Circadian rhythm of QT interval: Is there a genuine circadian rhythm or is it due to changes in heart rate?

摘要

Background: There is a diurnal variation in heart rate. As QTinterval varies inversely with heart rate (HR), it is corrected toderive an estimated QT interval for a HR of 60 bpm. The correctedQT interval (QTc) obtained using Bazett's (QTcB) or Fridericia's(QTcF) formulae is not perfect. Consequently, previousstudies evaluating circadian variation in QTc interval haveshown conflicting depending on the correction formula used.We, therefore, studied the diurnal changes in QT interval bymeasuring it in only ECGSs with a stable HR of 60 bpm during the24 h period to eliminate the confounding effect of QT correctionformulae.Methods: From 12-lead digital Holters recorded in 32 healthy subjects(22 males, 10 females) in a Phase I clinical trial, all ECGs at astable HR of 60 bpm were identified for each hour of the 24 h. Inaddition, 6 replicate ECGs in a window period of 5 min wereextracted at 24 time-points 1 h apart, regardless of the prevailingHR and QTcB and QTcF were calculated. Circadian pattern for QTinterval at a heart rate of 60 bpm (QT60), QTcB and QTcF werecompared at hourly intervals.Results: Circadian pattern of QT60 and QTcF were comparable(Figure). QTcB showed greater variation especially at timepointswith increased HR. QT60 was shortest (383–393 ms) between1 pm and 9 pm and longest between 4 AM to 7 AM (403–407 ms). Mean HR was lowest between 1 AM to 12 noon (range60–66 bpm) and remained high between 2 PM to 11 PM (range 70–80 bpm).Conclusions: A diurnal variation in QT60 was observed with anincrease after 8 PM. Although mean HR varied from 60 to 80 bpmover 24 h, QTcF closely matched QT60. QTcB varied with changes inHR and masked diurnal changes in QT60. Absence of diurnalvariation in some previous studies could be because HR changesblunt the circadian pattern when QTcB is used.