Physiological background underlying short-term heart rate variability

Abstract

A large number of papers has been published on heart rate variability (HRV) based on the assumption that the specific components of HRV provide specific information about cardiac parasympathetic or sympathetic efferent nerve activity. However, neural control of the cardiorespiratory system is very complex, and the physiological phenomenon underlying HRV in different conditions are far from being fully understood. This review summarizes, in the light of current literature, a series of studies focused on the mechanisms by which fluctuations in neural outflows are transferred into HRV. In the interpretation of HRV analyses, it should be taken into account that: (1) HRV seems to be strongly influenced by the parasympathetic nervous system at all the frequency components; (2) due to sympathovagal interactions, sympathetic outflow is able to reduce the variations generated by vagal modulation also in the high frequency band; and (3) the variations in heart rate reflect fluctuations in the neural activity rather than the mean level of sympathetic or parasympathetic neural activity. Thus, we should be cautious in interpreting a specific component of HRV as a specific marker of sympathetic or parasympathetic cardiac control. Furthermore, due to the complexity of the cardiorespiratory control system, the analysis of short- term HRV should be performed in well-controlled conditions, in which the behavior of the autonomic nervous system is well documented.