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]).