Magnesium ion inhibits spontaneous and induced contractions of isolated uterine muscle

Investigation of Intrapartum Parenteral Magnesium Sulfate as an Independent Risk Factor for Postpartum Hemorrhage Using Quantitative Blood Loss Assessment

BACKGROUND

Magnesium sulfate is used for seizure prophylaxis in preeclampsia and for fetal neuroprotection when delivery is anticipated before 32 weeks of gestation. Existing risk assessment tools for postpartum hemorrhage often identify the use of magnesium sulfate as an intrapartum risk factor. Previous studies examining the association between the use of magnesium sulfate and postpartum hemorrhage have relied largely on qualitative estimates of blood loss rather than quantitative estimates of blood loss.

OBJECTIVE

This study aimed to determine whether intrapartum administration of magnesium sulfate is associated with an increased risk of postpartum hemorrhage using a quantitative blood loss assessment via the use of graduated drapes and weight differences in surgical supplies.

STUDY DESIGN

This case-control study was conducted to test the hypothesis that intrapartum parenteral administration of magnesium sulfate is not independently associated with postpartum hemorrhage. All deliveries at our tertiary-level academic medical center between July 2017 and June 2018 were reviewed. Of note, 2 categories of postpartum hemorrhage were defined: the traditional definition (>500 mL for vaginal delivery and >1000 mL for cesarean delivery) and the contemporary definition (>1000 mL regardless of delivery mode). Statistical analyses using the chi-square test, Fisher exact test, t test, or Wilcoxon rank-sum test were performed to compare the patients who did and did not receive magnesium sulfate concerning the rates of postpartum hemorrhage, pre- and postdelivery hemoglobin level, and rates of blood transfusion.

RESULTS

A total of 1318 deliveries were included, with postpartum hemorrhage rates of 12.2% (traditional definition) and 6.2% (contemporary definition). Multivariate logistic regression did not find the use of magnesium sulfate as an independent risk factor by either definition (odds ratio, 1.44 [95% confidence interval, 0.87-2.38] and 1.34 [95% confidence interval, 0.71-2.54]). The only significant independent risk factor was cesarean delivery, by both definitions (odds ratio, 2.71 [95% confidence interval, 1.85-3.98] and 19.34 [95% confidence interval, 8.55-43.72]).

CONCLUSION

In our study population, intrapartum administration of magnesium sulfate was not found to be an independent risk factor for postpartum hemorrhage. Cesarean delivery was determined as an independent risk factor, consistent with previous reports.

Postpartum Hemorrhage-Epidemiology, Risk Factors, and Causes

The incidence of postpartum hemorrhage (PPH) is increasing worldwide and in the United States. Coinciding, is the increased rate of severe maternal morbidity with blood transfusion in the United States over the past 2 decades. Consequences of PPH can be life-threatening and carry significant cost burden to the health care system. This review will discuss the current trends, distribution, and risk factors for PPH. Causes of PPH will be explored in detail.

Calculated blood loss at cesarean delivery in patients with preeclampsia with severe features on magnesium sulfate

INTRODUCTION

Magnesium sulfate decreases the risk of eclampsia in patients with severe preeclampsia. There is a theoretical risk that magnesium sulfate increases the risk of intrapartum hemorrhage. We evaluated whether there was a quantitative difference in blood loss in patients receiving magnesium sulfate at the time of cesarean delivery (CD) compared to those who were not.

MATERIALS AND METHODS

A retrospective cohort study was performed using a database of patients with preeclampsia with severe features or eclampsia. The primary outcome was calculated estimated blood loss (cEBL) compared between patients who received magnesium sulfate during CD and those who did not. cEBL was derived through a validated equation by multiplying the patient's blood volume by percent of blood volume loss. Secondary outcomes were hematocrit change, visual EBL (vEBL), hemorrhage, cEBL >1500mL, Apgar <7 at 5 min, and NICU admission. Exclusion criteria were incomplete records or negative cEBL, as well as blood transfusion prior to collection of a postpartum hematocrit.

RESULTS

We identified 124 patients with preeclampsia with severe features or eclampsia at time of CD. There were 57 (46%) that received magnesium sulfate during CD and 67 (54%) in which magnesium was stopped during the CD. The mean differences for hematocrit value (0.8, 95% confidence interval (CI) -0.3 to 1.8) and cEBL (108, 95% CI -102 to 318) were not significantly different after adjusting for obesity and history of CD.

CONCLUSION

Magnesium sulfate administration for seizure prevention in patients with preeclampsia with severe features at the time of CD does not appear to be associated with an increase in the cEBL.

Landscape of Preterm Birth Therapeutics and a Path Forward

Preterm birth (PTB) remains the leading cause of infant morbidity and mortality. Despite 50 years of research, therapeutic options are limited and many lack clear efficacy. Tocolytic agents are drugs that briefly delay PTB, typically to allow antenatal corticosteroid administration for accelerating fetal lung maturity or to transfer patients to high-level care facilities. Globally, there is an unmet need for better tocolytic agents, particularly in low- and middle-income countries. Although most tocolytics, such as betamimetics and indomethacin, suppress downstream mediators of the parturition pathway, newer therapeutics are being designed to selectively target inflammatory checkpoints with the goal of providing broader and more effective tocolysis. However, the relatively small market for new PTB therapeutics and formidable regulatory hurdles have led to minimal pharmaceutical interest and a stagnant drug pipeline. In this review, we present the current landscape of PTB therapeutics, assessing the history of drug development, mechanisms of action, adverse effects, and the updated literature on drug efficacy. We also review the regulatory hurdles and other obstacles impairing novel tocolytic development. Ultimately, we present possible steps to expedite drug development and meet the growing need for effective preterm birth therapeutics.

Interrupted versus continuous magnesium sulfate and blood loss at cesarean delivery

OBJECTIVE

Magnesium sulfate is standard of care for prevention of eclampsia in women with preeclampsia with severe features. The American College of Obstetrics and Gynecology endorses its use throughout labor, delivery and the immediate postpartum period. Some providers pause magnesium sulfate infusion preoperatively due to concern for increased risk of uterine atony and postpartum hemorrhage. Using a non-inferiority analysis, we investigated the effect of interrupted versus continuous infusion of magnesium sulfate on postpartum hemorrhage in women with preeclampsia with severe features undergoing cesarean delivery.

STUDY DESIGN

Retrospective non-inferiority cohort study of women with preeclampsia with severe features treated with magnesium sulfate undergoing cesarean delivery with singleton pregnancies at tertiary care hospital from 2013 to 2018. The primary outcome was postpartum hemorrhage. Secondary outcomes included estimated blood loss, change in hematocrit and a composite of postpartum hemorrhage severity, including transfusion of blood products, use of more than one uterotonic and additional surgical interventions.

RESULTS

Of 249 women, magnesium sulfate infusion was interrupted in 171 (69%) and continued in 78 (31%). Women with interrupted magnesium sulfate infusion were more likely to be Caucasian (73% vs 67%, p = .024), have chronic hypertension (23% vs 1%, p < .001), labor prior to cesarean delivery (84% vs 55%, p < .001), undergo primary cesarean delivery (86% vs 67%, p = .005), and experience shorter surgical time (50 vs 55 min, p = .026). The rate of postpartum hemorrhage for those receiving interrupted magnesium sulfate infusion (9.9%) and continuous magnesium sulfate infusion (10.2%) was similar, falling within the non-inferiority margin (absolute difference 0.3%, 95% CI -7.8 to 8.4%, p = .88). There were no significant differences in the secondary outcomes.

CONCLUSION

Interrupted magnesium sulfate infusion is non-inferior to continued magnesium sulfate infusion for rates of postpartum hemorrhage in women with preeclampsia with severe features undergoing cesarean delivery.

Magnesium sulfate and risk of postpartum uterine atony and hemorrhage: A meta-analysis

OBJECTIVES

Magnesium sulfate (MgSO₄) is among the most commonly used medications in labor and delivery units. It has been used as a mean to protect against eclampsia and a neuroprotective agent for fetuses at risk of preterm birth. In the present study we investigated its impact in the occurrence of postpartum uterine atony and hemorrhage.

METHODS

We searched the Medline, Scopus, EMBASE, Cochrane Central Register of Controlled Trials CENTRAL, Clinicaltrials.gov and Google Scholar databases for randomized trials and observational studies. Statistical analysis was performed with the Hartung-Knapp-Sidik-Jonkman model in RStudio using the meta package.

RESULTS

Twelve studies fitted the predetermined criteria and these involved 41,190 women of whom 10,565 (25.6 %) received MgSO_4. The meta-analysis revealed that the risk of postpartum uterine atony was similar among patients that received MgSO₄ and those that did not (OR 1.93, 95 % CI 0.78, 4.81). Estimated blood loss (SMD 0.04, 95 % CI -0.10, 0.18) as well as the risk of postpartum hemorrhage (OR 1.82, 95 % CI 0.99, 3.35) also did not differ. Subgroup analysis revealed that evidence drawn from observational studies indicates a significant effect of MgSO₄ on the odds of postpartum uterine atony and hemorrhage; however, randomized trials do not support this.

CONCLUSIONS

The results of our meta-analysis suggest that it is reasonable to consider MgSO4 in women at risk of delivering before the completion of its elimination half-life. However, physicians should be vigilant in cases at risk of postpartum hemorrhage as current data are very heterogeneous and should not be considered as definitive.

Gestational and Hormonal Effects on Magnesium Sulfate's Ability to Inhibit Mouse Uterine Contractility

Magnesium sulfate is used as a tocolytic, but clinical efficacy has been seriously questioned. Our objective was to use controlled ex vivo conditions and known pregnancy stages, to investigate how 2 key factors, hormones and gestation, affect magnesium's tocolytic ability. We hypothesized that these factors could underlie the varying clinical findings around magnesium's efficacy. Myometrial strips were obtained from nonpregnant (n = 10), mid-pregnant (n = 12), and term-pregnant (n = 11) mouse uterus. The strips were mounted in organ baths superfused with oxygenated physiological saline at pH 7.4 and 37 °C. The effect of different concentrations of MgSO₄ (2-20 mM) was examined on spontaneous and oxytocin-induced (0.5-1 nM) contractions. Contractile properties (amplitude, frequency, and area under the curve) were measured before and after application of magnesium. Magnesium sulfate had a dose-dependent inhibitory effect on both spontaneous and oxytocin-induced contractions but was less effective in the presence of oxytocin. In spontaneous contractions, magnesium was more potent as gestation progressed (P < .0001). In the presence of oxytocin, however, there were no significant gestational differences in its effects on contraction. The rapid onset and reversal of magnesium's effects suggest an extracellular action on calcium entry. Taken together, we conclude that magnesium's actions are influenced by both gestational state and hormones, such that, at least in mice, it is least effective in early gestation with oxytocin present and most effective at term in the absence of oxytocin. That magnesium is least effective preterm and oxytocin decreases its effectiveness throughout gestation, may explain its disappointing clinical effects as a tocolytic.

Study of Magnesium Formulations on Intestinal Cells to Influence Myometrium Cell Relaxation

Background: Magnesium is involved in a wide variety of physiological processes including direct relaxation of smooth muscle. A magnesium imbalance can be considered the primary cause or consequence of many pathophysiological conditions. The smooth muscle tissue of the uterus, i.e., the myometrium, undergoes numerous physiological changes during life, fundamental for uterine activities, and it receives proven benefits from magnesium supplementation. However, magnesium supplements have poor absorption and bioavailability. Furthermore, no data are available on the direct interaction between intestinal absorption of magnesium and relaxation of the myometrium. Methods: Permeability in human intestinal cells (Caco-2 cells) and direct effects on myometrial cells (PHM1-41 cells) of two different forms of magnesium, i.e., sucrosomial and bisglycinate, were studied in order to verify the magnesium capacity of modulate contractility. Cell viability, reactive oxygen species (ROS) and nitric oxide (NO) production, magnesium concentration, contractility, and pathways involved were analyzed. Results: Data showed a better influence of buffered chelate bisglycinate on intestinal permeability and myometrial relaxation over time with a maximum effect at 3 h and greater availability compared to the sucrosomial form. Conclusions: Magnesium-buffered bisglycinate chelate showed better intestinal absorption and myometrial contraction, indicating a better chance of effectiveness in human applications.

Gestational and Hormonal Effects on Magnesium Sulfate's Ability to Inhibit Mouse Uterine Contractility

Magnesium sulfate is used as a tocolytic, but clinical efficacy has been seriously questioned. Our objective was to use controlled ex vivo conditions and known pregnancy stages, to investigate how 2 key factors, hormones and gestation, affect magnesium's tocolytic ability. We hypothesized that these factors could underlie the varying clinical findings around magnesium's efficacy. Myometrial strips were obtained from nonpregnant (n = 10), mid-pregnant (n = 12), and term-pregnant (n = 11) mouse uterus. The strips were mounted in organ baths superfused with oxygenated physiological saline at pH 7.4 and 37°C. The effect of different concentrations of MgSO4 (2-20 mM) was examined on spontaneous and oxytocin-induced (0.5-1 nM) contractions. Contractile properties (amplitude, frequency, and area under the curve) were measured before and after application of magnesium. Magnesium sulfate had a dose-dependent inhibitory effect on both spontaneous and oxytocin-induced contractions but was less effective in the presence of oxytocin. In spontaneous contractions, magnesium was more potent as gestation progressed ( P < .0001). In the presence of oxytocin, however, there were no significant gestational differences in its effects on contraction. The rapid onset and reversal of magnesium's effects suggest an extracellular action on calcium entry. Taken together, we conclude that magnesium's actions are influenced by both gestational state and hormones, such that, at least in mice, it is least effective in early gestation with oxytocin present and most effective at term in the absence of oxytocin. That magnesium is least effective preterm and oxytocin decreases its effectiveness throughout gestation, may explain its disappointing clinical effects as a tocolytic.

Maternal Magnesium Sulfate Supplementation in a Pre-Farrow Diet Improves Factors Important for Piglet Viability

Piglet hypoxia during farrowing is common and can lead to increased stillbirth rates, reduced piglet vitality, and an increased risk of preweaning mortality. Magnesium sulfate (MgSO₄) has successfully been used as a neuroprotectant and readily crosses the placenta in humans. Based on this human data, the aim of this study was to determine if maternal dietary supplementation with MgSO₄ prior to farrowing would reduce the impact of piglet hypoxia during the peri-natal period. Five days prior to farrowing, Large White × Landrace sows were fed either standard lactation sow diet (Cont; n = 30) or lactation diet supplemented to deliver 21 g/day MgSO₄ (Mg; n = 31). There was no effect of treatment on the percentage of stillborn piglets (p > 0.05). However, Mg piglets tended to have higher vitality scores immediately after birth (p < 0.10), were quicker to suck, and had higher day one blood glucose concentrations when compared with Cont piglets (p < 0.05). Furthermore, hypoxic piglets born to Cont sows did not gain weight from birth to 24 h, but Mg piglets did (p < 0.05), suggesting improved colostrum ingestion. In conclusion, MgSO₄ may reduce the negative impacts of birth hypoxia, improving piglet vitality, and colostrum intake during the peri-natal period.

The combination tocolytic effect of magnesium sulfate and an oxytocin receptor antagonist in myometrium from singleton and twin pregnancies

BACKGROUND

Preterm birth at <37 weeks of gestation is the most common and costly complication of pregnancy and remains the leading cause of neonatal morbidity, death, and reduced achievement in surviving infants. Magnesium sulfate is 1 class of tocolytics for threatened preterm labor; however, its clinical efficacy has been questioned. Twin pregnancies are at increased risk of preterm delivery compared with singleton gestations, which suggests that there is twin-specific risk to preterm delivery in twins. The prevention strategies that are applied to singleton pregnancies, however, have not been shown to be effective in twin pregnancies.

OBJECTIVE

The purpose of this study was to compare the relaxant effect of magnesium sulfate on spontaneous and oxytocin-augmented contractions of human myometrium from singleton and twin pregnancies and to examine whether the effect of oxytocin on magnesium sulfate's potency could be reversed with the use of the oxytocin receptor antagonist, atosiban.

STUDY DESIGN

Myometrium was obtained at the time of prelabor cesarean section (36-40 weeks of gestation) from women with singleton (n=23) or twin (n=12) pregnancy. Isometric tension recordings were made on myometrial strips that were mounted in organ baths that were superfused with physiologic saline solution. Strips were exposed to rising concentrations of magnesium sulfate, and the effect on spontaneous contractions or stimulated with oxytocin (0.5 nmol/L) and in the presence or absence of atosiban (100 nmol/L) was recorded. The contractile characteristics after each application of magnesium sulfate, which included amplitude of contraction and activity integral, were measured. Concentration-response curves were fitted with the use of nonlinear regression and comparison of the negative logarithm of the 50% reduction in activity values.

RESULTS

Magnesium sulfate exerted an equal concentration-dependent inhibitory effect on spontaneous myometrial contractions from both singleton and twin myometrium (P>.05). The application of oxytocin produced a significant rightward shift in the concentration-response curves (P<.0001), but no differences were found between pregnancy groups (P>.05). The addition of atosiban shifted concentration-response curves significantly back to the left for amplitude of contraction and activity integral in singletons (P<.0001). However, only activity integral was significantly reversed in twins (P<.01).

CONCLUSION

Magnesium sulfate is equipotent in suppressing contractions in singleton and twin myometrium. Oxytocin (0.5 nmol/L) significantly reduces the tocolytic potency of magnesium sulfate, which may explain, in part, magnesium sulfate's poor efficacy in vivo; however, this can be reversed partially by the use of an oxytocin receptor antagonist. Combination tocolysis that involves oxytocin receptor antagonists requires further investigation.

Blood loss at cesarean delivery in women on magnesium sulfate for preeclampsia

OBJECTIVE

To evaluate the effect of magnesium sulfate for prevention of eclampsia on blood loss at time of cesarean delivery (CD).

METHODS

We conducted an electronic based search using the following databases: MEDLINE, PUBMED and the Cochrane Library. The search terms were "magnesium sulfate", "preeclampsia" and "randomized". Inclusion criteria were randomized controlled trials of women with preeclampsia who delivered with or without magnesium sulfate therapy for seizure prophylaxis. Only trials with placebo or no treatment comparison groups were included. Primary outcome was postpartum hemorrhage. Secondary outcomes were estimated blood loss, change in hemoglobin, blood transfusion and eclampsia.

RESULTS

Five trials met inclusion criteria. The incidence of postpartum hemorrhage was similar between the two groups [magnesium sulfate: 754/4482 (17%); no magnesium sulfate: 775/4427 (18%); RR 0.97, 95% CI 0.88-1.06]. There was no statistical difference between any of the other blood loss outcomes reported in the included studies. The rate of eclampsia with magnesium sulfate was significantly lower than with placebo (42/5604, 0.7%, versus 107/5600, 1.9%; RR 0.40, 95% CI 0.28-0.57).

CONCLUSIONS

Magnesium sulfate does not appear to affect blood loss intrapartum and postpartum in women with preeclampsia. Magnesium sulfate, therefore, should be continued during CD, given the benefit of seizure prophylaxis without any increased risk of hemorrhage.

Does sildenafil citrate affect myometrial contractile response to nifedipine in vitro?

OBJECTIVE

We sought to test the hypothesis that sildenafil citrate (SC) at low concentrations potentiates the tocolytic effects of nifedipine in vitro.

STUDY DESIGN

Myometrial biopsies were obtained from 22 term nonlaboring women undergoing scheduled cesarean delivery. Tissue strips were suspended in organ chambers for isometric tension recording, and incubated for 30 minutes with either SC at 231 ng/mL or solvent. The effects of cumulative doses (10(-10) to 10(-5) mol/L) of nifedipine on spontaneous and oxytocin-induced uterine contractility were then determined. Areas under the contraction curve were compared using 1-way analysis of variance with Tukey post hoc test (significance: P < .05).

RESULTS

Nifedipine significantly inhibited spontaneous and oxytocin-induced myometrial contractility. Preincubation with SC increased response to nifedipine and significantly potentiated its inhibitory effect at 10(-8) mol/L, without affecting oxytocin-induced contractile response.

CONCLUSION

At concentrations within a therapeutic window, SC increases myometrial sensitivity to nifedipine.

Effect of maternal body mass index on in vitro response to tocolytics in term myometrium

OBJECTIVE

We sought to investigate the effect of body mass index (BMI) on in vitro response to tocolytics.

STUDY DESIGN

Myometrial biopsies were obtained at the time of scheduled cesarean deliveries from term nonlaboring women with BMI < or =29.9 (26.3 +/- 1.3; n = 7), 30-34.9 (31.8 +/- 1.2; n = 16), and > or = 35 (39.5 +/- 4.9; n = 9). Tissue strips were suspended in organ chambers for isometric tension recording. The effects of cumulative doses (10(-10) to 10(-5) mol/L) of nifedipine or indomethacin on spontaneous uterine contractility were determined. Areas under the contraction curve were compared using 1-way analysis of variance with Tukey post hoc test.

RESULTS

Myometrial response to tocolytics did not differ between the BMI groups. Nifedipine, but not indomethacin, significantly inhibited myometrial contractility independent of BMI.

CONCLUSION

BMI does not affect uterine response to tocolytics in isolated uterine tissue from term nonlaboring women.