Magnesium Sulfate Adversely Affects Fetal Lamb Survival and Blocks Fetal Cerebral Blood Flow Response During Maternal Hemorrhage

Magnesium sulfate has sex-specific, dose-dependent vasodilator effects on preterm placental vessels

Background

Women at risk of preterm delivery receive magnesium sulfate (MgSO₄) in the pre-delivery phase to reduce their child’s risk of neurodevelopmental complications associated with preterm birth. However, the mechanisms underpinning its placental vascular role remain uncertain.

Methods

The aim of this study was to examine MgSO₄ action on vascular tone in male and female human placental vessels from term and preterm deliveries. Vessels were obtained from placental biopsy following birth at term (37–41 weeks) or preterm gestation (<36 weeks of gestation). The vessels were mounted on a pressure myograph, pre-constricted with synthetic endoperoxide prostaglandin PGH₂ (U46619) (0.1–100 μmol/l), and percentage of relaxation was calculated following incubation with bradykinin. Experiments were carried out in the presence of MgSO₄ (0.2 mmol/l), N_Ψ-nitro-L-arginine methyl ester (L-NAME) (0.1 mmol/l), indomethacin (10 μmol/l), Ca²⁺-activated K⁺ channel blocker TRAM-34 (1 μM) and apamin (3 μM) to assess mechanisms of vascular function. Vascular [calcium ions (Ca²⁺)] was analysed using a colorimetric calcium assay.

Results

Vasodilation in vessels from preterm males was significantly blunted in the presence of MgSO₄ when compared to preterm female and term male and female vessels. Overall, MgSO₄ was observed to differentially modulate placental vascular tone and vascular calcium concentrations in a sex-specific manner.

Conclusions

As MgSO₄ regulates human placental blood flow via specific pathways, foetal sex-specific MgSO₄ treatment regimes may be necessary. In an era of increasing awareness of individualised medicine, sex-specific effects may be of importance when developing strategies to optimise care in high-risk patients.

Magnesium sulphate and cardiovascular and cerebrovascular adaptations to asphyxia in preterm fetal sheep

Magnesium sulphate is a standard therapy for eclampsia in pregnancy and is widely recommended for perinatal neuroprotection during threatened preterm labour. MgSO4 is a vasodilator and negative inotrope. Therefore the aim of this study was to investigate the effect of MgSO4 on the cardiovascular and cerebrovascular responses of the preterm fetus to asphyxia. Fetal sheep were instrumented at 98 ± 1 days of gestation (term = 147 days). At 104 days, unanaesthetised fetuses were randomly assigned to receive an intravenous infusion of MgSO4 (n = 6) or saline (n = 9). At 105 days all fetuses underwent umbilical cord occlusion for 25 min. Before occlusion, MgSO4 treatment reduced heart rate and increased femoral blood flow (FBF) and vascular conductance compared to controls. During occlusion, carotid and femoral arterial conductance and blood flows were higher in MgSO4-treated fetuses than controls. After occlusion, fetal heart rate was lower and carotid and femoral arterial conductance and blood flows were higher in MgSO4-treated fetuses than controls. Femoral arterial waveform height and width were increased during MgSO4 infusion, consistent with increased stroke volume. MgSO4 did not alter the fetal neurophysiological or nuchal electromyographic responses to asphyxia. These data demonstrate that a clinically comparable dose of MgSO4 increased FBF and stroke volume without impairing mean arterial pressure (MAP) or carotid blood flow (CaBF) during and immediately after profound asphyxia. Thus, MgSO4 may increase perfusion of peripheral vascular beds during adverse perinatal events.

Effect of magnesium sulfate on Doppler parameters of fetal umbilical and middle cerebral arteries in women with severe preeclampsia

Objective:

To assess the effect of injecting magnesium sulfate on Doppler parameters of fetal umbilical and middle cerebral arteries (MCA) in women with severe preeclampsia.

Materials and Methods:

A total of 21 patients with severe preeclampsia admitted to Imam Reza Hospital, Kermanshah (Iran), were evaluated. Before and after administration of magnesium sulfate, Doppler ultrasound scan was carried out to measure fetal middle cerebral artery and umbilical artery blood flow. Paired t-test was used for statistical analysis.

Results:

After injection of magnesium sulfate, the mean resistivity index (RI)-umbilical, and pulsatility index (PI)-cerebral showed a statistically significant reduction (P < 0.001). The cerebroumbilical C/U ratio increased after the intervention (P < 0.001). The PI-umbilical (P = 0.1) and pre- and post-RI-cerebral (P = 0.96) did not have statistically significant changes.

Conclusions:

Infusion of magnesium sulfate significantly decreases the flow in the fetus RI-umbilical and PI-MCA, and it increases C/U ratio indices in color Doppler ultrasound.

Magnesium sulfate exposure increases fetal blood flow redistribution to the brain during acute non-acidemic hypoxemia in goats

BACKGROUND

It is still controversial that intrapartum exposure to magnesium may or may not reduce brain damage in premature infants in human and animal models.

AIMS

We investigated the effect of hypoxemia alone under magnesium exposure on fetal cardiovascular changes in chronically catheterized goat fetuses.

STUDY DESIGN

We performed a 3-day experimental protocol with control (10% glucose) on day 1, recovery on day 2, and magnesium on day 3. Magnesium sulfate was directly infused to fetuses in a bolus dose of 270 mg/kg followed by 80 mg/kg/h. Hypoxemia was induced by maternal inhalation of nitrogen gas on day 1 and on day 3. Cerebral blood flow was measured by colored microsphere techniques. Repeated measure ANOVA and Bonferroni's/Dunn's test were used for comparison.

SUBJECTS

Six Japanese Saanen goats at 0.85 gestation.

OUTCOME MEASURES

Fetal heart rate, blood pressure, and cerebral blood flow.

RESULTS

Ionized magnesium concentrations were significantly increased. Fetal PO2 decreased significantly from 30 mmHg to 14 mmHg without acidemia. Magnesium exposure significantly attenuated hypoxemia-induced bradycardia but did not affect blood pressure. Hypoxemia significantly increased fetal brain blood flow from the pre-hypoxic levels on day 1. Magnesium exposure further increased hypoxemia-induced brain blood flow on day 3, but statistical significance was limited to the cerebral cortex.

CONCLUSION

In near-term, initially healthy goat fetuses, brain blood flow during acute hypoxemia was significantly increased with magnesium sulfate exposure.

New Data on the Importance of Gestational Mg Deficiency

Chronic primary Mg deficiency is frequent. About 20% of the population consumes less than two-thirds of the RDA for Mg. Women, particularly, have low intakes. For example, in France, 23% of women and 18% of men have inadequate intakes. Mg deficiency during pregnancy can induce maternal, fetal, and pediatric consequences that might last throughout life. Studies of gestational Mg deficiency in animals show that Mg deficiency may have marked effects on parturition and postuterine involution. It has interfered with fetal growth and development, and caused morbidity from hematological effects and disturbances in temperature regulation, to teratogenic effects. Emphasis, here, is on effects of chronic clinical gestational Mg deficiency as it affects the infant. Premature labor, contributed to by uterine hyperexcitability caused by chronic maternal Mg deficiency, that can be intensified by stress, gives rise to preterm birth. If the only cause of uterine overactivity is Mg deficiency, its supplementation constitutes nontoxic tocolytic treatment, as an adjuvant treatment, that is devoid of toxicity and enhances efficacy and safety of tocolytic drugs such as beta-2 mimetics. Evidence is considered that Mg deficiency or Mg depletion can contribute to the Sudden Infant Death Syndrome (SIDS). SIDS may be a fetal consequence of maternal Mg deficiency through impaired control of Brown Adipose Tissue (BAT) thermoregulation mechanisms leading to a modified temperature set point. SIDS can result from dysthermias: hypo- or hyperthermic forms. Possibly, simple nutritional Mg supplements might be preventive. Various stresses in an infant can transform simple Mg deficiency into Mg depletion. For example, lying prone can be stressful for the baby, as can parental smoking. The role of chronopathological stress appears to be often neglected, as it constitutes a clinical form of primary hypofunction of the biological clock [with its anatomical and clinical stigma such as reduced production of melatonin (MT) and of its urinary metabolite: 6 Sulfatoxy-Melatonin (6 SMT)]. SIDS might be linked to impaired maturation of both the photoneuroendocrine system and BAT. Prophylaxis of this form of SIDS should include atoxic nutritional Mg therapy for pregnant women with total light deprivation at night for the infant. Consequences of maternal primary Mg deficiency have been inadequately studied. To determine ultimate outcomes of gestational Mg deficiency in infants, a long-term multicenter placebo-controlled prospective study should undertaken on effects of maternal nutritional Mg supplementation on lethality/morbidity in fetus, neonates, infants, children and adults, not only during pregnancy and the baby's first year, but throughout life.

The effect of intrapartum magnesium sulfate therapy on fetal cardiac troponin I levels at delivery

OBJECTIVE

To determine whether intrapartum magnesium sulfate (MgSO4) therapy for seizure prophylaxis in pre-eclampsia-eclampsia is associated with biochemical evidence of subacute fetal myocardial damage at delivery.

STUDY DESIGN

Troponin I, a cardiac-specific protein used to detect myocardial injury, was measured from the umbilical vein at delivery in term pregnancies complicated by pre-eclampsia and uncomplicated control pregnancies. Women with pre-eclampsia received intravenous MgSO4 as a 6-g load followed by 2 g/hour until delivery. Clinical characteristics and fetal troponin levels were compared between groups.

RESULTS

There was no difference in troponin I concentrations between term patients with intrapartum MgSO4 therapy and controls who did not receive MgSO4 (median 0.86 ng/ml, range 0.72-1.10 vs. 0.89 ng/ml, range 0.68-1.50; p = 1.0). There was also no statistically significant difference in the number of patients with a troponin I level of > or = 1.0 ng/ml between groups (30.8% (4/13) vs. 15.4% (4/26); p = 0.4).

CONCLUSIONS

Our findings suggest that, in term fetuses that are not growth impaired, exposure to intrapartum MgSO4 is not associated with subacute myocardial injury.

Comparison of the effects of antenatal magnesium sulphate and ritodrine exposure on circulatory adaptation in preterm infants

We examined the effects of maternal magnesium sulphate (MgSO4) and ritodrine treatments on the autonomic cardiovascular control in preterm neonates with respiratory distress syndrome during the first 2 days of life. Serial measurements of heart rate (HR), blood pressure (BP) and respirogram were performed during the first 2 days of life in 28 preterm infants below 33 weeks of gestation with antenatal exposure to MgSO4 (n = 13) or ritodrine (n = 15), and in 12 nonexposed preterm controls. Spectral analysis was used for the quantification of HR and BP variability. Although antenatal MgSO4 exposure had no effect on HR or the systolic, diastolic or mean BP, it was associated with significant decreased beat-to-beat changes in BP. In contrast, ritodrine exposure had no consistent effects on the autonomic cardiovascular control during the first 2 days of life. Our data suggest that maternal MgSO4 treatment decreases the neonatal high frequency changes in BP. This early vascular stabilizing effect of antenatal MgSO4 exposure may contribute to a lowered risk of cerebral vascular catastrophes, in the vulnerable areas of the brain, among the preterm infants with respiratory distress syndrome.

Adverse and beneficial effects of tocolytic therapy

In addition to questions raised about the efficacy of many tocolytics, appropriate concern has been voiced about the safety of these potent drugs. Although some degree of risk for adverse effects with drugs promising a strong therapeutic effect can be accepted, caution needs to be exercised when benefits are marginal or unproven. Unfortunately, some of the tocolytics, most notably the betamimetics and magnesium sulfate, have been found to have considerable potential for adverse maternal cardiovascular and respiratory effects. Although less clearly established, the use of indomethacin appears to be associated with increased fetal and neonatal risks. Concerning magnesium sulfate, in addition to the well-known maternal effects, the accumulating evidence showing an increased frequency of adverse outcomes in the fetus and neonate has led to the recommendations to abandon its use entirely as a tocolytic. Given the limitations of our current state of knowledge, nifedipine would appear to be among the more efficacious and safer tocolytics available to use when properly indicated.

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.

Increased extracellular magnesium modulates proliferation in fetal neural cells in culture

Retrospective studies have shown that antenatal magnesium may decrease the risk of cerebral injury in preterm infants, leading to several ongoing trials of tocolytic magnesium as a neuroprotective agent. However, other studies have indicated that antenatal magnesium actually increases neonatal mortality, leaving it unclear if magnesium is protective or dangerous to preterm infants. This controversy may be secondary to our limited understanding about the mechanisms of magnesium's action on the fetal brain. We therefore investigated the effect of increasing extracellular magnesium on cultures of neurons from embryonic day 6 telencephalon. Conversion of MTT (3-(4,5-dimethyl, thiazol-2-yl)-2,5-diphenyltetrazolium bromide) by intact mitochondria was taken as a measure of cell viability. Nuclear incorporation of BrdU (5-bromo-2'-deoxyuridine) was taken as a measure of cell proliferation. Exposure of cultures for 24 h to a 4-fold increase in magnesium (3.3 mM) increased both overall cell viability (P<0.002) and proliferation (P<0.02) by approximately 50%. Proliferating cells showed characteristics of glial cell precursors but magnesium had no effect on mature astrocyte proliferation. Increased Akt activation was observed following magnesium treatment, comparable to that observed with the growth factor insulin, suggesting one mechanism for proliferation. However, when apoptosis was induced in these cultures with the phosphatidylinositol-3-kinase inhibitor wortmannin, magnesium significantly enhanced cell death. Thus under normal conditions in the fetus, magnesium may be a positive factor but during stress it may exacerbate cell injury. This is the first time increased extracellular magnesium has been shown to increase cell proliferation in neural cells in culture or suggested to induce Akt activation.

Antenatal therapies and the developing brain

This chapter presents a review of basic science and human studies of two commonly used pharmacologic agents (antenatal steroids and magnesium sulfate), in pregnancies at risk of preterm delivery, and examines the effects of these therapies on the developing brain. Very low birthweight (VLBW) infants are known to be at risk of both short-term and long-term neurodevelopmental sequelae; therefore, an understanding of the mechanisms contributing to both neuroprotective and neurotoxic effects of antenatal therapies on the immature brain and potential effects on long-term outcome are critical. Although the short-term beneficial effects of a single course of antenatal steroids are well documented, the experimental animal literature suggests detrimental effects on neurodevelopment of multiple doses. In addition, clinical studies of repeat doses suggest a negative impact on head and brain growth. The animal and human data on the effects of MgSO(4)are also mixed with both beneficial effects or no effects on neurodevelopment. This review will discuss the potential impact of single versus multiple doses and timing of doses on the brain.

Acute cord occlusion increases blood ionized magnesium concentration in preterm fetal sheep during maternal magnesium sulfate exposure

This study tested the hypothesis that a pathophysiologic insult to the fetus that decreases pH (umbilical cord occlusion) produces an increase in physiologically active (i.e., ionized) magnesium concentration. Preterm pregnant sheep (n = 7) were instrumented with maternal and fetal catheters and an inflatable vascular occluder was placed around the umbilical cord. After a 2-day recovery period, each ewe received a 4-g loading dose, followed by continuous intravenous infusion of 1 g magnesium sulfate/h. After 48 h, an episode of acute fetal distress was produced by inflation of the umbilical occluder for 10 min. Maternal and fetal arterial blood samples were collected at regular intervals to quantitate ionized magnesium concentration and monitor physiologic status. Magnesium sulfate infusion increased maternal and fetal blood ionized magnesium concentration. In vitro blood analysis demonstrated that there was a linear inverse correlation (r2= 0.99) between fetal sheep blood pH and ionized magnesium concentration. In vivo, 10 min of umbilical cord occlusion produced an increase in fetal blood ionized magnesium concentration in all animals (P = 0.02) that was temporally related to the decrease in fetal blood pH. Whether this increase in physiologically active magnesium concentration is beneficial (via neuroprotection) or deleterious (via suppression of stress response) to the distressed fetus remains to be determined.Key words: fetal, hypoxia, ionized magnesium, magnesium sulfate, sheep.

Intravenous infusion of magnesium sulfate and regional redistribution of fetal blood flow during maternal hemorrhage in late-gestation gravid ewes

OBJECTIVES

Even though magnesium sulfate is commonly prescribed for women with preeclampsia as prophylaxis against seizure and for women with preterm labor as a tocolytic agent there is limited information about its effects on the fetus. It is of particular concern that women with preeclampsia or in premature labor are at high risk for abruptio placentae with consequent compromise of fetal oxygenation. Magnesium sulfate is a vasodilator and thus may exert cardiovascular effects on the fetus. The goal of this study was to evaluate the effects of magnesium sulfate on fetal organ blood flow, especially regional cerebral blood flow, during the stressful condition of maternal hemorrhage.

STUDY DESIGN

Studies were performed with 11 long-term instrumented pregnant ewes and their fetuses at 121 to 128 days' gestation (term, 147 days' gestation). Animals were randomly allocated to either the experimental (n = 5) or the control (n = 6) group. After a 60-minute baseline period, experimental fetuses received intravenous magnesium sulfate diluted in 0.9% sodium chloride (0.3 g loading dose, then 0.3 g/h at a rate of 3 mL/h) and control fetuses were infused with an equivalent volume of intravenous 0.9% sodium chloride. After 60 minutes of this infusion-only period, the infusions were continued and ewes were intermittently bled 4 times at a rate of 5 mL/kg for 10 minutes with 5 minutes between hemorrhages. The total blood lost at the end of the hemorrhage-plus-infusion hour was 20 mL/kg. The infusions were continued and the sheep were observed for 1 hour after this period (posthemorrhage period). At the end of baseline, infusion-only, and hemorrhage-plus-infusion periods, fetal and maternal blood pressures and blood gas values were measured and fetal organ blood flows were determined through a fluorescent microsphere technique. Repeated-measures analysis of variance and Wilcoxon tests were used to determine the significance of changes in hemodynamic, blood gas, and organ blood flow parameters between different time points within each group. Comparisons between groups were made with rank sum tests (Mann-Whitney tests).

RESULTS

There were no significant differences between groups or within groups for baseline and infusion-only measurements in any measured hemodynamic or hematologic factor. Mean maternal blood pressure decreased significantly (P <.05) after hemorrhage, with similar median decrements in both control and experimental groups of 41 mm Hg (interquartile range, 24-57 mm Hg) and 41 mm Hg (interquartile range, 12-43 mm Hg), respectively. There were no significant differences between groups in fetal blood gas values or hemodynamic parameters. Fetal arterial PO(2) decreased significantly after hemorrhage plus infusion, with similar mean (+/-SEM) decreases in control and experimental groups of 5.9 +/- 1.4 mm Hg and 4.5 +/- 1.5 mm Hg, respectively. Fetal pH also decreased significantly in both groups. After hemorrhage plus infusion there were significant increases in fetal regional cerebral and myocardial blood flows in both groups. Adrenal blood flow increased significantly from baseline (214%, 183%-294%) in the control group after hemorrhage plus infusion but not in the experimental group. No other difference in organ blood flow between control and experimental groups was observed. Significant regional variations in cerebral blood flow were not observed in either group at any time.

CONCLUSIONS

In these initially healthy, late-gestation fetal lambs magnesium sulfate exposure did not impair cardiac output redistribution, nor did it cause fetal death in response to maternal hemorrhage.

Fetal carotid blood flow during videofetoscopy

BACKGROUND/PURPOSE

Intracranial bleeding has been reported as one of the complications of both open and minimally invasive fetal surgery and putatively attributed to intraoperative fluctuations of carotid blood flow. The aim of this study was to look at fetal carotid blood flow and its relationship with umbilical blood flow, blood pressure, oxygen delivery, and acid-base status in the fetus at various intraamniotic pressures with both liquid and gas media during fetoscopic surgery.

METHODS

Six 115- to 130-day-gestation ewes underwent continuous invasive systemic blood pressure monitoring in the descending aorta. A hysterotomy was performed. A 6-mm ultrasonic blood flow probe was placed around the common umbilical artery at its origin from the fetal aorta. This was followed by placement of a double-lumen, 4F catheter in the fetal descending aorta through a femoral artery. A 4-mm ultrasonic blood flow probe was then placed around the fetal left common carotid artery. A pressure-monitoring, multiperforated catheter was placed inside the amniotic cavity. The fetus was repositioned inside the uterus, which was then closed. The abdominal wall was closed loosely. No further manipulation was performed for 1 hour. Intraamniotic pressure was raised from 0 to 30 mm Hg at 5-mm Hg intervals by infusing either warmed saline or medical air. Common umbilical artery and left carotid artery blood flows, blood pressure, blood gases, bicarbonate, sodium, and hematocrit were recorded in all fetuses at each 5-mm Hg interval. Maternal systemic blood pressure, O2 saturation, and temperature were kept constant.

RESULTS

Carotid blood flow remained stable within the intra-amniotic pressure range studied (0 to 30 mm Hg), despite the significant drop in common umbilical artery blood flow uniformly observed above 20 mm Hg when saline was infused and above 15 mm Hg when air was infused. There was fetal hypoxemia and hypercarbia concomitant with decreased common umbilical artery blood flow (however, without fetal acidosis, because of compensatory elevation of bicarbonate). Within the intraamniotic pressure range studied, fetal aortic blood pressure, sodium, and hematocrit did not vary significantly, even when there was decreased umbilical blood flow.

CONCLUSIONS

Fetal carotid blood flow is protected, possibly autoregulated, remaining stable even after umbilical blood flow decreases as a consequence of elevated intrauterine pressures up to 30 mm Hg during videofetoscopy. These data suggests that perioperative intracranial bleeding related to videofetoscopic surgery is caused by factors other than fluctuations of cerebral blood flow.

Magnesium sulfate tocolysis and risk of neonatal death

OBJECTIVE

Our purpose was to evaluate the association between in utero exposure to magnesium sulfate and neonatal death.

STUDY DESIGN

Case-control comparison was performed of singleton infants who died at 0 to 28 days and control infants who survived to age 3 years and did not have disabling cerebral palsy. All subjects were born between 1983 and 1985 to mothers resident in one of four northern California counties.

RESULTS

Magnesium sulfate tocolysis was associated with a decreased risk of neonatal mortality (odds ratio 0.25, 95% confidence interval 0.6 to 1.1). Adjustment for birth weight and gestational age increased the inverse association (odds ratio 0.09, 95% confidence interval 0.01 to 0.93). The association between neonatal mortality and magnesium sulfate was not altered by maternal infection, gender, maternal race or ethnicity, maternal age, level of care, breech presentation, surgical delivery, corticosteroids, abruptio placentae, placenta previa, or bleeding on admission.

CONCLUSIONS

Magnesium sulfate tocolysis was not associated with increased neonatal mortality in premature infants. Thus any association of magnesium with reduced long-term neurologic morbidity is unlikely to be the result of selective mortality of vulnerable infants.