Study population
This was a cross-sectional study comprising 228 neonates from Henan Children’s Hospital during the five-year period 2015–2020 in China. All neonatal basic demographic information and clinical records were documented from the neonatal HIE database. The neonatal HIE database was set up in Henan Children’s Hospital and known to be a province-wide, clinically-oriented computerized disease-specific database. We obtained information on the neonatal age at diagnosis, gender, residential region, birth weight and delivery mode from their birth certificate cards or vaccination cards and also collected related medical data from their medical records in the hospital. To ensure the reliability of the information, we routinely examined the items and logical errors in the database. When data was found to be inconsistent, we would list the numbers of the patients, record names, variable name and error categories to facilitate future checks and corrections.
For the current study, to focus on moderate and severe HIE, cases of HIE were defined by the following criteria:(a) all the 228 HIE cases diagnostic criteria were set by the Neonatal Group of the Pediatric Society of Chinese Medical Association; (b) all 228 neonates resided in the Neonatal Intensive Care Unit (NICU) of Henan Children’s Hospital and had typical clinical manifestations (paroxysmal cyanosis, agitation, increased muscle tone, drowsiness, full anterior fontanelle, low muscle tone, binocular gaze, abnormal emission, screaming, convulsions, etc.), which were confirmed by head CT, magnetic resonance imaging (MRI) and other examinations;(c) the parents of the neonates were able to cooperate closely with the investigation and the data were complete. According to clinical classification, neonatal HIE was divided into three categories: mild mainly manifested as agitation, normal or slightly active primitive reflex, no convulsion; moderate (mainly with inhibition symptoms, manifested as drowsiness, decreased muscle tone, weakened primitive reflex, partially with increased intracranial pressure and convulsion), severe (mainly with coma, the manifestations are loss of muscle tone of limbs, no spontaneous movement, disappearance of primitive reflex, most of them have increased cranial pressure and convulsion). The exclusion criteria for neonates included patients with incomplete clinical data and with congenital cardiopulmonary diseases, genetic or metabolic diseases.
Ascertainment of variables
Background data and exposure to considered potential risk factors were extracted from the neonatal HIE database. The possible risk factors studied included neonatal birth weight, gestational age at birth, parity(one, two, three or more), feeding pattern(breastfeeding, artificial feeding or mixed feeding),delivery mode(natural delivery or cesarean section),1-min Apgar’s score (≥ 7 or < 7), 5-min Apgar’s score(≥ 7 or < 7), preterm birth, intrauterine infection, intrauterine distress, amniotic fluid contamination, amniotic fluid anomaly and abnormal labor stage. In this study, neonatal birth weight was categorized as low birth weight (1500-2499 g),normal birth weight (2500-4000 g) and macrosomia(> 4000 g). Neonatal gestational age at birth were classified as premature infant (32–36 weeks),term infant (37–42 weeks) and postterm infant (> 42 weeks). Intrauterine distress was referred to neonatal heart less than 120 beats/min or more than 160 beats/min. Amniotic fluid anomaly was defined as the amniotic fluid volume ≥ 2000 ml or ≤ 300 ml, and the amniotic fluid dark area > 7 cm or ≤ 2 cm by B-ultrasound examination. Abnormal labor stage was referred to the total labor course ≤ 3 h or ≥ 24 h, the second labor stage ≥ 2 h for primipara and ≥ 1 h for parturient. Neonatal preterm birth, intrauterine infection, intrauterine distress, amniotic fluid contamination, amniotic fluid anomaly and abnormal labor stage were all classified as a dichotomous variable “Yes” or “No”.
Statistical analysis
Maternal and neonatal baseline descriptive characteristics with continuous variables and categorical variable were all presented as mean ± standard deviation value and frequencies and percentages respectively. Comparisons between different levels neonatal HIE were conducted with the t-test or Wilcoxon rank-sum test for continuous variables and the Chi-square test for categorical variables. Considering that there still were nonmatching variables among different levels neonatal HIE apart from age, gender, and residence region, we carried out unconditional multiple logistic regression models to estimate the independent risk factors of interest for predicting neonatal moderate or severe HIE. Collinearity between potential confounding variables was examined using Spearman rank-order correlation for continuous variables or Chi-square test for category variables. All variables identified as statistically significant (P < 0.05) predictors in the univariate analysis were included as candidate predictors in the multivariate model. Odds ratios(ORs) of mild HIE vs. moderate or severe HIE were reported along with the corresponding 95% confidence intervals (CIs).First, we estimated the unadjusted ORs by each maternal and neonatal characteristic in model 1. Thereafter, we adjusted ORs for the different maternal and neonatal characteristics for each other, based on associations in present data and in previous reports in model 2.
To further explore the characteristics of 228 males and females neonatal HIE in different group, we conducted trend test analysis stratified by the neonatal birth weight and gestational age at birth. In addition, we also used an additive model to test for possible biological interactions between neonatal birth weight and gestational age at birth. OR was calculated for each category after adjustment for covariates and Relative Excess Risk of Interaction (RERI) was calculated based on the additive model. All statistical analyses were conducted with IBM software SPSS (version 21, Chicago, IL, USA). All statistical tests were two-tailed and considered to be statistically significant at P value less than 0.05.