Time-related heart autonomic balance characteristics in healthy subjects

Changes of the heart autonomic balance between morning (8-9 h) and afternoon (14-15 h) measurements were studied in 22 healthy subjects. The selection of these two daytime periods was substantiated by the established higher risk of cardiovascular incidents in the morning and the relative balance of the vegetative nervous system in the afternoon hours. The changes were analysed by RR-variability indices from ECG recordings in the resting state and with vegetative nervous system stimulation by the handgrip test and Valsalva manoeuvre. It was shown that there were no significant differences between the morning and afternoon values of the respective indices, between morning and afternoon handgrip tests and between morning and afternoon Valsalva manoeuvres. However, there were significant differences in comparison of the index values between resting state and handgrip test and resting state and Valsalva manoeuvre, both from morning and afternoon measurements. Moreover, the significantly differing indices were clustered in different groupings, when comparing resting state recordings with morning and afternoon stimulation tests. For this reason, the authors introduced an indicator for time-related autonomic balance changes. The indicator evaluates the power of each RR-variability index to respond to changes in the autonomic control, in comparisons between resting state and stimulation data in the morning and afternoon measurements. These evaluations showed low power of the frequency-domain indices to respond to time-related autonomic balance changes in stimulation, with respect to the resting state. The time-domain indices have considerably higher power to react to relative morning and afternoon changes in the two vegetative nervous system components. Based on the estimations of the RR-variability indices obtained by the introduced indicator, a profile was constructed to represent time-related heart autonomic balance changes in healthy subjects. It was established that in spite of the relative stability of the vegetative nervous system in healthy subjects, hypersympatheticotonia and relatively lower parasympathetic tone were present in the risky morning hours.