Pharmacologic management of preterm labor

Pharmacology for Preterm Labor

Preterm birth occurs with 10% of deliveries and yet accounts for more than 85% of perinatal morbidity and mortality. Management of preterm labor prior to delivery includes a multipronged pharmacologic approach targeting utilization of reproductive hormones for continuation of pregnancy, advancement of fetal lung maturity, and the decrease of uterine contractility (tocolysis). This article will review and compare guidelines on pharmacologic management of preterm labor as recommended by the American College of Obstetricians and Gynecologists and the European Association of Perinatal Medicine. The classifications of drugs discussed include exogenous progesterone, corticosteroids, and tocolytics (β-adrenergic agonists, magnesium sulfate, calcium channel blockers, prostaglandin inhibitors, nitrates, and oxytocin receptor blockers). For each of these drug classes, the following information will be presented: mechanism of action, maternal/fetal side effects, and nursing implications.

Molecular signaling through G-protein-coupled receptors and the control of intracellular calcium in myometrium

Cellular mechanisms regulating myometrial intracellular free calcium (Ca2+(i)) are addressed in this review, with emphasis on G-protein-coupled receptor pathways. An increase in myometrial Ca2+(i) results in phosphorylation of myosin light chain, an increase in myosin adenosine monophosphatase (ATPase) activity and contraction. Dephosphorylation of myosin light chain and a decline in Ca2+(i) are associated with relaxation. Increases in Ca2+(i) are controlled by multiple signaling pathways, including receptor-mediated activation of phospholipase Cbeta (PLCbeta), leading to release of Ca2+ from intracellular stores. Ca2+ also enters myometrial cells through plasma membrane Ca2+ channels. Conversely, adenosine triphosphate (ATP)-dependent Ca2+ pumps lower Ca2+(i) concentrations and potassium channels promote hyperpolarization that can decrease Ca2+ entry. Receptor-coupled pathways that promote uterine relaxation primarily involve activation of cyclic adenosine monophosphate (cAMP)- or cyclic guanosine monophosphate (cGMP)-stimulated protein kinases that phosphorylate proteins regulating Ca2+ homeostasis. cAMP has inhibitory effects on myometrial contractile activity, agonist-stimulated phosphatidylinositide turnover and increases in Ca2+(i). Some of these effects require association of protein kinase A (PKA) with a plasma membrane-associated A-kinase-anchoring-protein (AKAP). Near term in the rat, there is a decline in the plasma membrane localization of PKA associated with this anchoring protein. This correlates with changes in the regulation of signaling pathways controlling Ca2+(i). L-type voltage-operated Ca2+ entry is an important regulator of myometrial contraction. In addition, putative signal-regulated or capacitative Ca2+ channel proteins, TrpCs, are expressed in myometrium, and signal-regulated Ca2+ entry is observed in human myometrial cells. This Ca2+ entry mechanism may play a significant role in the control of myometrial Ca2+(i) dynamics and myometrial contraction. The regulation of myometrial Ca2+(i) is complex. Understanding the mechanisms involved may lead to design of tocolytics that target multiple pathways and achieve improved suppression of premature labor.

Nifedipine therapy for preterm labor: effects on placental, fetal cerebral and atrioventricular Doppler parameters in the first 48 hours

OBJECTIVE

To assess the effect of nifedipine tocolysis on Doppler parameters of the uterine, umbilical and fetal middle cerebral arteries and atrioventricular valves in the first 48 h of therapy.

METHODS

Doppler waveforms of uterine, umbilical and middle cerebral arteries and both atrioventricular valves were measured from 28 pregnant women and fetuses prior to and during nifedipine therapy for preterm labor. Maternal and fetal heart rates (FHR), maternal systolic and diastolic blood pressure, and the Doppler pulsatility index (PI) of the uterine, umbilical and middle cerebral arteries were measured. The cerebroplacental ratio (middle cerebral artery PI/umbilical artery PI) was calculated. The total time velocity integrals (TVIs) of tricuspid and mitral valves and their E- and A-wave peak velocity ratio (E/A) were measured. Friedman repeated-measures analysis of variance was used to compare the variables before and after nifedipine therapy. If significant differences were found, Wilcoxon's signed ranks test was used to analyze the difference between the two variables. A P-value of < 0.05 was considered significant.

RESULTS

Nifedipine maintenance was associated with a significant decline in maternal systolic and diastolic blood pressure after 24 h, while maternal heart rate and FHR were unaffected. The uterine artery PI had decreased significantly at 24 and 48 h, while the umbilical artery PI did not change significantly. The middle cerebral artery PI had decreased significantly at 24 and again at 48 h. A significant fall in the cerebroplacental Doppler ratio was maintained beyond 24 h. The mean E/A values, TVIs and TVI x FHR values at 24 and 48 h were unchanged from the baseline values.

CONCLUSIONS

Nifedipine maintenance tocolysis is associated with a significant decline in uterine artery and middle cerebral artery Doppler indices 24 h after the first dose. Fetal cardiac diastolic function is unaffected and the significant redistribution observed after 24 h is likely to be attributable to altered cerebral blood flow.

The short-term effect of nifedipine tocolysis on placental, fetal cerebral and atrioventricular Doppler waveforms

OBJECTIVE

To evaluate the effect of nifedipine on placental and fetal middle cerebral and atrioventricular Doppler waveforms.

METHODS

Doppler waveforms of uterine (UtA), umbilical (UA) and middle cerebral (MCA) arteries and both atrioventricular valves were measured from 21 pregnant women/fetuses prior to and during nifedipine therapy for preterm labor. Maternal and fetal heart rates (FHR), maternal systolic and diastolic blood pressure, the Doppler pulsatility index and systolic/diastolic ratio of the UtA, UA and MCA were measured. The total time velocity integrals (TVI) of tricuspid and mitral valves and their E-wave/A-wave (E/A) TVI ratios were measured. Wilcoxon signed pairs test was used to compare the differences in Doppler parameters before and at 3 h after nifedipine loading up to a maximum dose of 40 mg.

RESULTS

Fetal arterial and UtA Doppler parameters were not different before and after nifedipine therapy. Blood flow across the atrioventricular valves and the TVI were equally unaffected by nifedipine. The TVI x FHR product was also unchanged following nifedipine therapy.

CONCLUSIONS

In women with otherwise uncomplicated pregnancies, nifedipine loading and tocolysis are generally well tolerated by the mother. Placental and fetal cerebral arterial blood flow, fetal systolic and diastolic cardiac function and downstream distribution of fetal cardiac output are unaffected by nifedipine loading. These results apply to women with unchanged vital parameters. Further studies are necessary to show long-term effects of nifedipine therapy and may help to refine choice of tocolytic agents.

Regulation of matrix metalloproteinases (type IV collagenases) and their inhibitors in the virgin, timed pregnant, and postpartum rat uterus and cervix by prostaglandin E(2)-cyclic adenosine monophosphate

OBJECTIVE

Our purpose was to examine whether the type IV collagenases (metalloproteinase [MMP]-2 and MMP-9) and their inhibitors (TIMP-1 and TIMP-2) are regulated by a prostaglandin E(2) (PGE(2))-cyclic adenosine monophosphate (cAMP) mechanism in nonpregnant virgin, preterm and term pregnant and postpartum rats.

STUDY DESIGN

Sprague-Dawley rats were infused with either saline solution or PGE(2) over 24 hours or were noninfused. Plasma and tissue were analyzed for cAMP, MMP-2 and MMP-9, and TIMP-1 and TIMP-2.

RESULTS

PGE(2) evoked elevations in plasma and tissue levels of cAMP and MMP-2 in the preterm and term pregnant rats. MMP-9 levels were elevated in the preterm plasma and uterus, whereas in the term pregnant and postpartum rats, MMP-9 levels were increased in the cervix.

CONCLUSIONS

MMP levels in the pregnant and postpartum rat uterus and cervix are regulated in part by a PGE(2)-cAMP mechanism. Specifically, MMP-9 may be involved in preterm labor, cervical maturity, and involution of the postpartum uterus.

Oral nicardipine versus intravenous magnesium sulfate for the treatment of preterm labor

OBJECTIVE

The aim of this study was to compare the efficacy and safety of oral nicardipine in acute therapy for preterm labor with those of parenteral magnesium sulfate.

STUDY DESIGN

Patients between 24 and 34 weeks' gestation with documented preterm labor were randomly assigned to receive oral nicardipine (n = 57) or intravenous magnesium sulfate (n = 65) as initial tocolytic therapy. Patients in the nicardipine group received a 40-mg loading dose and then 20 mg every 2 hours as needed to stop contractions (total 80 mg). Patients in the magnesium sulfate group received a 6-g bolus followed by 2 to 4 g/h to provide uterine quiescence. Patients could be switched to another tocolytic regimen if they continued to have contractions after 6 hours of therapy. The main outcome variables examined were time to uterine quiescence, time gained in utero, recurrence of preterm labor, failure of tocolysis, and pertinent maternal and neonatal outcomes.

RESULTS

There were no significant differences in maternal demographic characteristics between the groups. Among patients who responded with uterine quiescence within 6 hours, there was a significant decrease in the time to uterine quiescence in the nicardipine group (P <.01). Patients in the magnesium sulfate group were more likely to have recurrence of preterm labor necessitating further tocolytic attempts (P =.048). The patients in the magnesium sulfate group had more adverse side effects, mainly nausea and vomiting (P =.004). There were no differences in birth weight, estimated gestational age at delivery, or neonatal complications between the 2 groups.

CONCLUSIONS

Oral nicardipine is an effective, safe, and well-tolerated tocolytic agent. In this prospective clinical trial patients randomly assigned to receive oral nicardipine had arrest of preterm labor more rapidly than did those randomly assigned to receive parenteral magnesium sulfate. Patients who received magnesium sulfate were more likely to have adverse medication effects and recurrent preterm labor.

Oxytocin-stimulated capacitative calcium entry in human myometrial cells

OBJECTIVE

Our purpose was to investigate the relative contribution of extracellular calcium recruitment and release of calcium from intracellular stores in an immortalized myometrial cell line derived from a pregnant woman (PHM1-41) and to determine the importance of capacitative calcium entry in the oxytocin-stimulated rise in intracellular free calcium.

STUDY DESIGN

The PHM1-41 immortalized myometrial cell line, which retains smooth muscle phenotype, estrogen, and oxytocin receptors and responds to oxytocin with an increase in intracellular free calcium, was used for this study. Intracellular free calcium was measured directly in cells loaded with Fura 2-AM.

RESULTS

The oxytocin-stimulated rise in intracellular free calcium decreased in the absence of extracellular calcium or in the presence of phospholipase C inhibitors, suggesting mobilization of calcium from both extracellular and intracellular sources to increase intracellular free calcium. Phospholipase C inhibitors resulted in greater inhibition of the oxytocin-stimulated rise in intracellular free calcium than expected on the basis of experiments performed in the absence of extracellular calcium. This implies interdependence of the intracellular and extracellular pathways for elevation of intracellular free calcium and suggests some capacitative entry of calcium as a consequence of depletion of intracellular stores. The oxytocin-stimulated intracellular free calcium increase resulting from calcium entry was blocked by store depletion by thapsigargin or cyclopiazonic acid, consistent with a capacitative calcium entry mechanism.

CONCLUSION

Oxytocin stimulates both capacitative and noncapacitative calcium entry in a pregnant human myometrium cell line.

The effect of relaxation therapy on preterm labor outcomes

OBJECTIVE

To examine the effect of relaxation on preterm labor outcome.

DESIGN

Quasi-experimental, with women who experienced preterm labor randomly assigned to a control or experimental group. The experimental group was to do a daily relaxation exercise. A third group was added to the study: women who were originally assigned to the relaxation group but were unable to adhere to the daily practice. Final data were analyzed for three groups: control (n = 40), experimental (n = 44), and nonadherent (n = 23) participants.

SETTING

Women were referred to the study from physician offices and a hospital-based obstetric triage clinic in the Northwest.

PARTICIPANTS

Total sample was comprised of 107 women with singleton gestations, documented contractions with cervical change, and intact membranes.

INTERVENTIONS

The experimental group was instructed in a progressive relaxation exercise. The participants were given tapes of the exercise and instructed to do it daily.

OUTCOME MEASURES

Study outcomes included gestational age at birth, rate of pregnancy prolongation, and birth weight.

RESULTS

The outcome variables were analyzed using analysis of covariance, with the preterm labor risk score entered as a covariate to compensate statistically for group differences. A positive response to the relaxation intervention was found: The experimental group had significantly longer gestations and larger newborns when compared to the control and nonadherent groups.

CONCLUSIONS

Relaxation therapy made a difference in preterm labor outcome. Women who practiced relaxation had larger newborns, longer gestations, and higher rates of pregnancy prolongation. Given the low cost of the intervention, it should be offered to all women at risk for preterm labor.

Regulation of uterine smooth muscle function during gestation

The uterus is unique among smooth muscular organs in that, during pregnancy, it undergoes profound, largely reversible, changes orchestrated by the ovarian hormones. These changes facilitate uterine adaptation to the stretch induced by the growing fetus such that a state of myometrial contractile quiescence can be maintained. This quiescent state usually is maintained until fetal development is sufficient for extrauterine life, at which point unknown mechanisms precipitate conversion to a highly contractile state. Throughout pregnancy, signaling mechanisms for myometrial contractility are altered--first to promote quiescence and then again to promote contractions. The mechanisms responsible for these changes are only partially understood. This review attempts to summarize salient features of many of the changes in uterine contractile signaling and the current state of ongoing investigations of their mechanisms. We have also highlighted some newer information and concepts from nonuterine tissues, which we believe may provide insight into the control of uterine smooth muscle function. Some detail has been omitted, and can be found in the many excellent reviews cited. We hope that this discussion may stimulate the interests of other investigators. The diverse areas of inquiry offer hope that this decade will lead to a fuller understanding of myometrial function and the development of vastly improved approaches for the control of preterm labor.

Adjustment of magnesium sulfate infusion rate in patients with preterm labor

OBJECTIVE

Our purpose was to investigate factors that might influence serum magnesium levels during intravenous magnesium sulfate tocolytic therapy.

STUDY DESIGN

Thirty-three women receiving magnesium sulfate for preterm labor participated in this prospective, observational study. Gestational ages were 24 to 34 weeks. Four groups of women were identified according to the maintenance magnesium infusion rate required for arresting preterm labor after 5 g of therapy induction: 1.5, 2, 2.5, and 3 g/h. Serum magnesium samples were drawn after a predefined period of at least 18 hours of arrested preterm labor, at a minimum of every 6 hours. Variables examined included serum albumin; serum protein; serum ionized calcium; serum creatinine; creatinine clearance; 24-hour urine output; maternal height, weight, body surface area; and body mass index.

RESULTS

By use of a multivariate stepwise regression model we identified four variables that independently and significantly contributed to the model: magnesium infusion rate (P < .001); total serum protein level (P < .001); serum creatinine level (P = .009); and maternal weight squared (P = .026). Seventy-two percent of the variance was accounted for by use of these parameters. A predictive linear model, developed to relate these factors, produced the following formula: Suggested magnesium infusion rate = 0.89 x Serum magnesium concentration (mg/dL) - 3.16 x Serum creatinine (mg/dL) - 0.66 x Serum total proteins (g/dL) + 0.0001 x (maternal weight)2 (kg) + 2.30.

CONCLUSIONS

Serum creatinine, serum protein, and maternal weight can be used to adjust the dose of magnesium sulfate in patients with premature labor to achieve therapeutic serum levels of magnesium more rapidly and safely.

A prospective, randomized trial of nifedipine vs. ritodrine in threatened preterm labor

OBJECTIVES

To compare the tocolytic efficacy and maternal tolerance of nifedipine with ritodrine in the management of threatened preterm labor.

METHODS

Prospective randomized study of 52 singleton pregnancies with preterm labor between 26 and 34 week's gestation. The capacity to delay delivery 48 h, 7 days, until week 36 or until fetal weight reached 2500 g were the outcome variables assessed. Doppler ultrasound studies were performed on the fetal umbilical artery as control.

RESULTS

No significant differences were found in the delay of delivery, but significantly fewer maternal side-effects were found in the nifedipine group. Doppler ultrasound results were similar in both groups.

CONCLUSIONS

Nifedipine is a valid and well-tolerated alternative among the tocolytic drugs, and apparently does not significantly alter fetal vascular blood flow.

Autonomic cardiovascular control in pregnancy

Pregnancy is associated with profound adaptive changes in the maternal hemodynamics. Although the autonomic nervous system plays a central role in the adaptation of the cardiovascular system to various needs, its role in the adaptation of the circulation to the demands of pregnancy is poorly understood. This paper reviews the literature of autonomic cardiovascular control in pregnancy as studied with the cardiovascular reflex tests. A Medline search and manual cross-referencing for prior publications were used. All papers found on the hemodynamic effects of the Valsalva maneuver, the orthostatic test, the deep breathing test, the isometric handgrip test and maternal heart rate variability in pregnancy were reviewed and all publications that studied short-term changes in maternal heart rate and blood pressure were included. The beginning of pregnancy is associated with sympathetic reactivity, whereas the latter half of pregnancy is characterized by increased hemodynamic stability during orthostatic stress. The heart rate response to the Valsalva maneuver is blunted in mid-pregnancy, possibly due to changes in the baroreflex and increased maternal blood volume. Heart rate variability is significantly reduced in the second trimester. Cardiovascular reflex tests can be used to study drug effects on maternal circulation non-invasively.

Potassium channel activation: a potential therapeutic approach?

The physiological role of K+ channel opening by endogenous substances (e.g., neurotransmitters and hormones) is a recognised inhibitory mechanism. Thus, the identification of novel synthetic molecules that 'directly' open K+ channels has led to a new direction in the pharmacology of ion channels. The existence of many different subtypes of K+ channels has been an impetus in the search for new molecules demonstrating channel and, thus, tissue selectivity. This review focuses on the different classes of openers of K+ channels, the intracellular mechanisms involved in the execution of their effects, and potential therapeutic targets.