Decoupling of QT interval variability from heart rate variability with ageing.

Ageing has been associated with changes in cardiac electrophysiology that result in QT interval prolongation. The effect of age on rate-adaptation dynamics of the QT interval is less well understood. The aim of this study was to assess age-related changes in the temporal relationship between QT and RR interval variability. Resting ECG of 20 young and 20 elderly healthy subjects were analyzed. Beat-to-beat RR and QT interval time series were automatically extracted. Coupling between QT and RR was assessed by means of the QT variability index, coherence in the frequency domain, rate-corrected QT interval, cross-multiscale entropy, information based similarity index and joint symbolic dynamics. In addition to QT interval prolongation (433 ± 31 versus 405 ± 33 ms, p = 0.008), elderly subjects were characterized by a significantly increased QT variability index (-1.26 ± 0.28 versus -1.52 ± 0.22 ms, p < 0.0001), reduced coherence in high (0.11 ± 0.09 versus 0.29 ± 0.14 ms, p = 0.003), and low frequency bands (0.20 ± 0.16 versus 0.49 ± 0.15 ms, p < 0.0001), reduced information domain synchronization index (0.13 ± 0.07 versus 0.19 ± 0.05 ms, p = 0.001) as well as increased entropy and disparity in joint symbolic dynamics of QT and RR interval time series. In conclusion, ageing is associated with decoupling of QT variability from heart rate variability. Complexity analysis in addition to standard metrics may provide additional insight.