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Failure to set a volume limit for a magnesium bolus dose leads to harm

From the June 3, 2010 issue

Problem: A 27-year-old pregnant woman was admitted to a labor and delivery unit with cramping, abdominal pain, vaginal bleeding, and leaking amniotic fluid. Although she was only 27 weeks pregnant, an exam showed that she was in the early stages of labor with a breech baby. In an attempt to stop the preterm labor from progressing, the obstetrician prescribed IV magnesium sulfate, with a bolus dose of 6 g over 30 minutes followed by a continuous infusion of 2 g/hour.

The patient’s nurse obtained a 20 g/500 mL bag of magnesium sulfate from an automated dispensing cabinet. She was unfamiliar with programming a bolus dose using the software in the smart pump. The bolus dose feature allows the user to program the pump to deliver a bolus dose (6 g/30 minutes) and then automatically switches to deliver a continuous infusion (2 g/hour) once the bolus dose has been administered. Not knowing how to use this feature, the nurse programmed the bolus to be delivered as a continuous infusion at 12 g/hour. The pump did not provide a hard stop to guard against an excessive dose of magnesium during continuous infusion. The nurse intended to return to the patient’s room in 30 minutes to reprogram the pump to deliver 2 g/hour. However, she was distracted by other responsibilities and failed to return to the patient’s room in 30 minutes to adjust the infusion rate.

When the patient became flushed and short of breath, she called for a nurse. When the nurse arrived, the patient also complained of dizziness and was found to be hypotensive. She quickly became unresponsive, requiring brief cardiopulmonary resuscitation. Magnesium toxicity was suspected, so the infusion was stopped and the patient was given a rescue dose of IV calcium. Lab results confirmed a supratherapeutic magnesium blood level. To avoid serious adverse effects from IV magnesium sulfate, bolus doses should not exceed 6 g over 15-20 minutes, and continuous infusion rates should not exceed 3 g/hour. In this case, the infusion ran at 12 g/hour for more than 1 hour.

Fortunately, the patient responded to emergency treatment. The baby was delivered by C-section several days after the event due to unrelated preterm complications. Both the mother and child were eventually discharged with no permanent harm. However, we have published descriptions of more than a dozen events related to magnesium overdoses (February 12, 1997; June 30, 1999; October 20, 2005; June 15, 2006 newsletters). In these cases, two mothers died and two suffered anoxic encephalopathy, leaving them in a vegetative state. Most of the errors were due to misprogrammed pumps, unfamiliarity with safe doses and signs of magnesium toxicity, inadequate monitoring, and mix-ups between magnesium and oxytocin.       

Safe Practice Recommendations: Never infuse an IV magnesium sulfate bolus dose from the maintenance solution UNLESS ALL of the following criteria are CONSISTENTLY met:

  1. The bolus is delivered using the bolus dose feature with a smart infusion pump that allows programming of both the bolus dose and continuous infusion rate, and then automatically switches to the continuous infusion rate once the bolus dose has been administered
  2. Separate dose limits have been set for bolus doses (e.g., 6 g/30 minutes) and maintenance doses (e.g., 3 g/hour) of magnesium sulfate
  3. These dose limit alerts remain operational at all times
  4. These dose limit alerts are configured as a “hard stop,” which forces the user to reprogram the infusion if the dose exceeds safe limits
  5. A qualified nurse remains at the bedside during infusion of the bolus dose to monitor the patient for signs of magnesium toxicity.

If these five conditions are not consistently met, ISMP recommends administering the bolus dose from a separate container prepared by pharmacy; a commercially available 4 g mini-bag can also be used if a 4 g bolus dose is prescribed. If the above five conditions are consistently met, ensure that all smart pump users have been trained to program the pump—including bolus doses—and can demonstrate ongoing competency. Bypassing the bolus dose functionality or employing a “generic” setting on a smart pump is unsafe. 

For maintenance solutions, ISMP recommends using only the 20 g/500 mL bags of magnesium sulfate, not the 40 g/1,000 mL bags. The 500 mL bag is easier to differentiate from liter bags of hydrating solutions and medications used during labor and delivery. The smaller volume also helps limit the amount of magnesium a patient could receive if a rapid infusion occurs accidentally. For added safety, some organizations only use the 4 g mini-bags for maintenance infusions, which require frequent bag changes but limits the amount of drug the patient could receive if an error occurs. Some facilities also require an independent double-check of pump settings before starting magnesium infusions (bolus and maintenance). 
During IV magnesium administration, frequently monitor the patient’s vital signs, oxygen saturation, deep tendon reflexes, level of consciousness, and fetal heart rate characteristics/maternal uterine activity. While giving a bolus dose, a nurse should remain at the bedside to monitor the patient and watch for clinical manifestations of magnesium toxicity. Generalized neuromuscular abnormalities, visual changes, facial flushing, nausea, somnolence, and weakness are often initial presenting symptoms, which progress to hypotension (refractive to vasopressors or volume expanders), muscle paralysis, coma, hypoactive tendon reflexes, respiratory failure, paradoxical bradycardia, prolonged PR, QRS, and QT interval, complete heart block, and/or cardiac arrest. If symptoms of magnesium toxicity are present, immediately treat the patient and then investigate the possibility of an error. Stock calcium gluconate nearby with directions for use during respiratory depression. Ensure that staffing patterns allow time for proper monitoring. Protocols should require periodic monitoring of magnesium blood levels. When magnesium sulfate is discontinued, immediately remove and discard the bag and tubing to avoid potential mix-ups with other IV fluids and medications.
Don’t be misled by a solid safety record in your labor and delivery unit. Complacency with using IV magnesium sulfate could eventually lead to a tragedy. All hospitals and birthing centers that offer obstetrical services should not delay in implementing these error-reduction strategies.

See our October 20, 2005 article ( for additional recommendations to prevent magnesium toxicity in antepartum patients.
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