Autonomic nervous system in newborns: a review based on heart rate variability.

PURPOSE: Heart rate variability (HRV) has been a relevant tool in the assessment of the autonomic nervous system (ANS). How autonomic control normally develops in newborns and how it is affected by gestational age (GA) is not fully understood. We aimed to review the current evidence on HRV in preterm (PT) and term neonates (TN) and investigate the relation between GA and the maturation of ANS.

METHODS: Electronic databases (Pubmed, World of Science, and Scopus) were searched for studies from 1997 to 2017 examining HRV (time and frequency domain) in PT and TN who followed to the Task Force (1996) guidelines. Ten studies met our inclusion criteria and were analyzed.

RESULTS: An increasing postnatal age was related to a significant rise of HRV parameters. Several significant differences were established between PT and TN (lower values on PTN), also found when PTN are evaluated at their theoretical term age. In general, there were no relevant results on LF/HF (low frequency/high frequency) ratio, as being an adequate marker of sympathovagal balance, but this was not a universal finding of this review. Frequency parameters that were more often used to evaluate newborns and HF showed the most relevant increase with GA.

CONCLUSIONS: HRV is an important tool to assess the maturation of ANS in newborns and there is a progressive increasing on cardiac parasympathetic activity, according to GA. HF appears as a relevant parameter in measurements of vagal maturation. HRV is higher in TN when compared with PTN and is more studied in newborns in terms of frequency domain. Standard recommendations in newborns remain to be fully defined.