MgSO4 — Monitoring & Reversal
Magnesium sulfate can stop seizures and contractions — but it can also stop breathing. The difference between therapeutic and lethal comes down to three assessments and one antidote you must have at the bedside.
Core Concept
When a client is on a magnesium sulfate infusion, nursing monitoring follows the triad: respiratory rate, deep tendon reflexes (DTRs), and urine output. Respirations must stay ≥12 breaths/min — respiratory depression is the earliest directly lethal sign of toxicity. DTRs (typically patellar) must remain present; loss of reflexes is the earliest clinical warning that the serum level is climbing toward danger (usually >9 mg/dL). Urine output must be ≥30 mL/hr because magnesium is renally excreted — oliguria causes rapid accumulation. Therapeutic serum magnesium range is 4–7 mg/dL. Monitoring frequency: vitals and DTRs every 1–2 hours, continuous pulse oximetry, and strict I&O. If toxicity develops (absent DTRs, respirations <12, cardiac arrest), the nurse stops the infusion immediately and administers the antidote: calcium gluconate 1 g IV push over 3 minutes. Calcium gluconate must be kept at the bedside before the infusion even begins — not retrieved from the pharmacy when trouble starts. Fetal monitoring is also continuous because magnesium crosses the placenta; the fetus may show decreased variability and reduced tone at delivery.
Watch Out For
Don't confuse calcium gluconate (the mag antidote) with calcium chloride — calcium gluconate is the form used in obstetric magnesium reversal because it is less irritating to veins and less likely to cause tissue necrosis. Students often think urine output monitoring is secondary — it is not; impaired renal function is the fastest route to accumulation. Loss of DTRs precedes respiratory arrest; if reflexes are gone, the next domino is breathing.
Clinical Pearl
Before you hang mag, put calcium gluconate at the bedside. Think: 'No reflexes, no respirations, no urine — no more mag.'
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