Magnesium sulfate heptahydrate: Application, Dosages and Toxicity

Magnesium sulfate heptahydrate, also known as MAGNESIUM SULFATE 7HYD XTL, is a white or colorless needle-shaped or inclined columnar crystal. When the temperature is at 200 ℃, all crystalline water will be lost and become anhydrous. It is easy to be weathered into powder in the air (dry). When heated, the crystalline water can be gradually removed and the magnesium sulfate heptahydrate becomes anhydrous magnesium sulfate. Because magnesium sulfate heptahydrate is not easy to dissolve, it is easier to weigh than anhydrous magnesium sulfate, which is convenient for quantitative control in industry. The Magnesium sulfate heptahydrate is mainly used in the manufacture of fertilizer, leather making, printing and dyeing, catalyst, paper making, plastics, matches, explosives and fireproof materials. Furthermore, it can be used for printing and dyeing thin cotton cloth and silk, as a weighting agent for cotton silk and a filler for kapok products; Besides, it is used medicinally as a cathartic salt. Its appearance is as follows:

Figure 1 Appearance of Magnesium sulfate heptahydrate

Initially, Magnesium sulfate heptahydrate was given in very low doses, although it is now administered in relatively high doses. Common regimens are an initial intravenous loading dose of 4 grams: followed by maintenance intravenous infusions of 1 gram per hour; or by 10 grams by intramuscular injection and then 5 grams intramuscularly every 6 hours [6].

Toxicity and safety

There are concerns regarding the possibility of hypermagnesemia toxicity in eclampsia treatment. Normal serum concentrations of Mg²⁺ are 1.5 to 2.5 mEq/L (1.8 to 3.0 mg/dL), with one-third to one-half bound to plasma proteins. Total magnesium serum concentrations advocated for the treatment of eclamptic convulsions are 3.5 to 7 mEq/L (4.2 to 8.4 mg/dL), which can be obtained by administering it intramuscularly (6 g loading dose followed by 2 g/h), intravenously (2 to 4 g dose up to 1 g/min), or a combination of both. Areflexia, particularly loss of the patellar deep tendon reflex, has been observed at 8 to 10 mEq/L, and respiratory paralysis seen at >13 mEq/L. Progressively higher serum magnesium levels can ultimately lead to cardiac arrest [7].Some suggest that using standard infusion protocols may not lead to therapeutic serum magnesium levels in all patients, with 36.2% of patients found to have total serum magnesium lower than 4 mEq/L at 30 minutes after treatment initiation in one study, though no eclamptic seizures were reported during Magnesium sulfate heptahydrate treatment. In addition, there are reports that in some patients eclamptic seizures do not cease even with elevated levels of Magnesium sulfate heptahydrate, suggesting that Magnesium sulfate heptahydrate is not effective in treating all cases of eclampsia.

[2]Witlin et al. Magnesium sulfate therapy in preeclampsia and eclampsia. Obstet Gynecol. 1998; 92: 883–889.

[3]Altman et al. The Magpie Trial Collaboration Group. Do women with pre-eclampsia, and their babies, benefit from magnesium sulphate? The Magpie Trial: A randomised placebo-controlled trial. Lancet. 2002; 359: 1877–1890.

[4]The Eclampsia Trial Collaborative Group. Which anticonvulsant for women with eclampsia? Evidence from the collaborative eclampsia trial. Lancet. 1995; 345: 1455–1463.

[5]Gülmezoglu AM, Duley L. Use of anticonvulsants in eclampsia and pre-eclampsia: Survey of obstetricians in the United Kingdom and Republic of Ireland. BMJ. 1998; 316: 975–976.

[6]Duley et al. Magnesium sulphate versus diazepam for eclampsia (Review). The Cochrane database of systematic reviews, 2000, 2: CD000127.

[7] Euser et al. Magnesium Sulfate for the Treatment of Eclampsia. Stroke. 2009;40:1169–1175.