Methods
This was a secondary analysis of a prospective cohort study of singleton term deliveries from 2010-2014. Patients were included in the analysis if they were ≥37 weeks gestation, had ruptured membranes and an IUPC in place for at least 30 minutes prior to delivery. Individual clinician decision making determined which patients received an IUPC for contraction monitoring. The patients were not involved in the study design. The Institutional Review Board and Human Research Protection Office approved this study. Patients with multiple gestations and those presenting for scheduled cesarean delivery were excluded. Trained obstetric research nurses blinded to outcomes abstracted intrauterine pressure measurements in 10 minute segments. Intrauterine resting tone was defined as the average baseline pressure between contractions during the 30 minutes prior to delivery. Elevated intrauterine resting tone was defined as intrauterine resting tone ≥75th percentile within this cohort. No established cut offs for defining elevated intrauterine resting tone have been published therefore the upper quartile was chosen.
The primary outcome was composite neonatal morbidity which included: Hypoxic ischemic encephalopathy, hypothermia treatment, intubation, seizures, umbilical arterial (UA) pH ≤ 7.1, neonatal respiratory support, and neonatal death. Secondary outcomes included lactate ≥ 4 mmol/l, NICU admission, 5 min Apgar <7 and mild acidemia defined as UA pH <7.2.[7, 8]. Mild acidemia was defined as UA pH < 7.2 as it has been associated with increased neonatal morbidity compared to neonates with UA pH ≥ 7.2[7]. Our institution performs universal umbilical artery cord gases. These outcomes were compared between patients with and without elevated intrauterine resting tone.
Baseline demographics and outcomes were compared usingX 2 for categorical variables and the studentt test or Mann-Whitney U test for continuous variables, as appropriate. Normality was tested using the Shapiro-Francia test. Multivariable logistic regression was used to estimate odds ratios (OR) while adjusting for potential confounders. Initial confounders included in the models were selected based on the results of univariate analyses and those that had biologic plausibility [9-13] [14]. Initial models included obesity, amnioinfusion, tachysystole, induction, parity and fetal growth restriction. A backwards step-wise selection was then performed, keeping only covariates that remained significant (P < 0.1). Regression models assessing the association between neonatal outcomes and elevated intrauterine resting tone included amnioinfusion as this has been previously associated with changes in uterine tone and neonatal outcomes, however, this covariate was not significant in the final model [13, 15]. Model fit was confirmed with the Hosmer-Lemeshow goodness of fit test[16].
A sub-group analysis was performed to evaluate the association between persistent elevated intrauterine pressure and primary and secondary outcomes. Persistent elevated intrauterine pressure was defined as present at both 30 and 120 min prior to delivery. Multivariable regression models were used to estimate the OR for primary and secondary outcomes for patients with normal uterine pressure, elevated resting tone only for the last 30 minutes prior to delivery and for patients with persistently elevated intrauterine pressure were performed. To account for elevated tone related to tachysystole, we performed an additional sensitivity analysis excluding those with uterine tachysystole during the monitoring period. We also performed a sensitivity analysis of patients who had elevated resting intrauterine pressure only in the second stage of labor.
All patients meeting inclusion criteria were included and an a priori sample size estimation was not performed. STATA Version 16 (STATA Corp., College Station, TX) was used to perform all analyses. STROBE guidelines were followed throughout the study [17].