A woman with severe pre-eclampsia at 34 weeks develops oliguria (25 mL/hour) after receiving IV magnesium sulfate. Serum magnesium level is 9.2 mg/dL. Which early sign of magnesium toxicity is FIRST lost at approximately this serum level?

• A Respiratory drive
• B Cardiac conduction
• C Level of consciousness
• D Patellar reflexes ✓

Explanation

Patellar (deep tendon) reflexes are lost at serum magnesium levels of 7–10 mg/dL, making them the earliest clinical indicator of magnesium toxicity and the primary bedside monitoring tool. Respiratory depression occurs at levels of 10–13 mg/dL, cardiac arrest follows at levels above 15 mg/dL. Since this patient has a level of 9.2 mg/dL and oliguria (reducing renal clearance of magnesium), immediate clinical monitoring of patellar reflexes and respiratory rate is essential, and magnesium infusion should be held. Calcium gluconate 1 g IV is the antidote.

Reference: Williams Obstetrics, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.