Spontaneous variability of the QTc interval in healthy subjects

Little is known to what extent spontaneous beat-to-beat changes in QT duration can contribute to its variations. Thus, the authors defined the amount of rate-independent changes in QT duration on continuous 24-hour electrocardiograms in 11 healthy subjects (6 M, 5 F, aged between 19 and 80 years). Mean daily QTc was 419 msec, with a mean daily standard deviation of 24 msec. Mean daily QTc variability was 29/spl plusmn/10 msec; the mean coefficient of variation for QTc was 5.6/spl plusmn/.8% Marked hour-to-hour variations in QT value at the same RR cycle length were evident specially at higher heart rates. These results indicate that there is a considerable beat to beat variability of the QTc interval. The casual importance of interventions inducing QT variations cannot be accurately assessed, unless the amount of the heart rate-independent QT variability is considered QT variations /spl les/39 msec (mean+1 SD) should be considered physiologic.