Subject-specific profiles of QT/RR hysteresis

The time lag of the QT interval adaptation to heart rate changes (QT/RR hysteresis) was studied in 40 healthy subjects (18 females; mean age, 30.4 ± 8.1 yr) with 3 separate daytime (>13 h) 12-lead electrocardiograms (ECG) in each subject. In each recording, 330 individual 10-s ECG segments were measured, including 100 segments preceded by 2 min of heart rate varying greater than ±2 beats/min. Other segments were preceded by a stable heart rate. In segments preceded by variable rate, QT/RR hysteresis was characterized by λ parameters of the exponential decay models. The intrasubject SDs of λ values were compared with the intersubject SD of the individual means. The λ values were also correlated to individually optimized parameters of heart rate correction. Intrasubject SDs of λ were substantially smaller than the population SD of individual means (0.390 ± 0.197 vs. 0.711, P < 0.0001). The λ values were unrelated to the QT/RR correction parameters. When compared with the corrected QT (QTc) for averaged RR intervals in 10-s ECGs and with the averaged RR intervals in 2-min history, QTc for QT/RR hysteresis led to a substantially smaller SD of QTc values (11.4 ± 2.00, 6.33 ± 1.31, and 4.66 ± 0.85 ms, respectively, P < 0.0001). Thus the speed with which the QT interval adapts to heart rate changes is highly individual with intra-subject stability and intersubject variability. QT/RR hysteresis is independent of the static QT/RR relationship and should be considered as a separate physiological process. The combination of individual heart rate correction with individual hysteresis correction of the QT interval is likely to lead to substantial improvements of cardiac repolarization studies.