Interdependence of sympathovagal balance and irreversibility of heart rate oscillations

Introduction: Time irreversibility is a characteristic feature of non-equilibrium, complex systems including cardiovascular control mediated by the autonomic nervous system. Its analysis in heart rate variability (HRV) signal represents a new approach to assess cardiovascular regulatory mechanisms. Aim: The purpose of this paper was to assess the changes in time irreversibility during an orthostatic test (a balance shifted towards sympathetic predominance). We also tested the behaviour of the irreversibility indices in relation to different lengths of analysed data and their mutual dependence. Methods: We examined the HRV time series from 28 healthy young subjects recorded during 20 minutes in the supine position followed by 15 minutes in the standing position. We used three different time irreversibility indices - Porta's, Guzik's and Costa's indices (P%, G% and A, respectively). The irreversibility indices were derived from data segments containing 300, 600 and 1000 RR intervals. Results: Indices P% and A were sensitive to the shift in sympathovagal balance during the orthostatic challenge. The indices were relatively insensitive to data length. The indices P% and A are closely mathematically related despite the difference in the calculation and they do not provide mutually independent information. Conclusion: The heart rate irreversibility indices are sensitive to the changes in autonomic tone after the orthostatic challenge even when derived from the 300 interbeat intervals. The Porta's index seems to be more sensitive to the autonomic nervous balance shift compared to the Guzik's index. The Costa's index is closely related to P% and does not provide any additional information.