The strength of QT-RR coupling decreases during graded head-up tilt.

A cross-conditional entropy approach was applied to evaluate the degree of coupling between the beat-to-beat series of heart period (RR interval) and ventricular repolarization duration (QT interval). The strength of the QT-RR coupling was measured during graded head-up tilt test with table inclination randomly chosen in the set {0,15,30,45,60,75,90} in 17 healthy subjects. We found that RR and QT variabilities are significantly coupled during the entire experimental protocol and the strength of the QT-RR variability interactions progressively decreases as a function of the tilt angles. Results suggest that the fraction of QT interval variability independent of RR interval changes is not constant but depends on the level of sympathetic activity and/or amplitude of sympathetic modulation.