Circadian changes in the QT variability index in the beagle dog.

1. QT variability is a non-invasive marker of cardiac repolarization lability and a higher QT variability is associated with sudden death. No data exist as to the circadian fluctuations in QT variability and the QT variability index (QTvi) in the canine. The purpose of the present investigation was to explore QT interval variability over 24 h in the healthy dog. 2. Continuous lead II electrocardiogram and blood pressure data were collected for 24 h from three beagles instrumented with radiotelemetry devices. The mean heart rate (HR), detrended HR variance, mean QT interval and detrended QT variance were calculated from the instantaneous HR and QT time series of 1024 points (256 s), as described previously, and a normalized QTvi was derived. 3. The dog has a diurnal pattern of QTvi similar to healthy humans. Both dogs and humans exhibit a significantly higher QTvi during active waking hours, with more negative values during deep sleep. 4. These findings suggest QTvi may serve as an additional non-invasive tool to assess ventricular repolarization lability in dogs in relation to any conditions or drugs that are known to be associated with increased cardiac mortality.