Received 11 April 2009 published online 03 August 2009.
Abstract
Background
Previous studies have shown that increased temporal variability of repolarization, as reflected by QT interval variability measured for 10 minutes, predicted spontaneous ventricular arrhythmias in implantable cardioverter defribrillator patients, but it is unclear how these measures perform in 24-hour recordings.
Methods
Twenty-four-hour digital Holter recordings from 372 subjects with chronic heart failure enrolled in Gruppo Italiano per lo Studio della Sopravvivenza nell'Insufficienza Cardiaca, (GISSI) Heart Failure study were analyzed using a template-matching, semiautomatic algorithm to measure QT and heart rate time series in sequential 5-minute epochs for 24 hours. QT variability was expressed as normalized QT variance (QTVN) or as the log ratio of the QTVN over normalized heart rate variance (QT variability index, or QTVI).
Results
A pronounced diurnal variation was seen in both QTVI and QTVN. Both were lowest in the midnight to 6 am time frame and increased throughout the day, peaking at noon to 6 pm, then decreasing 6 pm to midnight. For QTVI, all 4 time points were significantly different (P < .0001). QT variability index correlated with heart rate (r = 0.38, P < .0001) and was significantly higher for those in higher New York Heart Association (NYHA) classes (r = 0.22, P = .0003). Normalized QT variance did not correlate with heart rate or NYHA but correlated negatively with serum potassium (r = −0.22, P = .0002) and manifested the greatest increase during midmorning hours.
Conclusions
Repolarization lability as reflected in QT variability has a pronounced diurnal variation and increases significantly after 6 am, the time of greatest arrhythmic risk. QT variability for 24 hours might improve risk prediction in chronic heart failure patients and should be tested in appropriate trials.
fOspedale Civile S. Giuseppe, Albano Laziale (Roma), Italy
gGVM Hospitals of Care and Research, Cotignola (Ravenna), Italy
Corresponding author. Division of Cardiology, Department of Medicine, Uniformed Services University of the Health Sciences, A3060, USUHS, 4301 Jones Bridge Road, Bethesda, MD 20814, USA.
☆ Disclaimer: The views expressed in this article reflect the opinions of the authors only and not the official policy of the United States Army, Uniformed Services University, or the Department of Defense.
ABSTRACT