Dynamic correlation between QT and RR intervals: how long is QT adaptation to heart rate?

Abnormalities in the adaptation of QT to variations of heart rate may be associated with a predisposition to ventricular arrhythmics, more than the prolongation of the QT interval at rest. A new automatic method is presented to assess the dynamic correlation between the QT and RR intervals over the entire duration of 24 h Holter monitoring. In order to separate short from long term adaptation and to investigate on selected portions of the circadian sequences (sleep, awake, waking up and falling asleep) both frequency and time windows are used. Sequences of RR and QT values were measured over 24 h recordings and interpolated to provide a continuous time scale. By wavelet transform four RR and QT band filtered sequences were originated. From them the correlation coefficient and their fiducial limits were calculated within the chosen frequency and time windows for time lags from 1 to 300, thus evaluating the lag corresponding to the maximum correlation.