AN IMPROVED METHOD FOR ADJUSTING THE QT INTERVAL FOR HEART-RATE (THE FRAMINGHAM HEART-STUDY)

Several formulas have been proposed to adjust the QT interval for heart rate, the most commonly used being the QT correction formula (QT(c) = QT/square-root RR) proposed in 1920 by Bazett. The QT(c) formula was derived from observations in only 39 young subjects. Recently, the adequacy of Bazett's formula has been questioned. To evaluate the heart rate QT association, the QT interval was measured on the initial baseline electrocardiogram of 5,018 subjects (2,239 men and 2,779 women) from the Framingham Heart Study with a mean age of 44 years (range 28 to 62). Persons with coronary artery disease were excluded. A linear regression model was developed for correcting QT according to RR cycle length. The large sample allowed for subdivision of the population into sex-specific deciles of RR intervals and for comparison of QT, Bazett's QT(c) and linear corrected QT (QT(LC)). The mean RR interval was 0.81 second (range 0.5 to 1.47) heart rate 74 beats/min (range 41 to 120), and mean QT was 0.35 second (range 0.24 to 0.49) in men and 0.36 second (range 0.26 to 0.48) in women. The linear regression model yielded a correction formula (for a reference RR interval of 1 second): QT(LC) = QT + 0.154 (1 - RR) that applies for men and women. This equation corrects QT more reliably than the Bazett's formula, which overcorrects the QT interval at fast heart rates and undercorrects it at low heart rates. Lower and upper limits of normal QT values in relation to RR were generated. A simple linear equation was developed that is more accurate than Bazett's correction at different cycle lengths and more convenient for clinical practice. This formula alleviates the need to apply secondary corrections to Bazett's formula. Additional investigation is warranted to determined whether QT(LC) improves the identification of subjects at high risk for malignant arrhythmias or sudden death.