Judicious use of magnesium sulfate for eclampsia

Serum magnesium levels can guide therapy in patients with signs of toxicity, renal insufficiency, or recurrent seizures, but offer no advantage over close clinical scrutiny in typical patients. Steady-state plasma magnesium levels are about the same in the fetus as in the mother to whom magnesium is administered.²¹ High fetal levels can impair fetal breathing movements, which could lower biophysical profile scores in the absence of significant fetal hypoxia.

• Pulmonary function. Onset of a dry cough should raise suspicion of incipient pulmonary edema. Pulmonary auscultation detects rales that can accompany disease- or therapy-related pulmonary edema. Pulse oximetry, which provides continuous arterial oxygen saturation levels, can be very helpful in alerting the health-care team to both magnesium-induced respiratory depression and significant limitations in pulmonary gas exchange that accompany pulmonary edema.
  
• Urine output. In the presence of oliguria (less than 100 mL in 4 hours), the rate of magnesium administration should be reduced by 50%.
  

The effects of magnesium toxicity can be rapidly reversed with 1 g IV calcium chloride or calcium gluconate. Seriously affected patients, however, may require dialysis to lower maternal magnesium concentrations, due to the long half-life of magnesium in plasma (approximately 4 hours in normal gravidas).

Also give IV calcium chloride or calcium gluconate for respiratory depression or other signs of cardiorespiratory toxicity. Immediate intubation with assisted ventilation is necessary in cases of cardiorespiratory failure. Fortunately, this phenomenon occurs very rarely with proper patient selection and rigorous surveillance.

TABLE 4

Monitoring guidelines for patients receiving magnesium

FUNCTION	WHEN TO MONITOR	SUSPECT MAGNESIUM TOXICITY AND COMPLICATIONS WHEN…
Patellar reflexes	Every 1–4 h*	Reflexes are absent
Respiratory rate	Every 1–4 h*	Rate is
Pulmonary auscultation	Every 12 h or with development of respiratory symptoms or signs	Rales are present
Pulse oximetry	Every 1–4 h*
Urine output	Every 1–4 h*
Serum magnesium levels	In presence of oliguria, persistent seizures, or signs of toxicity	>8 mEq/L
*Depending on clinical state

The authors report no affiliations or financial arrangements with any of the manufacturers of products mentioned in this article.