Transfer Function Gain Between Heart Period and QT Variabilities Increases During Sympathetic Activation Induced by Head-up Tilt

The variability of the duration of the cardiac electrical activity, approximated by the onset of the Q-wave to the end of the T-wave (QT), computed in the low frequency (LF, from 0.04 to 0.15 Hz) band, is utilized as a marker of sympathetic modulation directed to the ventricles. However, the magnitude of QT variability might be affected by heart period (HP) variability as a consequence of the QT-HP relation. We hypothesize that the gain of the relationship between HP and QT in the LF band could be better suited than QT variability to infer the cardiac sympathetic control as a consequence of its intrinsic normalization by the magnitude of HP changes. In this study we assessed transfer function gain (TFG) from HP to QT as well as spectral QT and HP markers in 23 healthy young subjects (age $26\pm 6$ years, 11 males) undergoing head-up tilt test at 45° (T45) and 90° (T90). Results showed that QT variability in the LF band increased during T90. TFG increased as well but the raise was evident also at a lower tilt table inclination, namely during T45. H, the gain of the TFG in the LF band could be a more sensitive marker of the increased sympathetic drive than the LF power of QT variability.