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There are literally one hundred possible heart rate-correction formulas to derive a heart rate corrected QTc value with the most common formulas:

1) Bazett – QTcB=QT/RR
2) Fridericia – QTcFri=QT/RR

3) Hodges – QTcH=QT+0.00175 ([60/RR]−60)
4) Framingham – QTcFra=QT+0.154 (1−RR)

Depending on your institution, the formula used in the 12-lead ECG laboratory varies.  For example, at Mayo Clinic, the QTc is reported using Bazett’s formula.  For Thorough QT Drug Studies, the FDA prefers Fridericia.

 

In general, among otherwise healthy people, the average Bazett’s-derived QTc is about 410 ms pre-puberty, 410 ms in post-pubertal males, and 420 ms in post-pubertal females.  In contrast, in the Mayo Clinic Windland Smith Rice Genetic Heart Rhythm Clinic, the average QTc among patients with genetically-confirmed long QT syndrome (LQTS) is 465 ms.  Among patients being seen at Mayo Clinic as either an outpatient or inpatient where an ECG was ordered, 1% of those patients had a QTc > 500 ms with less than 10% stemming from patients with congenital LQTS.  As shown in the Algorithm, we estimate that 90% of patients will have a baseline QTc less than their respective age- and gender-predicted 99th percentile values: < 460 ms before puberty, < 470 ms in men, and < 480 ms in women.  In contrast, 1% of patients will have a QTc > 500 ms which is regarded as the established torsadogenic threshold.

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