Validity of stress assessment using heart-rate variability in newborns

Background The process of birth causes stress for neonates, but additional stressors for sick neonates are a matter of concern. As analysis of heart-rate variability (HRV), which reflects autonomic activity, has demonstrated that low-frequency (LF) activity reflects overall autonomic activity, high-frequency (HF) activity reflects parasympathetic activity, and the LF/HF ratio reflects sympathetic activity, HRV has been clinically applied as a non-invasive index of physical stress. In this study, we evaluated whether HRV is useful as a stress index for neonates by analyzing it in comparison with their salivary cortisol level. Methods We measured the salivary cortisol level and HRV in 12 healthy neonates and 37 neonates born during between 2014 and 2016 and admitted to the neonatal intensive care unit. These examinations were performed at birth and after approximately 1 week. The changes in parameters with time were examined. Results The LF and HF values in both groups exhibited significant negative correlations with the salivary cortisol level. In those admitted to the neonatal intensive care unit, the LF and HF values were correlated with gestational age and height. In the healthy neonates, a reduced salivary cortisol level and increase in the LF and HF values were observed approximately 1 week after birth compared with the values at birth, whereas the LF/HF ratio was not correlated with the salivary cortisol level and did not change over time. Conclusions The LF and HF values were significantly correlated with the cortisol level, suggesting their usefulness as physiological indices of stress in clinical neonatal care.