A woman with eclampsia is maintained on an intravenous magnesium sulfate infusion. Her obstetrician prescribes sublingual nifedipine for a BP of 165/112 mmHg. What is the principal concern with combining these two agents?

• A Nifedipine accelerates clearance of magnesium, lowering its protective level
• B Nifedipine triggers uterine tachysystole and fetal distress
• C The combination precipitates acute pulmonary oedema through sodium retention
• D Additive calcium-channel blockade potentiates magnesium-induced neuromuscular weakness and hypotension ✓

Explanation

Magnesium itself blocks calcium channels, and nifedipine blocks L-type calcium channels in vascular smooth muscle. Given together, their effects add, producing exaggerated maternal hypotension and potentiation of magnesium's neuromuscular blockade, which can depress reflexes and respiration. Many units therefore prefer labetalol or hydralazine for acute BP control while magnesium is running, or monitor very closely if nifedipine must be used. Nifedipine does not alter magnesium clearance and is otherwise safe in pregnancy.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.