use. [1]
The nomogram described in the article has been developed so the heart rate and the uncorrected QT can be plotted. A point plotted above a pre-defined line is associated with a higher risk of torsades de pointes (TdP). The nomogram, if available, is said to be the simplest method of QT interval interpretation, as a specific formula does not need to be applied. Compared to the Bazette formula, the nomogram has been shown to have equivalent sensitivity and specificity in a systematic review of 129 cases of TdP and was less likely to generate false positive results in two retrospective studies. The nomogram, however, lacks robust prospective data supporting its use; thus, it is not referenced in major guidelines. The nomogram can be found in Figure 3 within the following link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7080915/. [1]
A retrospective study aimed to determine which QT correction formula to use in an automated-QT monitoring algorithm for the electronic medical record. All electrocardiograms (ECGs) in patients > 18 years with sinus rhythm, normal QRS duration, and rate <90 beats/minute at a single center in Belgium during a 2-month period were included. The Bazette, Fridericia, Framingham, Hodges, and Rautaharju formulas were used to perform QT correction. A total of 6,609 patients were included (average heart rate 68.8 beats/minute). Using a population‐based approach, the relation between QTc and RR interval was determined using scatterplots for QTc/RR pairs per subject. QTc/RR linear regression slopes were calculated with the template: QTc=BxRR+intercept, and given that optimal QTc correction should be independent of the RR interval, the slope of the linear regression (B) and R^2 should be zero. The QTc/RR analysis identified the Fridericia and Framingham correction formulas as the best rate correction in this population, with slopes of 0.004 and −0.005, respectively. Bazzett’s correction formula, with a slope of -0.071, performed the worst, indicating the potential for significant over- and underestimation of QTc at high or low heart rates, respectively. Based on multivariate Cox regression, which included age, heart rate, and prolonged QTc, Framingham (hazard ratio [HR] 7.31; 95% confidence interval [CI] 4.10 to 13.05) and Fridericia (HR 5.95; 95% CI 3.34 to 10.60) were found to be significantly better predictors of 30-day all-cause mortality than Bazett (HR 4.49; 95% CI 2.31 to 8.74). Based on these results, it was suggested that the Fridericia formula may be an appropriate clinical standard for replacing the Bazett formula for hospital-based QT monitoring. [2]
Another retrospective study that evaluated four formulas of QT interval correction in individuals with sinus tachycardia included 6,723 patients without a history of heart failure with a baseline sinus rate ≥ 100 beats/minute. QT prolongation was diagnosed in 39% of the cohort using the Bazett formula, 6.2% using the Fridericia formula, 3.7% using the Framingham formula, and 8.7% using the Hodges formula. Only the Hodges formula was determined to be an independent predictor for death across the range of QT values (highest tertile HR 1.26; 95% CI 1.03 to 1.55). Despite all formulas showing an association between QTc values and cardiovascular events, only the Hodges formula was able to identify a significant number of individuals with tachycardia that are at higher risk for all-cause mortality. Additionally, it is suggested that the Bazett formula may overestimate the number of patients with a prolonged QT and was not associated with mortality. [3]