Magnesium sulphate increased prostacyclin production and that is a potent vasodilator. Some people think this might be the reason for prevention of pre eclampsia and eclampsia where cerebral vasospams and decreased blood flow are thought to be contributory. It prevents seizures by binding to Ca sites and not letting the muscles work as they are supposed to.
Cardiac output usually increases following magnesium administration, compensating for the vasodilatation and minimising hypotension. It is the first-line anticonvulsant for the management of pre-eclampsia and eclampsia, and it should be administered to all patients with severe pre-eclampsia or eclampsia. Magnesium is a moderate tocolytic but the evidence for its effectiveness remains disputed.
What is the data?
1. Two studies randomized showed Mg sulfate over placebo to be preventive of seizures in severe pre eclampsia.
2. Overview of all controlled randomized trials comparing mg sulfate in pre eclampsia as an anticonvulsants show its superiority versus placebo
3. ACOG recommends that use of mg sulfate in women with severe pre eclampsia and that there is lack of consensus as to women in mild pre eclampsia require such treatment or not given small data on that.
Can acute magnesium toxicity occur in the obstetric literature and patients? Magnesium levels when checked are high in these patients getting doses of magnesium but no toxicity is seen. It is rare. Literature has few cases reported but mostly were due to accidental overdosing of the agent.
The normal plasma concentration of magnesium is 0.8-1.0 mmol/L or 1.7 to 2.2 mg/dL. and the suggested therapeutic range in pregnancy 1.7 to 3.5 mmol/L or 4.8-8.4mg/dl. Complicating the interpretation of serum magnesium is that it can be reported in milligrams per decilitre (mg/dL), milliequivalents per litre (mEq/L) or millimoles per litre. As a divalent ion, the latter two are not the same. Deep tendon reflexes are diminished or lost between 3.5 and 5.0 mmol/L, with respiratory paralysis thought to occur at >= 7.5 mmol/L, although significant ventilatory changes occur at lower concentration. Central nervous system depression in conjunction with serious cardiac conduction abnormalities is seen at 7.5 mmol/L and cardiac arrest possible at >=12.5 mmol/L. It also depends on when you draw the level and initially you might get a very high value.
The most common regimen for prevention in pregnant females is a loading dose of 6 g intravenously over 15 to 20 minutes followed by 2 g per hour as a continuous infusion. If someone is in renal failure, lower doses are suggested as main route of clearance is kidney. Following serum magnesium levels is not required if the woman's clinical status is closely monitored for evidence of potential magnesium toxicity. So we don't need to be chasing mg values.