Judicious use of magnesium sulfate for eclampsia
The landmark Magpie study confirmed magnesium’s effectiveness in treating and preventing pregnancy-related seizures. Some Ob/Gyns fear side effects and toxicity, however. This practical guide tells how to assess risk and select the appropriate regimen.
IM injection remains an option. Intravenous infusion of magnesium sulfate may not always be practical—for example, when infusion pumps or close patient supervision are unavailable, or when a patient is transported to another facility. Intramuscular injections can be used in these situations.
We give an initial dose of 5 g (10 mL as a 50% solution) of magnesium sulfate with 1 mL of 2% xylocaine deep in the upper outer quadrant of each buttock (10 g total magnesium sulfate). The magnesium solution is injected in several different sites as the needle (3 inches long, 20 gauge) is advanced in muscle. Each injection should be preceded by aspiration to ensure that the needle tip is not in a blood vessel. Massaging the buttock after the injection will help disperse the magnesium in the tissue. Five grams of magnesium sulfate (10 mL as a 50% solution with 1 mL of 2% xylocaine) is subsequently administered as a single intramuscular injection every 4 hours to maintain circulating magnesium levels, provided there is no evidence of magnesium toxicity. Patients with severe preeclampsia, prodromal symptoms of eclampsia, or eclampsia should be given 4 g of magnesium sulfate intravenously (20 mL as a 20% solution) over 5 minutes to more rapidly establish therapeutic magnesium levels immediately prior to the initial intramuscular injection of 10 g of magnesium sulfate.
Convulsions may occur even at therapeutic levels. Even with therapeutic serum magnesium concentrations, seizures are possible.18,23 Recurrent convulsions in patients already receiving magnesium should be treated with an additional 2 g IV magnesium sulfate administered over 5 minutes. Another 2-g dose (4 g total) can be given, but the patient must be carefully watched for signs of respiratory depression. If magnesium fails to control the seizures, additional measures are needed, such as IV anticonvulsants or muscular paralysis in conjunction with intubation and mechanical ventilation.24
Continue administration postpartum. Magnesium sulfate generally is administered for 24 hours after delivery or after the last postpartum seizure, although optimal length of treatment is not firmly established. The clinical state of the patient may be a useful index for individualizing duration of magnesium infusion,25,26 but the risks and benefits of this approach have not been examined in a large patient population.
Know the contraindications. Contraindications to magnesium therapy include myasthenia gravis and myocardial ischemia/failure. Magnesium, which is excreted almost exclusively by the kidneys, should be administered with extreme caution to patients in renal failure (because of the risk of cardiorespiratory depression) and with care to those receiving other calcium-channel antagonists, such as nifedipine (though the incidence of significant maternal hypotension under these circumstances is low).13
The effects of magnesium toxicity can be rapidly reversed with 1 g intravenous calcium chloride or calcium gluconate.
Magnesium can interact with other cardiovascular drugs to elicit arrhythmias or reduce myocardial contractility, and can potentiate the action of muscle relaxants and anesthetics. Thus, use the drug cautiously under these conditions.
TABLE 2
Magnesium sulfate administration*
| INTRAVENOUS INFUSION | |
| Normal renal function | |
| Loading dose: | 4-6 g over 10-15 min |
| Maintenance infusion: | 2 g/h |
| Oliguria† | |
| Loading dose: | 4 g over 15 min |
| Maintenance infusion: | 1 g/h |
| INTRAMUSCULAR INJECTION | |
| Loading dose: | 5 g in each buttock (10 g total) |
| Maintenance injection: | 5 g in 1 buttock (5 g total) every 4 h |
| *All dosages refer to the hydrated form (MgSO 4 7H 2 O) | |
| †Must continuously monitor pulse oximetry and measure magnesium levels 2 hours after starting the infusion. Calcium chloride or calcium gluconate and emergency intubation should be immediately available, and fluids should be restricted provided renal failure is not due to hypovolemia. | |
Risks of magnesium
At therapeutic concentrations of magnesium, about one quarter of pregnant women experience nausea, emesis, flushing, or weakness.1 Magnesium therapy also can be associated with lethargy, blurred vision, and urinary retention.24 Toxic effects, which vary in a dose-dependent manner (TABLE 3), include loss of reflexes, respiratory depression, cardiac arrhythmias, and cardiac arrest.14
Theoretical concerns include prolonged labor or increased blood loss at delivery, but these have not posed a significant problem in actual practice.1,27
Magnesium-induced neonatal depression, as evidenced by hypotonia and low Apgar scores, also may occur, but have not been observed in all studies.28,29 Obviously, pediatricians should attend these deliveries in case such complications are encountered.
TABLE 3
Maternal serum magnesium concentrations associated with toxicity
| MMOL/L | MEQ/L | MG/DL | |
|---|---|---|---|
| Loss of patellar reflexes | 3.5–5 | 7–10 | 8.5–12 |
| Respiratory depression | 5–6.5 | 10–13 | 12–16 |
| Altered cardiac conduction | >7.5 | >15 | >18 |
| Cardiac arrest | >12.5 | >25 | >30 |
| Source: Lu and Nightingale14 | |||
Preventing magnesium toxicity
Closely monitor the patient. Safe administration requires vigilant monitoring of reflexes, respiratory status, and urine output. Prior to initiating therapy, document deep tendon reflexes, respirations of 16 or more per minute, and urinary excretion exceeding 25 mL/h.14 During magnesium infusion, regularly assess respiratory rate, patellar reflexes, and urine output (TABLE 4).
