Objective
We sought to determine if uterine tachysystole, ≥6 contractions per 10 minutes, within the first 4 hours of labor induction, is associated with adverse infant outcomes.
Study Design
This was a prospective cohort study of 584 women ≥37 weeks’ gestation undergoing induction of labor with 100 μg of oral misoprostol. Fetal heart rate tracings were analyzed for contractions per 10 minutes during the initial 4 hours after misoprostol administration. Patients were analyzed based on the maximum number of contractions per 10 minutes. Infant condition at birth was assessed using the fetal vulnerability composite.
Results
Adverse infant outcomes showed no association with increasing number of contractions per 10 minutes. Six or more contractions in 10 minutes were significantly associated with fetal heart rate decelerations ( P ≤ .001). Analysis was performed using the maximum number of contractions per 30 minutes with similar results.
Conclusion
Uterine tachysystole, as currently defined, when occurring remote from delivery is not associated with adverse infant outcomes.
In 2008, the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) proposed definitions for the interpretation of fetal heart rate (FHR) tracings. Within these definitions, uterine activity was quantified as the number of contractions present in a 10-minute window, averaged over 30 minutes. Uterine tachysystole was defined as ≥6 contractions in 10 minutes. Subsequently, in July 2009, the American Congress of Obstetricians and Gynecologists (ACOG) affirmed this definition of uterine tachysystole.
This definition of excessive uterine activity is the standard for current clinical practice, despite the fact that little evidence exists as to the clinical utility of this definition. Current clinical concerns regarding uterine tachysystole include the possibility of decreased fetal oxygenation due to inadequate relaxation time between contractions. It has been suggested that this decreased oxygenation during excessive contractions would result in a progressive decline in fetal oxygenation to a critical level. Based on fetal oxygenation studies some investigators have advocated changing the definition of uterine tachysystole to a more restrictive ≥5 contractions in a 10-minute epoch.
The purpose of our study was to determine if uterine tachysystole, as currently defined by ACOG, ≥6 contractions in 10 minutes, when occurring within the first 4 hours of labor induction, is associated with adverse infant outcomes. We also sought to determine at what threshold of uterine activity these adverse infant outcomes occurred.
Materials and Methods
This is a prospective cohort study of women undergoing misoprostol induction of labor at Parkland Hospital from March 17, 2009, through December 31, 2010. This study was approved by the institutional review board of the University of Texas, Southwestern Medical Center. Written informed consent was waived because this study was limited to observations during standard clinical care.
During the study period, our standard practice of misoprostol induction was to give 100 μg of oral misoprostol for a maximum of 2 doses, 4 hours apart. All consecutive women who qualified for misoprostol induction according to our standard criteria were eligible for this study. Those women with a fetal demise or those who had incomplete data were excluded from study. A woman qualified for the initial dose of misoprostol if the pregnancy was a singleton, cephalic, term gestation (≥37 weeks) without evidence of active labor, defined a cervical dilatation of ≥4 cm, with 30 minutes of reassuring FHR tracing without decelerations and <6 contractions per 10 minutes prior to receiving misoprostol. After 4 hours the woman was evaluated to assess if she qualified to receive the second dose of misoprostol. She did not receive the second dose if during the preceding 4 hours there were ≥6 contractions in any 10-minute epoch, any FHR decelerations, or if she had progressed to active labor. Those women who were unable to receive a second dose of misoprostol but needed further stimulation of labor received oxytocin infusion. Misoprostol has previously been shown to be an effective induction agent, with a reported rate of uterine tachysystole of 25% (≥6 contractions in 10 minutes without decelerations spanning 20 minutes). Our standard practice for treatment of tachysystole with associated prolonged decelerations was cesarean delivery for nonreassuring FHR. In those women with tachysystole and late decelerations, the woman was placed in the lateral decubitus position with administration of oxygen. If the late decelerations persisted, cesarean delivery was performed for nonreassuring FHR. If the tachysystole was associated with variable decelerations, the woman was placed in the lateral decubitus position with oxygen administration if needed. In both of these circumstances, the second dose of misoprostol would not be administered. If the woman had uterine tachysystole without associated decelerations, we would continue to monitor for evidence of fetal distress, however the second dose of misoprostol would not be administered.
At the conclusion of labor, the paper copy of the FHR tracing was collected. The contraction patterns were recorded with tocodynamometer unless an intrauterine pressure catheter was placed for obstetrical indications. These tracings were subsequently analyzed by visual assessment for the number of contractions per each 10-minute epoch during the initial 4 hours of misoprostol induction. In addition, for each 10-minute epoch the presence of uterine hypertonus, defined as a contraction lasting >120 seconds, was recorded. Similarly, variable, late, or prolonged decelerations and fetal tachycardia or bradycardia were recorded, using standard definitions as outlined by ACOG. The time and mode of delivery was also recorded. All FHR tracing analysis was conducted by 1 investigator (R.D.S.) who was blinded to the infant outcomes.
Obstetric and infant clinical outcome data were obtained using the preexisting Parkland Hospital obstetric database. Nurses attending each delivery complete an obstetric data sheet, and research nurses assess the data for completeness and consistency before electronic storage. Data on infant outcomes are also abstracted from discharge records and entered into a separate database. The outcome of interest for this study was infant condition at birth assessed using a composite outcome termed the fetal vulnerability composite, which included: 5-minute Apgar scores ≤3, umbilical artery blood pH <7.1, intubation in the delivery room, neonatal seizures, admission to intensive care, or perinatal death.
These results were electronically linked to the previously collected FHR tracing analyses. Patients were then divided into 4 groups based on the greatest number of contractions within any 10-minute epoch during the initial 4 hours of labor induction: ≤4, 5, 6, ≥7 contractions per 10 minutes. Analysis was also performed using the maximum number of contractions per 10 minutes averaged over 30 minutes.
Prior to commencing the study now reported, we assessed the rate of the primary outcome composite using a pilot study of 187 cases meeting the criteria for this study now reported. The fetal vulnerability composite occurred in 5% of the pilot cohort. These 187 cases were not included in the analysis of outcomes now reported because inclusion of this previously analyzed cohort could potentially bias our final results.
Using 80% power for a 2-sided test of < .05 significance, we estimated that 584 women receiving misoprostol would need to be examined to detect a significant difference in the fetal vulnerability composite. Statistical analysis included Pearson χ 2 , Cochran-Mantel-Haenszel χ 2 for trend, and analysis of variance. P values < .05 were considered significant. Analysis was performed using SAS 9.2 (SAS Institute Inc, Cary, NC).
Results
A total of 584 women undergoing induction of labor with misoprostol were analyzed. Maternal demographic characteristics and pregnancy complications are shown in Table 1 .
| Characteristic | No. of women, n = 584 |
|---|---|
| Parity | |
| 0 | 288 (49) |
| 1 | 112 (19) |
| ≥2 | 184 (32) |
| Epidural | 370 (63) |
| Maternal age, y | 25.9 ± 6.3 |
| Race/ethnicity | |
| Hispanic | 465 (80) |
| African American | 70 (12) |
| White | 32 (5) |
| Other | 17 (3) |
| Pregnancy complications | |
| Diabetes | 63 (11) |
| Hypertension | 141 (24) |
| Severe preeclampsia | 89 (15) |
| Abruption | 0 |
| Premature ruptured membranes | 164 (28) |
| Postterm induction | 151 (26) |
Of the women undergoing induction, 253 (43%) had at least one 10-minute epoch with ≥6 contractions during the initial 4 hours of induction; however when averaged over 30 minutes, 129 (22%) of the 584 women met this criteria for uterine tachysystole. Of the 584 women within the cohort, 253 (43%) required oxytocin infusion. Intrauterine pressure catheters were placed in 519 women (89%) during their labor. Infant outcomes, both the composite and individual components of the composite, according to number of uterine contractions per 10 minutes during the first 4 hours of induction are shown in Table 2 . The fetal vulnerability composite showed no association with increasing number of uterine contractions, and no individual component was significantly associated with increasing number of uterine contractions. Route of delivery similarly showed no association with the number of contractions per 10 minutes. When the infant outcomes were analyzed using the number of contractions averaged over 30 minutes, there was still no significant association between an increasing number of contractions and infant outcome, as reflected by the fetal vulnerability composite or any individual component of the composite ( Table 3 ). However, admission to neonatal intensive care was associated with decreasing uterine contractions when analyzed per 10 minutes or per 30 minutes ( P = .03 and P = .04). Of those infants without uterine tachysystole admitted to intensive care, 1 was due to a previously undiagnosed palate abnormality, 1 due to a hypoplastic left heart, and 1 for observation for an attempted maternal naproxen overdose prior to presentation to labor and delivery. The remaining cases were admitted to intensive care for unanticipated reasons, including sepsis evaluation and respiratory distress syndrome. Route of delivery was found to be associated with the number of contractions per 30 minutes, with vaginal delivery being more likely with increasing number of contractions per 30 minutes ( P = .04). Overall, 109 (19%) of the women underwent cesarean delivery. The most common reason for cesarean delivery was labor dystocia (n = 54, 50%), followed by FHR abnormalities (n = 52, 47%). Time from misoprostol administration to delivery was significantly less with increasing numbers of contractions per 30 minutes ( P = .03).
| Outcome | Maximum contractions per 10 min | ||||
|---|---|---|---|---|---|
| ≤4, n = 152 | 5, n = 179 | 6, n = 134 | ≥7, n = 119 | P value | |
| Fetal vulnerability composite: | 5 (3) | 6 (3) | 2 (1) | 6 (5) | .86 |
| 5-min Apgar ≤3 | 0 | 1 (1) | 0 | 0 | .86 |
| Umbilical artery pH <7.1 | 1 (1) | 4 (2) | 2 (1) | 6 (5) | .06 |
| Seizures | 0 | 0 | 0 | 0 | — |
| Intubation at delivery | 1 (1) | 0 | 0 | 0 | .11 |
| Stillborn | 0 | 0 | 0 | 0 | — |
| Neonatal death | 0 | 0 | 0 | 0 | — |
| NICU admission | 5 (3) | 3 (2) | 0 | 1 (1) | .03 |
| a Misoprostol to delivery time, h | 10.3 [6.7, 15.5] | 10.6 [5.9, 15.0] | 9.5 [4.9, 15.0] | 8.9 [4.3, 13.2] | .18 |
| Route of delivery | |||||
| Cesarean | 31 (20) | 35 (19) | 26 (19) | 17 (14) | .34 |
| Vaginal | 121 (80) | 144 (81) | 108 (81) | 102 (86) | .28 |
a Kruskal-Willis test. Data shown as median [1st quartile, 3rd quartile].
| Outcome | Maximum contractions per 30 min | P value | |||
|---|---|---|---|---|---|
| ≤13, n = 292 | 14-16, n = 163 | 17-19, n = 83 | ≥20, n = 46 | ||
| Fetal vulnerability composite | 10 (7) | 3 (2) | 2 (1) | 4 (3) | .82 |
| 5-min Apgar ≤3 | 1 (1) | 0 | 0 | 0 | .33 |
| Umbilical artery pH <7.1 | 4 (3) | 3 (2) | 2 (1) | 4 (3) | .06 |
| Seizures | 0 | 0 | 0 | 0 | — |
| Intubation at delivery | 1 (1) | 0 | 0 | 0 | .33 |
| Stillborn | 0 | 0 | 0 | 0 | — |
| Neonatal death | 0 | 0 | 0 | 0 | — |
| NICU admission | 8 (3) | 0 | 0 | 1 (2) | .04 |
| a Misoprostol to delivery time, h | 10.8 [6.7, 15.7] | 9.7 [5.0, 14.8] | 8.8 [4.4, 15.7] | 8.8 [3.9, 11.8] | .03 |
| Route of delivery | |||||
| Cesarean | 65 (22) | 26 (15) | 12 (14) | 6 (13) | .02 |
| Vaginal | 227 (78) | 137 (85) | 71 (86) | 40 (87) | .04 |
a Kruskal-Willis test. Data shown as median [1st quartile, 3rd quartile].
FHR decelerations were associated with increasing number of contractions ( Table 4 ). Analysis of decelerations of any configuration in relation to ≤4 through ≥7 contractions per 10 minutes showed a significant trend. A significant similar trend was also observed when contractions ≤13 through ≥20 per 30 minutes were analyzed. We then reanalyzed these data looking for a “cutpoint,” a threshold that provides the best discrimination for decelerations. We used the minimum P value method for this analysis. We then used a Bonferroni correction to adjust the P values. Based on this statistical technique, we found that ≥6 contractions per 10 minutes, as well as 17-19 per 30 minutes, were the cutpoints at which the most significant P value for all possible cutpoints was observed.
