The therapeutic value of maintenance tocolysis: an overview of the evidence

Maintaining and repeating tocolysis: A reflection on evidence

It is inherent to human logic that both doctors and patients want to suppress uterine contractions when a woman presents in threatened preterm labor. Tocolysis is widely applied in women with threatened preterm labor with a variety of drugs. According to literature, tocolysis is indicated to enable transfer to a tertiary center as well as to ensure the administration of corticosteroids for fetal maturation. There is international discrepancy in the content and the implementation of guidelines on preterm labor. Tocolysis is often maintained or repeated. Nevertheless, the benefit of prolonging pregnancy has not yet been proven, and it is not impossible that prolongation of the pregnancy in a potential hostile environment could harm the fetus. Here we reflect on the use of tocolysis, focusing on maintenance and repeated tocolysis, and compare international guidelines and practices to available evidence. Finally, we propose strategies to improve the evaluation and use of tocolytics, with potential implications for future research.

Care for women with prior preterm birth

Women who have delivered an infant between 16 and 36 weeks' gestation have an increased risk of preterm birth in subsequent pregnancies. The risk increases with more than 1 preterm birth and is inversely proportional to the gestational age of the previous preterm birth. African American women have rates of recurrent preterm birth that are nearly twice that of women of other backgrounds. An approximate risk of recurrent preterm birth can be estimated by a comprehensive reproductive history, with emphasis on maternal race, the number and gestational age of prior births, and the sequence of events preceding the index preterm birth. Interventions including smoking cessation, eradication of asymptomatic bacteriuria, progestational agents, and cervical cerclage can reduce the risk of recurrent preterm birth when employed appropriately.

Maintenance nifedipine tocolysis compared with placebo: a randomized controlled trial

OBJECTIVE

To estimate whether maintenance nifedipine tocolysis after arrested preterm labor prolongs pregnancy and improves neonatal outcomes.

METHODS

A prospective, randomized double-blind, multicenter study was conducted. After successful tocolysis, patients were randomly assigned to receive 20 mg nifedipine or an identical-appearing placebo every 4-6 hours until 37 weeks of gestation. The primary outcome was attainment of 37 weeks of gestation. Patients were enrolled between 24 weeks and 34 weeks if they had six or fewer contractions per hour, intact membranes, and less than 4 cm cervical dilation. Exclusion criteria were placental abruption or previa, fetal anomaly incompatible with life, or maternal medical contraindication to tocolysis. Sixty-six patients were required for 80% power to detect a 50% reduction in birth before 37 weeks, with a two-tailed alpha of 0.05. Data were analyzed by intent to treat.

RESULTS

Seventy-one patients were randomly assigned. Two patients were excluded after randomization and one was lost to follow-up. Thirty-five patients received placebo, and 33 received nifedipine. There were no maternal demographic differences between groups; the placebo group was significantly more dilated and effaced at study entry. There was no difference in attainment of 37 weeks (39% nifedipine compared with 37% placebo, P>.91), mean delay of delivery (33.5+/-19.9 days nifedipine compared with 32.6+/-21.4 days placebo, P=.81) or delay of delivery for greater than 48 hours or 1, 2, 3, or 4 weeks. Neonatal outcomes were similar between groups.

CONCLUSION

When compared with placebo, maintenance nifedipine tocolysis did not confer a large reduction in preterm birth or improvement in neonatal outcomes.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov, www.clinicaltrials.gov, NCT00185952

LEVEL OF EVIDENCE

I.

Do screening-preventative interventions in asymptomatic pregnancies reduce the risk of preterm delivery—A critical appraisal of the literature

Recent research has suggested that women who experience preterm delivery (PTD) may be identified earlier in pregnancy and before onset of symptoms. Interventions commenced at this earlier asymptomatic stage may offer an opportunity to prevent PTD or lengthen gestation sufficiently to reduce adverse perinatal outcome. Our objective was to examine the evidence that supports or refutes this approach to preventing PTD. We therefore conducted a systematic search and critical appraisal of the identified literature. We found evidence that introducing screening-preventative strategies for asymptomatic pregnancies may reduce the rate of PTD. Evidence for screening and selective treatment exists for: asymptomatic bacteriuria; bacterial vaginosis in low-risk population groups; elective cervical cerclage in high-risk pregnancies; indicated cervical cerclage in women with short cervical length on ultrasound; prophylactic progesterone supplementation in high-risk pregnancies, and smoking cessation. However, for most other strategies, such as increased antenatal attendance, or routine administration of prophylactic micronutrients, the evidence is inconsistent and conflicting. Information on neonatal outcomes apart from PTD (such as serious neonatal morbidity and mortality) was found to be lacking in most studies. It was therefore not possible to establish whether preventing PTD or prolonging gestation would correlate to improved perinatal outcome, and this lessened the potential clinical usefulness of any proposed preventative strategy. No studies were found that evaluated the effectiveness of combining screening-preventative strategies. The review concludes with a suggested an antenatal management plan designed to prevent PTD based on current practice and the evidence presented in this article.

Terbutaline increases the cervical resistance of the pregnant rat in vitro

Cervical ripening is a crucial process leading to delivery. Early dilation of the pregnant cervix can contribute to premature labour. The maturity of the cervix can be characterized by its resistance to mechanical stretching. Although a number of compounds are considered to increase cervical resistance (e.g., progesterone, nitric oxide synthase inhibitors and nonsteroidal anti-inflammatory drugs), none of them seem to be safe for clinical application. Other compounds, such as beta(2)-adrenergic receptor (beta(2)-AR) agonists, have been used for several decades to stop premature myometrium contractions, but their cervical action has never been investigated. The aim of this study was to detect the effects of the beta(2)-AR agonist terbutaline on nonpregnant and late-pregnant (day 18, 20, 21 or 22) cervices isolated from Sprague-Dawley rats. Cervical resistance was measured by means of a mechanical stretching test in vitro, the beta(2)-AR density was determined by Western blot analysis, the beta(2)-AR mRNA was determined by RT-PCR, while the G-protein activation following cervical beta(2)-AR stimulation with terbutaline was evaluated via a [(35)S]GTPgammaS binding assay. Terbutaline at 10(-6) M increased the cervical resistance of the late-pregnant samples in vitro from day 18 to day 22, but did not alter the resistance of the nonpregnant samples. This cervical resistance-increasing effect was concentration dependent and antagonized with propranolol on day 21. Terbutaline was ineffective on cervical samples when gradual stretching was omitted. RT-PCR and Western blot studies revealed increased beta(2)-AR mRNA and beta(2)-AR levels respectively on day 18 of pregnancy compared with the nonpregnant cervix, but no further changes were detected up to the end of pregnancy. The [(35)S]GTPgammaS binding assay demonstrated a decreased G-protein activation on the days of pregnancy investigated, but no activation was found in the nonpregnant samples. The degree of decrease in G-protein activation by terbutaline was in harmony with its cervical resistance-increasing action. On day 21, the G-protein activation-decreasing effect of terbutaline was antagonized with propranolol. We presume that the cervical resistance-increasing effect of terbutaline is a consequence of its G-protein activation-decreasing property via beta(2)-ARs, which finally leads to an increased muscle resistance against mechanical stretching. This action of terbutaline seems unique among the smooth muscles, and may open up a new perspective in the prevention of premature labour. Clinical experience indicates that beta(2)-AR agonists will not be sufficient to stop the overall process, but their combination with more potent inhibitors of uterine contractions may be of clinical benefit.