Severe Preeclampsia on Magnesium Sulfate at 36 Weeks
A 35-year-old G2P1 at 36 weeks' gestation is admitted with a blood pressure of 168/112 mmHg, 3+ proteinuria, persistent frontal headache, and blurred vision. Laboratory results: platelets 96,000/mm³, AST 92 U/L, creatinine 0.9 mg/dL. The provider orders a 4 g magnesium sulfate loading dose over 20 minutes, then 2 g/hour, plus labetalol for blood pressure ≥160/110 mmHg. Two hours into the infusion her blood pressure is 158/104 mmHg, she reports that the headache is worse, and she has vomited once. Urine output over the last 2 hours was 40 mL. Deep tendon reflexes are 2+ and equal; respiratory rate is 16/min. The fetal heart rate tracing shows a baseline of 150 beats/min with moderate variability and no decelerations.
1. Identify which findings establish severe features and explain what magnesium sulfate is and is not treating in this patient. What is the immediate pharmacologic priority?
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Severe features are present: a systolic of 168 mmHg (≥160), platelets below 100,000/mm³, elevated AST, and cerebral symptoms (headache, visual change). This is severe preeclampsia regardless of the protein value.
The common error is treating magnesium sulfate as an antihypertensive. It is not: it is given for seizure prophylaxis. Her blood pressure is not being controlled by the magnesium infusion, and increasing the magnesium rate to lower the pressure would risk toxicity without benefit.
The immediate priority is antihypertensive therapy with labetalol (or hydralazine or oral nifedipine per protocol) for sustained severe-range pressures, with a goal of bringing the pressure down gradually, not abruptly. Severe hypertension in pregnancy should be treated within 30–60 minutes, so the question is not whether 158/104 qualifies as an emergency but whether it is rechecked in 15 minutes and treated if it stays severe. Simultaneously notify the provider, because at 36 weeks the definitive treatment is delivery, and the worsening headache suggests progression toward eclampsia.
2. Describe your monitoring plan during the magnesium infusion and the exact decision points at which you would stop the infusion and call the provider.
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Assess and document at least hourly, and more often after a rate change: respiratory rate, oxygen saturation, deep tendon reflexes, level of consciousness, urine output, and fetal heart rate. Magnesium is cleared renally, so urine output is a safety parameter, not a comfort measure.
Stop the infusion and notify the provider immediately if: respiratory rate falls below 12/min, deep tendon reflexes are absent or diminished from baseline, urine output falls below 30 mL/hour (or 100 mL over 4 hours), the patient becomes confused or somnolent, or there is a seizure. Keep calcium gluconate 1 g IV at the bedside as the antidote.
In this patient the 40 mL over 2 hours equals 20 mL/hour, which is below the threshold and must be investigated, not charted and ignored. Check for a kinked or displaced catheter, assess hydration, and report it. Note that her reflexes (2+) and respiratory rate (16) do not suggest magnesium toxicity yet, so the low output is a trigger to reassess renal perfusion and the line, not a reason to give calcium gluconate empirically. Calcium gluconate is given for toxicity, not for oliguria alone.
3. Her headache is worsening and she vomited. How do you interpret this, and what is your plan over the next hour?
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The worsening headache with visual symptoms in the setting of thrombocytopenia and elevated liver enzymes signals progressing severe disease and impending eclampsia, not a benign headache and not magnesium toxicity. Magnesium toxicity would show loss of reflexes, hyporeflexia, or respiratory depression, none of which are present.
Plan for the next hour: move her to a quiet, darkened room with seizure precautions and minimal stimulation; maintain continuous fetal monitoring and continuous blood pressure monitoring; recheck the blood pressure in 15 minutes and administer the ordered antihypertensive if it remains severe; verify patency of the magnesium infusion and the IV access; assess reflexes, respiratory rate, and oxygen saturation more frequently; keep suction, oxygen, and emergency delivery equipment available; and call the provider now with a structured report.
Anticipate that the provider will plan delivery, since at 36 weeks the risks of continuing outweigh the risks of delivery. Prepare for induction or cesarean, ensure blood products are available given the platelet count, and confirm that the anesthesia team is aware. Document the timing of every assessment and every call, because the clinical picture can change within minutes.