This was a sub-study nested in the Baby-DUCC randomised controlled trial (RCT) and cohort study at the Royal Women’s Hospital (RWH) site.17 Approval was obtained from the RWH Human Research Ethics Committee (RWH ethics 17/19). Antenatal consent was obtained from parents in addition to consent for the Baby-DUCC trial. Recruitment was between March 2019 and April 2021. This study was prospectively registered: ACTRN12619000277145.

Infants were eligible for participation in the Baby-DUCC RCT and cohort study if they fulfilled the following inclusion criteria: ≥32+0 weeks’ gestation at birth, paediatric doctor requested to attend an at-risk birth, researcher present. Infants with any of the following criteria were excluded: known congenital anomalies compromising cardiorespiratory transition, high risk of obstetric complications requiring early cord clamping, monochorionic twins and multiples >2.

For this study, additional inclusion criteria applied: availability of additional research resources to measure blood pressure (BP) and cerebral oxygenation, antenatal consent for the additional study measurements.

In the BabyDUCC RCT, infants assessed as requiring resuscitation following initial stimulation were randomly allocated 1:1 within 1 min of birth to either standard care, where cord clamping occurred early (ECC) prior to resuscitation, or physiologically based cord clamping (PBCC).18 With PBCC, resuscitation (defined as ongoing vigorous stimulation and/or respiratory support) was commenced prior to UCC. Infants in the PBCC group receiving positive pressure ventilation had UCC deferred until ≥2 min after birth and until ≥60 s after exhaled carbon dioxide was detected on a disposable colorimetric carbon dioxide detector (Pedicap, Medtronic, Minneapolis, Minnesota) placed between the facemask and T-Piece. Infants in the PBCC group who breathed without positive pressure ventilation had UCC at ≥2 min after birth. This was to ensure that the pulmonary circulation was established prior to cord clamping. Infants in the ECC group had UCC immediately after randomisation and were transferred to a resuscitation trolley prior to commencing resuscitation.

Infants who were vigorous immediately after birth were not randomised and received 2 min of deferred cord clamping (DCC). These infants were eligible for inclusion in the observational study arm. Non-randomised infants who went on to receive respiratory support in the delivery room were excluded.19

The decision to provide resuscitation and the type of support provided were at the discretion of the attending first-line doctor trained in the Australian and New Zealand Committee on Resuscitation Neonatal Resuscitation Guidelines.20,21 Respiratory support was commenced with a Giraffe stand-alone resuscitation system (GE Healthcare, Chicago, Illinois) set to pressures of 30/5 cmH2O in 21% FiO2.

The pre-specified primary outcome for this sub-study was mean BP at 3–4 min after birth measured on the right upper arm. Secondary outcomes were mean BP at 6–7 min after birth, systolic and diastolic BP at both timepoints, change in cerebral oxygenation (rStO2 and cFTOE) over the 10 min after birth, and rStO2 at 1 h after birth.

Immediately after birth, a researcher dried the infant and placed three ECG chest leads and a preductal pulse oximeter to monitor the infant’s heart rate (HR) and SpO2. At 3–4 min and 6–7 min after birth, pre-ductal BP was measured using a Non-Invasive BP cuff (Neonatal Single-Patient Non-Invasive Blood Pressure Cuff, size 4 for term infants, size 3 for preterm infants, Philips Healthcare, Andover, Massachusetts) at the right upper arm, consistent with recent recommendations.22 A neonatal NIRS sensor (8004CB-NA, SenSmart X-100, Nonin, Plymouth, Minnesota) was placed on the right forehead and secured/protected from ambient light with a hat. HR, SpO2 and BP were displayed on a portable Intellivue X2 (Philips Healthcare, Andover, Massachusetts) and cerebral oxygen saturation displayed on a portable SenSmart X-100 monitor, visible to the clinician. A GoPro Hero Session (GoPro, San Mateo, California) camera captured the monitor screens, T-piece manometer, oxygen blender dial and audio of the events after birth. The videos were downloaded for offline manual data extracted to ensure high fidelity.

Blinded data extraction was performed for randomised infants. The video recording was cropped to include only the monitor screens and muted to sound before being shared with an off-site researcher. For all infants, HR and SpO2 data were extracted every 10 s and rStO2 from NIRS every 20 s until 10 min after birth. SpO2 readings were only accepted if plethysmograph waveforms showed adequate signal quality. rStO2 readings were only accepted if there was no signal interference error shown and no rapid fluctuation suggesting loss of contact. A spot rStO2 reading was taken at 1 h after birth if it did not interfere with skin-to-skin contact and breastfeeding. This reading was unblinded to group allocation.

Based on previous studies,23,24 we estimated that infants in the ECC group would have a mean (SD) BP of 55(10) mmHg. To detect a mean difference of 10 mmHg between study groups, accepting a 2-sided alpha of 0.05 and 90% power, we calculated a sample size of 24 infants per group. To accommodate 10% attrition rate for detecting the primary outcome due to monitoring failure, we increased our total sample size to 26 infants in each group (n = 52).

All analyses were specified a priori based on intention-to-treat. For the primary outcome, we calculated the difference in group means, 95% confidence interval (CI) and p-value from an independent samples t-test. Subgroup analyses were planned for the randomisation strata of preterm (32+0–35+6 weeks’ gestation), non-emergency birth ≥36+0 weeks’ gestation, and emergency birth ≥36+0 weeks’ gestation. Emergency births were defined as instrumental and unplanned caesarean births.

Data from non-randomised infants who received ≥2 min DCC per protocol and remained vigorous after cord clamping (no respiratory support) until ≥10 min after birth were used to determine reference ranges of pre-ductal BP at 3–4 min and 6–7 min, as well as reference percentiles of rStO2 and cFTOE. For rStO2 and cFTOE, we used methodology originally proposed by Royston and recommended by Cole for longitudinal data.17,25,26 This involved fitting nonlinear regression models to the mean using fractional polynomials in minutes after birth. Mean values were estimated using mixed-effect regression using the same power variables of minutes after birth as fixed effects, as well as infant and time as random effects. Type of birth and interaction between time after birth and type of birth as covariates were included in the model based on the data and previous literature.17,27 The percentiles were then calculated by adding or subtracting standard deviation of values multiplied by z-scores based on standard normal distribution to the mean values.