OBJECTIVE:
To analyze the effect of age on the ECG QT interval, an important predictor of cardiovascular mortality and drug-induced cardiac arrhythmias, and determine whether QT-heart rate correction formulae (QTc) have differential relationships with age and sex.

METHODS:
Data were examined from the US National Health and Nutrition Examination Survey (NHANES) II and III, civilian population aged 25 to 90 years. QT weighted means and standard deviations were calculated for all ages. The QTc were evaluated for six QTc: proposed by Bazett (QTcBZT), Fridericia (QTcFRD), Hodges (QTcHDG), Dmitrienko (QTcDMT), Rautaharju (QTcRTHa) and Framingham (QTcFRM).

RESULTS:
QTc was strongly related to age and gender, for all formulae except for QTcBZT for women. The relationship between QTc and age was significant regardless of whether the relationship was approximated by a linear or non-linear (quadratic or cubic spline) model. QTc increased more dramatically with age in men. There was a significant (P < 0.001) positive relationship between QTc variance and age for each QTc formula for both men and women. There were a greater proportion of individuals with longer QTc with older ages especially age 80 years and above.

CONCLUSION:
QTc and its variance increase with age. Prolonged QTc is more prevalent in older individuals, especially men.