Magnesium Sulfate in Preeclampsia with Severe Features: Correct Statement
The correct answer is (b): The loading dose is 4-6 grams over 20-30 minutes. This is the standard, evidence-based dosing regimen endorsed by multiple international guidelines and the FDA drug label 1, 2, 3.
Why Option B is Correct
The FDA-approved loading dose for magnesium sulfate in severe preeclampsia/eclampsia is 4-5 grams IV over 20-30 minutes (or 3-4 minutes for more rapid administration in active seizures), followed by a maintenance infusion of 1-2 grams/hour 3. This dosing achieves therapeutic serum levels of 1.8-3.0 mmol/L almost immediately when given intravenously 4.
- The American Heart Association specifically recommends a 4-5g IV loading dose over 5 minutes for eclampsia, with maintenance of 1-2g/hour for 24 hours after the last seizure 1
- The European Society of Cardiology and multiple guidelines confirm the 4-6 gram loading dose over 20-30 minutes as standard practice 2
- Alternative regimens include 4g IV combined with 10g IM (5g in each buttock) for a total loading dose of 14g when IV access is limited 1, 3
Why the Other Options are Wrong
Option A: "It increases the risk of placental abruption" - FALSE
There is no evidence that magnesium sulfate increases placental abruption risk. In fact, one study noted placental abruption occurred in patients receiving magnesium sulfate, but this was a complication of the underlying severe preeclampsia itself, not caused by the medication 5. Magnesium sulfate is used for seizure prevention, not as a cause of obstetric complications.
Option C: "It is used primarily to lower blood pressure" - FALSE
Magnesium sulfate is NOT an antihypertensive agent. Its primary indication is seizure prevention and control in preeclampsia/eclampsia 6, 1, 2. It is the most effective anticonvulsant for this indication, superior to phenytoin and diazepam 6.
- Blood pressure control in severe preeclampsia requires separate antihypertensive therapy with agents like IV labetalol, oral nifedipine, or IV hydralazine 7, 1
- The target BP is <160/105-110 mmHg, achieved with dedicated antihypertensives, not magnesium sulfate 1, 2
Option D: "A magnesium level should be checked if urine output is < 60 mL/hour" - PARTIALLY CORRECT BUT NOT THE BEST ANSWER
While oliguria does increase magnesium toxicity risk (as magnesium is renally excreted), routine serum magnesium levels are NOT recommended 6. The threshold mentioned is also imprecise:
- The American College of Cardiology states that clinical monitoring (reflexes, respiratory rate, urine output) should guide therapy, not routine serum levels 6
- The European Society of Cardiology recommends maintaining urine output ≥30 mL/hour, not 60 mL/hour 6
- The FDA label specifies monitoring urine output >100 mL over 4 hours (25 mL/hour average) preceding each dose 3
- Serum magnesium levels should only be checked in specific high-risk situations like renal impairment with elevated creatinine 6
Critical Monitoring Parameters (Not Serum Levels)
Clinical examination is sufficiently sensitive to detect toxicity before serious complications 6:
- Loss of patellar reflexes occurs at 3.5-5 mmol/L (first warning sign) 4
- Respiratory depression occurs at 5-6.5 mmol/L 4
- Cardiac conduction abnormalities occur at >7.5 mmol/L 4
- Cardiac arrest occurs at >12.5 mmol/L 4
Monitor respiratory rate, deep tendon reflexes, and urine output clinically—serum levels are unnecessary in most cases 6, 3, 4.
Common Pitfalls to Avoid
- Never combine magnesium sulfate with calcium channel blockers (especially IV or sublingual nifedipine) without extremely careful monitoring—this causes severe myocardial depression and precipitous hypotension 6, 1, 2
- Do not use magnesium sulfate beyond 5-7 days in pregnancy, as continuous administration can cause fetal abnormalities 3
- Maximum dose is 30-40g per 24 hours in normal renal function; reduce to 20g/48 hours in severe renal insufficiency 3
- Have injectable calcium salt available to counteract magnesium toxicity 1