In a 34‑week pregnant woman with preeclampsia, normal non‑stress test, normal renal function, decreased urine output despite adequate hydration and a magnesium sulfate infusion, is it safe to continue the magnesium sulfate infusion and to give furosemide to increase urine output?

Magnesium Sulfate Safety and Furosemide Use in Preeclampsia at 34 Weeks

Continue the magnesium sulfate infusion with close monitoring of reflexes, respiratory rate (≥12–16/min), and urine output (≥25–30 mL/hr), but do NOT give furosemide to increase urine output—this approach is contraindicated and potentially dangerous in preeclamptic women.

Magnesium Sulfate: Safe to Continue with Proper Monitoring

Clinical Safety Parameters

Your patient can safely continue magnesium sulfate if you maintain vigilant monitoring:

• Check patellar reflexes before each dose—the presence of deep tendon reflexes is the earliest clinical indicator that magnesium levels remain safe 1, 2
• Maintain respiratory rate ≥12–16 breaths per minute—falling below this threshold signals magnesium toxicity and requires immediate cessation 1, 2
• Ensure urine output ≥25–30 mL/hour—this is essential for adequate renal clearance of magnesium 1, 2
• Monitor oxygen saturation continuously with pulse oximetry to detect early respiratory depression 1

Normal Creatinine Does Not Eliminate Risk

Even with normal creatinine, magnesium is removed solely by the kidneys, so decreased urine output despite adequate hydration is a red flag requiring heightened vigilance 2. The FDA label explicitly states that magnesium "should be used with caution in patients with renal impairment" and that "urine output should be maintained at a level of 100 mL or more during the four hours preceding each dose" 2.

Fluid Restriction is Mandatory

Limit total IV fluid intake to 60–80 mL/hour during magnesium infusion 3. The ISSHP guidelines emphasize that preeclamptic women have capillary leak and are at risk for pulmonary edema—there is "no rationale to 'run dry' a preeclamptic woman as she is already at risk of AKI," but excessive fluids are equally dangerous 3.

Furosemide: Contraindicated in This Clinical Context

Why Furosemide Should NOT Be Given

Furosemide is inappropriate and potentially harmful in the antepartum management of preeclampsia with decreased urine output. Here's why:

• Preeclamptic women are already intravascularly volume-depleted despite appearing edematous due to capillary leak 3. The ISSHP guidelines explicitly state that "plasma volume expansion is not recommended routinely in women with preeclampsia" and that fluid balance should "aim for euvolemia" 3

• Diuretics worsen the underlying pathophysiology—forcing diuresis in a patient who is already at risk of acute kidney injury can precipitate renal failure and compromise uteroplacental perfusion 3

• The decreased urine output likely reflects the disease severity, not fluid overload requiring diuresis. With adequate hydration and normal creatinine, the oliguria suggests renal vasospasm from preeclampsia itself 3

When Furosemide IS Appropriate

The only indication for furosemide in preeclampsia is pulmonary edema, where nitroglycerine is actually preferred as first-line therapy 4. Your patient has no mention of pulmonary edema, making diuretics inappropriate 4.

Postpartum Furosemide: Different Context

After delivery, furosemide may have a role in accelerating blood pressure recovery. A 2025 randomized trial found that 40 mg/day oral furosemide for 5 days postpartum reduced mean blood pressure and time to BP control in women with severe preeclampsia 5. However, this is a postpartum intervention only—not applicable to your antepartum patient 5.

Magnesium-Induced Diuresis: A Recognized Phenomenon

Interestingly, magnesium sulfate itself can cause a solute diuresis. A 2017 case report documented polyuria exceeding 18 liters in 48 hours during magnesium infusion, with magnesium contributing 30% of urinary solute 6. This means your patient's decreased urine output is particularly concerning and warrants close monitoring, but the solution is not adding furosemide—it's ensuring adequate (but not excessive) hydration within the 60–80 mL/hr limit and considering whether magnesium toxicity is developing 1, 6.

Critical Monitoring Algorithm

Before each magnesium dose, verify ALL of the following:

1. Patellar reflex present 1, 2
2. Respiratory rate ≥12–16/min 1, 2
3. Urine output ≥25–30 mL/hr (≥100 mL in preceding 4 hours) 1, 2
4. Oxygen saturation adequate 1

If ANY parameter is abnormal:

• Hold magnesium immediately 2
• Have IV calcium gluconate at bedside as antidote 2
• Consider serum magnesium level (therapeutic range 2.5–5 mEq/L; toxicity risk >4 mEq/L) 2

Duration of Therapy

Plan to continue magnesium for 24 hours postpartum after delivery 3, 7. While one Latin American study suggested shorter courses might suffice, a 2022 meta-analysis found the two cases of postpartum eclampsia occurred in women receiving <24 hours of magnesium, supporting the traditional 24-hour postpartum regimen 7.

Common Pitfalls to Avoid

• Do not use furosemide to "improve" urine output in antepartum preeclampsia—this worsens intravascular depletion 3
• Do not exceed 60–80 mL/hr total IV fluids even if urine output is low—pulmonary edema is a greater immediate risk than oliguria 3, 1
• Do not continue magnesium if reflexes are absent—this is the earliest sign of toxicity 1, 2
• Do not assume normal creatinine means normal renal function—magnesium clearance depends on GFR, which may be reduced in preeclampsia despite normal creatinine 2, 8