Heartbeat dynamics and its spontaneous fluctuations are directly controlled by autonomic nervous system (ANS) outflow to the heart. Specifically, the multipath feedback system for neural control of the heart is manifested by the complex interaction between the sympathetic and parasympathetic (vagal) limbs of the ANS.
It is known that Heart Rate Variability (HRV) oscillations LF (centred in 0.1 Hz) are mediated by both cardiac vagal and sympathetic nerves, as well as arterial blood pressure and heart rate variations (possibly through the baroreflex), whereas HRV-HF (greater than 0.15Hz) is modulated by parasympathetic (vagal) activity. To overcome the decades-lasting issue of disentangling HRV sympathetic and vagal components, we have proposed two novel HRV-based indices able to effectively identify the time-varying cardiac sympathetic and parasympathetic activity through the Sympathetic Activity Index (SAI) and the Parasympathetic Activity Index (PAI). Orthonormal Laguerre functions are convolved with heartbeat series and processed further to obtain finite SAI and PAI estimates within a given observation in time, e.g. by using least square and maximum likelihood estimation methods, to obtain beat-to-beat SAI and PAI estimates in time, e.g. by using Kalman filtering, or to obtain instantaneous SAI and PAI estimates in time by using point-process modelling.
In the Figure, the estimated power LF(t) and the SAI(t) are shown in the left panel. At each time, the median value is superimposed (black line) on the standard error of the median (gray area). Vertical red line indicates the beginning of the fast-tilt manoeuvre. The estimated power HF(t) and the PAI(t) are shown in the central panel, whereas the estimated instantaneous LF/HF(t) and the SAI/PAI(t) are shown in the right panel.
JP Saul, G. Valenza, "Heart Rate Variability and the Dawn of Complex Physiological Signal Analysis: Methodological and Clinical Perspectives". Philosophical Transactions of the Royal Society A, 2021
G Valenza, L Citi, JP Saul, R Barbieri, "Measures of Sympathetic and Parasympathetic Autonomic Outflow from Heartbeat Dynamics". Journal of Applied Physiology, vol. 125, num 1, pp. 19-39, 2018.