Analysis of QT interval during passive tilt test: comparison of different correction formula

The authors analyzed the QT and RR interval time series continuously recorded during 90 degrees passive Tilt test in 10 young healthy subjects (age 26/spl plusmn/5 years, 5 females). Changes in QT interval were assessed on 1 minute time related templates and on rate-independent waveforms obtained by averaging beats preceded by the same RR interval. Following Tilt, the mean RR interval shortened (from 927/spl plusmn/73 ms to 733/spl plusmn/118 ms, p<0.01), Bazett corrected QT increased (QTb; from 403/spl plusmn/12 ms to 413/spl plusmn/21 ms, p=0.058) whereas exponential QTe did not change significantly. Conversely, QTf and QTl (respectively Fridericia and linear rate-corrected QT), decreased after Tilt (e.g. QTf 398/spl plusmn/11 ms vs 391/spl plusmn/14 ms, p<0.05). Comparison at identical heart rate confirmed a QT shortening (QTrr: 387/spl plusmn/14 ms vs. 363/spl plusmn/15 ms, p<0.01). Thus, rate-correction formula are inconsistent and should be used with caution when assessing autonomic nervous system changes.