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Revista Brasileira de Anestesiologia

versión impresa ISSN 0034-7094

Rev. Bras. Anestesiol. vol.60 no.1 Campinas enero/feb. 2010 



Applications of magnesium sulfate in obstetrics and anesthesia


Usos del sulfato de magnesio en obstetricia y en anestesia



Fabiano Timbó Barbosa, TSA, M.D.I; Luciano Timbó Barbosa, M.D.II; Mário Jorge Jucá, M.D.III; Rafael Martins da Cunha, M.DIV

IMestre em Ciências; Docente da Universidade Federal de Alagoas
IIClínico do Hospital Geral do Estado Professor Osvaldo Brandão Vilela; Especialista em Clínica Médica pela Sociedade Brasileira de Clínica Médica
IIIDocente da Universidade Federal de Alagoas; Pós-doutorado em Coloproctologia pela Universidade do Texas, Dallas, EUA; Doutor em Gastroenterologia Cirúrgica na Área de Concentração em Coloproctologia; Especialista em Didática do Ensino Superior pela Universidade Ibirapuera
IVAnestesiologista do Hospital Unimed - Maceió

Correspondence to




BACKGROUND AND OBJECTIVES: Magnesium is predominantly an intracellular ion. Its blocking effects on NMDA receptors are responsible for the analgesic and sedative characteristics of this ion. The objective of this study was to review the physiology, pharmacology, and decreased plasma levels of magnesium, as well as its applications in obstetrics and anesthesia.
CONTENTS: Magnesium is an intracellular cation with multiple functions: it is a cofactor for enzymes of the glucose metabolism and those that participate in the degradation of nucleic acids, proteins, and fatty acids; it regulates the movements of transmembrane ions; and it intervenes in the activity of several enzymes. Critical patients have a tendency to develop hypomagnesemia, and the treatment consists in correcting the cause, whenever possible, and replacement of magnesium. A reduction in the minimum alveolar concentration (MAC) of inhalational agents in animals and the use of opioids in humans under anesthesia has been demonstrated.
CONCLUSIONS: Magnesium sulfate has been used in obstetrics with good results, inhibiting premature labor and in the treatment of eclampsia-associated seizures. It is potentially analgesic and sedative, and could be used as adjuvant during general anesthesia, attenuating the blood pressure response to tracheal intubation and decreasing the need of anesthetics.

Keywords: ANESTHESIA, General; DISEASES: pregnancy-related hypertension, eclampsia; DRUGS: magnesium sulfate; PHARMACOLOGY: magnesium sulfate, hypomagnesemia.


JUSTIFICATIVA Y OBJETIVOS: El magnesio es un ión predominantemente intracelular. Su efecto bloqueador del receptor NMDA le confiere características analgésicas y sedativas. El objetivo de este artículo, fue revisar la fisiología, la farmacología y la disminución de la concentración plasmática del magnesio, como también de algunas de sus aplicaciones en obstetricia y en anestesia.
CONTENIDO: El magnesio es un catión intracelular que posee múltiples funciones: es cofactor de enzimas del metabolismo glicídico y de enzimas de la degradación de los ácidos nucleicos, proteínas y ácidos grasos; regula el paso de los iones transmembrana e interviene en la actividad de varias enzimas. El paciente en estado crítico, presenta una tendencia a la hipomagnesemia, y el tratamiento consiste en corregir la causa cuando es posible, acompañada de la reposición del magnesio. Ya ha quedado demostrada la reducción de la concentración alveolar mínima (CAM), de los agentes inhalatorios en animales y el uso de opioides en humanos bajo anestesia.
CONCLUSIONES: El sulfato de magnesio, ha venido siendo utilizado en obstetricia con una buena efectividad para la inhibición del parto prematuro y para el tratamiento de las crisis convulsivas asociadas al cuadro de eclampsia. Es un fármaco con potencial analgésico y sedativo que puede ser utilizado como coadyuvante durante la anestesia general, atenuando la respuesta presórica a la intubación traqueal y disminuyendo la necesidad del uso de anestésicos.




Magnesium is the second most abundant intracellular cation1,2 and the fourth when the extracellular medium is also considered1. As a cofactor, it is involved in more than 300 known reactions, such as3 hormone binding to receptors, flow of transmembrane ions, regulation of the adenylate kinase system, muscle contraction, neuronal activity, vasomotor tone, cardiac excitability, release of neurotransmitters, and calcium binding to calcium channels.

In 1906, Haulbold and Meltzer reported sensorial and motor blockades in humans after the intrathecal administration of magnesium; in 1950, magnesium was used in anesthesia, mainly to control seizures in gravidas. Currently, it has several applications in anesthesia, obstetrics, and intensive care3-5.

The objective of this report was to review the physiology, pharmacology, and reduction in plasma levels of magnesium, as well as some of its applications in obstetric and anesthesia.



Magnesium is an intracellular cation with multiple functions: it participates in energy metabolism, since it is a cofactor in glucose metabolism, and a cofactor of nucleic acid, protein, and fatty acid degradation enzymes5,6; it regulates the flow of transmembrane ions5; and it mediates the activity of several enzymes5,7.

Magnesium is considered a natural physiologic calcium antagonist, having several regulatory mechanisms, such as1-3,8-11: competitive antagonist affecting type L calcium channels, inhibition of the enzyme Ca2+-ATPase, and it is a cofactor for all enzymes that participate in phosphate transferences that use ATP. In high concentrations, it inhibits the enzyme Na+/K+ -ATPase5.

Magnesium is absorbed in the jejunum and ileum at a proportion of 11% to 65% of the ingested amount12. The kidney is the main regulator of the levels of magnesium in the body, being capable of eliminating almost 100% of the filtered magnesium in case of overload5.

Medicine has not elucidated completely the mechanism of action of magnesium sulfate used therapeutically13. Some of the following propositions have been developed over the years:

a) Inhibition of the release of acetylcholine in the neuromuscular junction9,14 leading to muscle relaxation, which has been known since the 1950s3 Hypermagnesemia decreases the sensitivity of the motor plate to acetylcholine and the amplitude of terminal end-plate potential9.
b) It is an antagonist of NMDA glutamate receptors1,2,5,10,11; this receptor is responsible for central sensitization10. Binding of this receptor has analgesic, anticonvulsant, and sedative properties5.
c) It can increase the synthesis of prostacyclins and inhibit angiotensin converting enzyme, leading to vasodilation11.
d) It decreases the release of catecholamines after sympathetic stimulus1,5,9,11. It has been used in the treatment of pheochromocytoma-related hypertensive episodes during surgeries or outside the surgical environment5.
e) In asthma patients, it inhibits the release of histamine and acetylcholine5,16, and it potentiates the effects of beta-adrenergic agents5. It is indicated only in severe cases because it decreases the rate of hospitalizations and the length of stay in the intensive care unit, but it has little beneficial effects in moderate and mild cases16.



The human body has 21 to 28 grams of magnesium5. It is distributed as follows: 53% in the bones, 27% in the muscles, 19% in the soft tissues, 0.3% in the red blood cells, and 0.3% in the plasma5,6. Fifty-five per cent of plasma magnesium is ionized and 45% is bound to plasma proteins or broken into divalent anions, such as phosphate and sulfate6. Its plasma concentration ranges from 1.6 to 2.3 mg.dL-1 5,6. Since magnesium is an abundant intracellular ion and it is present in the plasma in very low amounts, measuring its plasma levels is not adequate to evaluate real deficiency or overload5.

Body magnesium stores are better assessed by measuring the urinary excretion in patients without renal failure5,6. Under normal circumstances, a small amount of magnesium is eliminated in the urine6. The urinary retention test is performed by collecting 24-hour urine after the intravenous infusion of 6 g of magnesium sulfate6. When more than 70% of the amount administered is recovered from the urine, the presence of deficiency is unlike5, but when less than 50% is recovered from the urine, body stores are probably deficient6.

Hypomagnesemia is seen in 10% to 20% of hospitalized internal medicine patients6, and 60% of patients in Intensive Care Units (ICU)6,7, 7% of admissions for ketoacidosis, 30% of admissions to the neonatal ICU, and up to 70% after coronary revascularization in adults5. The presence of hypomagnesemia in the surgical ICU has been associated with increased mortality7. Clinical signs of hypomagnesemia are non-specific6, and they are associated with cardiac arrhythmia8, reduction in cardiac index8, reduction in neuromuscular excitability7, disorientation9, seizures9, and psychosis9. It is the main cause of refractory hypokalemia7.

Critical patients have a tendency to develop hypomagnesemia for several reasons: deficient intake, increased losses, and redistribution in the body5. The main cause of hypomagnesemia is the use of diuretics and it is seen in 50% of chronic furosemide users6. Other possible causes include5: total parenteral nutrition; pancreatitis; burns; extracorporeal circulation; use of beta-agonists, aminoglycosides, and amphotericin B; diarrhea; acute tubular necrosis; and hypoparathyroidism.

Treatment consists on correcting the underlying cause, whenever possible, and replacement of magnesium5. Intestinal absorption of magnesium is erratic and the intravenous route should be preferred for therapeutic use5,7. Six grams should be administered in 24 hours5,7 and, in critical patients, serum levels should be maintained above 2.0 mg.dL-17.



Magnesium sulfate has been used in obstetrics since 1925 for prevention of seizures in eclampsia17,18, with the advantage of decreasing peripheral vascular resistance without changing uterine blood flow5.

It has been postulated that the anticonvulsant property of magnesium sulfate is due to the blockade of NMDA receptors5,14. Considerations on the real effects of magnesium in the treatment of eclampsia-related seizures have been made, since its effects in the neuromuscular junction can mask the real effects of magnesium in the central nervous system19. Doses used to depress the activity of the neuromuscular junction have been used in gravidas20, and small alterations or even no changes on the electroencephalogram have been reported in some studies with women without eclampsia21, with eclampsia22, and in animal models23. Studies with Doppler flowmetry have demonstrated cerebral vasodilation2,5 and reversion of cerebral vasospasm2,15 after the administration of magnesium.

The therapeutic serum level for the treatment of seizures ranges from 4.2 to 8.4 mg.dL-1 which can be achieved by the intramuscular administration of 6 g followed by 2 g/hour; intravenous administration of 3 to 4 g (up to 1 g/min) or a combination of both routes19. Two administration schedules of magnesium are widely used: Pritchard's and Zuspan's18. Pritchard's schedule starts with a 14-gram dose, 4 g IV and 5 g in each gluteal region18. Maintenance is achieved with 5 g every 4 hours in the gluteal region18. Zuspan's schedule begins with 4 g IV18, followed by continuous infusion of 1 g/hour18.

Elevated plasma levels are associated with adverse effects (Table I); therefore, it is necessary to observe some clinical parameters to guarantee the safety of its use17. Those parameters include: diuresis of 25 mL.h-1, positive patellar reflex, respiratory rate greater than 12 bpm, and unchanged vital signs (blood pressure, heart rate, and level of consciousness)17.



Magnesium decreases by 52% of the risk of seizures when compared to diazepam, and 67% when compared to phenytoin24. This study increased the use of magnesium from 2% to 40% in patients with preeclampsia in the United Kingdom19. Benzodiazepines are indicated for the treatment of seizures only postpartum17, in the absence of magnesium sulfate17, or when treatment with magnesium sulfate has failed2.

Further studies on this area will focus on aquaporin 419, a water channel-bound protein found in the final portion of astrocyte axon, whose levels are increased in cerebral edema19. Magnetic resonance imaging studies have documented cerebral edema of the white matter of the posterior region of the brain eclampsia patients25. This change has also been documented in animal models of eclampsia25. The expression of aquaporin 4 is increased in pregnancy26, and the use of magnesium sulfate decreases cerebral expression of this protein, which can attenuate cerebral edema in eclampsia patients27.

Magnesium has been used as the standard drug for tocolysis during treatment of premature labor, and other drugs have been compared to it28. The mechanism of action has not been completely elucidated, but it seems to be secondary to calcium antagonism by competing for the binding site of this ion28. The loading dose for tocolysis ranges from 4 to 6 g intravenous over 15 to 30 minutes, followed by maintenance with 2 to 6 g IV/hour28. Several patients treated with magnesium develop minor adverse reactions, such as: feeling hot, scotomata, nausea, vomiting, blurred or double vision, and lethargy5,28. Adverse effects can be reverted by the intravenous administration of 1 g of calcium gluconate5.



The indications of magnesium sulfate in anesthesia have been increasing over the years to include situations out of the gynecological field5. It has analgesic and sedative properties with potential neuro- and cardioprotective effects, although it is not know the mechanisms of those actions5,29.

During acute myocardial infarction (AMI), 80% of the patients develop hypomagnesemia in the first 48 hours, probably secondary to the high serum levels of catecholamines6. Magnesium deficiency leads to cell depolarization and promotes tachycardia6. Two studies using magnesium in patients with AMI, LIMIT 2 and ISIS 4, showed antagonic mortality results5. Only LIMIT 2 showed a reduction in mortality, but magnesium was used before spontaneous or pharmacologic recovery of the occluded vessel5. The prophylactic use to prevent hypomagnesemia during extracorporeal circulation is controversial, although reduction in the incidence of ventricular tachycardia and atrial fibrillation has been shown5.

Neuronal ischemia leads to the outflow of ATP from the cell and inflow of calcium, which triggers the release of toxic metabolites, culminating with cell death5. Blockade of glutamate NMDA receptors inhibits the cellular inflow of calcium and contributes for neuronal protection3,29-31. Other probable actions for cerebral protection include: reduction in the presynaptic release of excitatory neurotransmitters32, blockade of calcium channels32,33, suppression of anoxic depolarization32, antioxidant effects31,32, and an increase in cerebral blood flow29,30,32,33. Besides, cellular energy preservation is also seen, since magnesium is bound to ATP in the cytosol.

Two studies demonstrated antagonic results in patients with cerebral ischemia, IMAGE and FAST-MAG32. A 90 dose reduced infarction volume after middle cerebral artery embolus by 48% when administered in the first six hours32. It is possible that the doses used in the IMAGE study have not been enough to cause an increase in the concentration of magnesium in cerebral cells15.

Magnesium has been used to attenuate the cardiovascular response to tracheal intubation5. This effects is, probably, secondary to a reduction in the release of catecholamines after sympathetic stimulation1,3,5,11,34,35. A 40 dose has shown similar efficacy to that of 10 µ of alfentanil as well as greater effectivity than 1.5 of lidocaine5. It is a complementary drug in the treatment of hypertensive episodes during the surgical treatment of pheochromocytoma, since it inhibits the release of catecholamines from the adrenal glands5.

Magnesium inhibits the release of acetylcholine in the neuromuscular junction and behaves as a neuromuscular relaxant, potentiating the effects of non-depolarizing neuromuscular blockers5. A 40 dose of magnesium reduces the ED50 of vecuronium by 25%5. When magnesium is administered before induction, it prevents succinylcholine-induced increase in potassium levels9,36. This drug limits muscular fasciculation, but it does not interfere with the time of recovery of succinylcholine36.

The analgesic potential of magnesium is partially secondary to the blockade of NMDA receptors, but also to a reduction in the release of catecholamines11. The potential to reduce the MAC of volatile anesthetics has been confirmed in laboratorial studies with rat models1,5,37, and it can be as high as 60%1. Schutz-Stubner et al.3 demonstrated a reduction in the need of remifentanil and fentanyl when one intravenous dose of 50 of magnesium was used in humans. Collateral effects were not observed with this dose3.

The postoperative analgesia of magnesium has been analyzed in a systematic review that used qualitative evaluation methods38. Fourteen randomized clinical assays with 778 patients, 404 of which received magnesium, were included. Magnesium levulinate, sulfate, and gluconate were tested. Meta analysis could not be carried out due to methodological heterogeneity among the studies secondary to a wide variety of magnesium infusion schedules and among the final treatment objectives of each study38. Besides those factors mentioned by the authors of that study, different age groups and surgical procedures of different subspecialties were also included. The author of that review concluded that the studies included in the analysis did not demonstrate convincing evidence that magnesium was beneficial in the treatment of postoperative pain and reduction in analgesic consumption38.



Magnesium sulfate has been used in obstetrics with good results inhibiting of premature labor and in the treatment of eclampsia-related seizures. This drug is potentially analgesic and sedative and it could be used as adjuvant during general anesthesia, reducing the blood pressure response to tracheal intubation and decreasing the need of anesthetics.



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Correspondence to:
Dr. Fabiano Timbó Barbosa
Rua Comendador Palmeira, 113/202 Farol
Maceió, AL CEP: 57051-150

Submitted: 15 de junho de 2009
Accepted: 7 de outubro de 2009



Received from: Hospital da Agroindústria do Açúcar e do Álcool do Estado de Alagoas

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