A comparison of commonly used QT correction formulae: the effect of heart rate on the QTc of normal ECGs.

The corrected QT interval (QTc) is widely used in pharmaceutical studies and clinical practice. Bazett's QT correction formula is still the most popular, despite Simonson's warning in 1961 that it could not be recommended. Other QTc formulae, e.g. Fridericia, Framingham, and Hodges, are also used. This study compares these four formulae using 10,303 normal ECGs recorded from four US hospitals. QT intervals were measured by the same computer program on ECGs confirmed by physicians. The distributions of QTc based on Fridericia, Framingham, and Hodges formulae were similar but Bazett's was significantly wider. The global group QTc-heart rate (HR) correlation coefficients were calculated as Bazett 0.33, Fridericia 0.24, Framingham 0.26, and Hodges 0.11, with the uncorrected QT-HR correlation being 0.82. Overall by far, Hodges QTc is significantly less correlated with HR compared to the others. Certain subgroup correlations of gender and low, mid, or high HR show that one individual formula can out-perform the others, whereby automated selection of QT correction formula based on the patient's HR and gender could be implemented as another option in products. The upper normal limits of corrected QTc were determined by excluding the top 2% from the global distribution charts as follows: Bazett 483 ms, Fridericia 460 ms, Framingham 457 ms, and Hodges 457 ms. Whether for males and/or females, the middle range of HR from 60 to 99 bpm has similar upper normal limits of QTc for all formulae except Bazett. Numerous references recommend 420 to 440 ms as the threshold for reporting prolonged QTc when using Bazett's formula. Based on this database, 30% of apparently normal ECGs would be reported as having abnormal QT intervals for the 440 ms threshold, or 10% if 460 ms is chosen, compared to <2% for the other formulae. It was also noted that QT has a linear trend with HR but not with RR.