QT/RR curvatures in healthy subjects: sex differences and covariates

• Published in: American Journal of Physiology-Heart and Circulatory Physiology Vol. 305; no. 12; pp. H1798 - H1806
• Main Authors: Malik, Marek, Hnatkova, Katerina, Kowalski, Donna, Keirns, James J., van Gelderen, E. Marcel
• Format: Journal Article
• Language: English
• Published: United States American Physiological Society 15.12.2013
• Subjects: Adolescent; Adult; Cardiac Excitation and Contraction; Electrocardiography, Ambulatory; Female; Gender differences; Heart Rate; Heart Rate - physiology; Humans; Male; Middle Aged; Reference Values; Regression analysis; Reproducibility of Results; Sex Characteristics; intrasubject stability; QT heart rate correction; intersubject differences; QT/RR curvature profile
• ISSN: 0363-6135; 1522-1539; 1522-1539
• DOI: 10.1152/ajpheart.00577.2013

Data of a large clinical study were used to investigate how much are the QT/RR patterns in healthy subjects curved and whether these curvatures differ between women and men. Daytime drug-free 12-lead Holter recordings were repeated 4 times in each of 176 female healthy subjects and 176 male healthy subjects aged 32.7 ± 9.1 yr. In each of the subjects, up to 1,440 carefully verified QT interval measurements were obtained with QT/RR hysteresis-corrected RR intervals. Individual subject data were used to fit the following regression equation: QT = χ + (δ/γ)(1 − RR γ ) + ε, where QT and RR are QT and RR measurements (in s), χ is regression intercept, δ is the QT/RR slope, γ is the QT/RR curvature and provides the lowest regression residual, and ε represents normally distributed zero-centered errors. The bootstrap technique showed the intrasubject reproducibility of QT/RR slopes and curvatures. In women and men, QT/RR curvatures were 0.544 ± 0.661 and 0.797 ± 0.706, respectively ( P = 0.0006). The corresponding QT/RR slopes were 0.158 ± 0.030 and 0.139 ± 0.023, respectively ( P < 0.0001). QT/RR curvatures were related to QT/RR slopes but not to individually corrected mean QTc intervals or individual QT/RR hysteresis profiles. The individual heart rate correction formula derived from the curvilinear regression provided a significantly lower intrasubject variability of QTc interval than individual optimisation of linear or log-linear QT/RR heart rate corrections. The QT/RR curvature can be reliable measured and expressed numerically. The corresponding heart rate correction formula provides more compact data than the previously proposed approaches. There are substantial sex differences in QT/RR patterns. Women have a QT/RR pattern that is not only steeper than men but also more curved.