Loading dose: 6g IV over 20 min
Maintenance: 2-3 g/h IV
Monitoring of MgSO4 therapy
- Patellar reflexes
- Respiratory rate (>14/minl
- Urine output (100 cc in 4 h or 30 cc/h)
- Therapeutic range of magnesium is 4-7 mEq/L.
- Uterus stops contracting at 8-10 mEq/L
- Patellar reflexes disappear when the plasma magnesium level reaches 10 mEq/L, presumably because of a curariform action. This sign serves to warn of impending magnesium toxicity, because a further increase leads to respiratory depression.
- When plasma levels rise above 10 mEq/L, respiratory depression develops, and at 12 mEq/L or more, respiratory paralysis and arrest follow.
- Treatment with calcium gluconate, 1 g intravenously, along with withholding further magnesium sulfate usually reverses mild-to-moderate respiratory depression.
- Because magnesium is cleared almost exclusively by renal excretion, plasma magnesium concentration, using the doses described previously, is excessive if glomerular filtration is decreased substantively. The initial standard dose of magnesium sulfate can be safely administered without knowledge of renal function. Renal function is thereafter estimated by measuring plasma creatinine, and whenever it is 1.3 mg/ dL or higher, only half of the maintenance intramuscular magnesium sulfate dose should be administered.