24-Hour QT variability in heart failure

Comparison of mean QT variability for 24 hours in GISSI HF subjects. A, Mean ± SEM QTVI for 24 hours; all time points are significantly different by post hoc testing, P < .0001. B, Mean ± SEM QTVN for 24 hours; all time points are significantly different from 0 to 6 am by post hoc testing, P < .0001. C, Mean ± SEM in normalized HRv for 24 hours; there was significant diurnal variation (P < .001). After adjusting for multiple comparisons using Tukey procedure, no significant difference was found between the midnight to 6 am and the 6 am to noon time periods (P = .979), or between the noon to 6 pm and the 6 pm to midnight time periods (P = .683). All other differences were statistically significant (P < .001).