A nurse is monitoring a client with severe preeclampsia receiving magnesium sulfate. Which assessment finding requires immediate intervention?

Magnesium sulfate toxicity is a serious complication in obstetric care, especially in clients with severe preeclampsia. Magnesium sulfate is used to prevent seizures by depressing neuromuscular transmission and central nervous system activity. Toxicity manifests with hyporeflexia, respiratory depression, and cardiac arrest. Therapeutic serum magnesium levels range from 4.8 to 8.4 mg/dL. Toxicity risk increases with renal impairment, as magnesium is excreted renally. Early signs include loss of deep tendon reflexes and respiratory rate below 12 breaths per minute. Late signs include hypotension and bradycardia.

Rationale for correct answer

1. A respiratory rate of 10 breaths per minute indicates respiratory depression, a hallmark of magnesium sulfate toxicity. Magnesium inhibits acetylcholine release at neuromuscular junctions, leading to muscle weakness and decreased respiratory drive. Immediate intervention is required to prevent progression to apnea and cardiac arrest.

Rationale for incorrect answers

2. The presence of 2+ deep tendon reflexes is a normal finding and suggests that magnesium levels are within the therapeutic range. Loss of reflexes is an early sign of toxicity, so preserved reflexes indicate no immediate concern. Reflexes are monitored to assess neuromuscular function during magnesium therapy.

3. Urine output of 40 mL/hour is above the minimum threshold of 30 mL/hour required for safe magnesium excretion. Although renal function must be monitored closely, this output does not indicate renal compromise or toxicity. Magnesium clearance depends on glomerular filtration, and this rate is adequate.

4. Mild flushing of the skin is a common and benign side effect of magnesium sulfate due to vasodilation. It does not indicate toxicity and typically resolves spontaneously. Flushing is related to peripheral vascular effects and does not require intervention.

Take home points

• Magnesium sulfate toxicity presents with respiratory depression, hyporeflexia, and cardiac arrest.
• Therapeutic magnesium levels range from 4.8 to 8.4 mg/dL.
• Deep tendon reflexes are the earliest clinical indicator of toxicity.
• Renal function must be monitored to prevent magnesium accumulation.

Severe preeclampsia is a hypertensive disorder of pregnancy characterized by blood pressure ≥160/110 mmHg, proteinuria ≥0.3 g/24h, and signs of end-organ dysfunction. It poses a high risk for eclampsia, defined by new-onset seizures. The cornerstone of seizure prophylaxis is magnesium sulfate, which acts as a central nervous system depressant by blocking neuromuscular transmission and reducing cerebral vasospasm. It is not an antihypertensive but is essential for preventing convulsions. Therapeutic serum magnesium levels range from 4.8 to 8.4 mg/dL. Toxicity presents with hyporeflexia, respiratory depression, and cardiac arrest, especially in renal impairment.

Rationale for correct answer

3. Magnesium sulfate is the drug of choice for seizure prophylaxis in severe preeclampsia. It reduces the risk of progression to eclampsia by stabilizing neuronal membranes and decreasing excitability. It does not lower blood pressure but is critical in preventing maternal morbidity from seizures.

Rationale for incorrect answers

1. Labetalol is a beta-blocker used to manage acute hypertension in preeclampsia. It reduces systemic vascular resistance but has no anticonvulsant properties. It is not used for seizure prevention and does not affect neuronal excitability.

2. Hydralazine is a direct vasodilator used to lower diastolic blood pressure in hypertensive emergencies. It is effective for blood pressure control but lacks central nervous system depressant effects. It does not prevent seizures and is not the standard for eclampsia prophylaxis.

4. Nifedipine is a calcium channel blocker used for blood pressure reduction in pregnancy. It relaxes vascular smooth muscle but does not cross the blood-brain barrier to exert anticonvulsant effects. It is not indicated for seizure prevention in preeclampsia.

Take home points

• Magnesium sulfate is the first-line agent for seizure prophylaxis in severe preeclampsia.
• Antihypertensives like labetalol, hydralazine, and nifedipine manage blood pressure but do not prevent seizures.
• Eclampsia is defined by new-onset seizures in a preeclamptic patient.
• Magnesium toxicity presents with hyporeflexia and respiratory depression.