A 29-year-old primigravida at 36 weeks develops sudden-onset tonic-clonic convulsions preceded by headache and visual disturbances. BP is 170/112 mmHg. The patient is managed with magnesium sulphate (loading dose 4 g IV over 15 minutes). The maintenance infusion of MgSO₄ is 1 g/hour IV. Six hours later, the patient develops loss of deep tendon reflexes (DTR). The first sign of magnesium toxicity and the antidote are:

• A Loss of DTR (first sign); antidote is calcium gluconate 1 g IV ✓
• B Respiratory depression (first sign); antidote is neostigmine IV
• C Cardiac arrest (first sign); antidote is calcium chloride IV
• D Oliguria (first sign); antidote is furosemide IV

Explanation

Magnesium toxicity occurs in a predictable sequence: loss of deep tendon reflexes (patellar reflex disappears at serum Mg ~7–10 mEq/L, first sign), followed by respiratory depression (Mg ~10–13 mEq/L), and then cardiac arrest (Mg > 15 mEq/L). The antidote for MgSO₄ toxicity is calcium gluconate 1 g IV (10 mL of 10% solution) given slowly over 2–3 minutes, which competes with magnesium at calcium channels. Monitoring should include hourly DTR checks, urine output (> 25 mL/hour), and respiratory rate (> 12/min).

Reference: Williams Obstetrics, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.