In a patient with pre-eclampsia with severe features at 33 weeks, magnesium sulfate is being given for seizure prophylaxis. The infusion rate is 2 g/hour. Which clinical sign is the EARLIEST indicator of magnesium toxicity before respiratory depression occurs?
- A Respiratory rate <12/min
- B Loss of deep tendon reflexes ✓
- C Urine output <25 mL/hour
- D Serum magnesium >7 mEq/L
Correct answer: B. Loss of deep tendon reflexes
Explanation
Loss of patellar deep tendon reflexes is the first clinical sign of magnesium toxicity, occurring at serum magnesium levels of approximately 7–10 mEq/L. Respiratory depression occurs at higher levels (>10 mEq/L) and cardiac arrest at >15 mEq/L. Checking reflexes hourly is the standard bedside monitoring tool; if reflexes are absent, the infusion must be stopped immediately.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.