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].