Magnesium Sulfate Administration in Severe Preeclampsia
The correct answer is (d): A magnesium level should be checked if the urine output is < 60 mL/hour, though the more critical threshold is actually < 30 mL/hour, as oliguria increases the risk of magnesium toxicity since the drug is renally excreted. 1, 2
Why Each Answer is Right or Wrong
Option (a): Placental Abruption Risk - FALSE
- Magnesium sulfate does not increase the risk of placental abruption 3
- In fact, placental abruption is listed as an indication for delivery in preeclampsia, not as a complication of magnesium therapy 3
Option (b): Loading Dose of 4-6 Grams Over 20-30 Minutes - TRUE
- The standard loading dose is indeed 4-6 grams IV over 20-30 minutes 2, 4
- The FDA label specifies: "4 to 5 g in 250 mL of 5% Dextrose Injection or 0.9% Sodium Chloride Injection may be infused" or "the initial IV dose of 4 g may be given by diluting the 50% solution to a 10 or 20% concentration...injected IV over a period of three to four minutes" 4
- This is followed by maintenance infusion of 1-2 grams per hour 2, 4
Option (c): Used Primarily to Lower Blood Pressure - FALSE
- Magnesium sulfate is NOT used for blood pressure control 1
- Its primary indication is seizure prevention and control in severe preeclampsia and eclampsia 1, 2
- Hydralazine, labetalol, and nifedipine are used for acute blood pressure control, not magnesium sulfate 3, 1
Option (d): Magnesium Level Check with Low Urine Output - TRUE (Most Clinically Important)
- Urine output monitoring is critical because magnesium is almost exclusively renally excreted 5
- The European Society of Cardiology recommends maintaining urine output ≥30 mL/hour, as oliguria increases toxicity risk 1
- The FDA label states: "In the presence of severe renal insufficiency, the maximum dosage of magnesium sulfate is 20 grams/48 hours and frequent serum magnesium concentrations must be obtained" 4
- However, routine serum magnesium monitoring is NOT recommended in patients with normal renal function 1, 6
Clinical Monitoring Protocol (Not Routine Lab Monitoring)
The American College of Cardiology emphasizes that magnesium levels should NOT be routinely drawn; instead, clinical monitoring should guide therapy: 1
Essential Clinical Parameters to Monitor:
- Patellar reflexes - Loss occurs at 3.5-5 mmol/L (first sign of toxicity) 5
- Respiratory rate - Respiratory paralysis occurs at 5-6.5 mmol/L 5
- Urine output - Must maintain ≥30 mL/hour 1
- Oxygen saturation - Keep >90% 1, 2
When to Check Serum Magnesium Levels:
Laboratory monitoring is indicated ONLY in specific high-risk situations: 1
- Renal impairment (elevated creatinine)
- Urine output < 30 mL/hour (oliguria)
- Loss of patellar reflexes
- Respiratory rate < 12 breaths/minute
Critical Safety Considerations
Absolute Contraindication:
- NEVER combine magnesium sulfate with calcium channel blockers (especially IV or sublingual nifedipine) - this can cause severe myocardial depression and precipitous hypotension 3, 1, 2, 7
Fluid Management:
- Limit total fluid intake to 60-80 mL/hour to avoid pulmonary edema 3, 2, 7
- Preeclamptic women have capillary leak and are at risk for both pulmonary edema and acute kidney injury 3
Duration of Therapy:
- Continue for 24 hours postpartum in most cases 3, 2
- Do NOT exceed 5-7 days of continuous administration as this can cause fetal abnormalities 4
Common Clinical Pitfall
The question tests whether you understand that magnesium sulfate is for seizure prophylaxis, NOT blood pressure control. Many clinicians mistakenly believe it lowers blood pressure, but this is incorrect. Antihypertensive agents (nifedipine, labetalol, hydralazine) are used separately for blood pressure management when BP ≥160/110 mmHg. 3, 1