Results
Of the 8580 patients in the cohort, 2210 (25.8%) had an IUPC with at
least 30 minutes of pressure data prior to delivery and were included in
this analysis. The median intrauterine resting tone was 9.7 mmHg (IQR
7.3-12.3 mmHg; Figure 1). A total of 567 patients had an average
elevated resting intrauterine tone ≥
12.3 mmHg (≥75th percentile).
Admission body mass index was higher for patients with elevated resting
tone (33 mmHg [IQR 29-39] vs 32 mmHg [28-38], p <
0.01.) Patients with elevated resting tone were also older and more
likely to be nulliparous (Table 1). Incidence of maternal comorbidities
including chronic hypertension, pregnancy associated hypertension, and
pregestational diabetes were not significantly different between groups
(p < 0.05). Neonatal birth weight and incidence of
intrauterine growth restriction did not differ for patients with
elevated resting tone (Table 1).
Patients with elevated resting tone were less likely to undergo
induction of labor (52.9 vs 60.4%, p<0.05) and were less
likely to deliver by cesarean (0.4% vs 2.4%, p< 0.05).
Oxytocin use (84.0 vs 89.9%, p<0.01) and maximum oxytocin
dose achieved (12 mu/min [IQR 6-18] vs 14 mu/min [8-20],
p<0.01) were lower for patients with elevated resting tone
compared to those with normal resting tone. Chorioamnionitis was more
common among patients with high resting tone (6.2% vs 3.7%, p = 0.01).
While the median durations of 1st and
2nd stage of labor differed slightly between patients
with elevated and normal resting tone, overall prolonged labor duration
> 24hrs and prolonged 2nd stage ≥ 3hrs
were similar (Table 2). Incidence of tachysystole is more common for
patients with elevated resting tone then patients with normal tone
(15.3% vs 1.3%, p<0.01).
Composite neonatal morbidity was significantly higher in the group with
elevated resting tone (5.1% vs 2.9%, p = 0.01; Table 3). After
adjusting for amnioinfusion and chorioamnionitis, elevated intrauterine
resting tone was associated with increased odds of neonatal morbidity
(aOR 1.70 95% [CI 1.06-2.74]; Table 3). Elevated intrauterine
resting tone was associated with mild acidemia and elevated lactate (aOR
1.81 [95%CI 1.38-2.37] and aOR 1.45 [95% CI 1.17-1.80]
respectively; Table 3). The incidence of low Apgar score at 5 minute and
NICU admission was similar between groups (Table 3). Among women without
evidence of tachysystole, high intrauterine resting tone remained
significantly associated with increased risk of neonatal composite
morbidity (aOR 1.64 [95%CI 1.01-2.78]).
A sub-group analysis stratified patients with intrauterine pressure for
30 minutes and 120 minutes prior to delivery. Persistently elevated
intrauterine tone for 120 minutes prior to delivery was seen in 234
patients, representing 41% of all patients with elevated intrauterine
resting tone. Increasing duration of
elevated resting tone was associated with increased risk of composite
neonatal morbidity (2.9% for normal resting tone vs 3.9% for 30
minutes elevated resting tone vs 6.8% for 120 minutes elevated resting
tone; p for trend 0.03). After adjusting for amnioinfusion and
chorioamnionitis, persistently elevated intrauterine tone was associated
with increased risk of neonatal morbidity while elevated tone for only
30 minutes did not (aOR 2.21[95% CI 1.22-4.01] and aOR 1.34 [95%
CI 0.71-2.50], respectively). A similar pattern was noted for lactate
≥ 4 mmol/l, 5-minute Apgar score < 7, and umbilical artery pH
< 7.2 (Table 4). The majority of patients included in the
analysis were in the second stage of labor (98.1%). In a sensitivity
analysis excluding those who delivered in the first stage of labor,
elevated intrauterine resting tone remained significantly associated
with composite neonatal morbidity (aOR 1.77 [95% CI 1.09-2.86]).