Relationship of QT interval variability to heart rate and RR interval variability.

The study investigated whether the beat-to-beat QT interval variability relationship to the mean heart rate and the RR interval variability depended on the cardiovascular autonomic status changed by postural positioning. Repeated long-term 12-lead Holter recordings were obtained from 352 healthy subjects (mean age 32.7 ± 9.1 years, 176 females) while they underwent postural provocative tests involving supine, unsupported sitting and unsupported standing positions. Each recording was processed as a sequence of overlapping 10-second segments. In each segment, the mean RR interval, the coefficients of variance of the RR intervals (RRCV) and the QT intervals (QTCV) were obtained. In each subject, these characteristics, corresponding to different postural positions, were firstly averaged and secondly used to obtain within-subject correlation coefficients between the different characteristics at different postural positions. While the within-subject means of RRCV generally decreased when changing the position from supine to sitting and to standing (4.53 ± 1.95%, 4.12 ± 1.51% and 3.26 ± 1.56% in females and 3.99 ± 1.44%, 4.00 ± 1.24% and 3.53 ± 1.32% in males respectively), the means of QTCV systematically increased during these position changes (0.96 ± 0.40%, 1.30 ± 0.56% and 1.88 ± 1.46% in females and 0.85 ± 0.30%, 1.13 ± 0.41% and 1.41 ± 0.59% in males, respectively). The intra-subject relationship between QTCV, RRCV and mean RR intervals was highly dependent on postural positions. The study concludes that no universally applicable normalization of the QT interval variability for the heart rate and/or the RR interval variability should be assumed. In future studies of the QT variability, it seems preferable to report on the absolute values of QT variability, RR variability and mean heart rate separately.