Objective
Recent recommendations called for obstetricians to abandon the terms of “hyperstimulation” and “hypercontractility” in favor of the more rigidly defined term, “tachysystole” (TS). The aim of the current study is to describe incidence of and risk factors for TS, describe fetal heart rate (FHR) changes associated with TS, and investigate maternal and neonatal outcomes associated with TS.
Study Design
For this retrospective cohort study, we reviewed and analyzed the intrapartum FHR and tocometric characteristics of all patients with a singleton, nonanomalous fetus in term labor in a single hospital system over a 28-month period. Univariate association testing was done using χ 2 and t tests, comparing demographics, pregnancy characteristics, outcomes, and TS events. Multivariable association testing between risk factors and TS events were tested using generalized estimating equations, adjusting for multiple pregnancies during the study period for the same woman.
Results
There were a total of 50,335 deliveries from 48,529 women during the 28-month period. Of these, there were a total of 7567 TS events in 5363 deliveries among 5332 women. Use of oxytocin or misoprostol, an epidural, hypertension, and induction of labor were associated with an increased risk of TS. We found a doubling of TS events with any oxytocin, a dose-response correlation between oxytocin and TS, FHR changes occurring in a quarter of TS events and, finally, that presence of TS increases the chance of composite neonatal morbidity.
Conclusion
TS is associated with specific risk factors and impacts FHR and neonatal morbidity.
See Journal Club, page 68
In 2008, the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), with the American Congress of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine, convened a workshop to examine fetal heart rate (FHR) monitoring. Workshop participants proposed an updated classification system and terminology for describing FHR patterns. This document calls for obstetricians to abandon the terms of “hyperstimulation” and “hypercontractility” when describing uterine activity. The preferred term, “tachysystole” (TS), is described as >5 contractions in 10 minutes, averaged over a 30-minute window. Prior to this consensus, excessive uterine activity was poorly defined and, thus, understudied.
Excessive uterine activity has been associated with abnormal FHR patterns, presumably because of inadequate uterine relaxation time. Shortened relaxation time leads to excess constriction of the spiral arterioles that provide oxygen delivery to the placenta and fetus and increased intervillous space pressure, effectively reducing perfusion pressure and thus placental blood flow. However, few data exist in the literature to confirm an association between TS and abnormal FHR patterns, increased cesarean section rates, or adverse neonatal outcome. Furthermore, most of the research that does exist was mainly published prior to the 2008 NICHD definitions and is consequently difficult to interpret and apply. A very recent study found that TS was associated with decelerations but not adverse infant outcomes. However, that study was focused on TS during early labor of women who received misoprostol.
One of the research priorities identified by the NICHD workshop was to define the effect of uterine contraction characteristics on clinical outcomes. The aim of the current study is to describe incidence of and risk factors for TS, to describe FHR changes associated with TS, and to investigate maternal and neonatal outcomes associated with TS.
Materials and Methods
The Intermountain Healthcare Institutional Review Board approved this investigation. For this retrospective cohort study, the intrapartum FHR and tocometric characteristics of all patients with a singleton, nonanomalous fetus in term labor at 10 Intermountain Healthcare hospitals from March 1, 2007, through June 30, 2009, were reviewed and analyzed. The patient inclusion criteria for this study included: singleton pregnancy; gestational age ≥37.0 weeks; in labor (either spontaneous or induced) with FHR monitoring for at least 120 minutes; and delivery during the monitoring period. Patients admitted for a scheduled cesarean delivery were excluded.
The data were extracted from the computerized clinical database maintained by the health care system. The input of FHR data, the training of labor staff in FHR interpretation, the frequency and type of FHR assessments, and the conversion of both uterine contraction and FHR data into appropriate NICHD categories have been described previously. Briefly, FHR and contraction characteristics are entered into a bedside workstation at least every 20 minutes in labor and delivery, or sooner if a change in either occurs. This information was stored in a systemwide enterprise data warehouse and linked to maternal and short-term neonatal data. Software was developed to convert the data on FHR characteristics into the appropriate NICHD FHR category at each reading. Training of the labor and delivery nursing staff included 2 formal instructional sessions developed and sponsored by the Association of Women’s Health, Obstetric and Neonatal Nurses, and monthly continuing education in FHR interpretation. To identify the subset of patients who had TS, the system was queried for nursing entry that noted any of the following: TS, contraction frequency 2 minutes apart or shorter, or >5 contractions in 10 minutes. To assess the validity of data entry, a random subset of cardiotocographic data from 100 patients was manually examined by the authors.
For the current study, TS was defined using the NICHD definition of contractions occurring >5 times in 10 minutes averaged over a 30-minute window. For this analysis, any continuous episode of TS, regardless of duration, was considered to be 1 episode. For example, a continuous 60 minutes of TS was counted as 1 event and not 2 shorter events. The overall incidence of TS was calculated, as were the number of TS events experienced by each patient and the total time of TS present during monitoring. We also assessed risk factors associated with TS alone (TS); with TS with a fetal heart category change (TS-F); with TS necessitating intervention by terbutaline administration, change in oxytocin dose, and/or position change (TS-I); and with TS leading to expedited delivery by cesarean or instrumented vaginal delivery within 30 minutes (TS-D).
Univariate association testing was done using χ 2 tests and t tests, comparing maternal demographics, pregnancy characteristics, delivery outcomes, and TS events. Multivariable association testing between risk factors and TS events were tested using generalized estimating equations (GEE), adjusting for repeat pregnancies during the study period for the same woman. The risk factors included in the final GEE models were chosen based on backwards selection from the risk factors of age, marital status, parity, diabetes, hypertension, smoking history, alcohol use history, drug use history, intrauterine growth restriction, preeclampsia, placenta abruption, oligohydramnios, oxytocin usage, misoprostol usage, epidural usage, and labor induction. The GEE model accounts for the nonindependence in the observations.
We assessed the likelihood of outcomes, including delivery methods and neonatal outcomes, based on TS, TS-F, TS-I, and TS-D as well as total number of TS events. Subjects without any TS were the referent group for TS, TS-F, TS-I, and TS-D, and all analyses controlled for oxytocin use, misoprostol use, age, epidural, multiparty, hypertension, marriage status, and preeclampsia (previously identified as significant risk factors for TS events). The delivery outcomes were examined using 2 models. First we examined the outcome of cesarean delivery (1 = cesarean delivery and 0 = vaginal delivery, either operative vaginal delivery [OVD] or spontaneous vaginal delivery [SVD]). Then for those women who had a vaginal delivery (n = 44,425), we examined the outcome of OVD (1 = OVD and 0 = SVD, and those with cesarean delivery were excluded from the analysis). Neonatal outcomes evaluated included 5-minute Apgar <7, neonatal intensive care unit (NICU) admission, sepsis, respiratory complications, intraventricular hemorrhage (IVH), necrotizing enterocolitis (NEC), and pneumothorax. We also assessed composite adverse neonatal outcome (sepsis, IVH, NEC, pneumothorax, low Apgar). Individual components of the composite outcome were chosen prior to any data analysis.
Results
There were a total of 50,335 deliveries from 48,529 women during the 28-month period. Of these, there were a total of 7567 TS events in 5363 deliveries among 5332 women. The maternal demographics, pregnancy features, and delivery outcomes are shown by TS event in Table 1 .
| Characteristic | No tachysystole | Tachysystole |
|---|---|---|
| n = 44,972 | n = 5363 | |
| Maternal age, mean y (SD) | 27.26 (5.09) | 25.06 (4.87) a |
| White | 36,510 (81.18%) | 4377 (81.61%) |
| Married | 37,528 (83.45%) | 4050 (75.52%) a |
| Parity | 16,166 (35.95%) | 3358 (62.61%) a |
| Tobacco use ever | 4591 (10.25%) | 670 (12.56%) a |
| Tobacco use current | 1437 (3.21%) | 199 (3.73%) b |
| Smoker in home | 3371 (7.53%) | 499 (9.36%) a |
| Alcohol use ever | 2378 (11.05%) | 293 (13.62%) b |
| Alcohol use current | 279 (1.30%) | 34 (1.58%) |
| Drug use ever | 536 (2.49%) | 75 (3.49%) b |
| Drug use current | 32 (0.15%) | 3 (0.14%) |
| Diabetes | 204 (0.45%) | 22 (0.41%) |
| Hypertension | 1871 (4.16%) | 394 (7.35%) a |
| IUGR | 550 (1.22%) | 74 (1.38%) |
| Preeclampsia | 924 (2.05%) | 202 (3.77%) a |
| Oligohydramnios | 728 (1.62%) | 129 (2.41%) a |
| Placenta abruption | 441 (0.98%) | 53 (0.99%) |
| Type of labor a | ||
| Elective induction | 10,950 (24.39%) | 1186 (22.16%) |
| Indicated induction | 16,364 (36.45%) | 2394 (44.72%) |
| Spontaneous labor | 17,582 (39.16%) | 1773 (33.12%) |
| Oxytocin use | 32,429 (72.11%) | 4591 (85.61%) a |
| Misoprostol use | 763 (1.70%) | 188 (3.51%) a |
| Foley cervical ripening | 158 (0.35%) | 41 (0.76%) a |
| Epidural c | 38,779 (89.2%) | 5074 (94.7%) a |
| Time in labor, mean h (SD) | 8.43 (5.44) | 12.01 (6.47) a |
| Type of delivery a | ||
| Vaginal delivery | 36,241 (80.59%) | 4011 (74.79%) |
| Cesarean delivery | 5240 (11.65%) | 670 (12.49%) |
| Operative vaginal delivery | 3491 (7.76%) | 682 (12.72%) |
| Gestational age, mean wk (SD) | 38.82 (2.09) | 39.00 (1.93) a |
| Birthweight, mean g (SD) | 3379.2 (431.8) | 3444.4 (437.3) a |
| Apgar <7 | 336 (0.75%) | 61 (1.14%) b |
| NICU admission | 2093 (4.65%) | 399 (7.44%) a |
| Neonatal sepsis | 484 (1.08%) | 114 (2.13%) a |
| Bronchopulmonary dysplasia | 2 (0.00%) | 0 (0.00%) |
| Interventricular hemorrhage | 6 (0.01%) | 1 (0.02%) |
| Necrotizing enterocolitis | 39 (0.09%) | 6 (0.11%) |
| Pneumothorax | 198 (0.44%) | 27 (0.50%) |
| Composite neonatal outcome d | 968 (2.15%) | 190 (3.54%) a |
c Epidural use data available for 43,472 of no tachysystole deliveries and 5356 tachysystole
d Composite outcome (sepsis, intraventricular hemorrhage, necrotizing enterocolitis, pneumothorax, or low Apgar score).
Of all deliveries, 5363 (10.65%) had at least 1 TS event; 1736 (3.72%) had at least 1 TS-F; 2017 (4.29%) had at least 1 TS-I; and 691 (1.51%) had a TS-D. These events were not mutually exclusive. There was no TS event in 44,972 (89.35%) labors; 3883 (7.71%) had 1 TS event; 1013 (2.01%) had 2 TS events; and 467 (0.93%) had ≥3 TS events. The average duration of a TS event was 58.50 minutes for those with only 1 TS event, 63.10 minutes for those with 2 TS events, and 62.54 minutes for those with ≥3 TS events.
By univariate analysis, maternal age, marital status, parity, history of hypertension, smoking history, alcohol history, and drug history were all significantly associated with TS events ( Table 1 ). Pregnancy and delivery factors associated with TS events were preeclampsia, oligohydramnios, use of oxytocin, use of misoprostol, use of a Foley catheter for cervical ripening, labor induction, time in labor, epidural, and mode of delivery.
The results of the multivariable analysis are detailed in Table 2 . Use of oxytocin, use of misoprostol, an epidural, and hypertension were associated with an increased risk of TS, TS-F, and TS-I. Preeclampsia and induction of labor (both elective and indicated) were associated with an increased risk of TS and TS-I. Multiparty, married status, and maternal age ≥30 years were associated with decreased risk of TS, TS-F, and TS-I. For TS-D, the use of oxytocin (increased risk), multiparty (decreased risk), and age (decreased risk) were the only significant risk factors in the multivariable analysis.
| Variable | TS | TS-F | TS-I | TS-D | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| RR | (95% CI) | P value | RR | (95% CI) | P value | RR | (95% CI) | P value | RR | (95% CI) | P value | |
| Any oxytocin | 1.69 | (1.56–1.83) | < .0001 | 1.89 | (1.65–2.17) | < .0001 | 2.62 | (2.22–3.07) | < .0001 | 1.76 | (1.45–2.13) | < .0001 |
| Any misoprostol | 1.39 | (1.22–1.58) | < .0001 | 1.76 | (1.40–2.22) | < .0001 | 1.57 | (1.27–1.93) | < .0001 | NA a | ||
| Epidural | 1.55 | (1.37–1.74) | < .0001 | 2.02 | (1.59–2.56) | < .0001 | 2.06 | (1.63–2.60) | < .0001 | NA a | ||
| Indicated induction vs spontaneous labor | 1.16 | (1.10–1.24) | < .0001 | NA a | 1.31 | (1.18–1.46) | < .0001 | NA a | ||||
| Elective induction vs spontaneous labor | 1.20 | (1.12–1.30) | < .0001 | NA a | 1.42 | (1.25–1.61) | < .0001 | NA a | ||||
| Multiparty | 0.45 | (0.42–0.47) | < .0001 | 0.41 | (0.37–0.45) | < .0001 | 0.39 | (0.36–0.43) | < .0001 | 0.73 | (0.63–0.85) | < .0001 |
| Hypertension | 1.33 | (1.21–1.46) | < .0001 | 1.50 | (1.27–1.78) | < .0001 | 1.63 | (1.41–1.89) | < .0001 | NA a | ||
| Married | 0.81 | (0.76–0.85) | < .0001 | 0.83 | (0.75–0.93) | .001 | 0.76 | (0.68–0.83) | < .0001 | NA a | ||
| Preeclampsia | 1.25 | (1.10–1.41) | .0007 | NA a | 1.31 | (1.06–1.62) | .012 | NA a | ||||
| Age (<30 vs ≥30 y) | 0.71 | (0.66–0.76) | < .0001 | 0.69 | (0.61–0.79) | < .0001 | 0.69 | (0.61–0.77) | < .0001 | 0.56 | (0.46–0.68) | < .0001 |
a Variables eliminated from model in backwards generalized estimating equation procedure.
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