The salient finding of our study is that the term newborn’s sleeping position is associated with HRV as analysed by frequency domain spectral analysis. TP and LF in term newborns are all higher when lying supine in comparison to prone position which might imply an increased responsiveness of the ANS in supine position. To the best of our knowledge, this is the first study that evaluated the effect of sleeping position on HRV analysed by frequency domain spectral analysis in term newborns.
Only little information is available regarding the impact of ANS on the cardiovascular regulation in newborns. Findings show that the activity of the ANS increases with PMA mainly in terms of an increase of reflex vagal activity [25–28]. On the other hand, clinical studies conducted in preterms show greater sympathetic activity, higher HR and less expressed vagal activity as compared to term newborns [26,27,29,30].
In our study, newborns had higher TP HRV when lying supine compared to prone position. Also, the LF was significantly higher in supine in comparison to prone position. Since LF spectrum is supposed to reflect the sympathetic activity , we can assume that the cardiac sympathetic modulation is less pronounced in prone position. Similarly, Gabai et al. found reduced HRV parameters analysed in time domain in three-day old term newborns when lying prone. In prone position, they showed a decrease in SDNN (standard deviation of normal R-R) which correlated with TP and also a decrease in short term variability (assessed by pNN50) which correlated with HF. No effect of birth-weight or gestational age on HRV was noted in their study. Similar to our study, Jean-Luis et al. also found both, TP and LF to be significantly higher in supine compared to prone position. Also similar to our observation, no significant difference was seen in HF . Moreover, Galland et al. also showed lower HRV assessed by the point dispersion of Poincaré plots in prone position in term infants . On the other hand, Ariagno et al. found lower HRV in prone position only in the time domain, but not in the frequency domain: RMSSD (the square root of the mean of the sum of the squares of differences between adjacent R-R), which corresponds to HF was significantly greater in the supine position at both 1 and 3 months’ corrected age, whereas the SDNN was significantly higher in the supine position, but only at 1 month corrected age . These results on increased HF are in agreement with the results of our study and, besides the above speculated sympathetic influence, imply also an important contribution of vagal baroreflex modulation.
Besides assessing the parameters of HRV, we have simultaneously measured the arterial oxygen saturation and BF what was not performed in other available studies. Blood oxygen saturation was significantly lower in prone compared to supine although not clinically important, since in both positions, the measured saturation was above 94%. On the contrary, it has been reported that preterms receiving nasal continuous positive airway pressure (nCPAP) for mild respiratory failure had better arterial blood oxygenation when lying prone . In newborns who were without non-invasive support, we have also observed lower BF when lying prone. Yet, both physiological parameters were within normal limits (BF of the newborn 30—60/min) in both positions and could imply greater impact of the parasympathetic nervous system in prone position [34,35].
We did not find any correlation between gender and either parameter of the HRV which is in accordance with the findings of Yang et al. and Javorka et al [25, 26]. On the other hand, Nagy et al. found significantly lower HR and lower HRV, expressed as the standard deviation of the HR, in boys . Male newborns had a significantly decreased pNN50 (namely HF) compared to females when lying prone .
In our study, newborn’s age positively correlated with TP and HF power and negatively with LF power when lying supine but not when lying prone. Our observations are in accordance with the finding of the Cardoso et al.  who have shown higher HRV in older newborns. Gestational age as well as PMA have been shown to be positively correlated with HF and negatively with LF [3,27,28,40]. These findings might implicate that the vagal activity increases with PMA, while the sympathetic modulation of HRV in neonates seems to be less expressed.
Friedman et al. showed that at term, the cardiovascular system is not fully mature yet, and the development continues for several weeks after birth. Bar-Haim et al. found an increase in HF power spectral density also in the period between 4 and 48 months of postnatal age . We might imply that during the period between 37th to 41st weeks the vagal influence becomes more expressed. Interestingly, the correlation between PMA and the parameters of HRV was significant only in supine position in our study.
Impaired regulation of the cardiovascular system is one of the most important risk factors for SIDS. In infants who later suffered from SIDS, a higher HR and lower HRV were found . Decreased ANS responsiveness has been suggested to contribute to an increased risk for SIDS in infants sleeping in the prone position . In our study, we did not find any significant differences in basal HR in different sleeping positions. On the other hand, we found higher TP and LF in supine position. According to our results we may speculate that sleeping in a supine position could have some advantages in prevention of SIDS.
Besides parameters of HRV, increased arterial blood oxygen saturation in supine additionally speaks in favour of supine over prone position, which is in concordance with some previous studies [12,15]. Fyfe et al. discovered that cerebral perfusion in preterm infants was significantly lower when lying prone compared with supine in both—active and quiet sleep stages. In accordance with our study, they found lower blood oxygen saturation in newborns when lying prone during a quite sleep at 2 to 4 weeks and at 5 to 6 months of age (P <0,05) .
A potential limitation of our study is an intermittent and not a continuous measuring of the BF, MAP and lack of electroencephalographic data of the sleep stages. Second limitation is a rather small sample size: had we had a larger sample, we could have compared more variables such as the Apgar score, and the type of the childbirth to the variables of HRV. Additional limitation is a rather heterogeneous PMA of the included newborns at the time of the HRV measurement. Yet, as the newborns were all older than 5 days, we might assume that they have already overcome the transitional period of hemodynamic adaptations. It might be possible that the sequence in which the positions were applied could be a confounding variable, but to test this hypothesis too many possible variants should have been tested so we decided on the decribed protocol.