QT interval in normal infants during sleep with concurrent evaluation of QT correction formulae.

BACKGROUND: Physiological changes in the QT interval caused by sleep are important in the evaluation of the normal QT interval. Traditionally, Bazett's formula is used for correcting the QT interval for heart rate in childhood. We set out to establish the QT-RR relationship in infants during sleep and concurrently test the performance of four QT correction formulae to determine their comparability and clinical utility.

MATERIAL/METHODS: 130 healthy term born infants undergoing routine 8-hour polysomnography were enrolled. Continuous EKG limb lead II recording, saturation monitoring, and electroencephalography were conducted. QT intervals and heart rate were measured manually digitally on-screen at hourly intervals. The QT-RR relationship was plotted and QTc calculated using the Bazett, Hodges, Fridericia and Framingham formulae. QTc values for each formula were plotted against age and RR interval. Analysis of variance and regression analysis were used to compare the four formulae and the QT-RR relationship.

RESULTS: Mean age was 17.7 weeks (range 4-72) and mean weight 6.3 kg (range 2.68-12.7). The QT-RR relationship during sleep is curvilinear and appears to be slightly shifted. With increasing heart rate and younger age the Bazett and Hodges formulae overcorrect the QTc whereas the Fridericia and Framingham formulae undercorrect. The Hodges formula correlated best with the RR interval.

CONCLUSIONS: These formulae are strongly dependent on heart rate. In the setting of a borderline QTc Bazett prolongation, Hodges formula may be a useful adjunct. Normal infants may have a slightly longer QTc Bazett during sleep.