**1. Introduction**

Maintaining optimal body temperature remains a challenge in neonatal intensive care. In 1997, the World Health Organization (WHO) defined the normal range of body temperature in newborn infants between 36.5–37.5 degrees Celsius (◦C) [1]. However, Knobel et al. observed that the maximized normal heart rate (HR) observations occurred when the neonates body temperature was 36.8–37.0 ◦C [2], and Lyu et al. described the lowest rates of adverse neonatal outcomes in preterm infants with body temperatures of 36.5–37.2 ◦C [3]. Generally, the body temperature of newborn infants varies widely and depends on di fferent variables such as postnatal age, location of measurement, and the method to determine body temperature [4–6]. Both hypothermia and hyperthermia are associated with increased

mortality and morbidity [3,7]. The incidence of neonatal hypothermia is up to 84% in developing countries [8] and up to 53% in Europe [9], whereby preterm neonates are more often a ffected compared to term neonates because of di fferences in body temperature managemen<sup>t</sup> [10]. Pichler et al. reported that body temperature correlated with peripheral muscle tissue oxygenation, measured by near-infrared spectroscopy (NIRS), in newborn infants who were admitted to the neonatal intensive care unit [11]. Whether body temperature a ffects both peripheral and cerebral tissue oxygenation in newborn infants during fetal-to-neonatal transition remains unclear.

The primary aim of this study is to analyze whether there is an association between (i) body temperature and cerebral (crSO2/cTOI) and peripheral regional tissue oxygen saturation (prSO2) and between (ii) body temperature and the HR and arterial oxygen saturation (SpO2) in neonates born by Caesarean section during fetal-to-neonatal transition. Our secondary aim is to (iii) describe and compare body temperatures between term and preterm neonates of the same cohort in minute 15 after birth. We hypothesize that body temperature will not a ffect crSO2/cTOI, due to auto-regulative processes. For prSO2, we expect a negative correlation with body temperature, due to a consequently increased metabolic rate. Further on, we hypothesize that the HR and SpO2 will correlate positively with body temperature.

### **2. Materials and Methods**

This study is a post-hoc analysis of secondary outcome parameters of five prospective single-center observational studies that were performed between 2008 and 2016 at the Division of Neonatology, Department of Pediatrics and Adolescent Medicine, Medical University of Graz, Austria. All studies were approved by the Ethics Committee of the Medical University of Graz, Austria (EK-numbers: 19-291ex07/08, 23-302ex10/11, 25-342ex12/13, 25-592ex12/13, and 27-465ex14/15), with written parental consent being obtained prior to study inclusion.
