Magnesium sulfate-induced relaxation of uterine arteries from pregnant and nonpregnant patients

Intrapartum magnesium sulfate exposure and obstetric hemorrhage risk

BACKGROUND

The gold standard intrapartum treatment for preeclampsia with severe features is magnesium sulfate in order to provide prophylaxis against eclampsia. However, though magnesium sulfate is known to have a relaxant effect on uterine muscle, there have been variable reports in the literature in regard to the association between magnesium and obstetric hemorrhage (OBH).

OBJECTIVE

We aim to compare OBH incidence in patients with hypertensive disease of pregnancy (HDP) with or without exposure to intrapartum magnesium sulfate.

METHODS

We performed a retrospective cohort study of all deliveries at our institution associated with a diagnosis of hypertensive disease of pregnancy (HDP) (e.g. chronic and gestational hypertension, preeclampsia with or without severe features, eclampsia, or HELLP) from January 1, 2018 to December 31, 2019. The category of HDP diagnosis was determined by a detailed chart review by trained chart abstractors. The primary outcome was total quantitative blood loss (QBL) and the rate of obstetric hemorrhage. Secondary outcomes included a composite of obstetric hemorrhage-related maternal morbidity outcomes (OBH-M), the individual composite components and the incidence of additional hemorrhage-related interventions (e.g. uterotonics and surgical interventions). We also examined the same primary and secondary outcomes in a stratified analysis based on delivery mode (i.e. vaginal deliveries only and cesarean deliveries only).

RESULTS

Of 791 patients with a diagnosis of HDP, 411 patients received magnesium sulfate for eclampsia prophylaxis and 380 patients did not receive magnesium sulfate. For all delivery modes, there was a significantly higher QBL (p < .01), increased rate of OBH (p = .04) and increased OBH-M (p < .01) in deliveries associated with intrapartum exposure to magnesium compared to those without. However, our stratified analysis by delivery mode demonstrated that magnesium-related hemorrhage risk only persisted for vaginal deliveries (QBL p < .01; OBH aOR 1.47, 95% CI: 0.75-2.85; OBH-M aOR 1.47, 95% CI 1.00-7.55) with no significant hemorrhage-related differences among cesareans with or without magnesium exposure (QBL p = .51; OBH aOR 1.45, 95% CI: 0.85-2.47; OBH-M 1.50 95% CI: 0.70-3.23).

CONCLUSION

Intrapartum exposure to magnesium sulfate use was associated with an increase in QBL and risk of OBH-M in vaginal deliveries, but not associated with any hemorrhage-related outcome differences in cesarean deliveries. More research is needed to explore the effects of hypertensive disease, magnesium exposure, and delivery mode on obstetric hemorrhage risk.

Association between Receipt of Intrapartum Magnesium Sulfate and Postpartum Hemorrhage

Objective  The aim of the study is to investigate the association between intrapartum administration of magnesium sulfate in women with hypertensive disorders of pregnancy and postpartum hemorrhage. Study Design  This was a retrospective cohort study of women diagnosed with a hypertensive disorder of pregnancy who delivered singleton gestations >32 weeks at a single, large volume tertiary care center between January 2006 and February 2015. Women who received intrapartum magnesium sulfate for seizure prophylaxis were compared with women who did not receive intrapartum magnesium sulfate. The primary outcome was frequency of postpartum hemorrhage. Secondary outcomes included estimated blood loss, uterine atony, and transfusion of packed red blood cells. Bivariable analyses were used to compare the frequencies of each outcome. Multivariable logistic regression models examined the independent associations of magnesium sulfate with outcomes. Results  Of 2,970 women who met inclusion criteria, 1,072 (36%) received intrapartum magnesium sulfate. Women who received magnesium sulfate were more likely to be nulliparous, publicly insured, of minority race or ethnicity, earlier gestational age at delivery, and undergo labor induction. The frequency of postpartum hemorrhage was significantly higher among women who received magnesium sulfate compared with those who did not (12.4 vs. 9.3%, p  = 0.008), which persisted after controlling for potential confounders. Of secondary outcomes, there was no difference in estimated blood loss between women who did and did not receive magnesium sulfate (250 mL [interquartile range 250-750] vs. 250 mL [interquartile range 250-750], p  = 0.446). However, compared with women who did not receive magnesium sulfate, women who received magnesium sulfate had a greater frequency of uterine atony (8.9 vs 4.9%, p  < 0.001) and transfusion of packed red blood cells (2.0 vs. 0.8%, p  = 0.008). These differences persisted after controlling for potential confounders. Conclusion  Intrapartum magnesium sulfate administration to women with hypertensive disorders of pregnancy is associated with increased odds of postpartum hemorrhage, uterine atony, and red blood cell transfusion.

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.

Maternal magnesium supplementation reduces intrauterine growth restriction and suppresses inflammation in a rat model

OBJECTIVE

Intrauterine growth restriction (IUGR) is associated with increased inflammatory responses. We sought to investigate whether magnesium (Mg) attenuates inflammation and IUGR in a rat model.

STUDY DESIGN

Pregnant Wistar rats (12 weeks, gestational day 18) were randomly assigned to 1 of 4 groups: normal diet with bilateral uterine artery ligation (BL) (n = 6) or sham surgery (SH) (n = 5); and Mg chloride (MgCl2) 1% (wt/vol) in the drinking water throughout gestation + BL (MgBL) (n = 6) or SH (MgSH) (n = 5). Dams were euthanized 24 hours postsurgery (gestational day 19). Maternal plasma, fetal plasma (pooled), individual amniotic fluid (AF) samples, and placentas (PL) were collected and assessed from live fetal pups only (BL, n = 36; SH, n = 20; MgBL, n = 20; MgSH, n = 20). All samples were analyzed for cytokines/chemokines (interleukin [IL]-6, IL-1β, chemokine [C-X-C motif] ligand 1 [CXCL1], chemokine [C-C motif] ligand 2 [CCL2], and tumor necrosis factor [TNF-α] sensitivity <3 pg/mL) using a multiplex platform. Data were analyzed using Mann Whitney, analysis of variance, and Fisher exact tests.

RESULTS

The incidence of IUGR (pup weight <10th percentile of SH) in the MgBL group was significantly lower (31%) than the BL group (86.3%) (relative risk, 0.36; 95% confidence interval, 0.2-0.6; P < .0001). BL significantly increased AF levels of IL-6, IL-1β, TNF-α (P < .05), and CCL2 (P < .001) vs SH and PL levels of IL-6, IL-1β, CCL2 and CXCL1 (P < .001), and TNF-α (P < .05) vs SH. Maternal MgCl2 supplementation significantly decreased IL-1β, TNF-α, and CCL2 levels in AF and IL-1β in PL tissues of MgBL vs BL rats (P < .0001).

CONCLUSION

Maternal oral MgCl2 supplementation reduced BL-induced IUGR by 64% and suppressed cytokine/chemokine levels in the AF and PL.

High-end arteriolar resistance limits uterine artery blood flow and restricts fetal growth in preeclampsia and gestational hypertension at high altitude

The reduction in infant birth weight and increased frequency of preeclampsia (PE) in high-altitude residents have been attributed to greater placental hypoxia, smaller uterine artery (UA) diameter, and lower UA blood flow (Q(UA)). This cross-sectional case-control study determined UA, common iliac (CI), and external iliac (EI) arterial blood flow in Andeans residing at 3,600-4,100 m, who were either nonpregnant (NP, n = 23), or experiencing normotensive pregnancies (NORM; n = 155), preeclampsia (PE, n = 20), or gestational hypertension (GH, n = 12). Pregnancy enlarged UA diameter to ~0.62 cm in all groups, but indices of end-arteriolar vascular resistance were higher in PE or GH than in NORM. Q(UA) was lower in early-onset (≤34 wk) PE or GH than in NORM, but was normal in late-onset (>34 wk) illness. Left Q(UA) was consistently greater than right in NORM, but the pattern reversed in PE. Although Q(CI) and Q(EI) were higher in PE and GH than NORM, the fraction of Q(CI) distributed to the UA was reduced 2- to 3-fold. Women with early-onset PE delivered preterm, and 43% had stillborn small for gestational age (SGA) babies. Those with GH and late-onset PE delivered at term but had higher frequencies of SGA babies (GH=50%, PE=46% vs. NORM=15%, both P < 0.01). Birth weight was strongly associated with reduced Q(UA) (R(2) = 0.80, P < 0.01), as were disease severity and adverse fetal outcomes. We concluded that high end-arteriolar resistance, not smaller UA diameter, limited Q(UA) and restricted fetal growth in PE and GH. These are, to our knowledge, the first quantitative measurements of Q(UA) and pelvic blood flow in early- vs. late-onset PE in high-altitude residents.

The effects of ritodrine and magnesium sulfate on maternal and fetal Doppler blood flow patterns in women with preterm labor

OBJECTIVE

To determine the effects of ritodrine and magnesium sulfate on maternal-fetal blood flows.

STUDY DESIGN

A total of 85 pregnant women between 26th and 36th weeks with preterm labor, and 83 healthy pregnant women were included. Patients in the study group were randomly assigned to receive either ritodrine (with the addition of verapamil) (n=46) or magnesium sulfate (n=39). Blood flow examinations on the umbilical artery (UA), middle cerebral artery (MCA), bilateral uterine arteries (Ut.A) and ductus venosus (DV) were performed before and 48 h after initiating therapy.

RESULTS

UA pulsatility index (PI) significantly differed in women receiving tocolysis compared to controls after 48 h. DV PI increased in women receiving MgSO(4), whereas it decreased in the ritodrine and control groups. Ut.A values did not significantly change after 48 h in the groups. In women between the 26th and 32nd weeks, UA, MCA and DV PI did not significantly change after 48 h in the three groups. However, in women between the 32nd and 36th weeks UA and MCA PI significantly differed in the treatment groups compared to controls after 48 h. DV PI increased in women receiving MgSO(4), whereas it decreased in the ritodrine and control groups.

CONCLUSIONS

MgSO(4) and ritodrine affect blood flow patterns after 48 h in some maternal-fetal vessels. These effects on blood flow are particularly significant in women between 32nd and 36th weeks. The effects of both drugs on fetal and maternal Doppler flows seem similar, except the increased resistance to flow in DV in women receiving MgSO(4).

Magnesium sulfate for the treatment of eclampsia: a brief review

BACKGROUND AND PURPOSE

Magnesium sulfate is used extensively for prevention of eclamptic seizures. Empirical and clinical evidence supports the effectiveness of magnesium sulfate; however, questions remain as to its safety and mechanism. This review summarizes current evidence supporting the possible mechanisms of action and several controversies for magnesium sulfate treatment.

SUMMARY OF REVIEW

Several mechanisms are presented, including the effects of magnesium sulfate on peripheral and cerebral vasodilation, blood-brain barrier protection, and as an anticonvulsant.

CONCLUSIONS

Though the specific mechanisms of action remain unclear, the effect of magnesium sulfate in the prevention of eclampsia is likely multi-factorial. Magnesium sulfate may act as a vasodilator, with actions in the peripheral vasculature or the cerebrovasculature, to decrease peripheral vascular resistance or relieve vasoconstriction. Additionally, magnesium sulfate may also protect the blood-brain barrier and limit cerebral edema formation, or it may act through a central anticonvulsant action.

Endothelium dependence and gestational regulation of inhibition of vascular tone by magnesium sulfate in rat aorta

OBJECTIVE

The aim of this study was to investigate the role of nitric oxide in the vasorelaxant effect of magnesium sulfate during pregnancy.

STUDY DESIGN

Segments of 3 mm of the aorta, with or without intact endothelium, from 16- or 22-day-pregnant rats were mounted in organ chambers with standard Krebs solution or low-magnesium Krebs solution for measurement of isometric tension. The rings were contracted with phenylephrine, and cumulative concentration-response curves for magnesium were determined after incubation with various inhibitors.

RESULTS

Magnesium relaxed the aortic rings from pregnant rats in a concentration-dependent manner. The relaxation was significantly lower on day 22 of gestation than on day 16 of gestation. Removal of the endothelium or incubation with 10(-4)-mol/L N omega-nitro-L -arginine methyl ester (a nitric oxide synthase inhibitor), 10(-5)-mol/L 6-anilino-5,8-quinolinedione (a guanylate cyclase inhibitor), or 10(-5)-mol/L indomethacin (a cyclooxygenase inhibitor) significantly decreased the relaxant effect of magnesium on aortic rings from 16-day-pregnant but not 22-day-pregnant rats. Treatment with minimally effective concentrations of a nitric oxide donor (3 x 10(-10)-mol/L sodium nitroprusside) or a cyclic guanosine monophosphate analog (10(-6)-mol/L 8-bromo-cyclic guanosine monophosphate) restored the response to magnesium.

CONCLUSIONS

The relaxant effect of magnesium on rat aortic rings was dependent on both endothelium and gestational age and was lower at term than during late pregnancy. The endothelium appears to potentiate the vasorelaxant effects of magnesium through the nitric oxide-cyclic guanosine monophosphate and cyclooxygenase systems.

Modern management of eclampsia

Eclampsia, the occurrence of a seizure in association with pre-eclampsia, remains an important cause of maternal mortality and morbidity. Despite being recognised since antiquity, consistent management practices are still lacking. Given that the aim of good care is to prevent seizures, it is disappointing that in the majority of cases the first eclamptic convulsion occurs after admission to hospital. This indicates that either the women who are likely to have a convulsion were not identified accurately, or the treatment given was ineffective. The answer to poor management of eclampsia lies in better education and training of all obstetricians, anaesthetists, midwives, and general practitioners in the diagnosis and treatment of severe pre-eclampsia and eclampsia. Protocols for the management of fluid balance, antihypertensive and anticonvulsant therapies should be available and reviewed regularly. The universal adoption of such guidelines in all obstetric units would substantially reduce elements of substandard care which have repeatedly been identified in the triennial reports of the confidential enquiries into maternal deaths in the UK.

Magnesium sulfate therapy during asphyxia in near-term fetal lambs does not compromise the fetus but does not reduce cerebral injury

OBJECTIVE

Our purpose was to investigate (1) the safety of fetal magnesium sulfate treatment and (2) possible beneficial effects on the brain during perinatal asphyxia.

STUDY DESIGN

In 20 chronically instrumented fetal lambs (gestational age 125.8 +/- 3.5 days) four total umbilical cord occlusions for 5 minutes were repeated at 30-minute intervals. Fetuses received either saline solution (n = 11) or magnesium sulfate (n = 9) as a bolus of 300 mg intravenously 2 hours before occlusions, followed by an infusion of 100 mg/hr until 1 hour after occlusions.

RESULTS

In the treated fetuses plasma magnesium levels rose from 0.85 +/- 0.20 to 2.23 +/- 0.40 mmol/ L. Occlusions induced asphyxia, associated with mortality; 4 of 11 fetuses in the control group versus 1 of 9 in the magnesium-treated group died (not significant). Fetal electroencephalographic activity decreased and cerebral impedance increased during occlusions. Maximum spike and seizure activity occurred 5 to 10 hours after asphyxia. Neuronal loss was primarily localized in the corpus striatum. Magnesium caused no alterations in blood pressure, heart rate, or cerebral and peripheral blood flow, nor did it influence electrophysiologic responses or neuronal loss.

CONCLUSIONS

Administration of magnesium sulfate was safe but did not offer significant cerebral protection from asphyxia in the near-term fetal lamb.

Pregnancy-induced alterations of relaxation in response to magnesium in the rat aorta may be due to plasma-borne agents

The influence of plasma from pregnant and nonpregnant humans was examined, using magnesium-induced relaxation of precontracted rat aortic rings. The results showing magnesium-induced relaxation of aortic rings from pregnant and nonpregnant rats were compared. In rat aortic rings incubated in plasma from pregnant patients, magnesium was more potent in relaxing the precontractions induced by potassium chloride than in relaxing those induced by phenylephrine. The magnesium-induced relaxation of rings incubated in plasma from normal pregnant subjects was similar to that in unincubated rings from normal pregnant rats. Neither the removal of endothelium nor pretreatment with indomethacin affected the pattern of responses in the rings. The results suggest that the effects of pregnancy on magnesium-induced relaxation of the rat aorta may be mediated by plasma-borne agents, and the mechanisms by which the agents alter the relaxation do not involve either the vascular endothelium or prostaglandin synthesis.

[Indications for the use of magnesium in anesthesia and intensive care]

Magnesium (Mg), a cofactor in numerous enzymatic reactions, is often ignored by clinicians, as the symptomatology of Mg depletion is not specific and usually associated with that of the cause of the depletion. Furthermore, the plasma Mg concentration (0.8 to 1.1 mmol.L-1) is only equivalent to one percent of the total body content. A Mg deficit may exist while plasma Mg concentration is normal. Therefore other techniques for Mg assessment, such as the repletion test, as well as red blood cell and lymphocyte concentrations have been used. A renewed interest for Mg occurred as numerous studies have shown the therapeutic efficiency of Mg and as the mechanisms of its haemodynamic effects have been recognized. Mg regulates Na-K-ATPase activity, K channels activity and, most of all, it is a natural calcium channel blocking agent. These properties explain its important place in electrophysiology of myocardial cells and the effects on the tension of smooth muscles, resulting in a vasodilation and a bronchodilation respectively. The antagonistic effect of Mg on calcium decreases the presynaptic release of acetylcholine at the neuromuscular junction and the release of epinephrine at the peripheral sympathetic nerves and the adrenals. Mg potentiates the effect of non-depolarizing muscle relaxants. A Mg deficiency occurs often in ICU patients, in alcoholics and during use of diuretics. Simultaneous administration of Mg is often required for treatment of potassium deficiency. Mg has an anti-arrhythmic effect towards digoxin-mediated dysrhythmias and torsades de pointes, and can be efficient in other arrhythmias. Systematic use of Mg seems to decrease mortality of acute myocardial infarction and is justified during cardiac surgery, often associated with hypomagnesemia, because of vasodilation of coronary arteries and in order to prevent occurrence of arrhythmias. Mg, because of its calcium channel blocking properties and as it lowers the release of epinephrine, is indicated for surgery of pheochromocytoma. In eclamptic and pre-eclamptic patients, the use of Mg is valuable, but not as an anti-epileptic agent. Other clinical uses of Mg have been proposed, but they are either anecdotal or of uncertain efficiency.

Influence of endothelium on Mg(2+)-induced relaxation in noradrenaline-contracted aorta from DOCA-salt hypertensive rat

The aim of this study was to examine the influence of vascular endothelium on the relaxation induced by increased extracellular Mg2+ concentrations on isolated and noradrenaline-precontracted aorta from deoxycorticosterone acetate-salt (DOCA-salt) hypertensive and normotensive rats. In Mg(2+)-free physiologic salt solution (PSS), addition of Mg2+ (0.1-6.0 nM) caused concentration-dependent relaxation of noradrenaline-precontracted aorta with intact or disrupted endothelium. Mg(2+)-induced relaxation in intact aorta, however, was less in DOCA-salt hypertensive rats than in normotensive rats. When endothelium was disrupted, Mg(2+)-induced relaxation was depressed in aorta from both DOCA-salt hypertensive and normotensive rats. The same observations were made in presence of N-nitro-L-arginine methyl ester (L-NAME), an inhibitor of endothelium-derived relaxing factor nitric oxide (EDRF/NO) biosynthesis. Mg(2+)-induced relaxation following contraction with noradrenaline was significantly less in intact aorta treated with L-NAME from DOCA-salt hypertensive rats than in intact aorta from normotensive rats. Indomethacin did not affect Mg(2+)-induced relaxation in intact aorta from normotensive rats whereas indomethacin significantly increased it in DOCA-salt hypertensive rats. It is concluded that (1) Mg(2+)-induced relaxation can be mediated by endothelium-dependent mechanisms implicating EDRF/NO; (2) the influence of EDRF/NO is more pronounced on the impaired Mg(2+)-induced relaxation of aorta from DOCA-salt hypertensive rats; (3) Mg(2+)-induced relaxation seems masked by vasoconstrictor prostaglandin release in DOCA-salt hypertensive rats; (4) these differences between normotensive and hypertensive rats could be related to the impaired endothelial function in aorta from DOCA-salt hypertensive rats.

Lack of reactivity of uterine arteries from patients with obstetric hemorrhage

Obstetric hemorrhage may occur throughout pregnancy and the puerperium. The purpose of this study was to investigate the reactivity of isolated, suffused uterine arteries from obstetric patients with uncontrollable uterine bleeding and to compare those blood vessels with uterine arteries from patients undergoing cesarean hysterectomy for other medical reasons (control patients). The uterine arteries from the control patients (n = 9) responded with maximal or near-maximal constriction to norepinephrine (30 mumol/L, 3.6 +/- 1 gm), potassium chloride (75 mmol/L, 10.2 +/- 3 gm), prostaglandin F2 alpha (30 mumol/L, 1.8 +/- 1 gm), and arginine vasopressin (1 mumol/L, 18.8 +/- 2.6 gm). In uterine arteries from five patients with uncontrollable bleeding, the constrictor responses to the same drugs were markedly depressed: norepinephrine (30 mumol/L, 0.5 +/- 0.2 gm), potassium chloride (75 mmol/L, 1.9 +/- 0.8 gm); prostaglandin F2 alpha (30 mumol/L, 0 gm), and arginine vasopressin (1 mumol/L, 0.2 +/- 0.05 gm). Uterine arteries from two patients exhibited no constrictor responses to norepinephrine (30 mumol/L), potassium chloride (75 mmol/L), prostaglandin F2 alpha (30 mumol/L), or arginine vasopressin (1 mumol/L). The impaired responses to the vasoconstrictor drugs were not reversed by indomethacin (1 mumol/L), which is an inhibitor of prostaglandin synthetase; methylene blue (10 mumol/L), which is a blocker of endothelium-derived relaxing factor activation of guanylate cyclase; or propranolol (1 mumol/L), a beta-adrenergic receptor antagonist. The levels of adenosine 3':5'-cyclic monophosphate were not elevated in the uterine arteries from the patients with obstetric hemorrhage. The impaired reactivity to the multiple vasoconstrictors implies that a mechanism involved in constriction common to all of the constrictors is depressed or blocked. Furthermore, the depression or lack of reactivity of these isolated uterine arteries is not mediated by vasodilatory prostaglandins, endothelium-derived relaxing factor, beta-adrenergic receptors, or elevated levels of adenosine 3':5'-cyclic monophosphate. The results suggest that obstetric hemorrhage involves, in part, a lack of constrictor reactivity of the uterine vasculature.