Tocolytics for preterm labor: a systematic review

Maternal magnesium sulfate treatment is associated with reduced brain-blood flow perfusion in preterm infants

OBJECTIVE

To examine the influence of antenatally administered magnesium sulfate (MgSO4) and ritodrine on cerebral blood flow and systemic hemodynamics in preterm infants.

DESIGN

Prospective, observational study.

SETTING

Neonatal intensive care unit of a university central hospital.

PATIENTS

Fifty-five preterm infants age <33 wks of gestation.

INTERVENTIONS

Serial Doppler examinations of the brain circulation, heart rate, systemic blood pressure, and echocardiographic assessment of ductus arteriosus shunting were performed during the first week of life in infants exposed antenatally to maternal MgSO4 (n = 19) or ritodrine treatment (n = 17), and in 19 nonexposed preterm controls.

MEASUREMENTS AND MAIN RESULTS

Cerebral blood flow velocity measurements were obtained from the anterior cerebral artery and internal carotid artery. Perfusion pressure and indices of resistance and blood flow in both vessels were subsequently derived. Maternal MgSO4 had no effect on neonatal cerebral blood flow velocity or resistance, but was associated with decreased (p <.05) perfusion pressure and blood flow in the anterior cerebral artery and internal carotid artery during the first day of life. Systolic blood pressure and pulse pressure were also lower (p <.05) during the whole study period in the MgSO4-exposed infants when compared with the controls. Maternal ritodrine treatment, on the other hand, had no consistent effects on either neonatal cerebral or systemic hemodynamics.

CONCLUSIONS

Our data indicate that maternal MgSO4 treatment, in contrast to antenatal ritodrine, is associated with lowered cerebral perfusion in preterm infants on the first day of life.

Medical and obstetric problems complicating pregnancy

The present review addresses several medical and obstetric factors/problems, and their interactions during pregnancy. These include the following: maternal age; morbidity and mortality during pregnancy; thromboembolism; gestational diabetes; haemolysis, elevated liver enzymes, low platelets syndrome; human immunodeficiency virus; hepatitis C; preterm labour; antenatal administration of corticosteroids; intrapartum surveillance; breech delivery; and caesarean section.

Beta-3 versus beta-2 adrenergic agonists and preterm labour: in vitro uterine relaxation effects

OBJECTIVE

1. To investigate the effects of the selective beta-3 adrenoreceptor agonist, BRL 37344, on human pregnant myometrial contractility in vitro. 2. to compare these effects with those of the beta-2 adrenoreceptor agonist, ritodrine.

METHODS

Isometric tension recording was performed under physiological conditions in isolated myometrial strips from biopsies obtained at elective caesarean section. Following pre-incubation with oxytocin (10(-9) M), the effects of cumulative additions of BRL 37344 or ritodrine (10(-8)-10(-3.5) M) on myometrial contractility were investigated. Results were expressed as -log EC50 (pD2) and mean maximal inhibition achieved for both drug compounds.

RESULTS

BRL 37344 exerted a concentration dependant relaxant effect on myometrial contractions in all strips exposed [pD2, 7.26 (0.48) (SEM); mean maximal inhibition 61.98 (4.89%); n = 6]. Similarly, ritodrine exerted a concentration dependant inhibition of myometrial contractility in all strips exposed [pD2 = 7.40 (0.28); mean maximal inhibition 59.49 (3.97%); n = 6]. There was no significant difference between calculated pD2 values (P = 0.65) or mean maximal inhibition achieved (P = 0.79).

CONCLUSIONS

The beta-3 adrenoreceptor agonist BRL 37344 induced relaxation of human myometrial contractions with similar potency to that of the most commonly used tocolytic agent ritodrine. This raises the possibility that the novel beta-3 adrenoreceptor agonists may have potential as therapeutic agents for human preterm labour. In view of their reported reduced cardiovascular side effects their potential clinical use requires further evaluation.

Abnormality of calcium channel inhibitor released from fetal membranes in preterm labor

OBJECTIVE

This study was undertaken to test the hypothesis that an inhibitor of uterine contractions acting at the level of the dihydropyridine receptor of the uterine L -type uterine calcium channel is released in greater amounts from fetal membranes before term than at term.

STUDY DESIGN

Endogenous calcium channel inhibitor activity was generated with standardized 25-cm2 surface area fetal membrane samples from the following 4 categories of women: preterm in labor, preterm not in labor, term in labor, and term not in labor. The amount of inhibitor in each membrane category was quantified by means of a competitive binding assay. Inhibition of uterine contractions induced by Bay K 8644 (an L -type calcium channel agonist) was used as another test of endogenous calcium channel inhibitor activity released from fetal membranes of all 4 groups of patients.

RESULTS

Endogenous calcium channel inhibitor activity was most variable but present in the greatest amount in fetal membranes of women who were preterm not in labor followed by those in women at term not in labor and at term in labor. Fetal membranes from women in preterm labor had the least amount of measured endogenous calcium channel inhibitor activity. Consistent with the competitive binding assay, endogenous calcium channel inhibitor activity from fetal membranes from women who were preterm not in labor, at term not in labor, and at term in labor inhibited Bay K 8644-induced uterine contractions. Fetal membranes from women in preterm labor did not inhibit Bay K 8644-induced contractions. Endogenous calcium channel inhibitor activity was present in the chorion, the decidua, and the placenta, with little activity in the amnion.

CONCLUSION

The down-regulation of endogenous calcium channel inhibitor activity with advancing gestation is consistent with a potential role for this inhibitor in maintaining uterine quiescence and in regulating the transition into labor. One possible cause of idiopathic preterm labor may be an abnormally low amount of endogenous calcium channel inhibitor activity in fetal membranes.