This was a retrospective observational cohort study, carried out at the NICU of the Fondazione Policlinico Universitario A. Gemelli, IRCCS in Rome—Italy, between March, 1 2018 and April, 30 2024.

We included all surgical interventions performed in preterm and full-term newborn infants admitted to the NICU who required, during their hospitalization, one or more major elective or urgent/emergency surgical interventions.

We excluded surgeries performed on infants with complex malformations associated with genetic syndromes undergoing palliative surgery and those for which data relevant to the study were missing. Furthermore, surgical procedures performed on infants for which the parents had not provided written informed consent for participation in the study were also excluded.

Major surgical interventions were considered to be those performed for the following conditions: congenital malformations, such as diaphragmatic hernia, oesophageal atresia, intestinal atresia, lymphatic malformations, and acquired surgical conditions, such as necrotizing enterocolitis, spontaneous intestinal perforation, intestinal volvulus, intestinal recanalization after abdominal surgery, intracranial haemorrhage.

The study was carried out in compliance with the Declaration of Helsinki and does not contain any personal information that could lead to the identification of the patient; all data analysed were collected as part of routine diagnosis and treatment; and patient medical care was not set up for research purposes but was part of standard clinical procedure. The study was approved by institutional review boards (Study ID 6601) and parents gave written consent to the processing of personal data.

Primary outcome of the study was the incidence of postoperative hypothermia, defined as a rectal temperature below 36.5 °C [11]. Secondary outcomes were the mortality rate within 30 days of surgery and the occurrence of post-operative infection within 48 h of surgery.

The following data were collected: gestational age (GA), determined by the best obstetric estimate based on the first day of the last menstrual period, prenatal ultrasound and postnatal physical examination; birth weight (BW); gender; post-menstrual age (PMA) and day of life (DOL) at the time of surgery; anatomical compartment where the major surgery was performed; planning modality of the surgery (elective or urgent/emergency surgery); duration of surgery. Before and after surgery we recorded body temperature and weight, blood pH, need for inotrope therapy (in the 48 h before and in the 48 h after surgery). The extent of body weight, body temperature and blood pH variation (Δ-weight, Δ-body temperature and Δ-pH) before and after surgery was calculated.

Duration of parenteral nutrition, duration of invasive and non-invasive ventilation, length of hospital stay, occurrence of systemic infection (defined as positive blood or cerebrospinal fluid culture within 48 h after surgery), grade 3–4 intraventricular haemorrhage (IVH) [12], periventricular leukomalacia (PVL) > grade 2 [13], moderate and severe forms of bronchopulmonary dysplasia (BPD) [14], and occurrence of death within 30 days of surgery were also registered.

All data were collected from electronic clinical records.

Our NICU is an open space with 16 beds. One bed is reserved for bedside surgical procedures. This bed is located in a 16 square meters, negative pressure (−6 Pascal) room adjacent to the open space. The room is equipped with a central air flow system and with an area for hand washing and disinfection. In the room, there are an open infant warmer incubator (Giraffe Incubator Carestation, GE Healthcare), one ventilator (Dräger Babylog VN500), one docking stations for the syringe pumps, two monitors with alarming equipment software, one mobile surgical lamp. The average room temperature during procedures is set at 26 °C. The BS team consists of two surgeons, an anaesthesiologist, a neonatologist, a scrubbed nurse, and an OR technician. The neonatologist intubates the neonate and attends the surgery for adjustment of the ventilator. Intravenous anaesthesia is given throughout the procedure.

The conventional ORs are located 3 or 4 floors below our NICU. The newborn infants are transferred to the OR with a neonatal transport incubator (Baby Shuttle, Ginevri). In the OR, infants are warmed using a blanket connected to a forced air system (Calima™ Temperature Management Unit, Ajalvir, Madrid). The temperature of the OR is set at approximately 27 °C before the start of surgery and decreased to 23 °C to 25 °C after the patient is covered by surgical drapes. During ORS, a conventional anaesthesia ventilator (AISYS CS2, GE Healthcare, Fairfield, CT, USA) is used to perform volume-controlled ventilation, maintaining the same parameter previously set during ventilation in the NICU.

The ORS team consists of two surgeons, an anaesthesiologist, a scrubbed nurse, and an OR technician. Both inhalation and intravenous anaesthesia are administered in OR.

The body temperature is measured continuously during both BS and ORS.

The sample size calculation was based on the observation that the incidence of hypothermia following ORS in our neonatal population is 50%. To detect a reduction of up to 25% in this incidence, it was necessary to enroll in each study group (BS-Group and ORS-Group) at least 55 surgical interventions, for a total sample size of at least 110 surgical interventions, considering a power of the study of 80% and an alpha error of 0.05 [15].

The demographic, clinical and laboratory characteristics of the sample were summarized using the main descriptive statistic.

Continuous variables were expressed as mean and standard deviation (SD) if distributed as a normal, otherwise as median and interquartile range (IQR); categorical variables were expressed as absolute and relative frequency (percentage).

Continuous variables, distributed as a normal, were compared using the independent samples t-test otherwise with the Mann–Whitney test. The normality in the distribution of the quantitative variables was verified using the Shapiro-Wilks test. Categorical variables were compared using the Chi-square test and/or Fisher’s exact test.

To identify the odds ratio of the different risk factors, a multivariable logistic regression analysis was performed using 30-day mortality and hypothermia as dependent variables. A p value < 0.05 was considered statistically significant.

Data were analysed using SPSS (statistical package for the social sciences) version 25.0 (SPSS, Inc, Chicago, IL, USA).