Magnesium sulfate is a common agent used in obstetrics for tocolysis and fetal neuroprotection, although no evidence supports its benefit for these purposes. It is also used in the treatment of preeclampsia and preeclampsia. Its use has many implications for anesthesiologists including the potential need to assist in managing overdosage as well as interactions with other anesthetic drugs.
Serum levels of magnesium (1%) reflect only a small portion of the body’s total magnesium reserve (24 grams). While magnesium is involved in numerous physiologic processes including respiratory, cardiac, neurological, gastrointestinal, and metabolic, it is also a widely used pharmacologic agent in obstetrics. As a calcium antagonist, magnesium sulfate is a widely used tocolytic and treatment for preeclampsia. As an N-methyl-d-aspartate (NMDA) antagonist, magnesium is used in fetal neuroprotection and as an anticonvulsant in the prevention and treatment of eclampsia. It also results in decreased minimum alveolar concentration during general anesthesia.
Side effects of magnesium therapy include flushing, nausea/vomiting, muscle weakness, lethargy, and respiratory depression. It can also result in significant cardiovascular aberrations that ranges from hypotension and bradycardia to complete heart bock and cardiac arrest (Table 155-1). Electrocardiogram (EKG) findings include a widened QRS complex and prolonged PR interval. It can cause postpartum hemorrhage by increasing uterine atony. Finally, neonates born to mothers receiving magnesium sulfate therapy can suffer from floppy baby syndrome, characterized by flaccidity and respiratory depression or apnea. Magnesium sulfate toxicity is treated with calcium.
TABLE 155-1Magnesium Blood Levels and Effect ||Download (.pdf) TABLE 155-1 Magnesium Blood Levels and Effect
|mg/dL ||mEq/L ||Effect |
|1.8–2.4 ||1.2–2 ||Normal |
|4.8–9.6 ||4–8 ||Therapeutic |
|6–12 ||5–10 ||EKG changes (prolonged PR interval, widened QRS) |
|12 ||10 ||Muscle weakness; loss of deep tendon reflexes |
|18 ||15 ||Sinoatrial/atrioventricular node block; respiratory paralysis |
|24 ||20 ||Cardiac arrest |
Magnesium sulfate is the most commonly used tocolytic agent in the United States. As a calcium antagonist, it decreases the availability of intracellular calcium by blocking extracellular calcium influx and intracellular calcium release. Therefore, it results in decreased myosin activity, thereby leading to muscle relaxation. In addition, in animal studies, it has been shown to cause uterine artery vasodilation and increased uterine blood flow. While it has been shown to decreases uterine activity, its benefit in delaying preterm labor has not been proven.
Magnesium sulfate is frequently used as a tocolytic agent in preterm labor because of its potential for neuroprotection in premature neonates. Although animal studies have suggested that magnesium sulfate can reduce ischemic cellular injury—potentially by acting as an NMDA receptor antagonist and decreasing intracellular calcium release—its exact mechanism is unknown. Its use for fetal neuroprotection has not been associated with improved neonatal outcomes.