MgSO4 — Toxicity Recognition

Magnesium sulfate toxicity follows a dose-dependent, predictable sequence. Therapeutic serum levels for seizure prophylaxis are 4–7 mEq/L. The first warning sign is loss of deep tendon reflexes (DTRs), which disappears around 7–10 mEq/L. This is the clinical tripwire — if patellar reflexes are absent, the infusion must stop immediately. As levels climb to 10–13 mEq/L, respiratory depression sets in; respiratory rate drops below 12 breaths/min. Above 15 mEq/L, cardiac conduction fails — bradycardia, widened QRS, and cardiac arrest become the threat. The progression is always reflexes first, then respirations, then cardiac. You don't need a serum level to recognize early toxicity — absent DTRs and slowing respirations are your bedside alarms. Urine output below 30 mL/hr signals impaired renal clearance because magnesium is excreted entirely by the kidneys; oliguria means the drug is accumulating even if the dose hasn't changed. Other signs include flushing, lethargy, slurred speech, and muscle weakness. The patient may describe feeling heavy or unable to lift her arms. These subjective complaints are real warnings, not anxiety.