Methods have been previously described in a case–control study matching premature infants with CCA to controls without CCA [16]. In brief, a retrospective cohort study was performed on infants born in the Leiden University Medical Centre (LUMC) at < 30 weeks’ gestation between January 2016 and April 2021. Data were collected from respiratory function monitors (RFMs; Advanced Life Diagnostics resuscitation monitor, Weener, Germany) and electronic health records.

Infants were included in the study when RFM data was available and they were compared at three different levels (Fig. 1). Comparison 0: infants from pregnancies with available placental pathology reports were compared to infants from pregnancies without available placental pathology reports. Comparison 1: infants with histological chorioamnionitis and/or funisitis (HCA + FUN) were compared to infants without HCA + FUN. Comparison 2: infants with subclinical HCA + FUN were compared to infants without any chorioamnionitis (i.e. without HCA + FUN or CCA) (Fig. 1). In the LUMC, placental pathology is ordered by the attending obstetrician upon indications listed in Supplementary file 1 when it might contribute to obstetrical evaluation of the pregnant woman. HCA + FUN was defined according to the placental clinical pathology report where anatomical localisation of inflammation was recorded (i.e. chorion, amnion and umbilical vessels) without grading or staging the inflammation.

Fig. 1

Flowchart diagram. Flowchart of comparisons between different groups

Respiratory support in the delivery room was provided using a Neopuff™ infant T-piece resuscitator with a facemask (Fisher & Paykel Healthcare, Auckland, New Zealand) in combination with a visible RFM. Respiratory support started with CPAP 5–8 cm H2O, but in case of apnoea and/or bradycardia, initial inflations of 3–15 s or iPPV were administered with a frequency of 40–60 inflations per minute, a positive end-expiratory pressure of 5–8 cm H2O and a peak inspiratory pressure (PIP) of 20–25 cm H2O. Fraction of inspired oxygen (FiO2) was commenced at 0.3 and titrated based on the 25th percentile of a published normogram [17]. Collected RFM data consisted of physiological measurements in the first 5 min after birth using a Masimo SET pulse oximeter probe (Masimo Radical, Masimo Corporation, Irvine, California, USA), a portable oxygen analyser (AX300-I Teledyne Analytical Instruments, CA, USA) and a variable orifice flow sensor (Avea Varflex Flow Transducer, Carefusion, Yorba Linda, CA, USA) recorded digitally using the Polybench physiological software (Applied Biosignals, Weener, Germany). Pulmochart software (Applied Biosignals) was used to calculate respiratory outcomes corrected for birth weight. All physiological data was assessed blinded to the groups.

The primary outcome was measurable breathing effort in the first 5 min after birth, measured as minute volume of spontaneous breathing. Minute volume was calculated as the product of the average inspiratory tidal volume on CPAP with mask leak < 75% and respiratory rate per minute of breathing independent of respiratory support provided during this time period. The primary outcome was calculated per minute. Additional physiological parameters assessed in the first 5 min after birth consisted of average: inspiratory drive (tidal volume corrected by inspiratory time; a reflection of depth of breathing [18]), incidence and duration of apnoea, cessation of breathing > 10 s, inter-breath interval coefficient of variation (COV, a higher COV indicates a less stable breathing pattern [19]), CPAP and PIP levels, heart rate (HR), oxygen saturation (SpO2) and FiO2. Infants were considered stable according to the following criteria: HR ≥ 100, SpO2 ≥ 85%, FiO2 ≤ 40% during spontaneous breathing. Data on respiratory support in the delivery room and NICU were also collected (i.e. initial inflations, caffeine, iPPV, surfactant administration and intubation rate).

Demographics comprised of maternal characteristics: maternal age, parity, multiple pregnancy, mode of birth, type of anaesthesia, antenatal corticosteroids, maternal intrapartum antibiotics,Footnote 1 CCAFootnote 2 and anatomical location of inflammation, and neonatal characteristics: gestational age, birth weight, sex, Apgar scores and umbilical pH.

Statistical analysis

Statistical analyses were performed with IBM SPSS Statistics V.29.0 (IBM Software, Chicago, Illinois, USA, 2022) and R version 4.4.0 (R: The R Project for Statistical Computing (r-project.org)) within Studio version 4.4.0 (R Studio, Boston, MA, USA, 2024). In comparison 0, baseline characteristics (i.e. demographic data) were compared between infants with vs infants without available placental pathology (Fig. 1 and Supplemental file 2). Baseline characteristics were also compared between infants with and without HCA + FUN (comparison 1) and between infants with subclinical HCA + FUN and infants without any chorioamnionitis (comparison 2) (Fig. 1).

Respiratory, additional physiological and respiratory support parameters were compared between infants with and without HCA + FUN (comparison 1) and between infants with subclinical HCA + FUN and infants without any chorioamnionitis (comparison 2) after adjusting for potential confounding factors described below (Fig. 1).

Categorical data were described as n (%) and analysed with a Pearson chi2-test or Fisher’s exact test. Normality for continuous data was assessed via inspection of histograms and using the Shapiro-Wilkinson test. Parametric data was presented as mean ± standard deviation and analysed with an Independent samples T-test, while non-parametric data was presented as median (interquartile range) and analysed with a Mann–Whitney U test. Outcomes for “time until a physiological eventFootnote 3” were presented as median (interquartile range) and analysed with a Log-rank test of the Kaplan–Meier survival curve. Missing data is noted in the corresponding table.

Taking into account that gestational age, birthweight, antenatal corticosteroids, mode of delivery and general anaesthesia could affect spontaneous breathing at birth, multiple regression analyses were performed to adjust for these covariates when baseline differences between groups in these variables were significant [16, 20,21,22]. Birthweight was exempted as a covariate, if gestational age was also different between groups, due to the likely collinearity between birthweight and gestational. Likewise, the mode of delivery was exempted if general anaesthesia was significant. Raw data of respiratory and additional physiological outcomes are presented and compared for comparisons 1 and 2 in Supplementary file 3.

The primary outcome, measurable breathing effort, was compared over time in comparisons 1 and 2 using a linear mixed-effects model accounting for multiple measurements in the same infant. Fixed effects in the regression analyses consisted of (subclinical) HCA + FUN, time, (subclinical) HCA + FUN*time and the other aforementioned covariates. An unstructured variance–covariance matrix was used. The outcome of the linear mixed-effects model was described as estimated marginal mean ± standard error for the (subclinical) HCA + FUN fixed effect. The p-value for the (subclinical) HCA + FUN covariate assesses the difference in average breathing effort between groups. The p-value for the (subclinical) HCA + FUN*time fixed effect informs on differences in breathing effort between groups over time in the first 5 min after birth.

All other outcomes were compared with generalised linear models, binary logistic regression or Cox regression and presented as estimated marginal means ± standard error, odds ratios (95% confidence interval; 95% CI) and median (interquartile range), respectively, with adjusted p-values. For generalised linear models, only adjusted p-values are displayed in the results section.

Post hoc analysis was employed for inspiratory tidal volume, respiratory rate, inspiratory drive and inter-breath variability to analyse temporal changes.

A two-sided p-value < 0.05 was considered significant.

Results

Resuscitation data were available for 321 infants born < 30 weeks’ gestation in the LUMC between 2016 and 2021; 186 placentas were examined for pathology of which 75 (40%) placentas had evidence of HCA + FUN and 111 (60%) did not (Fig. 1).

Baseline Characteristics Comparison 1

Infants with HCA + FUN had lower gestational ages (infants with HCA + FUN vs. infants without HCA + FUN; 26+5 (25+0–28+1) vs. 28+4 (27+0–29+1) weeks, p < 0.001), lower birthweights (943 ± 264 vs. 1023 ± 270, p = 0.049) and lower Apgar scores at 1 min (5 (2–7) vs. 6 (3–7), p = 0.006) compared to infants without HCA + FUN (Table 1). Infants with HCA + FUN had a lower incidence of caesarean deliveries (13 (28%) vs. 70 (69%), p < 0.001) and general anaesthesia (10 (13%) vs. 28 (25%), p = 0.048), while they had higher exposure to maternal intrapartum antibiotics (31 (41%) vs. 23 (21%), p = 0.002) and antenatal corticosteroids (55 (73%) vs. 64 (58%), p = 0.043) (Table 1).

Comparison 2

Infants with subclinical HCA + FUN (n = 46) had lower gestational ages (infants with subclinical HCA + FUN vs. infants without any chorioamnionitis; 26+6 (25+1–28+3) vs. 28+4 (27+2–29+1) weeks, p < 0.001) were less often born after caesarean Sect. (13 (28%) vs. 70 (69%), p < 0.001) and had lower Apgar scores at 1 min (3 (2–6) vs. 5 (4–7), p = 0.005) compared to infants without any chorioamnionitis (n = 102) (Table 1).

Table 1 Baseline characteristics of infants with and without chorioamnionitis and funisitis Respiratory parameters in the first 5 min after birth Comparison 1

After adjustment for the covariates (see above), HCA + FUN was associated with lower minute volume (38 ± 20 vs. 74 ± 18 mL/kg/min, p = 0.029) without a significant change in minute volume over time (p = 0.496). HCA + FUN was associated with lower tidal volume (p = 0.006), lower inspiratory drive (p < 0.001) and a greater variability (assessed as COV) of the inter-breath interval (p = 0.019), while the respiratory rate was not significantly affected by the presence of HCA + FUN (Table 2).

Comparison 2

After adjustment, subclinical HCA + FUN was not associated with lower minute volume (64 ± 17 vs. 91 ± 12 mL/kg/min, p = 0.220) nor with a significant change in minute volume over time (p = 0.541). Subclinical HCA + FUN was also associated with lower tidal volume (p = 0.033), lower inspiratory drive (p = 0.014), a greater variability of the inter-breath interval (p = 0.001) and no significant differences in respiratory (p = 0.157) (Table 2).

Table 2 Respiratory and additional physiological parameters of infants with and without chorioamnionitis and funisitis Post-hoc analyses Comparison 1

In post-hoc analyses, HCA + FUN was solely associated with a significant change in respiratory rate over time (p = 0.034) (Supplementary File 3).

Comparison 2

Subclinical HCA + FUN was associated with a non-significant change in tidal volume (p = 0.053) and respiratory rate (p = 0.069) over time (Supplementary File 3).

Additional physiological parameters in the first 5 min after birth Comparison 1

After adjustment for potential confounders, HCA + FUN was associated with lower SpO2 levels at 5 min, lower SpO2/FiO2 ratio and higher FiO2 requirements (p = 0.009) (Table 2).

Comparison 2

Likewise, subclinical HCA + FUN was associated with lower SpO2 at 5 min (p = 0.036), lower SpO2/FiO2 ratio (p = 0.040) and higher FiO2 requirements after adjustment (p = 0.026) (Table 2).

Respiratory support in the delivery room and NICU Comparison 1

After adjustment, HCA + FUN was not associated with any delivery room outcomes, but HCA + FUN was significantly associated with a decreased incidence of surfactant administration in the NICU (0.28 (0.11–0.62) and intubation in the NICU (0.34 (0.14–0.80)) (Table 3).

Comparison 2

Similarly, subclinical HCA + FUN was not associated with any delivery room outcomes and was negatively associated with the incidence of surfactant administration in the NICU (0.27 (0.10–0.71)) (Table 3).

Table 3 Respiratory support in the delivery room and NICU provided to infants with and without chorioamnionitis and funisitis