Administration and Dosing
Administration Routes and Preparation
- Administered intravenously (IV) in maternal care settings, typically as a loading dose followed by continuous infusion. Intramuscular (IM) administration may be used in some emergency situations but is less common due to pain and inconsistent absorption.
- Available in concentrations of 10%, 12.5%, 25%, and 50% solutions; must be diluted appropriately before administration.
Never administer undiluted magnesium sulfate as a rapid IV push! Always use controlled infusion devices for administration.
- Verify order and patient using two identifiers
- Prepare solution according to facility protocol (typically 40g in 1000mL solution)
- Use controlled infusion device with dedicated IV line
- Monitor vital signs per protocol before and during administration
- Have calcium gluconate (antidote) readily available
Key Points
- For preeclampsia/eclampsia: Typical loading dose is 4-6g IV over 20-30 minutes, followed by maintenance infusion of 1-2g/hour.
- For tocolysis: Initial bolus of 4-6g over 20-30 minutes, followed by maintenance infusion of 2-4g/hour for 12-24 hours.
Clinical Scenario:
A 32-year-old G2P1 at 30 weeks gestation is admitted with BP 168/110, proteinuria 3+, and complaints of headache and visual changes. After the physician diagnoses severe preeclampsia, magnesium sulfate is ordered. The nurse prepares to administer a 6g loading dose over 20 minutes, followed by a maintenance infusion of 2g/hour.
Question: What assessments must be completed before initiating magnesium sulfate therapy?
Answer: Before initiating therapy, the nurse must assess: baseline vital signs including respiratory rate, deep tendon reflexes, level of consciousness, urinary output (should be >30 mL/hr), and laboratory values including magnesium, calcium, renal function, and liver function tests.
Monitoring and Nursing Considerations
Essential Monitoring Parameters
- Monitoring focuses on identifying therapeutic effects versus toxicity. Therapeutic serum magnesium levels range from 4-7 mEq/L (4.8-8.4 mg/dL) for seizure prophylaxis and treatment.
Magnesium Toxicity Monitoring (Remember: STOP MgSO4)
- Serum levels (therapeutic: 4-7 mEq/L)
- Tendon reflexes (should be present)
- Output (urinary output >30 mL/hr)
- Pulmonary function (respiratory rate >12/min)
Signs of Magnesium Levels
| Serum Level (mEq/L) |
Clinical Manifestations |
Nursing Action |
| 4-7 |
Therapeutic range, normal DTRs |
Continue monitoring |
| 7-10 |
Diminished DTRs, feeling of warmth, flushing, lethargy |
Notify provider, consider rate reduction |
| 10-12 |
Absent DTRs, somnolence, slurred speech |
Stop infusion, notify provider immediately |
| 12-15 |
Respiratory depression, hypotension |
Stop infusion, prepare to administer calcium gluconate |
| >15 |
Respiratory arrest, cardiac arrest |
Stop infusion, administer calcium gluconate, initiate resuscitation |
Key Points
- Monitor maternal vital signs, deep tendon reflexes, and urinary output every 1-4 hours during administration.
- Calcium gluconate 1g IV (10mL of 10% solution) is the antidote for magnesium toxicity and should be readily available.
- Magnesium is primarily excreted by the kidneys; use with extreme caution in renal impairment.
Maternal and Fetal Effects
- Maternal Effects: Peripheral vasodilation leading to decreased blood pressure, decreased central nervous system irritability, decreased uterine contractility, and potential for magnesium toxicity.
- Fetal/Neonatal Effects: Crosses the placenta leading to potential hypotonia, respiratory depression, and decreased intestinal motility in the neonate. Prolonged exposure may cause bone demineralization.
Newborns of mothers who received magnesium sulfate should be monitored for respiratory depression, hypotonia, and hypocalcemia for at least 24-48 hours after birth!
Key Points
- Magnesium therapy is typically continued for 24 hours postpartum in preeclampsia/eclampsia cases.
- Fetal heart rate should be continuously monitored during administration in antepartum patients.
Commonly Confused Points
Magnesium Sulfate vs. Other Medications
Magnesium Sulfate vs. Other Anticonvulsants/Tocolytics
| Aspect |
Magnesium Sulfate |
Diazepam (Valium) |
Nifedipine (Procardia) |
| Primary Use in Maternity |
Anticonvulsant in preeclampsia; tocolytic |
Alternative anticonvulsant in eclampsia |
Tocolytic; antihypertensive |
| Mechanism |
CNS depression; blocks calcium |
GABA receptor agonist |
Calcium channel blocker |
| Monitoring Focus |
DTRs, RR, UO, BP |
Respiratory depression, sedation |
BP, pulse, fetal heart rate |
| Antidote |
Calcium gluconate |
Flumazenil |
None specific |
Never administer calcium channel blockers (like nifedipine) concurrently with magnesium sulfate as this can cause profound hypotension and neuromuscular blockade!
Key Points
- Magnesium sulfate is preferred over diazepam for eclamptic seizures because it causes less respiratory depression in both mother and neonate.
- Unlike betamimetics (terbutaline), magnesium has minimal effect on maternal heart rate and blood glucose.
Common Medication Errors
- Confusion between percentage concentration and dose is a common error. For example, a 50% solution contains 500mg/mL, while a 10% solution contains 100mg/mL.
- Failure to distinguish between magnesium sulfate and other medications with similar names (e.g., morphine sulfate) can lead to serious medication errors.
Key Points
- Always use infusion pumps and smart technology with programmed dose limits.
- Double-check dosage calculations with another nurse before administration.