Estimation of the QT/RR hysteresis lag.

The process of QT interval adaptation to heart rate (HR) changes was evaluated by considering weighted averages of RR intervals to characterize the influence of previous cardiac cycles. An optimum adaptation pattern was individually derived for each patient and several descriptors of the QT/RR hysteresis were subsequently calculated. The values of these parameters showed that the QT adaptation to HR changes is highly individual and, consequently, any generalized approach may lead to inappropriate conclusions.