title: Autonomic indexes based on the analysis of heart rate variability: a view from the sinus node.
abstract: OBJECTIVE
  Clinical indexes of autonomic activity are based on the analysis of sinus cycle length and of its variability. A common assumption underlying this practice is that neural activity and cycle length may be linearly related. Recent experimental evidence suggests that such an assumption may not be correct; indeed, the relation linking autonomic agonist concentration to cycle length was found to be strongly non-linear in single sinoatrial myocytes. In the present work, we (i) test compatibility of non-linearity of neural modulation of cycle length (CL) with previous experimental and clinical observations; (ii) evaluate its implications for what concerns the interpretation of time- and frequency-domain parameters of heart rate variability (HRV) and baroreflex sensitivity (BRS).
  
  
  CONCLUSIONS
  Non-linearity of neural modulation of CL may result in an intrinsic rate-dependency of autonomic indexes, with the exception of normalised frequency-domain indexes (e.g. the low frequency/high frequency (LF/HF) ratio), which appear to be devoid of intrinsic rate-dependency. This characteristic may not limit the value of HRV indexes and BRS in risk stratification, but has to be taken into account in their pathophysiological interpretation.
authors.forename: A
authors.initials: A
authors.lastname: Zaza
authors.forename: F
authors.initials: F
authors.lastname: Lombardi
mesh_terms: D001341:Autonomic Nervous System / Q000502:physiology*; D017704:Baroreflex / Q000502:physiology; D001683:Biological Clocks / Q000502:physiology; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D012849:Sinoatrial Node / Q000502:physiology*